U.S. patent application number 10/115965 was filed with the patent office on 2003-09-18 for exposure control in scanning-based detection of ionizing radiation.
Invention is credited to Francke, Tom, Thunberg, Stefan.
Application Number | 20030174806 10/115965 |
Document ID | / |
Family ID | 20287226 |
Filed Date | 2003-09-18 |
United States Patent
Application |
20030174806 |
Kind Code |
A1 |
Francke, Tom ; et
al. |
September 18, 2003 |
Exposure control in scanning-based detection of ionizing
radiation
Abstract
An apparatus for recording a 2D image of an object comprises a
plurality of 1D detector units, each exposed to ionizing radiation,
as transmitted through or scattered off the object, and being
arranged for 1D imaging of the radiation, to which it is exposed.
The detector units are distributed in an array such that the 1D
images of the radiation from the detector units are distributed
over a substantial portion of the 2D image. The apparatus includes
a device for moving the detector units relative the object while
the detector units repeatedly detect to create the 2D image of the
object; and a control device for controlling the detector units to
detect ionizing radiation during a short period of time before or
during an initial part of the movement; calculating an optimum
exposure time for the repeated detection based on the short period
of time detection; and controlling the repeated detection to
automatically obtain the optimum exposure time.
Inventors: |
Francke, Tom; (Sollentuna,
SE) ; Thunberg, Stefan; (Lidingo, SE) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 8910
RESTON
VA
20195
US
|
Family ID: |
20287226 |
Appl. No.: |
10/115965 |
Filed: |
April 5, 2002 |
Current U.S.
Class: |
378/37 |
Current CPC
Class: |
A61B 6/032 20130101;
A61B 6/488 20130101; A61B 6/502 20130101; A61B 6/544 20130101; A61B
6/542 20130101 |
Class at
Publication: |
378/37 |
International
Class: |
A61B 006/04 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 12, 2002 |
SE |
0200731-8 |
Claims
1. A scanning-based radiation detector apparatus for recording a
two-dimensional image of an object comprising a plurality of
one-dimensional detector units, each exposed to ionizing radiation,
as transmitted through or scattered off said object, and being
arranged for one-dimensional imaging of the respective ionizing
radiation, to which it is exposed, wherein said plurality of
one-dimensional detector units are distributed in an array such
that the one-dimensional images of the ionizing radiation from said
plurality of one-dimensional detector units are distributed over a
substantial portion of said two-dimensional image of an object to
be recorded; and said scanning-based detector apparatus further
includes: a device for moving said array of one-dimensional
detector units relative said object while the plurality of
one-dimensional detector units are arranged to repeatedly detect to
thereby create a two-dimensional image of the object; and a control
device for controlling the movement of and the repeated detections
by said array of one-dimensional detector units, said control
device being adapted to control said array of one-dimensional
detector units to detect ionizing radiation during a short period
of time before or during an initial part of the movement, to
calculate an optimum exposure time for each one of the repeated
detections based on said detection of ionizing radiation before or
during an initial part of the movement and said short period of
time, and to control the repeated detections by said array of
one-dimensional detector units to automatically obtain said optimum
exposure time for each one of the repeated detections to thereby
achieve optimum image quality.
2. The scanning-based radiation detector apparatus of claim 1
wherein said control device is adapted to calculate said optimum
exposure time based on a minimum or average signal value as
obtained from said detection of ionizing radiation before or during
an initial part of the movement.
3. The scanning-based radiation detector apparatus of claim 1
wherein said control device is adapted to calculate said optimum
exposure time based on a minimum or average signal value within a
region of said array of one-dimensional detector units as obtained
from said detection of ionizing radiation before or during an
initial part of the movement.
4. The scanning-based radiation detector apparatus of claim 1
wherein said control device is adapted to calculate said optimum
exposure time based on an integrated signal value of one or some of
the one-dimensional detector units of said array of one-dimensional
detector units, preferably the unit(s) with lowest integrated
signal value, as obtained from said detection of ionizing radiation
before or during an initial part of the movement.
5. The scanning-based radiation detector apparatus of claim 1
wherein said control device is adapted to control said device for
moving to move said array of one-dimensional detector units
relative said object continuously, while the plurality of
one-dimensional detector units repeatedly detect to create the
two-dimensional image of the object.
6. The scanning-based radiation detector apparatus of claim 5
wherein said control device is adapted to control said device for
moving to move said array of one-dimensional detector units
relative said object at a speed, which is based on said calculated
optimum exposure time for each one of the repeated detections.
7. The scanning-based radiation detector apparatus of claim 1
wherein said control device is adapted to control said device for
moving to move said array of one-dimensional detector units
relative said object stepwise; and to control the plurality of
one-dimensional detector units to detect while said array of
one-dimensional detector units is kept still with respect to said
object.
8. The scanning-based radiation detector apparatus of claim 1
wherein said short period of time, during which said array of
one-dimensional detector units detects ionizing radiation before or
during an initial part of the movement, is in the interval 100
ns-10 s, preferably in the interval 1 .mu.s-100 ms, and most
preferably 10 .mu.s-10 ms.
9. The scanning-based radiation detector apparatus of claim 1
wherein said control device contains a look-up table of desired
signal strengths for various object characteristics or an algorithm
for determining desired signal strengths for various object
characteristics, and is adapted to receive a characteristic of the
object, of which a two-dimensional image is to be recorded; and to
calculate the optimum exposure time for each one of the repeated
detections by means of multiplying said short period of time with
the ratio of the desired signal strength and a signal strength as
obtained from said detection of ionizing radiation before or during
an initial part of the movement.
10. The scanning-based radiation detector apparatus of claim 9
wherein said various object characteristics include object
thicknesses, wherein e.g. the object is a compressed breast and the
thicknesses are defined as thicknesses at a particular compression
force.
11. The scanning-based radiation detector apparatus of claim 9
wherein said various object characteristics include estimated
densities, wherein e.g. the object is a breast and the estimated
densities are defined as fat content versus content of other
tissue.
12. The scanning-based radiation detector apparatus of claim 9
wherein said object is a compressed breast and said various object
characteristics include compressed breast thickness changes as
caused by a change in compression force.
13. The scanning-based radiation detector apparatus of claim 9
wherein said object is a compressed breast and said various object
characteristics include the change of absorption as caused by a
change in compression force.
14. The scanning-based radiation detector apparatus of claim 1
comprising a collimator with a controllable variable aperture
arranged in the path of said ionizing radiation upstream of said
object, wherein said control device is adapted to deduce from said
detection of ionizing radiation before or during an initial part of
the movement an outer shape of said object; and to control the
variable aperture of the shielding device to shield radiation not
transmitted through or scattered off said object, and said
shielding device is arranged to be fixed with respect to said array
of one-dimensional detector units during movement of said array of
one-dimensional detector units relative said object.
15. The scanning-based radiation detector apparatus of claim 1
comprising a filter device with a controllable variable spectral
transmission characteristics arranged in the path of said ionizing
radiation upstream of said object, wherein said control device is
adapted to deduce from said detection of ionizing radiation before
or during an initial part of the movement a measure indicative of
the contrast of the detection; and to control the variable spectral
transmission characteristics of the filter device in response to
said measure indicative of the contrast of the detection.
16. The scanning-based radiation detector apparatus of claim 1
comprising an X-ray tube for producing the ionizing radiation, said
X-ray tube having a cathode which emits electrons and an anode on
which said electrons impinge and which is a source of X-rays, said
tube having an controllable variable operating voltage which is the
voltage drop between said anode and said cathode, a tube current
which is the current between said anode and said cathode, and a
focal spot size which is the area of said anode on which said
electrons impinge, wherein said control device is adapted to
control the variable operating voltage of the X-ray tube in
response to said detection of ionizing radiation before or during
an initial part of the movement.
17. The scanning-based radiation detector apparatus of claim 1
comprising an X-ray tube for producing the ionizing radiation, said
X-ray tube having a cathode which emits electrons and an anode on
which said electrons impinge and which is a source of X-rays, said
tube having an controllable variable operating voltage which is the
voltage drop between said anode and said cathode, a controllable
variable tube current which is the current between said anode and
said cathode, and a focal spot size which is the area of said anode
on which said electrons impinge, wherein said control device is
adapted to control the variable tube current of the X-ray tube in
response to said detection of ionizing radiation before or during
an initial part of the movement.
18. The scanning-based radiation detector apparatus of claim 1
comprising an X-ray tube for producing the ionizing radiation, said
X-ray tube having a cathode which emits electrons and an anode on
which said electrons impinge and which is a source of X-rays, said
tube having an controllable variable operating voltage which is the
voltage drop between said anode and said cathode, a controllable
variable tube current which is the current between said anode and
said cathode, and a controllable variable focal spot size which is
the area of said anode on which said electrons impinge, wherein
said control device is adapted to control the variable focal spot
size of the X-ray tube in response to said detection of ionizing
radiation before or during an initial part of the movement.
19. The scanning-based radiation detector apparatus of claim 1
wherein said plurality of one-dimensional detector units is
distributed in a two-dimensional pattern on a common support
structure.
20. The scanning-based radiation detector apparatus of claim 1
wherein said plurality of one-dimensional detector units are sited
in rows and stacks, the rows being parallel with the
one-dimensional detector units and the stacks being essentially
orthogonal thereto, where the one-dimensional detector units in
each row are together capable of detecting completely the object in
one dimension.
21. The apparatus of claim 20 wherein the one-dimensional detector
units of each row are staggered with an overlap between adjacent
one-dimensional detector units in the direction of the row.
22. The scanning-based radiation detector apparatus of claim 1
wherein said plurality of one-dimensional detector units are
arranged in a circle, each oriented essentially radially with
respect to said circle.
23. The scanning-based radiation detector apparatus of claim 1
wherein each of said plurality of one-dimensional detector units is
a gaseous-based ionizing radiation detector, wherein electrons
released by interactions between radiation photons and the gas can
be extracted in a direction essentially perpendicular to the
ionizing radiation entered into that one-dimensional detector
unit.
24. The scanning-based radiation detector apparatus of claim 1
comprising a collimator of a radiation-absorbing material arranged
in the path of said ionizing radiation upstream of said object,
which collimator includes a plurality of radiation transparent
slits, the number of the radiation transparent slits corresponding
to the number of one-dimensional detector units, wherein the
radiation transparent slits are aligned with the one-dimensional
detector units, such that essentially planar ray bundles as
transmitted through the radiation transparent slits of the
collimator irradiate the respective one-dimensional detector units,
and wherein said collimator is arranged to be fixed with respect to
said array of one-dimensional detector units during movement of
said array of one-dimensional detector units relative said
object.
25. A method for recording a two-dimensional image of an object
comprising the steps of: providing a scanning-based radiation
detector apparatus comprising a plurality of one-dimensional
detector units, each being arranged for one-dimensional imaging of
the respective ionizing radiation, to which it is exposed, wherein
the plurality of one-dimensional detector units are distributed in
an array such that the one-dimensional images of the ionizing
radiation from the plurality of one-dimensional detector units are
distributed over a substantial portion of the two-dimensional image
to be recorded; detecting ionizing radiation, as transmitted
through or scattered off said object, during a short period of
time; calculating an optimum exposure time for each one of repeated
detections based on said detection of ionizing radiation during a
short period of time; and moving the array of the plurality of
one-dimensional detector units relative said object while exposing
the plurality of one-dimensional detector units to ionizing
radiation, as transmitted through or scattered off said object, and
detecting repeatedly using said calculated optimum exposure time to
thereby create a two-dimensional image of the object.
26. The method of claim 25 wherein said optimum exposure time is
calculated based on a minimum or average signal value as obtained
from said detection of ionizing radiation during a short period of
time.
27. The method of claim 25 wherein said optimum exposure time is
calculated based on a minimum or average signal value within a
region of said array of one-dimensional detector units as obtained
from said detection of ionizing radiation during a short period of
time.
28. The method of claim 25 wherein said optimum exposure time is
calculated based on an integrated signal value of one or some of
the one-dimensional detector units of said array of one-dimensional
detector units, preferably the unit with lowest integrated signal
value, as obtained from said detection of ionizing radiation during
a short period of time.
29. The method of claim 25 wherein said array of one-dimensional
detector units is moved relative said object continuously, while
detecting repeatedly to create the two-dimensional image of the
object.
30. The method of claim 29 wherein said array of one-dimensional
detector units is moved relative said object at a speed, which is
based on said calculated optimum exposure time.
31. The method of claim 25 wherein said array of one-dimensional
detector units is moved stepwise relative said object, and said
repeated detection using said calculated optimum exposure time is
performed between each step of movement, while said array of
one-dimensional detector units is kept still with respect to said
object.
32. The method of claim 25 wherein said short period of time,
during which said array of one-dimensional detector units detects
ionizing radiation before or during an initial part of the
movement, is in the interval 100 ns-10 s, preferably in the
interval 1 .mu.-100 ms, and most preferably 10 .mu.s-10 ms.
33. The method of claim 25 wherein a characteristic of the object,
of which a two-dimensional image is to be recorded, is received; a
desired signal strength for the object, of which a two-dimensional
image is to be recorded, is established by means of referring to a
look-up table of desired signal strengths for various object
characteristics or by means of an algorithm; and said optimum
exposure time for each one of the repeated detections is calculated
by means of multiplying said short period of time with the ratio of
the desired signal strength and a signal strength as obtained from
said detection of ionizing radiation during a short period of
time.
34. The method of claim 25 comprising the steps of: arranging a
shielding device with a variable aperture in the path of said
ionizing radiation upstream of said object; deducing an outer shape
of said object from said detection of ionizing radiation during a
short period of time; and adjusting the variable aperture of said
shielding device to shield radiation not transmitted through or
scattered off said object, wherein said collimator device is fixed
with respect to said array of one-dimensional detector units during
the step of moving said array of one-dimensional detector units
relative said object.
35. The method of claim 25 comprising the steps of: arranging a
filter device with variable spectral transmission characteristics
in the path of said ionizing radiation upstream of said object;
deducing from said detection of ionizing radiation before or during
an initial part of the movement a measure indicative of the
contrast of the detection; and adjusting the variable spectral
transmission characteristics of said filter device in response to
said measure indicative of the contrast of the detection.
36. The method of claim 25 comprising the steps of: producing said
ionizing radiation by means of an X-ray tube having a cathode which
emits electrons and an anode on which said electrons impinge and
which is a source of X-rays, said tube having an variable operating
voltage which is the voltage drop between said anode and said
cathode, a tube current which is the current between said anode and
said cathode, and a focal spot size which is the area of said anode
on which said electrons impinge; and adjusting the variable
operating voltage of the X-ray tube in response to said detection
of ionizing radiation during a short period of time.
37. The method of claim 25 comprising the steps of: producing said
ionizing radiation by means of an X-ray tube having a cathode which
emits electrons and an anode on which said electrons impinge and
which is a source of X-rays, said tube having an operating voltage
which is the voltage drop between said anode and said cathode, a
variable tube current which is the current between said anode and
said cathode, and a focal spot size which is the area of said anode
on which said electrons impinge; and adjusting the variable tube
current of the X-ray tube in response to said detection of ionizing
radiation during a short period of time.
38. The method of claim 25 comprising the steps of: producing said
ionizing radiation by means of an X-ray tube having a cathode which
emits electrons and an anode on which said electrons impinge and
which is a source of X-rays, said tube having an operating voltage
which is the voltage drop between said anode and said cathode, a
tube current which is the current between said anode and said
cathode, and a variable focal spot size which is the area of said
anode on which said electrons impinge; and adjusting the variable
focal spot size of the X-ray tube in response to said detection of
ionizing radiation during a short period of time.
39. The method of claim 25 wherein said plurality of
one-dimensional detector units is provided in a two-dimensional
pattern on a common support structure.
40. The method of claim 25 wherein said plurality of
one-dimensional detector units are provided in rows and stacks, the
rows being parallel with the one-dimensional detector units and the
stacks being essentially orthogonal thereto, where the
one-dimensional detector units in each row are together capable of
detecting completely the object in one dimension.
41. The method of claim 40 wherein the one-dimensional detector
units of each row are staggered with an overlap between adjacent
one-dimensional detector units in the direction of the row.
42. The method of claim 25 wherein said plurality of
one-dimensional detector units are arranged in a circle, each
oriented essentially radially with respect to said circle.
43. The method of claim 25 wherein each of said plurality of
one-dimensional detector units is a gaseous-based ionizing
radiation detector, wherein electrons released by interactions
between radiation photons and the gas is extracted in a direction
essentially perpendicular to the ionizing radiation entered into
that one-dimensional detector unit.
44. The method of claim 25 comprising the steps of: arranging a
collimator of a radiation-absorbing material in the path of said
ionizing radiation upstream of said object, which collimator
includes a plurality of radiation transparent slits, the number of
the radiation transparent slits corresponding to the number of
one-dimensional detector units; and aligning the radiation
transparent slits with the one-dimensional detector units, such
that essentially planar ray bundles as transmitted through the
radiation transparent slits of the collimator irradiate the
respective one-dimensional detector units, wherein said collimator
is fixed with respect to said array of one-dimensional detector
units during the step of moving said array of one-dimensional
detector units relative said object.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to apparatuses and methods
for scanning-based two-dimensional radiation detection, and more
specifically the invention relates to automatic exposure control
therein for achieving optimum image quality.
BACKGROUND OF THE INVENTION AND RELATED ART
[0002] In digital medical X-ray diagnosis, the X-ray radiation
time, energy and flux employed must be carefully controlled to
achieve optimum quality of the images recorded.
[0003] The exposure has to be selected such that the images possess
high signal-to-noise ratio, and high dynamic range without being
overexposed, i.e. that the detector saturates. One method of
controlling the exposure of a sensitive two-dimensional detector
array, such as a CCD, is obviously to record an image, analyze it
with respect to signal strengths and contrast achieved, and then
adjust the exposure, whereafter a second high-quality image is
recorded. While such an approach provides for the recording of
high-quality images, it nevertheless suffers from a few drawbacks.
Firstly, the method is time consuming: two read-outs have to be
made for each object area to be imaged with intermediate analysis
and adjustments. Further, the radiation dose to the object area to
be imaged is higher, since it is exposed to radiation twice.
[0004] Another method, disclosed in U.S. Re. 33,634 by Yanaki,
samples the radiation passed through an object to be examined
during a short portion of the total exposure time by means of a
sensor and adjust exposure time and the voltage, current and focal
spot size of the X-ray source so that the radiation delivered by
the X-ray tube during the remainder of the exposure will produce
optimum contrast between structures within the object examined and
optimum darkening of a film, xerographic picture, fluoroscopic
image, or other recording medium. The method accounts for
variations in absorption coefficient between one object to be
radiographed and the next.
SUMMARY OF THE INVENTION
[0005] One drawback of the technique disclosed by Yanaki is that a
sensor is needed in addition to the recording medium for the
production of a two-dimensional image. Such solution is unnecessary
complicated and the sensor and the recording medium may have
different sensitivities, different dynamic ranges, and different
noise levels, which can make the calibration and the exposure
control more complicated.
[0006] A further drawback is that the sensor employed lacks
capabilities of sensing signal strengths at different positions of
the image simultaneously and/or capabilities of sensing a
differential signal with high spatial resolution, which are needed
in order to obtain the signal strength of the object region having
the highest density and thus highest absorption and the variations
in signal strengths across the image, and not only a spatially
integrated single value of the signal strength.
[0007] A main object of the invention is therefore to provide an
ionizing radiation detecting apparatus and method including an
automatic exposure control, which overcome the limitations
associated with the prior art.
[0008] In this respect there is a particular object to provide such
an apparatus and such a method, which are uncomplicated and can
still produce high-quality images with excellent signal-to-noise
ratios, dynamic range, and image contrast.
[0009] A further object of the invention is to provide such an
apparatus and such a method, which optionally incorporate a
shielding functionality integrated with said automatic exposure
control for automatically shielding radiation passing outside the
outer shape of an object to be recorded.
[0010] A yet further object of the invention is to provide such an
apparatus and such a method, which are reliable, accurate, precise
and inexpensive.
[0011] A still further object of the invention is to provide such
an apparatus, which is suitable for volume production and which has
a long lifetime.
[0012] These objects, among others, are attained by apparatuses and
methods as claimed in the appended claims.
[0013] The inventors have found that by arranging smaller
one-dimensional radiation detector units in an array, a
scanning-based detector apparatus for highly resolved
two-dimensional imaging of objects, such as e.g. breasts in
mammography examinations, is provided, which is extremely well
suited for fast and sophisticated automatic exposure control. The
detector units are distributed in the array such that the
one-dimensional images of the radiation from the plurality of
one-dimensional detector units are distributed over a substantial
portion of the two-dimensional image of the object, which is to be
recorded. The detector units may be arranged in a dense
two-dimensional array of rows and stacks, which reduces scanning
distance and provides macroscopic structure information of the
whole object area to be imaged without scanning. The detector units
may alternatively be arranged in other patterns, e.g. in a circle,
where each detector unit is oriented essentially radially with
respect to the circle.
[0014] By means of detecting ionizing radiation by the array of
one-dimensional detector units during a short period of time before
or during an initial part of a scan a picture of line images
distributed over a substantial portion of the picture is obtained
very fast, which is excellent for deriving information of the
object to be scanned, such as e.g. average, maximum and minimum
density of the object.
[0015] An optimum exposure time for each readout during the
subsequent scan or the remainder of the scan is then calculated
based on information deduced from the picture of line images and on
the short period of time, wherafter this optimum exposure time is
employed for each readout during the scan or the remainder thereof.
Hereby, an optimum image quality is achieved.
[0016] The exposure time can be calculated from a minimum or
average signal value in the picture of line images or in a limited
area thereof or from a sophisticated algorithm based on e.g. the
histogram of the picture of line images. Preferably, a minimum or
average signal value is deduced from a number of nearby line images
having the lowest average signal strength (corresponding to the
most absorbing part of the object to be scanned).
[0017] Preferably, a look-up table of desired signal strengths for
various object characteristics (e.g. different compressed breast
thicknesses) is provided, and a characteristic of the object, of
which a two-dimensional image is to be recorded, is received from
e.g. a sensor or an operator of the apparatus, wherafter the
optimum exposure time is calculated by means of multiplying the
short period of time with the ratio of the desired signal strength
and a signal strength as obtained from the picture of line
images.
[0018] Another inventive feature that may optionally be
incorporated is a collimator device with a variable aperture
arranged in the path of the ionizing radiation upstream of the
object. By means of the picture of line images, an outer shape of
the object can be determined, and the variable aperture is adjusted
to shield radiation not interacting with the object.
[0019] Further characteristics of the invention, and advantages
thereof, will be evident from the detailed description of preferred
embodiments of the present invention given hereinafter and the
accompanying FIGS. 1-6, which are given by way of illustration
only, and thus are not limitative of the present invention.
[0020] It shall be particularly emphasized that while the present
invention is described in detail as regarding X-ray radiation and
X-ray tubes the present invention is mutatis mutandis applicable
for other kinds of ionizing radiation and ionizing radiation
sources.
[0021] Further, the invention is primarily focused on medical
applications and mammography in particular, but it is nevertheless
useful for other kind of industrial applications including such as
non-destructive testing and inspection e.g. of printed circuit
boards and pipelines. Thus, while the object to be imaged will be
referred to as a breast in the description below, it shall be
appreciated that it can be exchanged for virtually any kind of
materia without departuring from the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 illustrates schematically, in a side view, a device
for X-ray examinations according to a preferred embodiment of the
present invention.
[0023] FIG. 2 is a schematic plan view of a fan beam collimator as
being comprised in the device of FIG. 1.
[0024] FIG. 3 illustrates schematically, in a front view, a
scanning-based detector arrangement as being comprised in the
device of FIG. 1.
[0025] FIG. 4 is a flow chart of a method for automatic exposure
control in the device of FIG. 1 according to a preferred embodiment
of the present invention.
[0026] FIG. 5 illustrates schematically, in a front view, the
scanning-based detector arrangement of FIG. 2, wherein shielding by
a collimator device, as being comprised in the device of FIG. 1, is
indicated.
[0027] FIG. 6 illustrates schematically, in a front view, a
scanning-based detector arrangement according to another preferred
embodiment of the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0028] With reference to FIG. 1, which illustrates schematically,
in a side view, a device preferably designed for mammography
examinations a preferred embodiment of the present invention will
be described.
[0029] From top to bottom the device comprises an X-ray source 81,
a filter device 82, a collimator device 83a, a fan beam collimator
83b, a compression plate 84 and an object table 85, and a
scanning-based detector arrangement 86 including a plurality of
one-dimensional detector units.
[0030] The X-ray source 81 is a conventional X-ray tube having a
cathode, which emits electrons, and an anode, on which said
electrons impinge, and which is a source of X-rays, said tube
having an operating voltage, which is the voltage drop between said
anode and said cathode, a tube current, which is the current
between said anode and said cathode, and a focal spot size, which
is the area of said anode on which said electrons impinge.
[0031] The operation voltage, tube current and/or focal spot size
may be adjustable. By applying a lower peak voltage to the X-ray
tube lower energy X-ray photons are produced, which are more easily
absorbed by any tissue. By increasing the current from cathode to
anode in the X-ray tube the X-ray flux is increased proportionally.
By increasing the size of the focal spot, the power rating of the
X-ray tube can be increased. For further details regarding the
effect of operation voltage, tube current and focal spot size on
radiography examinations reference is made to the Yanaki patent
(U.S. Re. 33,634), the content of which being hereby incorporated
by reference.
[0032] Just beneath the X-ray tube are placed a filter device 82
typically including thin metallic foils acting as filters to absorb
the lowest (and sometimes also the highest) energy photons, which
do not contribute significantly to the image quality but do
increase the radiation dose to the patient. The filter device may
have variable spectral transmission characteristics.
[0033] The collimator device 83a is radiation absorbing, but has a
radiation transparent controllable variable aperture, whereby large
amounts of radiation, which are not needed for the examination, may
be stopped before reaching the level of the examination object,
i.e. the breast in mammography examinations. Hereby, the amount of
scattered radiation, which increases the dose to the patient and
reduces the contrast in the image, is reduced.
[0034] The fan beam collimator 83b, schematically illustrated in
FIG. 2, may be a thin foil 51 of e.g. tungsten with multiple narrow
radiation transparent slits 52 etched away. The slits are arranged
in rows 53 and stacks 54 and are aligned with corresponding
line-shaped sensitive areas or entrance slits of the detector units
of the detector arrangement such that X-rays passing through each
slit 52 will reach a corresponding sensitive area or the detector
arrangement. The purpose of this collimator is to reduce the
radiation dose to the breast being examined.
[0035] The detector arrangement 86 is illustrated in FIG. 3 and
includes a plurality of one-dimensional detector units 41 arranged
on a common support structure 42 in a two-dimensional array of rows
44 and stacks 45 with their respective sensitive areas or entrance
slits 43 facing the front of the arrangement. For illustrative
purposes a matrix of only 4.times.10 detector units is illustrated,
i.e. each row 44 includes four detector units and each stack 45
includes ten detector units, even though it shall be appreciated
that the arrangement may include many more units. For instance if
the detector units are spaced apart by S.sub.1=5 mm (from detector
unit to detector unit) and an area of typically 20.times.20 to
50.times.50 cm.sup.2 shall be covered each stack may include 40-100
detector units. The width of each line detector unit may for
instance be 40-60 mm, and thus typically 5-12 detector units are
arranged in each row.
[0036] Further the detector arrangement of FIG. 3 may include side
and front covers (not explicitly illustrated).
[0037] During the mammography examination the breast is compressed
between the compression plate 84 and the object table 85, where the
compression plate 84 for that purpose is movable in the vertical
direction and lockable. If the device of FIG. 1 shall be used for
other kind of measurements than mammography examinations the two
compression plate 84 and object table 85 may be exchanged by a
holder or support for holding the particular object to be examined
(not illustrated).
[0038] The X-ray tube 81, the fan beam collimator 83b and the
detector arrangement 86 are attached to a common E-arm 87, which in
turn is rotatably attached to a vertical stand 88 by means of a
spindle 89 approximately at the height of the X-ray tube 81. In
this manner, the X-ray tube 81, the fan beam collimator 83b and the
detector arrangement 86 can be moved in a common pivoting movement
relative to the breast to scan the breast and produce a
two-dimensional image thereof. Assuming a distance of 5 mm between
the detector units in each stack 45 of the detector arrangement
(which correspond to the shortest possible scanning distance for
recording a complete two-dimensional image) and a distance of 65 cm
between the spindle 89 and the detector arrangement a scan
corresponds typically to a rotation of about 0.44.degree., which
typically may be performed in a few seconds. The scanning direction
is indicated by arrow 47 in FIG. 3.
[0039] The collimator device 83a is firmly attached to the vertical
stand 88, and the compression plate 84 and object table 85 are
firmly attached to a support 90, which in turn is firmly attached
to the vertical stand 88. For this purpose the E-arm 87 is provided
with two recesses or similar in the E-arm 87 (illustrated by the
dashed lines). During scanning, the collimator device 83a and the
breast are kept still.
[0040] It shall be appreciated that the device of FIG. 1 may be
modified and arranged for linear movement of the X-ray tube 81, the
fan beam collimator 83b and the detector arrangement 86 with
respect to the breast being examined.
[0041] It shall further be appreciated that the device of FIG. 1
may be modified such that the patient and the collimator device are
moved during scanning, while the X-ray tube 81, the fan beam
collimator 83b and the detector arrangement 86 are kept at
rest.
[0042] It shall be noted that the detector units 41 in each row 44
of the detector arrangement of FIG. 3 are staggered. Since the
detector units may not be capable of detecting at its extreme end
portions, the units are staggered to cover the complete distance of
20-50 cm, avoiding any "dead" zones. Where the sensitive area or
entrance slit 43 of one detector unit 41 ends, the sensitive area
or entrance slit of a further detector unit begins in each row 44.
This feature can be seen distinctly along dashed line 48 in FIG. 3
and calls for an overlap x.sub.1 between the detector units, where
x.sub.1 may typically be at least 0.05-10 mm or larger.
[0043] It shall be appreciated that the line detector units are not
necessarily arranged parallel with each other on a plane substrate,
but are arranged to point towards the radiation source used such
that radiation from the radiation source can enter the respective
detector unit.
[0044] For the same purpose the fan beam collimator 83b has slits
that are less spaced apart than the detector units and narrower
that the detector unit entrance slits. The alignment between the
radiation source (point source, line source or 2D source), the fan
beam collimator 83b and the detector arrangement 86 provides for
multiple planar radiation beams from the radiation source passing
through the fan beam collimator 83b 51 and into the individual
detector units 41 of the detector arrangement 86.
[0045] For further details regarding arrays of detector units and
the detector units themselves, reference is made to our pending
Swedish patent application No. 0200447-1 entitled Radiation
detector arrangement and filed on Feb. 15, 2001, the content of
which being hereby incorporated by reference.
[0046] Further, the device comprises a microprocessor or computer
91 provided with suitable software for controlling the device and
readout and post-processing of the signals from the line detector
units and a power supply 92 for supplying the detector units and
the microprocessor or computer 91 with power and for driving a step
motor or similar housed in the vertical stand 88 for driving the
spindle 89 and thus the E-arm 87.
[0047] In operation, X-rays are emitted from the X-ray tube 81 and
pass through the filter device 82. The collimator 83a and the fan
beam collimator 83b absorb most of the X-rays. Only x-rays passing
through the slits of the fan beam collimator 83b traverse the
breast. In the breast, the X-ray photons can be transmitted,
absorbed or scattered. The X-rays that are transmitted leave the
breast and enter into the detector units 41 of the detector
arrangement 86 and are detected.
[0048] During scanning, the E-arm 87, holding the X-ray source 81,
the fan beam collimator 83b and the detector arrangement 86, is
moved in a pivoting movement such that the detector arrangement
scans across the breast in a direction, which is essentially
parallel with the compression plate 84 and object table 85 and
parallel with the chest wall.
[0049] Each line detector unit is continuously detecting X-rays. At
regular movement intervals, typically every 10-50 micrometer, the
detected signals are read out and stored in a memory of the
microprocessor 91. In this way, each line detector unit gives a
number of line images of the breast. When the X-ray source and the
scanning are stopped, all these image segments are grouped together
by the microprocessor 91 to form a two-dimensional image.
[0050] In an alternative scanning technique the array of
one-dimensional detector units is moved relative the breast
stepwise, and the one-dimensional detector units are detecting
while the array of one-dimensional detector units is kept still
between the stepwise movements.
[0051] According to the present invention the device of FIG. 1 is
provided with an automatic exposure control preferably implemented
in microprocessor 91 by means of appropriate software. In the most
general version the microprocessor 91 is adapted to perform the
following actions:
[0052] (i) controlling the one-dimensional detector units to detect
X-rays during a short period of time before or during an initial
part of the scanning of the breast, where the short period of time
typically is in the interval 100 ns-10 s, preferably in the
interval 1 .mu.s-100 ms, and most preferably 10 .mu.s-10 ms;
[0053] (ii) calculating an optimum exposure time for each detection
of the scan based on the detection of X-rays before or during an
initial part of the scan; and
[0054] (iii) controlling the scan so as to obtain the optimum
exposure time for each of the detections during the scan to thereby
obtain a two-dimensional image of the breast having optimum
quality.
[0055] An important advantage of the exposure control implemented
in the scanning-based detector arrangement is that as a result of
the short period of time detection (pre-scan detection) a picture
is obtained, which includes a plurality of one-dimensional images
of the breast distributed over a substantial portion of the
two-dimensional image of the breast, which is to be recorded during
the subsequent scan. Thus, a very good knowledge of the breast and
its macroscopic structure can be obtained very quickly with a
minimum of dosage to the breast, which provides for an optimum
setting of the exposure time for the remaining scanning.
[0056] The optimum exposure time for the scanning-based detection
can be calculated based on a minimum or average signal value as
obtained from the detection of X-rays before or during an initial
part of the scan, or from a minimum or average signal value as
obtained within a particular region of the picture of line images,
e.g. within a centrally located region 56 as being illustrated in
FIG. 5 or from a certain number of nearby line images. Such
centrally located region 56 may have size of e.g. 2 cm.times.2 cm
or 3 cm.times.3 cm. The region is preferably located where the
signal strength is lowest (corresponding to the most absorbing
portion of the imaged breast).
[0057] Alternatively, the optimum exposure time for the
scanning-based detection can be calculated based on an integrated
signal value of one or some of the one-dimensional detector units,
e.g. of the unit with lowest integrated signal value.
[0058] Further, the speed, at which the array of one-dimensional
detector units is moved relative the breast during scanning, may be
adjusted depending on the detection of X-rays before or during an
initial part of the scan or more particularly on the optimum
exposure time calculated. If e.g. a very short optimum exposure
time is calculated, this may indicate that the scanning shall be
performed faster such that not an excessive amount of signal values
are recorded.
[0059] With reference now to FIG. 4, which is a flow chart of a
method for automatic exposure control, a preferred embodiment of
the present invention will be overviewed.
[0060] The method begins, in a step 61, with receiving a
characteristic of the breast (or other object) to be imaged. This
information may in the case of mammography be breast thickness in
compressed state with a certain force applied to the compression
plate. Alternatively, or complementary, to this the information may
relate to the estimated density of the breast tissue, e.g. defined
as its fat content versus glandular tissue. The information may be
received by microprocessor 91 by means of being entered by an
operator of the device or by means of being sensed by a sensor or
similar (not illustrated). For instance, the distance between the
compression plates would be easily measured by means of position
sensors as well as the applied force.
[0061] Alternatively, the fat content versus glandular tissue of a
breast may be determined from detections (by the arrangement 86 of
multiple one-dimensional detectors) of two short exposures of the
breast at two different compressed states (i.e. two different
forces applied to the compression plate), since the fat content and
glandular tissue have quite different absorption coefficients (not
illustrated in the flow chart).
[0062] Thereafter, in a step 62, a short exposure of the breast
under investigation is performed while the radiation transmitted is
measured by the arrangement 86 of multiple one-dimensional
detectors. The signals are, in a step 63, read out from the units
and transferred to the microprocessor 91. Due to the construction
of the detector arrangement, the detection and readout may be
performed extremely fast.
[0063] From the signals, which represent a number of well
distributed line images of the breast, a signal value, e.g. a count
rate, is, in a step 64 deduced. This signal value may be deduced in
a number of manners, e.g. as described above or by a sophisticated
method taking the complete histogram and/or spatial signal
information into account.
[0064] Next, a search is, in a step 65, performed in a look-up
table stored in microprocessor 91, or in an accessible memory (not
explicitly illustrated), which contains a table of desired signal
strengths for various breast characteristics, and optionally
thickness thereof, as being entered by the manufacturer of the
device or by an operator. The desired signal strengths may be
determined from calculations to achieve an optimum or acceptable
signal-to-noise level, dynamic range or contrast of the
subsequently recorded two-dimensional image, or they may be
established by regulations.
[0065] The search is based on the input in step 61 and a desired
signal strength for the scan is determined. Alternatively, instead
of using a look-up table, the desired signal strength may be
determined by means of employing an appropriate algorithm.
[0066] Then, in a step 66, an optimum exposure time for the breast
at current settings is calculated based on the desired signal
strength, the signal value deduced in step 64 and the exposure time
used in step 62, and the exposure time setting is, in a step 67,
adjusted to the optimum exposure time calculated, whereafter the
method may be ended, and the device is ready to scan the
breast.
[0067] A further feature of the method is that radiation not used
for the scanning can be shielded. Thus, in a step 68 (which has to
be performed after step 63, but may be performed independently of
method steps 64-67) the picture elements (pixels) of the line
images having a "full" signal strength, i.e. where no absorption at
all has occurred, which in turn indicates that the X-rays are not
transmitted through the breast, are deduced. Hereby, the outer
shape of the breast may be determined. Then, in a step 69, the
variable aperture of the collimator device 83a of the device of
FIG. 1 is controlled to adjust to the outer shape of the breast,
such that radiation not transmitted through the breast is stopped
from passing through the collimator device. In such manner the
amount of scattered radiation, which may increase the dose to the
patient and reduce the image contrast, can med reduced.
[0068] A still further feature of the method is that the variable
spectral transmission characteristics of the filter device 82
and/or the operation voltage of the X-ray tube 81 can be adjusted.
Thus, in a step 70 (which has to be performed after step 63, but
may be performed independently of method steps 64-67 and 68-69) a
measure indicative of the contrast in the picture of the line
images. Such measure is preferably related to the variations of the
signal strengths of the picture elements (pixels) in the line
images, or the signal strength of detected X-rays for different
thicknesses of the breast as controlled by the compression
unit.
[0069] Next, a search is, in a step 71, performed in a look-up
table stored in microprocessor 91, or in an accessible memory (not
explicitly illustrated), which contains a table of desired contrast
levels e.g. for various breast characteristics. The search may be
based on the input in step 61 and a desired contrast level for the
scan is determined. Instead of using a look-up table, an
appropriate algorithm may be applied to determine a desired
contrast level.
[0070] The desired contrast level for the scan may in the case of
mammography alternatively, or additionally, be based on (i) the
change in compressed breast thickness caused by a change in
compression force, and/or (ii) the signal levels as obtained from
two exposures of the compressed breast at different compression
forces.
[0071] Then, in a step 72, the measure indicative of the contrast
in the picture of the line images is compared with the desired
contrast level and based on this comparison the variable spectral
transmission characteristics of the filter device 82 and/or the
operation voltage of the X-ray tube 81 can be adjusted to obtain
the desired contrast level in the subsequent scan.
[0072] Such adjustment may call for a further exposure time
adjustment to take into account the altered spectrum of the X-rays
transmitted through the breast and subsequently detected, and thus
steps 62-67 may have to be repeated, e.g. using different filters
and/or different compressions of the breast.
[0073] Yet further, if the optimum exposure time calculated in step
66 is very long an increase of the X-ray flux may be required. Very
long exposure times may be unpleasant to a patient being examined,
and further there is risk that the patient is moving and thus
blurring the image recorded. Hence, the method as described above
may be modified in the following manner.
[0074] If the optimum exposure time calculated in step 66 is longer
than a particular threshold value (as set by the manufacturer or
the operator possibly depending on the kind of measurement
performed) then the tube current of the X-ray tube is increased and
so is possibly also the focal spot size (not illustrated).
[0075] It shall be appreciated that the plurality of
one-dimensional detector units 41 may be distributed arbitrary in
an array as long as they are located such that the one-dimensional
images of the ionizing radiation from them are distributed over a
substantial portion of the two-dimensional image to be
recorded.
[0076] For instance, detector units 41 may be arranged in a circle
on a common circular support 42' as illustrated in FIG. 6, where
each detector unit 41 is oriented essentially radially with respect
to the circle and has its sensitive area or entrance slit 43 facing
the front of the arrangement. The illustrated arrangement has one
very wide detector unit arranged across the complete diameter of
the support 42', ten less wider detector units symmetrically
arranged with respect to the very wide unit, and twelve narrower
detector units 41, each symmetrically arranged between two adjacent
ones of the wider ones of the detector units.
[0077] This arrangement is during scanning rotated in the plane of
the support 42' with respect to the breast or other object to be
imaged as illustrated by arrow 47'. Preferably, the arrangement of
FIG. 6 is rotated by means of a centrally located spindle 89'
attached to the arrangement from the backside. One complete
two-dimensional image may be recorded by rotating the array an
angle corresponding to a circumferential distance s.sub.1. If a fan
beam collimator is to be used this has to be rotated together with
the detector arrangement to keep the alignment during scanning.
[0078] For further details regarding such circular arrays of
detector units, reference is made to our pending Swedish patent
application No. 0200446-3 entitled Radiation detector arrangement
and filed on Feb. 15, 2001, the content of which being hereby
incorporated by reference.
[0079] It shall further be appreciated that the present invention
is equally applicable for recording two-dimensional images of
radiation as scattered off an object instead of being transmitted
there through.
[0080] It shall still further be appreciated that the detector
units of the of the present invention may of virtually any kind as
long as they are one-dimensional detectors capable of recording
one-dimensional images of the ionizing radiation, to which they are
exposed.
[0081] However, a preferred line detector unit is the gaseous-based
ionization detector, optionally provided with an electron avalanche
amplifier, and particularly such gaseous-based ionization detector
wherein the freed electrons are drifted in a direction essentially
perpendicular to the direction of the incident ionization.
[0082] For further details regarding different kind of
gaseous-based detector units for use in the present invention,
reference is made to the following U.S. patent application Ser.
Nos. by Tom Francke et al. and assigned to XCounter AB, which
applications are hereby incorporated by reference: 08/969,554
(issued as U.S. Pat. No. 6,118,125); 09/443,292; 09/443,320;
09/443,321; 09/444,569; 09/550,288; 09/551,603; 09/552,692;
09/698,174; 09/708,521; 09/716,228; and 09/760,748.
* * * * *