U.S. patent application number 09/962140 was filed with the patent office on 2002-01-31 for radiation image information read-out method and system.
This patent application is currently assigned to FUJI PHOTO FILM CO., LTD.. Invention is credited to Arakawa, Satoshi, Karasawa, Hiroyuki, Yasuda, Hiroaki.
Application Number | 20020011577 09/962140 |
Document ID | / |
Family ID | 46257467 |
Filed Date | 2002-01-31 |
United States Patent
Application |
20020011577 |
Kind Code |
A1 |
Arakawa, Satoshi ; et
al. |
January 31, 2002 |
Radiation image information read-out method and system
Abstract
An image signal representing radiation image information on an
object stored on a stimulable phosphor sheet is read out by
scanning the stimulable phosphor sheet with a stimulating light
beam and photoelectrically detecting light emitted from the
stimulable phosphor sheet upon stimulation thereof by a
photodetector having a photoelectric surface. A variable
transmittance medium whose transmittance to the light emitted from
the stimulable phosphor sheet upon stimulation thereof is variable
continuously or stepwise is inserted into the optical path of the
light between the stimulable phosphor sheet and the photoelectric
surface of the photodetector The transmittance of the variable
transmittance medium is changed according to the amount of light
emitted from the stimulable phosphor sheet upon stimulation thereof
so that the photoelectric surface is not saturated by an excessive
amount of light impinging thereupon.
Inventors: |
Arakawa, Satoshi;
(Kanagawa-ken, JP) ; Karasawa, Hiroyuki;
(Kanagawa-ken, JP) ; Yasuda, Hiroaki;
(Kanagawa-ken, JP) |
Correspondence
Address: |
SUGHRUE MION ZINN MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, NW
Washington
DC
20037-3213
US
|
Assignee: |
FUJI PHOTO FILM CO., LTD.
|
Family ID: |
46257467 |
Appl. No.: |
09/962140 |
Filed: |
September 26, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09962140 |
Sep 26, 2001 |
|
|
|
09120304 |
Jul 22, 1998 |
|
|
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Current U.S.
Class: |
250/584 |
Current CPC
Class: |
G01T 1/2014
20130101 |
Class at
Publication: |
250/584 |
International
Class: |
G03B 042/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 22, 1997 |
JP |
195311/1997 |
Aug 25, 1997 |
JP |
228158/1997 |
Claims
What is claimed is:
1. A radiation image information read-out method for obtaining an
image signal representing radiation image information on an object
stored on a stimulable phosphor sheet by scanning the stimulable
phosphor sheet with a stimulating light beam and photoelectrically
detecting light emitted from the stimulable phosphor sheet upon
stimulation thereof by a photodetector having a photoelectric
surface, wherein the improvement comprises the steps of: inserting
a variable transmittance medium whose transmittance to the light
emitted from the stimulable phosphor sheet upon stimulation thereof
is variable continuously or stepwise into the optical path of the
light between the stimulable phosphor sheet and the photoelectric
surface of the photodetector, and changing the transmittance of the
variable transmittance medium according to the amount of light
emitted from the stimulable phosphor sheet upon stimulation thereof
so that the photoelectric surface is not saturated by an excessive
amount of light impinging thereupon.
2. A radiation image information read-out method as defined in
claim 1 in which the variable transmittance medium is an
electrochromic element.
3. A radiation image information read-out method as defined in
claim 1 in which the variable transmittance medium is a NCAP type
liquid crystal element.
4. A radiation image information read-out method as defined in
claim 1 in which the variable transmittance medium is a ND filter
system.
5. A radiation image information read-out method as defined in
claim 1 in which the amount of light emitted from the stimulable
phosphor sheet upon stimulation thereof is determined according to
the radiographing menu when the radiation image information on the
object was recorded on the stimulable phosphor sheet.
6. A radiation image information read-out method for obtaining an
image signal representing radiation image information on an object
stored on a stimulable phosphor sheet by scanning the stimulable
phosphor sheet with a stimulating light beam and photoelectrically
detecting light emitted from the stimulable phosphor sheet upon
stimulation thereof by a photodetector having a photoelectric
surface, wherein the improvement comprises the step of changing the
amount of the stimulating light impinging upon the stimulable
phosphor sheet continuously or stepwise according to the amount of
light emitted from the stimulable phosphor sheet upon stimulation
thereof so that the photoelectric surface is not saturated by an
excessive amount of light impinging thereupon.
7. A radiation image information read-out method as defined in
claim 6 in which the amount of light emitted from the stimulable
phosphor sheet upon stimulation thereof is determined according to
the radiographing menu when the radiation image information on the
object was recorded on the stimulable phosphor sheet.
8. A radiation image information read-out system for obtaining an
image signal representing radiation image information on an object
stored on a stimulable phosphor sheet by scanning the stimulable
phosphor sheet with a stimulating light beam and photoelectrically
detecting light emitted from the stimulable phosphor sheet upon
stimulation thereof by a photodetector having a photoelectric
surface, wherein the improvement comprises. a variable
transmittance medium which is variable continuously or stepwise in
transmittance to the light emitted from the stimulable phosphor
sheet upon stimulation thereof and is inserted into the optical
path of the light between the stimulable phosphor sheet and the
photoelectric surface of the photodetector, and a transmittance
changing means which changes the transmittance of the variable
transmittance medium according to the amount of light emitted from
the stimulable phosphor sheet upon stimulation thereof so that the
photoelectric surface is not saturated by an excessive amount of
light impinging thereupon.
9. A radiation image information read-out system as defined in
claim 8 in which the variable transmittance medium is an
electrochromic element.
10. A radiation image information read-out system as defined in
claim 8 in which the variable transmittance medium is a NCAP type
liquid crystal element.
11. A radiation image information read-out system as defined in
claim 8 in which the variable transmittance medium is a ND filter
system.
12. A radiation image information read-out system as defined in
claim 8 in which the amount of light emitted from the stimulable
phosphor sheet upon stimulation thereof is determined according to
the radiographing menu when the radiation image information on the
object was recorded on the stimulable phosphor sheet.
13. A radiation image information read-out system for obtaining an
image signal representing radiation image information on an object
stored on a stimulable phosphor sheet by scanning the stimulable
phosphor sheet with a stimulating light beam and photoelectrically
detecting light emitted from the stimulable phosphor sheet upon
stimulation thereof by a photodetector having a photoelectric
surface, wherein the improvement comprises a means for changing the
amount of the stimulating light impinging upon the stimulable
phosphor sheet continuously or stepwise according to the amount of
light emitted from the stimulable phosphor sheet upon stimulation
thereof so that the photoelectric surface is not saturated by an
excessive amount of light impinging thereupon.
14. A radiation image information read-out system as defined in
claim 13 in which the amount of light emitted from the stimulable
phosphor sheet upon stimulation thereof is determined according to
the radiographing menu when the radiation image information on the
object was recorded on the stimulable phosphor sheet.
15. A radiation image information read-out method for obtaining an
image signal representing radiation image information on an object
stored on a stimulable phosphor sheet by exposing the stimulable
phosphor sheet to stimulating light and photoelectrically detecting
light emitted from the stimulable phosphor sheet upon stimulation
thereof by a photodetector, wherein the improvement comprises the
step of setting the sensitivity of the photodetector so that the
amount of light emitted from a blank portion on the stimulable
phosphor sheet becomes larger than that corresponding to the upper
limit of the operable range of the photodetector, and controlling
the irradiation energy of the stimulating light so that the level
of a signal component obtained by the photodetector by reading the
light emitted from the blank portion is minimized in the range
higher than the upper limit of the read-out signal level range
corresponding to the operable range of the photodetector.
16. A radiation image information read-out method as defined in
claim 15 in which preliminary read-out is effected prior to final
read-out of the radiation image information on the object and the
sensitivity of the photodetector is set on the basis of the image
information obtained by the preliminary read-out.
17. A radiation image information read-out method as defined in
claim 15 in which a plurality of levels of the sensitivity of the
photodetector is registered in advance in relation to different
radiographing menus and the sensitivity of the photodetector is set
to the sensitivity level corresponding to the radiographing menu
for the radiation image information to be read out.
18. A radiation image information read-out system for obtaining an
image signal representing radiation image information on an object
stored on a stimulable phosphor sheet comprising a stimulating
light projecting means which exposes the stimulable phosphor sheet
to stimulating light thereby causing the stimulable phosphor sheet
to emit light in proportion to the amount of energy stored thereon
and a photodetector which detects the light emitted from the
stimulable phosphor sheet upon stimulation thereof, wherein the
improvement comprises a sensitivity setting means which sets the
sensitivity of the photodetector so that the amount of light
emitted from a blank portion on the stimulable phosphor sheet
becomes larger than that corresponding to the upper limit of the
operable range of the photodetector, and a stimulating energy
control means which controls the irradiation energy of the
stimulating light so that the level of a signal component obtained
by the photodetector by reading the light emitted from the blank
portion is minimized in the range higher than the upper limit of
the read-out signal level range corresponding to the operable range
of the photodetector.
19. A radiation image information read-out system as defined in
claim 18 further comprising a preliminary read-out means which
effects preliminary read-out prior to final read-out of the
radiation image information and the sensitivity setting means sets
the sensitivity of the photodetector on the basis of the image
information obtained by the preliminary read-out.
20. A radiation image information read-out system as defined in
claim 18 further comprising a means for registering a plurality of
levels of the sensitivity of the photodetector in relation to
different radiographing menus, wherein the sensitivity setting
means sets the sensitivity of the photodetector to the sensitivity
level corresponding to the radiographing menu for the radiation
image information to be read out.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a method of and a system for
reading out radiation image information stored on a stimulable
phosphor sheet in which the stimulable phosphor sheet is exposed to
stimulating rays, thereby causing it to emit light in proportion to
the amount of energy stored thereon during its exposure to the
radiation and the light emitted by the stimulable phosphor sheet
upon stimulation thereof is photoelectrically detected and
converted into an electric image signal representing the radiation
image information, and more particularly to an improvement for
preventing saturation of a photodetector having a photoelectric
surface.
[0003] 2. Description of the Related Art
[0004] When certain kinds of phosphors are exposed to radiation
such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode
rays or ultraviolet rays, they store a part of the energy of the
radiation. Then, when the phosphor which has been exposed to the
radiation is exposed to stimulating rays such as visible light,
light is emitted from the phosphor in proportion to the stored
energy of the radiation. A phosphor exhibiting such properties is
referred to as "a stimulable phosphor". It has been known to use
stimulable phosphors in radiation image recording and reproducing
systems (sometimes referred to as "computed radiography").
Specifically, a radiation image of an object, such as a human body,
is recorded on a stimulable phosphor sheet (a recording medium
provided with a layer of the stimulable phosphor). The stimulable
phosphor sheet, on which the radiation image has been stored, is
then exposed to stimulating rays, such as a laser beam, which cause
it to emit light in proportion to the amount of energy stored
thereon during its exposure to the radiation. The light emitted by
the stimulable phosphor sheet, upon stimulation thereof, is
photoelectrically detected and converted into an electric image
signal. The image signal is used for reproducing the radiation
image of the object as a visible image on a recording medium such a
photosensitive material or a display such as a CRT. See Japanese
Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395,
56(1981)-11397 and the like.
[0005] The radiation image recording and reproducing system is
practically advantageous in that as compared with conventional
radiographies using silver halide film, an image can be recorded
over an extremely wide radiation exposure range.
[0006] When reading out radiation image information from the
stimulable phosphor sheet in the radiation image recording and
reproducing system, for instance, a light beam such as a laser beam
is caused to two-dimensionally scan the stimulable phosphor sheet
storing thereon a radiation image, and the light emitted from the
stimulable phosphor sheet upon stimulation thereof is transmitted
to a photodetector through an optical guide having a light inlet
end face extending along the main scanning line. The photodetector
photoelectrically detects in time the light emitted from the
stimulable phosphor sheet upon stimulation thereof and an image
signal made up of image signal components for respective picture
elements is obtained.
[0007] The photodetectors generally employed in such systems
include those utilizing an internal photoelectric effect, e.g., a
phototransistor, a photodiode and the like, and those having an
photoelectric surface and utilizing a photoemissive effect on the
photoelectric surface, e.g., a photomultiplier. When those having a
photoelectric surface such as a photomultiplier are employed in the
above system, the following provision is generally made in order to
improve sensitivity of the photodetector.
[0008] That is, the photodetector of this type is generally
provided with an optical guide for collecting the light, emitted
from the stimulable phosphor sheet upon stimulation thereof, to the
photoelectric surface. The provision involves increasing light
collecting efficiency to the photoelectric surface by improving the
light collecting performance of the optical guide and/or employment
of a photoelectric surface made of a material such as bialkali,
e.g., Sb--K--Cs, which is high in quantum efficiency.
[0009] However, increasing the sensitivity of a photodetector with
a photoelectric surface gives rise to another problem that a
saturation phenomenon, that the sensitivity of the photodetector
deteriorates for a while after detection of a large amount of
light, is apt to occur at the photoelectric surface and when a
visible image is reproduced on the basis of an image signal
obtained from a saturated photodetector, a ghost image appears and
the image quality deteriorates.
[0010] The system may be employed in general industrial field as
well as a medical field. In the medical field, the object is the
human body and since the irradiation dose does not greatly vary
depending on the part whose radiation image is to be taken, the
amount of light emitted from the stimulable phosphor sheet upon
stimulation thereof does not greatly fluctuate.
[0011] To the contrast, in the general industrial field, where the
system is used in non-destructive inspection of products such as a
cast iron block, an iron block and the like, various kinds of
materials can be the object. Accordingly, in order to obtain
radiation image information suitable for the respective kinds of
materials, the irradiation dose varies depending on the kind of the
object over a very wide range (two to three figures in terms of
dose ratio).
[0012] Thus in the radiation image recording and reproducing system
for use in the general industrial field, the aforesaid saturation
phenomenon is apt to occur, and accordingly, there has been a
demand for a radiation image information read-out system in which
the saturation phenomenon at the photodetector is suppressed with
the sensitivity of the photodetector kept high.
[0013] The intensity of light emitted from the stimulable phosphor
sheet upon stimulation thereof rapidly increases from initiation of
exposure to the stimulating light and is maximized in a short time
(e.g., in several ns) and then is gradually weakened, with the
stimulable phosphor sheet keeping emitting light for a time unique
to the phosphors on the stimulable phosphor sheet after termination
of exposure to the stimulating light. The light emitted from the
stimulable phosphor sheet after termination of exposure to the
stimulating light is generally referred to as "afterglow".
Accordingly when the stimulable phosphor sheet is scanned by the
stimulating light and the light emitted from the stimulable
phosphor sheet is photoelectrically read out in time series, the
afterglow component of picture elements precedingly exposed to the
stimulating light is read in addition to the light emitted from a
given picture element upon stimulation thereof as the radiation
image information component for the picture element, which results
in incomplete separation of image signal components for the picture
elements and deterioration in sharpness of the reproduced image.
Accordingly, when the stimulable phosphors on the stimulable
phosphor sheet exhibit long afterglow, the sharpness of the image
deteriorates to an unacceptable level.
[0014] Such a phenomenon occurs substantially in proportion to the
irradiation dose (radiation energy) of the stimulating light per
unit area of the stimulable phosphor sheet. Accordingly, for
example, when a picture element where the amount of light emitted
upon stimulation is relatively small exists just behind a picture
element where the amount of light emitted upon stimulation is
relatively large, the afterglow component from the large emission
picture element is superposed on the light emitted from the small
emission picture element upon stimulation thereof, and the amount
of light read out as that emitted from the small emission picture
element is increased by the amount of the afterglow. This reduces
the difference between the image signal components for the large
emission picture element and the small emission picture element as
compared with the actual difference therebetween, and accordingly
the reproduced image deteriorates in contrast, i.e., the S/N ratio
of the image signal deteriorates.
[0015] In order to overcome this problem, there has been proposed a
method in which interference between image signal components for
the respective picture elements due to response properties (e.g.,
attenuation properties) of the light emitted from the picture
elements upon stimulation thereof is electrically corrected by
adding, to each of the image signal components in time series
obtained by scanning the stimulable phosphor sheet with the
stimulating light, a differential value of the image signal
component. (See Japanese Patent Publication No. 2(1990)-15154.)
[0016] However this approach is disadvantageous in the following
points. First this approach involves a large amount of calculation
since interference between image signal components for the
respective picture elements is corrected by calculation taking into
account the response properties (e.g., attenuation properties) of
the light emitted from the picture elements upon stimulation
thereof. Second though being able to avoid deterioration in
sharpness and/or contrast of the radiation image, the approach
cannot overcome the problem of deterioration in S/N ratio.
SUMMARY OF THE INVENTION
[0017] In view of the foregoing observations and description, a
first object of the present invention is to provide a radiation
image information read-out system and method in which the
saturation phenomenon at the photodetector is suppressed, thereby
preventing appearance of a ghost image, with the sensitivity of the
photodetector kept high.
[0018] A second object of the present invention is to provide a
radiation image information read-out system and method in which
said problems caused by the afterglow inclusive of deterioration in
S/N ratio can be overcome in a simple manner and the radiation
image information can be accurately read out at a high speed even
if the scanning speed is increased.
[0019] The first object of the present invention can be
accomplished by a radiation image information read-out method and a
radiation image information read-out system in which the absolute
value of the amount of light impinging upon the photoelectric
surface of the photodetector is suppressed by causing the light
emitted from the stimulable phosphor sheet upon stimulation thereof
to pass through a variable transmittance medium whose transmittance
is varied according to the amount of light emitted from the
stimulable phosphor sheet before impinging upon the photoelectric
surface or by changing the amount of stimulating light according to
the amount of light emitted from the stimulable phosphor sheet upon
stimulation thereof.
[0020] That is, in accordance with a first aspect of the present
invention, there is provided a radiation image information read-out
method for obtaining an image signal representing radiation image
information on an object stored on a stimulable phosphor sheet by
scanning the stimulable phosphor sheet with a stimulating light
beam and photoelectrically detecting light emitted from the
stimulable phosphor sheet upon stimulation thereof by a
photodetector having a photoelectric surface, wherein the
improvement comprises the steps of
[0021] inserting a variable transmittance medium whose
transmittance to the light emitted from the stimulable phosphor
sheet upon stimulation thereof is variable continuously or stepwise
into the optical path of the light between the stimulable phosphor
sheet and the photoelectric surface of the photodetector, and
[0022] changing the transmittance of the variable transmittance
medium according to the amount of light emitted from the stimulable
phosphor sheet upon stimulation thereof so that the photoelectric
surface is not saturated by an excessive amount of light impinging
thereupon.
[0023] The stimulating light beam may be visible light, a laser
beam or the like.
[0024] As the variable transmittance medium, an electrochromic
element whose transmittance to light varies according to the
direction of current, a NCAP type liquid crystal element or the
like can be employed as those whose transmittance can be
electrically changed. Further a ND filter system in which a
plurality of optical elements which are different in transmittance
are mechanically selectively inserted into said optical path can
also be employed.
[0025] As a material for electrochromic element, amorphous WO.sub.3
(colorless to blue), IrO.sub.2 (colorless to blue), viologen
(colorless to blue), anthraquinone (colorless to red) or the like
may be used, and may be selected according to the color of light
emitted from the stimulable phosphor sheet.
[0026] The NCAP (Nematic Curvilinear Aligned Phase) type liquid
crystal element is of encapsulated nematic liquid crystals.
Generally the liquid crystal has a rod-like molecule exhibiting
electrooptic anisotropy. The molecules are apt to orient along an
oriented film, and in a normal state where no electric field is
applied to the liquid crystal element, the molecules orient inward
of the capsules and incident light is scattered at the surface and
inside of the liquid crystals depending on the refractive
properties of the crystals, whereby the liquid crystal element
becomes opaque. When an electric field is applied to the liquid
crystal element, the liquid crystals which are positive in
dielectric anisotropy orient in the direction perpendicular to the
surface of the electrodes. When the liquid crystals are of the same
refractive index as the polymer which is outside the capsules and
in which the liquid crystals are dispersed, light travels straight
without being scattered and accordingly the liquid crystal element
becomes transparent.
[0027] The variable transmittance medium may be disposed at the
light inlet end face of the optical guide or at the connection
between the optical guide and the photodetector when the optical
guide is integrated with the photodetector. Though not necessary,
it is preferred that the optical guide be integrated with the
photodetector from the viewpoint of simplicity of handling. In the
case of a ND filter system, the filter must be moved to adjust the
transmittance and accordingly it is not preferred that the ND
filter system is integrated with the photodetector or the like to
such an extent that the ND filter cannot make a relative
movement.
[0028] The degree by which the transmittance of the variable
transmittance medium is to be changed according to the amount of
light emitted from the stimulable phosphor sheet upon stimulation
thereof may be empirically determined and may be tabulated with
respect to the amount of light so that the degree by which the
transmittance of the variable transmittance medium is to be changed
can be known by referring to the table.
[0029] The amount of light emitted from the stimulable phosphor
sheet upon stimulation thereof may be determined by actually
exposing a part of stimulable phosphor sheet, which is limited not
to affect reproduction of the radiation image, to the stimulating
light and detecting the amount of light emitted from the part or
may be determined by estimation based on the irradiation dose to
which the stimulable phosphor sheet was exposed to the radiation
upon taking the radiation image and the energy of the stimulating
light. Such estimation may be input from the outside. In the case
of the radiation image recording and reproducing system for
industrial use, the kinds of objects are larger than in the case of
that for medical use. Accordingly, the radiographing menu such as
the material and the thickness of the object, the radiographing
direction and the like is closely related to the irradiation dose
and on the basis of this fact, the amount of light emitted from the
stimulable phosphor sheet upon stimulation thereof may be
determined according to the radiographing menu.
[0030] Also in the case where the amount of light emitted from the
stimulable phosphor sheet upon stimulation thereof is determined
according to the radiographing menu, the degree by which the
transmittance of the variable transmittance medium is to be changed
may be empirically determined and may be tabulated in relation to
the radiographing menu so that the degree by which the
transmittance of the variable transmittance medium is to be changed
can be known by referring to the table on the basis of the
menu.
[0031] The above description may also be applied to the followings
inventions.
[0032] In accordance with a second aspect of the present invention,
there is provided a radiation image information read-out system for
carrying out the method in accordance with the first aspect of the
present invention. That is, in accordance with the second aspect of
the present invention, there is provided a radiation image
information read-out system for obtaining an image signal
representing radiation image information on an object stored on a
stimulable phosphor sheet by scanning the stimulable phosphor sheet
with a stimulating light beam and photoelectrically detecting light
emitted from the stimulable phosphor sheet upon stimulation thereof
by a photodetector having a photoelectric surface, wherein the
improvement comprises
[0033] a variable transmittance medium which is variable
continuously or stepwise in transmittance to the light emitted from
the stimulable phosphor sheet upon stimulation thereof and is
inserted into the optical path of the light between the stimulable
phosphor sheet and the photoelectric surface of the photodetector,
and
[0034] a transmittance changing means which changes the
transmittance of the variable transmittance medium according to the
amount of light emitted from the stimulable phosphor sheet upon
stimulation thereof so that the photoelectric surface is not
saturated by an excessive amount of light impinging thereupon.
[0035] In accordance with a third aspect of the present invention,
there is provided a radiation image information read-out method for
obtaining an image signal representing radiation image information
on an object stored on a stimulable phosphor sheet by scanning the
stimulable phosphor sheet with a stimulating light beam and
photoelectrically detecting light emitted from the stimulable
phosphor sheet upon stimulation thereof by a photodetector having a
photoelectric surface, wherein the improvement comprises the step
of
[0036] changing the amount of the stimulating light impinging upon
the stimulable phosphor sheet continuously or stepwise according to
the amount of light emitted from the stimulable phosphor sheet upon
stimulation thereof so that the photoelectric surface is not
saturated by an excessive amount of light impinging thereupon.
[0037] The amount of the stimulating light impinging upon the
stimulable phosphor sheet may be changed by directly controlling
the source of the stimulating light to change the amount of
stimulating light emitted from the source, or by inserting a
variable transmittance medium whose transmittance to the
stimulating light is variable continuously or stepwise into the
optical path of the stimulating light from the stimulating light
source to the stimulable phosphor sheet and changing the
transmittance of the variable transmittance medium with the amount
of stimulating light emitted from the source unchanged. The
variable transmittance medium may be an electrochromic element, a
NCAP type liquid crystal element, a ND filter system or the like.
When the amount of the stimulating light impinging upon the
stimulable phosphor sheet is changed, the levels of the signal
representing detection of the leading end of the sheet (in the
sub-scanning direction) and the signal representing detection of
the scanning starting point (in the main scanning direction) are
changed. Accordingly it is preferred that the gains of such signals
be automatically adjusted.
[0038] The above description may also be applied to the followings
inventions.
[0039] In accordance with a fourth aspect of the present invention,
there is provided a radiation image information read-out system for
obtaining an image signal representing radiation image information
on an object stored on a stimulable phosphor sheet by scanning the
stimulable phosphor sheet with a stimulating light beam and
photoelectrically detecting light emitted from the stimulable
phosphor sheet upon stimulation thereof by a photodetector having a
photoelectric surface, wherein the improvement comprises
[0040] a means for changing the amount of the stimulating light
impinging upon the stimulable phosphor sheet continuously or
stepwise according to the amount of light emitted from the
stimulable phosphor sheet upon stimulation thereof so that the
photoelectric surface is not saturated by an excessive amount of
light impinging thereupon.
[0041] In the radiation image information read-out method and
system of the first and second aspects of the present invention,
the level of the amount of light impinging upon the photoelectric
surface of the photodetector is suppressed not to saturate the
photoelectric surface by changing the transmittance of the variable
transmittance medium disposed in the optical path of the light
emitted from the stimulable phosphor sheet upon stimulation thereof
between the stimulable phosphor sheet and the photoelectric surface
according to an estimated or measured amount of the light emitted
from the stimulable phosphor sheet upon stimulation thereof.
[0042] For example, when the amount of light is large, the
transmittance of the variable transmittance medium is reduced so
that a smaller part of the light emitted from the stimulable
phosphor sheet can reach the photoelectric surface, whereby
generation of image signal components which can produce a ghost
image due to saturation of the photoelectric surface can be
suppressed.
[0043] On the other hand, when the amount of light is small, the
transmittance of the variable transmittance medium is kept high so
that an image signal can be obtained at a high sensitivity and
deterioration in S/N ratio can be suppressed. In this case, since
the amount of light emitted from the stimulable phosphor sheet upon
stimulation thereof is originally small, there is no fear that the
photoelectric surface is saturated.
[0044] In the radiation image information read-out method and
system of the third and fourth aspects of the present invention,
the level of the amount of light impinging upon the photoelectric
surface of the photodetector is suppressed not to saturate the
photoelectric surface by changing the amount of stimulating light
impinging upon the stimulable phosphor sheet according to an
estimated or measured amount of the light emitted from the
stimulable phosphor sheet upon stimulation thereof.
[0045] For example, when the amount of light expected to be emitted
from the stimulable phosphor sheet upon stimulation thereof is
large, the amount of stimulating light impinging upon the
stimulable phosphor sheet is reduced so that a smaller amount of
light is emitted from the stimulable phosphor sheet, whereby the
level of the amount of light impinging upon the photoelectric
surface of the photodetector is suppressed and generation of image
signal components which can produce a ghost image due to saturation
of the photoelectric surface can be suppressed.
[0046] On the other hand, when the amount of light expected to be
emitted from the stimulable phosphor sheet upon stimulation thereof
is small, the amount of stimulating light is not reduced (if
possible, may be increased, for instance, by controlling the
stimulating light source) and accordingly, the amount of light
emitted from the stimulable phosphor sheet is not reduced, whereby
an image signal can be obtained at a high sensitivity and
deterioration in S/N ratio can be suppressed. In this case, since
the amount of light emitted from the stimulable phosphor sheet upon
stimulation thereof is originally small, there is no fear that the
photoelectric surface is saturated.
[0047] In the radiation image information read-out methods and
systems of first to fourth aspects of the present invention, when
the amount of light emitted from the stimulable phosphor sheet upon
stimulation thereof is estimated on the basis of the radiographing
menu, necessity of actually detecting the amount of light is
eliminated and algorithm for the system can be simplified.
[0048] In accordance with a fifth aspect of the present invention,
there is provided a radiation image information read-out method for
obtaining an image signal representing radiation image information
on an object stored on a stimulable phosphor sheet by exposing the
stimulable phosphor sheet to stimulating light and
photoelectrically detecting light emitted from the stimulable
phosphor sheet upon stimulation thereof by a photodetector, wherein
the improvement comprises the step of
[0049] setting the sensitivity of the photodetector so that the
amount of light emitted from a blank portion on the stimulable
phosphor sheet bearing thereon no radiation image information of
the object becomes larger than that corresponding to the upper
limit of the operable range of the photodetector, and
[0050] controlling the irradiation energy of the stimulating light
so that the level of a signal component obtained by the
photodetector by reading the light emitted from the blank portion
is minimized in the range higher than the upper limit of the
read-out signal level range corresponding to the operable range of
the photodetector.
[0051] The "blank portion" on the stimulable phosphor sheet bearing
thereon no radiation image information of the object is a portion
which, when the radiation image was recorded, was directly exposed
to the radiation without passing through the object. Further "the
operable range of the photodetector" is a range of the amount of
light in which the photodetector can correctly convert the amount
of light into an electric signal component.
[0052] The expression "controlling the irradiation energy of the
stimulating light" means to control the effective energy of the
stimulating light to which the stimulable phosphor sheet is exposed
per unit area thereof. The irradiation energy of the stimulating
light can be controlled, for instance, by attenuating the
stimulating light from the stimulating light source such as a laser
by an acoustooptic modulator or the like provided on the optical
path of the stimulating light, or by, in the case where the
stimulating light source is a semiconductor laser, controlling the
input voltage to the semiconductor laser, or by increasing the
scanning speed of the stimulating light beam.
[0053] In the radiation image information read-out method of the
fifth aspect, it is preferred that "preliminary read-out" be
effected prior to "final read-out" and the sensitivity of the
photodetector be set on the basis of the image information obtained
by the preliminary read-out.
[0054] As disclosed, for instance, in Japanese Unexamined Patent
Publication Nos. 58(1983)-67241, 58(1983)-67243 and 58(1983)-83937,
the "preliminary read-out" is a well-known technique in which brief
of the radiation image information stored on a stimulable phosphor
sheet is read out prior to the "final read-out" by exposing the
stimulable phosphor sheet to stimulating light having a lower level
than that used in the final read-out and reading out light emitted
from the stimulable phosphor sheet upon stimulation by the lower
level stimulating light.
[0055] A plurality of levels of the sensitivity of the
photodetector may be registered in advance in relation to different
radiographing menus and the sensitivity of the photodetector may be
set to the sensitivity level corresponding to the radiographing
menu for the radiation image information to be read out.
[0056] The "radiographing menu" means the part of the object, the
method of radiographing and the like and includes, for instance,
chest radiographing, head radiographing, angiography and the
like.
[0057] In accordance with a sixth aspect of the present invention,
there is provided a radiation image information read-out system for
obtaining an image signal representing radiation image information
on an object stored on a stimulable phosphor sheet comprising a
stimulating light projecting means which exposes the stimulable
phosphor sheet to stimulating light thereby causing the stimulable
phosphor sheet to emit light in proportion to the amount of energy
stored thereon and a photodetector which detects the light emitted
from the stimulable phosphor sheet upon stimulation thereof,
wherein the improvement comprises
[0058] a sensitivity setting means which sets the sensitivity of
the photodetector so that the amount of light emitted from a blank
portion on the stimulable phosphor sheet bearing thereon no
radiation image information of the object becomes larger than that
corresponding to the upper limit of the operable range of the
photodetector, and
[0059] a stimulating energy control means which controls the
irradiation energy of the stimulating light so that the level of a
signal component obtained by the photodetector by reading the light
emitted from the blank portion is minimized in the range higher
than the upper limit of the read-out signal level range
corresponding to the operable range of the photodetector.
[0060] It is preferred that the radiation image information
read-out system of the sixth aspect be provided with a preliminary
read-out means which effects preliminary read-out prior to final
read-out of the radiation image information and the sensitivity
setting means sets the sensitivity of the photodetector on the
basis of the image information obtained by the preliminary
read-out.
[0061] Further it is preferred that the radiation image information
read-out system of the sixth aspect be provided with a means for
registering a plurality of levels of the sensitivity of the
photodetector in relation to different radiographing menus and the
sensitivity setting means sets the sensitivity of the photodetector
to the sensitivity level corresponding to the radiographing menu
for the radiation image information to be read out.
[0062] In the radiation image information read-out method and
system of the fifth and sixth embodiments of the present invention,
since the sensitivity of the photodetector is set in the aforesaid
manner and the irradiation energy of the stimulating light is
controlled in the aforesaid manner, the afterglow component of the
light emitted from the blank portion is reduced with the light
emitted from the portion other than the blank portion kept at the
normal level. Since the afterglow component itself is reduced,
noise due to the afterglow component can be reduced and the S/N
ratio of the read-out signal can be improved. Further since the
light emitted from the portion other than the blank portion is kept
at the normal level, the image information for the object necessary
for diagnosis can be obtained at a proper quality.
[0063] When the preliminary read-out is effected prior to the final
read-out and the sensitivity of the photodetector is set on the
basis of the image information obtained by the preliminary
read-out, setting of read-out condition is facilitated. Further
when the sensitivity of the photodetector is set on the basis of
the radiographing menu, setting of read-out condition is also
facilitated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0064] FIG. 1 is a schematic view for illustrating the basic
arrangement of a radiation image information read-out system,
[0065] FIG. 2 is a view showing an important part of a radiation
image information read-out system in accordance with a first
embodiment of the present invention,
[0066] FIG. 3 is a view showing an important part of a radiation
image information read-out system in accordance with a second
embodiment of the present invention,
[0067] FIG. 4 is a schematic view showing a radiation image
information read-out system in accordance with a third embodiment
of the present invention,
[0068] FIG. 5 is a schematic view showing a radiation image
information read-out system in accordance with a fourth embodiment
of the present invention,
[0069] FIG. 6 is a view for illustrating the operation of the
preliminary read-out means employed in the radiation image
information read-out system in accordance with the fourth
embodiment,
[0070] FIGS. 7A and 7B are views showing the relation between the
amount of light emitted from the stimulable phosphor sheet and the
level of the signal output from the photodetector, and
[0071] FIGS. 8A to 8E are views for illustrating the method of
controlling the energy of the stimulating light.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0072] In FIG. 1, a radiation image information read-out system to
which the present invention is to be applied comprises an endless
belt 40 which is driven by an electric motor (not shown) to convey
a stimulable phosphor sheet 30 placed thereon in the direction of
arrow Y (the sub-scanning direction), a laser 11 which is located
above the stimulable phosphor sheet 30 and emits a stimulating
laser beam L for stimulating the stimulable phosphor sheet, a
rotating polygonal mirror 12 which deflects the laser beam L, an
electric motor 13 for rotating the polygonal mirror 12 and a
scanning lens (f.theta. lens) 14 which focuses the laser beam L,
deflect by the polygonal mirror 12, on the stimulable phosphor
sheet 30 and causes it to scan the stimulable phosphor sheet 30 at
a constant speed.
[0073] An optical guide 21 is located above the stimulable phosphor
sheet 30 close to the scanning line of the laser beam L to collect
light M emitted from the upper surface of the stimulable phosphor
sheet 30 upon stimulation thereof by the laser beam L. A
photomultiplier 22 is connected to the optical guide 21 by way of a
stimulating light cut filter 23, which prevents the laser beam L to
enter the photomultiplier 22, and photoelectrically converts the
collected light M to an analog image signal y.
[0074] A logarithmic amplifier 24 is connected to the
photomultiplier 22 and outputs a logarithmic image signal by
logarithmic conversion of the analog image signal y. An A/D
convertor 25 is connected to the logarithmic amplifier 24 and
digitizes the logarithmic image signal g into a digital image
signal S. The digital image signal S is output to an external image
processing system.
[0075] The operation of the radiation image information read-out
system shown 1 will be briefly described hereinbelow.
[0076] A stimulable phosphor sheet 30 storing thereon radiation
image information is set on the endless belt 40 in a predetermined
position. Then the stimulable phosphor sheet 30 is conveyed
(sub-scanning) in the direction of arrow Y by the endless belt
40.
[0077] The laser beam L emitted from the laser 11 is deflected by
the polygonal mirror 12 which is rotated at a high speed in the
direction of arrow by the motor 13 and is focused on the surface of
the stimulable phosphor sheet 30 by the scanning lens 14 and is
caused to scan the surface of the stimulable phosphor sheet 30 at a
constant speed in the direction of arrow X (main scanning). Thus
the stimulable phosphor sheet 30 is exposed to the laser beam L
over the entire area thereof.
[0078] The portion of the stimulable phosphor sheet 30 exposed to
the laser beam L emits light M in proportion to the amount of
energy stored thereon.
[0079] The light M emitted from the exposed parts of the stimulable
phosphor sheet 30 in sequence is guided to the photomultiplier 22
by the optical guide 21. At this time also a part of the laser beam
L, e.g., a part of the laser beam L reflected at the stimulable
phosphor sheet 30, enters the optical guide 21.
[0080] The light M and the laser beam L entering the optical guide
21 impinge upon the stimulating light cut filter 23 between the
optical guide 21 and the photomultiplier 22, and the laser beam L
is cut by the filter 23 while the light M passes through the filter
23 to impinge upon the photomultiplier 22.
[0081] The photomultiplier 22 photoelectrically converts the light
M to an analog image signal y corresponding to the amount P of the
light M impinging upon the photomultiplier 22 and outputs the
analog image signal y to the logarithmic amplifier 24.
[0082] The logarithmic amplifier 24 outputs a logarithmic image
signal g by logarithmic conversion of the analog image signal y.
The logarithmic image signal g is input into the A/D convertor 25
and is sampled at predetermined sampling intervals in
synchronization with the scanning by the laser beam L, thereby
quantized into a digital image signal S made up of image signal
components for the respective picture elements.
[0083] A radiation image information read-out system in accordance
with a first embodiment will be described with reference to FIG. 2,
hereinbelow.
[0084] The radiation image information read-out system of this
embodiment mainly differs from that shown in FIG. 1 in that an
electrochromic element 51 is disposed between the stimulating light
cut filer 23 and the photomultiplier 22. The transmittance of the
electrochromic element 51 to the light M emitted from the
stimulable phosphor sheet 30 upon stimulation thereof can be
changed.
[0085] As the material for the electrochromic element 51, amorphous
WO.sub.3 (colorless to blue), IrO.sub.2 (colorless to blue),
viologen (colorless to blue), anthraquinone (colorless to red) or
the like may be used.
[0086] The radiation image information read-out system of this
embodiment further comprises a light amount determination means 53
which determines the amount of light M which is expected to impinge
upon the photomultiplier 22 through estimation on the basis of the
radiographing menu for the stimulable phosphor sheet 30 input from
the exterior, and a transmittance changing means 52 which changes
the transmittance of the electrochromic element 51 on the basis of
the amount of light M which is expected to impinge upon the
photomultiplier 22 determined by the light amount determination
means 53 so that the photoelectric surface of the photomultiplier
22 is not saturated by an excessive amount of light impinging
thereupon.
[0087] The operation of the radiation image information read-out
system of this embodiment will be described hereinbelow.
[0088] The radiographing menu for the stimulable phosphor sheet 30
to be read out is first input into the light amount determination
means 53. The radiographing menu includes the kind of the object,
the material of the object, the shape of the object, the size of
the object, the direction of radiographing and the like and the
irradiation dose of the radiations to which the object was exposed
can be determined according to the radiographing menu. The light
amount determination means 53 determines the amount of radiation
energy stored on the stimulable phosphor sheet 30 on the basis of
the radiographing menu input and calculates the amount of light M
expected to be emitted from the stimulable phosphor sheet 30 upon
stimulation thereof by the laser beam L on the basis of the amount
of radiation energy stored on the stimulable phosphor sheet 30 and
the stimulating energy of the laser beam L.
[0089] The radiographing menu may be input into the light amount
determination means 53 by reading information recorded on the
stimulable phosphor sheet 30, e.g., by reading a bar code
representing the menu by use of a bar code reader, or by way of
information on the radiographing menu of the stimulable phosphor
sheet associated with the ID information for the sheet 30 which is
read out from a server computer and automatically input into the
light amount determination means 53. Otherwise the operator may
manually input information on the radiographing menu to the light
amount determination means 53.
[0090] The amount of light M expected to be emitted from the
stimulable phosphor sheet 30 determined by the light amount
determination means 53 is input into the transmittance changing
means 52 and the transmittance changing means 52 changes the
transmittance of the electrochromic element 51 according to the
amount of light M expected to be emitted from the stimulable
phosphor sheet 30.
[0091] That is, when the amount of light M expected to be emitted
from the stimulable phosphor sheet 30 is large, the transmittance
of the electrochromic element 51 is set low so that the
photoelectric surface of the photomultiplier 22 is not saturated by
an excessive amount of light passing through the filer 23. On the
other hand, when the amount of light M expected to be emitted from
the stimulable phosphor sheet 30 is small, the transmittance of the
electrochromic element 51 is set high within a limit where the
photoelectric surface of the photomultiplier 22 is not
saturated.
[0092] After thus adjusting the transmittance of the electrochromic
element 51, the laser 11 (FIG. 1) is operated. The laser beam L
emitted from the laser 11 is deflected by the polygonal mirror 12
which is rotated at a high speed in the direction of arrow by the
motor 13 and is focused on the surface of the stimulable phosphor
sheet 30 by the scanning lens 14 and is caused to scan the surface
of the stimulable phosphor sheet 30 at a constant speed in the
direction of arrow X while the stimulable phosphor sheet 30 is
conveyed in the direction arrow Y. Thus the stimulable phosphor
sheet 30 is exposed to the laser beam L over the entire area
thereof.
[0093] The portion of the stimulable phosphor sheet 30 exposed to
the laser beam L emits light M in proportion to the amount of
energy stored thereon. The light M emitted from the exposed parts
of the stimulable phosphor sheet 30 enters the optical guide 21
together with a part of the laser beam L.
[0094] Since the laser beam L is cut by the stimulating light cut
filter 23, only the light M passes through the filter 23. (See FIG.
2)
[0095] The light M passing through the filter 23 further passes
through the electrochromic element 51, whose transmittance has been
adjusted according to the amount of light M expected to be emitted
from the stimulable phosphor sheet 30 upon stimulation thereof, and
impinges upon the photomultiplier 22. That is, when the amount of
light M is relatively large, the transmittance of the
electrochromic element 51 is reduced and accordingly the amount of
light M impinging upon the photomultiplier 22 cannot be so large
that the photoelectric surface of the photomultiplier 22 is
saturated.
[0096] On the other hand, when the amount of light M is small, the
transmittance of the electrochromic element 51 is set high. In this
case, however, since the amount of light M emitted from the
stimulable phosphor sheet upon stimulation thereof is originally
small, there is no fear that the photoelectric surface of the
photomultiplier 22 is saturated and the radiation image information
can be read out at a high sensitivity.
[0097] The photomultiplier 22 photoelectrically converts the light
M to an analog image signal y and outputs the analog image signal y
to the logarithmic amplifier 24.
[0098] The logarithmic amplifier 24 outputs a logarithmic image
signal g by logarithmic conversion of the analog image signal y.
The logarithmic image signal g is input into the A/D convertor 25
and is sampled at predetermined sampling intervals in
synchronization with the scanning by the laser beam L, thereby
quantized into a digital image signal S made up of image signal
components for the respective picture elements.
[0099] Thus in the radiation image information read-out system of
this embodiment, the phenomenon of saturation of the photoelectric
surface can be suppressed and generation of ghost image can be
suppressed while ensuring a high sensitivity of the photomultiplier
22.
[0100] A radiation image information read-out system in accordance
with a second embodiment of the present invention will be
described, hereinbelow. The radiation image information read-out
system of this embodiment mainly differs from that of the first
embodiment in that a ND filer system 54 is used in place of the
electrochromic element 51 as the variable transmittance medium, and
accordingly the elements analogous to those shown in FIGS. 1 and 2
are given the same reference numerals and will not be described
here.
[0101] The ND filter system 54 is disposed between the stimulating
light cut filer 23 and the photomultiplier 22. The transmittance of
the ND filter system 54 to the light M emitted from the stimulable
phosphor sheet 30 upon stimulation thereof can be changed
continuously or stepwise.
[0102] The radiation image information read-out system of this
embodiment further comprises a transmittance changing means 55
which changes the transmittance of the ND filter system 54 by
moving the system 54 in a direction, shown by the arrow in FIG. 3,
perpendicular to the optical path on the basis of the amount of
light M expected to be emitted from the stimulable phosphor sheet
30 determined by the light amount determination means 53 so that
the photoelectric surface of the photomultiplier 22 is not
saturated by an excessive amount of light impinging thereupon.
[0103] The operation of the radiation image information read-out
system of this embodiment will be described hereinbelow.
[0104] The radiographing menu for the stimulable phosphor sheet 30
to be read out is first input into the light amount determination
means 53. The radiographing menu includes the kind of the object,
the material of the object, the shape of the object, the size of
the object, the direction of radiographing and the like and the
irradiation dose of the radiations to which the object was exposed
can be determined according to the radiographing menu. The light
amount determination means 53 determines the amount of radiation
energy stored on the stimulable phosphor sheet 30 on the basis of
the radiographing menu input and calculates the amount of light M
expected to be emitted from the stimulable phosphor sheet 30 upon
stimulation thereof by the laser beam L on the basis of the amount
of radiation energy stored on the stimulable phosphor sheet 30 and
the stimulating energy of the laser beam L.
[0105] The radiographing menu may be input into the light amount
determination means 53 in any one of the manners described
above.
[0106] The amount of light M expected to be emitted from the
stimulable phosphor sheet 30 determined by the light amount
determination means 53 is input into the transmittance changing
means 55 and the transmittance changing means 55 changes the
transmittance of the ND filter system 54 by moving it in the
direction of the arrow according to the amount of light M expected
to be emitted from the stimulable phosphor sheet 30.
[0107] That is, when the amount of light M expected to be emitted
from the stimulable phosphor sheet 30 is large, the transmittance
is set low so that the photoelectric surface of the photomultiplier
22 is not saturated by an excessive amount of light passing through
the filer 23. On the other hand, when the amount of light M
expected to be emitted from the stimulable phosphor sheet 30 is
small, the transmittance is set high within a limit where the
photoelectric surface of the photomultiplier 22 is not
saturated.
[0108] After thus adjusting the transmittance of the ND filter
system 54, the laser 11 (FIG. 1) is operated. The laser beam L
emitted from the laser 11 is deflected by the polygonal mirror 12
which is rotated at a high speed in the direction of arrow by the
motor 13 and is focused on the surface of the stimulable phosphor
sheet 30 by the scanning lens 14 and is caused to scan the surface
of the stimulable phosphor sheet 30 at a constant speed in the
direction of arrow X while the stimulable phosphor sheet 30 is
conveyed in the direction arrow Y. Thus the stimulable phosphor
sheet 30 is exposed to the laser beam L over the entire area
thereof.
[0109] The portion of the stimulable phosphor sheet 30 exposed to
the laser beam L emits light M in proportion to the amount of
energy stored thereon. The light M emitted from the exposed parts
of the stimulable phosphor sheet 30 enters the optical guide 21
together with a part of the laser beam L.
[0110] Since the laser beam L is cut by the stimulating light cut
filter 23, only the light M passes through the filter 23. (See FIG.
3)
[0111] The light M passing through the filter 23 further passes
through the ND filter system 54, whose transmittance has been
adjusted according to the amount of light M expected to be emitted
from the stimulable phosphor sheet 30 upon stimulation thereof, and
impinges upon the photomultiplier 22. That is, when the amount of
light M is relatively large, the transmittance is reduced and
accordingly the amount of light M impinging upon the
photomultiplier 22 cannot be so large that the photoelectric
surface of the photomultiplier 22 is saturated. On the other hand,
when the amount of light M is small, the transmittance is set high.
In this case, however, since the amount of light M emitted from the
stimulable phosphor sheet upon stimulation thereof is originally
small, there is no fear that the photoelectric surface of the
photomultiplier 22 is saturated and the radiation image information
can be read out at a high sensitivity.
[0112] The photomultiplier 22 photoelectrically converts the light
M to an analog image signal y and outputs the analog image signal y
to the logarithmic amplifier 24.
[0113] The logarithmic amplifier 24 outputs a logarithmic image
signal g by logarithmic conversion of the analog image signal y.
The logarithmic image signal g is input into the A/D convertor 25
and is sampled at predetermined sampling intervals in
synchronization with the scanning by the laser beam L, thereby
quantized into a digital image signal S made up of image signal
components for the respective picture elements.
[0114] Thus in the radiation image information read-out system of
this embodiment, the phenomenon of saturation of the photoelectric
surface can be suppressed and generation of ghost image can be
suppressed while ensuring a high sensitivity of the photomultiplier
22.
[0115] A radiation image information read-out system in accordance
with a third embodiment of the present invention will be described
with reference to FIG. 4, hereinbelow.
[0116] The radiation image information read-out system of this
embodiment mainly differs from that shown in FIG. 1 in that the
amount of laser beam L emitted from the laser 11' is variable. That
is, the laser 11' is controlled by a light amount determination
means 53 which determines the amount of light M expected to be
emitted from the stimulable phosphor sheet 30 when the stimulable
phosphor sheet 30 is exposed to a given amount of laser beam L and
a light amount changing means 56 which controls the laser 11'
according to the amount of light M expected to be emitted from the
stimulable phosphor sheet 30 determined by the light amount
determination means 53 so that the amount of laser beam L emitted
from the laser 11' is limited to a level such that the light M
emitted from the stimulable phosphor sheet 30 when it is stimulated
by the laser beam L cannot saturate the photoelectric surface of
the photomultiplier 22.
[0117] The light amount determination means 53 may be the same as
that employed in the first embodiment shown in FIG. 2 and
accordingly will not be described here.
[0118] The operation of the radiation image information read-out
system of this embodiment will be described hereinbelow.
[0119] The radiographing menu for the stimulable phosphor sheet 30
to be read out is first input into the light amount determination
means 53. The light amount determination means 53 calculates the
amount of light M expected to be emitted from the stimulable
phosphor sheet 30 upon stimulation thereof by the laser beam L on
the basis of the radiographing menu and a given amount of the laser
beam L. Here it is assumed that the amount of light M expected to
be emitted from the stimulable phosphor sheet 30 when stimulated by
the given amount of the laser beam L is large enough to saturate
the photoelectric surface of the photomultiplier 22.
[0120] The amount of light M expected to be emitted from the
stimulable phosphor sheet 30 determined by the light amount
determination means 53 is input into the light amount changing
means 56 and the light amount changing means 56 controls the output
of the laser 11' according to the amount of light M expected to be
emitted from the stimulable phosphor sheet 30 to reduce the amount
of the laser beam L emitted from the laser 11' by a level .DELTA.L.
The level .DELTA.L is selected so that the amount of light M'
expected to be emitted from the stimulable phosphor sheet 30 when
stimulated by the laser beam L' having an amount of light reduced
by the .DELTA.L cannot saturate the photoelectric surface of the
photomultiplier 22.
[0121] After thus adjusting the output of the laser 11', the laser
11' is operated. The laser beam L' emitted from the laser 11' is
deflected by the polygonal mirror 12 which is rotated at a high
speed in the direction of arrow by the motor 13 and is focused on
the surface of the stimulable phosphor sheet 30 by the scanning
lens 14 and is caused to scan the surface of the stimulable
phosphor sheet 30 at a constant speed in the direction of arrow X
while the stimulable phosphor sheet 30 is conveyed in the direction
arrow Y. Thus the stimulable phosphor sheet 30 is exposed to the
laser beam L' over the entire area thereof.
[0122] The portion of the stimulable phosphor sheet 30 exposed to
the laser beam L' emits light M' in proportion to the amount of
energy stored thereon. The light M' emitted from the exposed parts
of the stimulable phosphor sheet 30 enters the optical guide 21
together with a part of the laser beam L'. Since the laser beam L'
is cut by the stimulating light cut filter 23, only the light M'
passes through the filter 23.
[0123] The light M' passing through the filter 23 impinges upon the
photomultiplier 22. Since the amount of the stimulating laser beam
L' is reduced as described above, the amount of light M' impinging
upon the photomultiplier 22 cannot be so large that the
photoelectric surface of the photomultiplier 22 is saturated.
Accordingly the photomultiplier 22 can photoelectrically convert
the amount p' of the light M' to an analog image signal y' without
fear that the photoelectric surface thereof being saturated and
outputs the analog image signal y' to the logarithmic amplifier
24.
[0124] The logarithmic amplifier 24 outputs a logarithmic image
signal g' by logarithmic conversion of the analog image signal y'.
The logarithmic image signal g' is input into the A/D convertor 25
and is sampled at predetermined sampling intervals in
synchronization with the scanning by the laser beam L, thereby
quantized into a digital image signal S' made up of image signal
components for the respective picture elements.
[0125] Thus in the radiation image information read-out system of
this embodiment, the phenomenon of saturation of the photoelectric
surface can be suppressed and generation of ghost image can be
suppressed while ensuring a high sensitivity of the photomultiplier
22.
[0126] Though, in the radiation image information read-out systems
of the first to third embodiments, the amount of light M' expected
to be emitted from the stimulable phosphor sheet upon stimulation
thereof is determined by use of the light amount determination
means, the radiation image information read-out systems need not be
limited to such an arrangement provided that the amount of light
impinging upon the photodetector can be controlled not to saturate
the photoelectric surface of the photodetector according to the
amount of light to be emitted which may be determined in any
manner, e.g., by actually measuring the amount light, by estimation
or on the basis of information input from the exterior.
[0127] A radiation image information read-out system in accordance
with a fourth embodiment of the present invention will be described
with reference to FIG. 5, hereinbelow.
[0128] In FIG. 5, the radiation image information readout system of
this embodiment comprises a stimulating light projecting means 110
for scanning a stimulable phosphor sheet 101 with a laser beam A, a
photoelectric convertor means 120 which photoelectrically reads
light B emitted from the stimulable phosphor sheet 101 upon
stimulation by the laser beam A and converts it to an electric
signal (an image signal) V0, a signal processing means 130 which
carries out a predetermined image processing on the image signal
V0, a sensitivity setting means 170 which sets the sensitivity of
the photoelectric convertor means 120, an irradiation energy
control means 140 which controls the amount of laser beam A
(irradiation energy) to which the stimulable phosphor sheet 101 is
exposed per unit area thereof, a preliminary read-out means 150 and
a conveyor means 160 which conveys back and forth the stimulable
phosphor sheet 101 in the directions of arrows X0 and X1 in
perpendicular to the direction of arrow Y (to be described
later).
[0129] The stimulating light projecting means 110 comprises a
semiconductor laser pumped solid state laser (SHG) 11 emitting a
laser beam A at a predetermined wavelength, a rotating polygonal
mirror 112 which deflects the laser beam A, an electric motor 113
which drives the polygonal mirror 112, an f.theta. lens 114 which
condenses the laser beam A and a reflecting optical element 115
which changes the direction of the condensed laser beam A.
[0130] The photoelectric convertor means 120 comprises an optical
guide 122 which collects light B emitted from the stimulable
phosphor sheet 101 upon stimulation by the laser beam A and a
photomultiplier 124 which amplifies and converts the light B into
an electric signal.
[0131] The signal processing means 130 comprises a logarithmic
amplifier 132, an A/D convertor 133 and an image processing circuit
134. The image signal output from the photoelectric convertor means
120 is subjected to a predetermined image processing by the signal
processing means 130 and then reproduced as a visible image on a
display (not shown) or stored in a memory (not shown) for
subsequent processing in the form digital image data.
[0132] The irradiation energy control means 140 comprises a monitor
circuit 142 which watches the output signal V1 of the logarithmic
amplifier 132, a stimulating system control circuit 144 and an AOM
(acoustooptic modulator) 146 which intensity-modulates the laser
beam A emitted from the laser 111.
[0133] The operation of the radiation image information read-out
system of this embodiment will be described hereinbelow.
[0134] The stimulable phosphor sheet 101 on which radiation image
information has been recorded by, for instance, a radiation image
recording system (not shown) is placed on the conveyor means 160
and conveyed in the direction of arrow X0 to a read-out section
formed by the stimulating light projection means 110 and the
photoelectric convertor means 120. In the read-out section, the
laser beam A emitted from the laser 111 travels through the AOM 146
and is focused on the surface of the stimulable phosphor sheet 101
through the rotating polygonal mirror 112, the f.theta. lens 114
and the reflecting optical element 115. The laser beam A is caused
to scan the stimulable phosphor sheet 101 in the direction of arrow
Y (main scanning) by virtue of rotation of the polygonal mirror 112
in the direction of arrow K while the stimulable phosphor sheet 101
is conveyed in the sub-scanning direction (the direction of arrow
X0 or X1), whereby the stimulable phosphor sheet 101 is
two-dimensionally scanned by the laser beam A.
[0135] The stimulable phosphor sheet 101 emits light B upon
stimulation by the laser beam A and the light B is guided to the
photoelectric convertor means 120 by the optical guide 22. The
photoelectric convertor means 120 converts the light B into an
image signal V0 representing the radiation image information stored
on the stimulable phosphor sheet 101.
[0136] The method of determining read-out conditions of the
radiation image information read-out system in this embodiment will
be described hereinbelow. The read-out conditions includes various
factors which affect on the relation between the amount of the
light B and the output of the radiation image information read-out
system, e.g., the read-out sensitivity of the photoelectric
convertor means 120 which governs the relation between input and
output, the scale factor, and the irradiation energy of the
stimulating light. The read-out conditions are determined so that
maximum and minimum amounts S1 and S2 of the light B within the
range representing the radiation image information respectively
correspond to maximum and minimum signal levels Q1 and Q2 within
the input signal level range in the signal processing means 130,
whereby the maximum and minimum amounts S1 and S2 of the light B
are reproduced in a proper density range.
[0137] In this embodiment, preliminary read-out is effected prior
to final read-out. In the preliminary readout, the radiation image
information stored on the stimulable phosphor sheet 101 is read out
by exposing the stimulable phosphor sheet 101 to a laser beam
having energy smaller than the laser beam A employed in the final
readout, and the read-out conditions for the final read-out are
determined on the basis of the image information obtained by the
preliminary read-out through analysis to be described later.
[0138] In this particular embodiment, the preliminary read-out is
effected in the same manner as the final readout except that the
irradiation energy of the laser beam A is reduced as compared with
in the final read-out. That is, the irradiation intensity of the
laser beam A to the stimulable phosphor sheet 101 is weakened by
intensity-modulating the laser beam A as emitted from the laser 111
by the AOM 146, whereby the irradiation energy of the laser beam A
per unit area of the stimulable phosphor sheet 101 is reduced. In
this state, the stimulable phosphor sheet 101 is scanned by the
laser beam A and light B emitted from the stimulable phosphor sheet
101 is read out by the photoelectric convertor means 120 and the
image signal V0 thus obtained is subjected to logarithmic
conversion by the logarithmic amplifier 132. Then the logarithmic
image signal V1 is converted into a digital image signal V2 by the
A/D convertor 133 and the digital image signal V2 is input into the
preliminary read-out means 150. The preliminary read-out may be
effected by use of a stimulating light projecting means, a conveyor
means and the like provided separately from those for the final
readout as disclosed, for instance, in Japanese Unexamined Patent
Publication No. 4(1992)-1745. Further as the method for reducing
the irradiation energy of the laser beam A per unit area of the
stimulable phosphor sheet 101, any method may be employed provided
that a brief of the radiation image information stored on the
stimulable phosphor sheet 101 can be detected and the read-out
conditions can be determined on the basis of the information
obtained.
[0139] FIG. 6 shows an example of the method for determining, in
the preliminary read-out means 150, the read-out conditions
(especially the read-out sensitivity) under which the photoelectric
convertor means 120 reads out the radiation image information in
the final read-out. The image signal V2 obtained by the preliminary
read-out is input into a histogram making means 154, which makes a
histogram for the levels of the image signal components for the
respective picture elements (i.e., a histogram for the amounts of
light emitted from the respective spots on the stimulable phosphor
sheet 101 exposed to the laser beam A). The histogram made by the
histogram making means 154 is input into a histogram analysis means
156 which analyzes the histogram. FIG. 7A shows a pattern of a
histogram for a radiation image of the chest. In FIG. 3A, F is the
pattern for the mediastinum, G is for the heart, H is for the lung,
I is for the skin or a soft part, and J is for the outside of the
object (i.e., a blank portion). By such analysis, a brief of the
energy stored on the entire stimulable phosphor sheet, e.g.,
maximum and minimum amounts S1 and S2 of light B in a desired image
information range (e.g., from the mediastinum to the soft part) can
be obtained, and the read-out sensitivity for the photoelectric
convertor means 120 can be set on the basis of the maximum and
minimum amounts S1 and S2 of light B in the following manner. FIG.
7B shows the histogram shown in FIG. 7A on a graph showing the
relation between the amount of light B and the output of the
photoelectric convertor means 120. In FIG. 7B, the abscissa
represents the amount of light B and the ordinate represents the
output of the photoelectric convertor means 120 in digital values
(i.e., the output signal V2 of the A/D convertor 33).
[0140] Thus the maximum and minimum amounts S1 and S2 of light B in
a desired image information range are obtained from a histogram
made by the preliminary read-out and the range (Q1 to Q2) of the
signal input into the signal processing means 130, that is, the
range of the signal output from the photoelectric convertor means
120, is determined corresponding to the values S1 and S2 so that
the values S1 and S2 are reproduced properly.
[0141] By changing the scale factor represented by the values S1,
S2, Q1 and Q2 according to the width of the desired range of the
amounts of light B to be emitted from the stimulable phosphor sheet
101 upon stimulation thereof, the width of the range of the signal
levels input into the signal processing means 130 can be normally
conformed to the width of the desired input signal level range (Q1
to Q2). Thereafter the read-out sensitivity is changed so that the
position of the range of the signal levels input into the signal
processing means 130 normally conforms to the position of the
desired input signal level range.
[0142] In this particular embodiment, the image signal V0 read out
by the photoelectric convertor means 120 is converted into a
logarithmic signal by the logarithmic amplifier 132 and the
logarithmic image signal V1 is converted into the digital signal V2
having components ranging over 10 bits (digital values of 0 to
1023). Then the read-out sensitivity is set so that the range (S1
to S2) of the amount of light B emitted from the stimulable
phosphor sheet 101 conforms to the 10-bit digital signal V2 and the
amount of light (S3 in FIG. 7B) emitted from a blank portion of the
radiation image information stored on the stimulable phosphor sheet
101 (e.g., the portion denoted by J in FIG. 7A) becomes larger than
the amount corresponding to the upper limit of the operable range L
of the photoelectric convertor means 120 (in this particular
embodiment, the range of the amounts of light B resulting in
digital values of 0 to 1023). The read-out sensitivity can be
adjusted by changing the highest voltage HV of the photoelectric
convertor means 120. Then a control signal V3 is obtained in the
manner described above on the basis of histogram analysis and the
control signal V3 is input into the sensitivity setting means 170,
which sets the read-out sensitivity of the photoelectric convertor
means 120 by changing the highest voltage HV imparted to the
photoelectric convertor means 120 on the basis of the control
signal V3.
[0143] The intensity (irradiation energy) P0 of the laser beam A
when scanning the image area other than the blank portion in the
final read-out (will be referred to as "the steady state",
hereinbelow) may be obtained in advance in the preliminary
read-out.
[0144] A method of controlling the intensity of the laser beam A to
the stimulable phosphor sheet 101 in the final read-out will be
described with reference to FIGS. 8A to 8E. After the read-out
sensitivity is set in the manner described above, the conveyor
means 60 is reversed to return (in the direction of arrow X1 in
FIG. 5) the stimulable phosphor sheet 101 to the original position
(the position before the preliminary read-out). The intensity of
the laser beam B is first set to the intensity P0 in the steady
state in the final read-out. The intensity P0 may be obtained
through analysis of the histogram obtained by the preliminary
read-out, or may be set to a predetermined value corresponding to
the radiographing menu. In this state, the radiation image
information stored on the stimulable phosphor sheet 101 is read out
by the photoelectric convertor means 120 while scanning the
stimulable phosphor sheet 101 with the laser beam A.
[0145] FIG. 8A schematically shows a radiation image K0 of an
object stored on a stimulable phosphor sheet 101. K1 denotes a
blank portion. FIG. 8B shows change in the amount of light B
emitted from the stimulable phosphor sheet 101 when the laser beam
A scans the stimulable phosphor sheet 101 along the dashed line in
FIG. 8A. FIG. 8C shows change in the irradiation dose or the
irradiation energy of the laser beam A to which the stimulable
phosphor sheet 101 is exposed per unit area thereof. FIG. 8D shows
change in the image signal read out by the photoelectric convertor
means 120 (in this embodiment, the output V1 of the logarithmic
amplifier 132). FIG. 8E shows change in the digital image signal V2
obtained by digitizing the image signal V1 by the A/D convertor
133. The solid lines in FIGS. 8C and 8D show the corresponding
properties when the control of the stimulating light in accordance
with the present invention is not performed.
[0146] The output signal V1 of the logarithmic amplifier 132 is
watched by the monitor circuit 142, and the stimulating system
control circuit 144 controls the AOM 146 to reduce the amount of
the laser beam A on the basis of the output of the monitor circuit
142 so that the output signal level V1 of the logarithmic amplifier
132 when the photoelectric convertor means 120 detects the light B
emitted from the blank portion K1 is minimized in the range higher
than the upper limit of the read-out signal level range of the
photoelectric convertor means 120. By changing the amount of the
laser beam A passing through the AOM 146, the irradiation dose to
which the stimulable phosphor sheet 101 is exposed per unit area
thereof, that is, the stimulating energy of the laser beam A can be
reduced. Since the amount of light emitted from the stimulable
phosphor sheet 101 upon stimulation by the stimulating light
depends upon the energy of the stimulating light (the laser beam A)
per unit area of the stimulable phosphor sheet 101 as described
above, the amount of light emitted from the blank portion K1 can be
reduced by reducing the amount of the laser beam A passing through
the AOM 146 when that the laser beam A is scanning the blank
portion K1 is detected through the level of the image signal V1 by
the stimulating system control circuit 144. When such scanning is
repeated, the amount of light emitted from the blank portion K1 is
rapidly reduced. Since the amount of afterglow becomes smaller as
the amount of light emitted from the stimulable phosphor sheet 101
upon stimulation thereof becomes smaller as described above, the
aforesaid problems caused by the afterglow can be overcome and the
radiation image information can be accurately read out at a high
speed. Further since generation of afterglow itself is prevented by
reducing the stimulating light, the S/N ratio can be improved. The
change in the irradiation energy of the laser beam A to which the
stimulable phosphor sheet 101 is exposed per unit area thereof and
the change in the image signal read out by the photoelectric
convertor means 120 are shown by the dashed line respectively in
FIGS. 8C and 8D.
[0147] In the portion KO other than the blank portion, the
irradiation energy of the laser beam A is kept at the value P0 in
the steady state, whereby the image signal V1 can be at a signal
level suitable for image information required in diagnosis and the
radiation image information can be reproduced as a visible image at
proper gradation.
[0148] In the above description, the irradiation energy of the
laser beam A to which the stimulable phosphor sheet 101 is exposed
per unit area thereof is reduced by the AOM 146, the irradiation
energy may be controlled in various manners. For example, a visible
region semiconductor laser is employed as the laser 111 and the
intensity of the visible laser beam may be directly
intensity-modulated. Further the irradiation energy may be
controlled by changing the scanning speed of the laser beam A
relative to the stimulable phosphor sheet 101, e.g., by changing
the conveying speed by the conveyor means 160 or the scanning speed
of the laser beam A.
[0149] Further though in the above description, the readout
sensitivity of the photoelectric convertor means 120 is set on the
basis of the information obtained by the preliminary read-out, the
preliminary read-out need not be effected. For example, since a
desired read-out sensitivity can be determined in advance according
to the radiographing menu, e.g., whether the object is the chest or
the heart, a plurality of levels of the read-out sensitivity of the
photoelectric convertor means 120 in relation to different
radiographing menus and the read-out sensitivity of the
photoelectric convertor means 120 may be set to the sensitivity
level corresponding to the radiographing menu for the radiation
image information to be read out.
[0150] When erasing residual energy on the stimulable phosphor
sheet after completion of read-out, energy for erasure is sometimes
calculated by carrying out a predetermined signal processing on the
read-out signal level. In such a case, there is a fear that a
correct erasing energy level cannot be obtained when the
conventional method of calculation is applied as it is to the
stimulable phosphor sheet which has been read out by the method of
the present invention since the signal level for the blank portion
is lowered. On the other hand, since in accordance with the present
invention, the light B emitted from the blank portion K1 is
minimized in the range higher than the upper limit of the read-out
signal level range of the photoelectric convertor means 120, the
problem that the read-out signal level for the blank portion
exceeds the range where the characteristics of the photoelectric
convertor means is linear, which makes it impossible to realize a
correct signal level, can be avoided. Accordingly, by taking into
account information on the reduction of the stimulating light
during read-out, erasing energy can be more accurately
calculated.
* * * * *