U.S. patent application number 09/745886 was filed with the patent office on 2001-08-23 for method and system of compensation of thickness of an organ.
Invention is credited to Lienard, Jean, Muller, Serge, Nicolas, Francois, Rick, Andreas, Soubelet, Elisabeth.
Application Number | 20010016030 09/745886 |
Document ID | / |
Family ID | 9553918 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010016030 |
Kind Code |
A1 |
Nicolas, Francois ; et
al. |
August 23, 2001 |
Method and system of compensation of thickness of an organ
Abstract
Method and system for compensating for the thickness of an organ
in a radiology instrument, in which an image of the radiological
thicknesses of the organ through which the X-ray beam has passed is
calculated on the basis of a digitized image, the thickness image
is filtered using a low-pass filter in order to obtain a
low-frequency image, the low-frequency image is subtracted from the
radiological thickness image in order to obtain a contrast image,
the lowfrequency image is processed using a pre-recorded table
taking into account a contract .chi. selected by a user in order to
obtain an image with reduced dynamic range, and the image with
reduced dynamic range is added to the contrast image in order to
obtain a compensated thickness image, the pixels having a level
below or above a predetermined threshold being returned at least to
the value of the said threshold, while preserving the differences
and real ratios between the anatomical structures.
Inventors: |
Nicolas, Francois;
(Palaiseau, FR) ; Lienard, Jean; (Clamart, FR)
; Muller, Serge; (Guyancourt, FR) ; Soubelet,
Elisabeth; (Meudon, FR) ; Rick, Andreas;
(Plaisir, FR) |
Correspondence
Address: |
Jay L. Chaskin
General Electric Company
3135 Easton Turnpike
Fairfield
CT
06431-0001
US
|
Family ID: |
9553918 |
Appl. No.: |
09/745886 |
Filed: |
December 21, 2000 |
Current U.S.
Class: |
378/98.11 ;
378/37; 378/98.8 |
Current CPC
Class: |
G06T 2207/10116
20130101; G06T 5/40 20130101; G06T 5/009 20130101; G06T 2207/30068
20130101 |
Class at
Publication: |
378/98.11 ;
378/98.8; 378/37 |
International
Class: |
H05G 001/64 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 1999 |
FR |
99 16586 |
Claims
What is claimed is:
1. A method of compensation of thickness of an object in a
radiological imaging apparatus having radiation source and a means
of detection of the radiation source after it has crossed the
object, the means of detection being capable of converting the
radiation source into a digital electronic signal, in which, from a
digitized image, an image of the radiological thicknesses of the
object crossed by the radiation source is calculated, the image of
radiological thicknesses is filtered by a low-pass filter in order
to obtain a low-frequency image, the low-frequency image of
radiological thicknesses is subtracted in order to obtain a
contrast image, the low-frequency image is processed by a
prerecorded table or calculated in real time taking into account a
contrast X chosen by a user in order to obtain an image with
reduced dynamics and the image with reduced dynamics and the
contrast image are added in order to obtain a compensated thickness
image, the pixels of level lower or higher than a predetermined
threshold having been brought back at least to the value of the
threshold, while preserving the differences and the real relations
between the internal structures of the object.
2. The method according to claim 1, in which calculation of the
image with reduced dynamics takes into account a range of gray
levels of width and center indicated by a user.
3. The method according to claim 2, in which processing of the
low-frequency image for obtaining the image with reduced dynamics
is carried out in accordance with the width and center of the range
of gray levels.
4. The method according to claim 1 in which processing of the
low-frequency image for obtaining the image with reduced dynamics
is carried out by means of a digital table or an analytical
law.
5. The method according to claim 2 in which processing of the
low-frequency image for obtaining the image with reduced dynamics
is carried out by means of a digital table or an analytical
law.
6. The method according to claim 3 in which processing of the
low-frequency image for obtaining the image with reduced dynamics
is carried out by means of a digital table or an analytical
law.
7. The method according to claim 4, in which processing of the
low-frequency image follows a monotone law.
8. The method according to claim 5, in which processing of the
low-frequency image follows a monotone law.
9. The method according to claim 6, in which processing of the
low-frequency image follows a monotone law.
10. The method according to claim 7, in which processing of the
low-frequency law follows a linear law of slope .alpha..
11. The method according to claim 8, in which processing of the
low-frequency law follows a linear law of slope .alpha..
12. The method according to claim 9, in which processing of the
low-frequency law follows a linear law of slope .alpha..
13. The method according to claim 6, in which the slope .alpha.
evolves in a manner inversely proportional to the contrast
.chi..
14. The method according to claim 11, in which the slope .alpha.
evolves in a manner inversely proportional to the contrast
.chi..
15. The method according to claim 12, in which the slope .alpha.
evolves in a manner inversely proportional to the contrast
.chi..
16. The method according to claim 1 in which a low-frequency image
is stored in a memory and, on a change of contrast, the
low-frequency image is read in the memory and the necessary
processings and calculations are carried out.
17. The method according to claim 1 in which, on a change of
contrast, the parameters of the filter applied to the image of
radiological thicknesses by a low-pass filter are adapted in order
to obtain a low-frequency image and the necessary processings and
calculations are carried out.
18. A system of compensation of thickness of an object in a
radiological imaging apparatus, of the type comprising a radiation
source and a means of detection of the radiation source after it
has crossed the object, the means of detection being capable of
converting the radiation source into a digital electronic signal,
including a means for calculating, from a digitized image, an image
of the radiological thicknesses of object crossed by the radiation
source, low-pass filter for obtaining radiological thicknesses from
the image, a low-frequency image, means for subtracting the
low-frequency image from the radiological thicknesses in order to
obtain a contrast image, means for processing the low-frequency
image according to a prerecorded table taking into account a
contrast .chi. chosen by a user in order to obtain an image with
reduced dynamics and means for adding the image with reduced
dynamics and the contrast image in order to obtain a compensated
thickness image, the pixels of level lower or higher than a
predetermined threshold having been brought back at least to the
value of the threshold, while preserving the differences and the
real relations between the features of the object
Description
BACKGROUND OF THE INVENTION
[0001] The present invention concerns the field of radiological
imaging to visualize an organ or part of an organ, generally of the
human body.
[0002] Radiography is conventionally carried out with sensitive
films exposed by X-rays after they cross the organs to be studied.
Radiologists have been trained in the interpretation of such
images. The new imaging technologies--solid state detector and
digital acquisition system--must be adapted to common practices and
must provide an equivalent perception of the pertinent information
that radiologists are accustomed to examining. In particular, one
of the requirements that digital systems must satisfy consists of
reducing the extent of the gray level dynamics in order to simulate
a conventional film as faithfully as possible. For this purpose,
the digital image is displayed on a screen that the radiologist
adjusts interactively in order to identify all the clinical signs
by perceiving the relations between the different image components.
But the precise perception of density information by means of the
image displayed on the screen is limited by the dynamics of the
screen. Now, one must pass automatically from the high contrast of
the original image, for example, in the order of 30 to 50, to the
low contrast offered by a video screen.
[0003] Physicians commonly apply configuration techniques to
patients with a view to limiting the dynamics of acquired images,
for example, by compression of thick regions and/or by addition of
absorbent substances in order to compensate for thin zones. In the
case of X-ray mammography, the breast is compressed to the smallest
and most constant possible thickness. In the field of cardiology,
contour filters are used to avoid problems associated with weakly
absorbent zones of the chest, such as the lungs. These filters
consist of plates of shape complementing that of the heart, made of
materials of given X-ray absorption coefficients.
[0004] However, these techniques prove insufficient for digital
image acquisition and processing and cumbersome to use, if not
uncomfortable for the patient.
[0005] The present invention is intended to remedy the
above-mentioned problems.
BRIEF SUMMARY OF THE INVENTION
[0006] The embodiments of the present invention compensate for
variations of thickness, in particular at the limit between
high-density regions and lowdensity regions of the organ X-rayed,
in an adaptive manner, depending on characteristics of the image
visualized, chosen by a user.
[0007] The present invention is intended to offer an image on which
tissues presenting absorptions different from one another are
naturally and exploitably represented.
[0008] The method of compensation of thickness of an organ is
designed for an X-ray apparatus of the type comprising an X-ray
source and a means of detection of the X-ray beam after it has
crossed the organ, the means of detection being capable of
converting the X-ray beam into a digital electronic signal. From a
digitized image, an image of the radiological thicknesses of organs
crossed by the X-ray beam is calculated, the image of radiological
thicknesses is filtered by a low-pass filter in order to obtain a
low-frequency image, the low-frequency image is subtracted from the
image of the radiological thicknesses in order to obtain a contrast
image, the low-frequency image is processed by a prerecorded table
taking into account a contrast .chi. chosen by a user in order to
obtain an image with reduced dynamics, and the image with reduced
dynamics and the contrast image are added to obtain a compensated
thickness image. The pixels of level lower or higher than a
predetermined threshold are brought back at least to the value of
the threshold, while preserving the differences and the real
relations between the anatomical structures.
[0009] The thickness of an organ measured by X-rays is called
radiological thickness, in other words, taking into account the
absorption of the materials crossed. For example, 1 cm of bone has
the same radiological thickness as 10 cm of water.
[0010] The thickness image can be obtained by means of Lambert's
law: I=I.smallcircle.e.sup.-.mu..sup..sub.t with I the number of
photons received at a given point of the means of detection,
I.smallcircle. the number of photons which would be received at a
given point of the means of detection, if the organ was not present
in the field of view, .mu. the coefficient of linear attenuation of
the X-rays by the material crossed, and t the thickness of material
crossed, from which one deduces:
[0011] In I.smallcircle.-1n I=.mu.t, the product .mu.t
corresponding to the gray level for a pixel of a radiological
thickness image.
[0012] Calculation of the image with reduced dynamics
advantageously takes into account a range of gray levels of width
and center indicated by a user.
[0013] In an embodiment of the invention, the low-frequency image
processing for obtaining the image with reduced dynamics is carried
out as a function of the width and center of the passband.
[0014] In an embodiment of the invention, the low-frequency image
processing for obtaining the image with reduced dynamics is carried
out by means of a digital table or an analytical law.
[0015] In an embodiment of the invention, the low-frequency image
processing follows a monotone law.
[0016] In an embodiment of the invention, the low-frequency image
processing follows a linear law of slope .alpha..
[0017] The slope cc advantageously evolves in a manner inversely
proportional to the contrast .chi..
[0018] In an embodiment of the invention, the low-frequency image
is stored in a memory and, on a change of contrast, the
low-frequency image is read in the memory and the necessary
processings and calculations are carried out.
[0019] In another embodiment of the invention, on a change of
contrast, the image of radiological thicknesses is filtered by a
low-pass filter in order to obtain a low-frequency image and the
necessary processings and calculations are carried out.
[0020] The present invention also concerns a system of compensation
of thickness of an organ in an X-ray apparatus. The X-ray apparatus
is of the type comprising an X-ray source and a means of detection
of the X-ray beam after it has crossed the organ, the means of
detection being capable of converting the X-ray beam into a digital
electronic signal. The system includes a means of calculating, from
a digitized image, an image of the radiological thicknesses of
organs crossed by the X-ray beam, a low-pass filter for obtaining
radiological thicknesses from the image, a low-frequency image, a
means for subtracting the low-frequency image from the radiological
thicknesses in order to obtain a contrast image, a means for
processing the low-frequency image according to a prerecorded table
taking into account a contrast .chi. chosen by a user in order to
obtain an image with reduced dynamics and a means for adding the
image with reduced dynamics and the contrast image in order to
obtain a compensated thickness image, the pixels of level lower or
higher than a predetermined threshold having been brought back at
least to the value of the threshold, while preserving the
differences and the real relations between the anatomical
structures.
[0021] In an embodiment of the invention, the low-pass filter is
calculated to eliminate the pixels corresponding to an organ loaded
with contrast medium in the compensation image and thus maintain
them in the compensated image.
[0022] Thus, the invention offers an image processing supplying an
image equivalent to that which would be obtained by using an
absorbent liquid on the edges of the organ over a part of their
height and makes it possible to simulate a physical phenomenon,
which provides a better understanding of the optimal adjustment of
the parameters that has to be made. This image processing confers a
natural and pleasing appearance to the different tissues of the
organ studied. Each of those tissues is seen at the same time and
on the same image with, on the one hand, a natural appearance and,
on the other, a precision and quality enabling the user to derive
important information from it, as if the user were interested only
in one particular tissue. In the field of mammography, the
glandular zones and the adipose zones are observed simultaneously
with the possibility of deriving information from the same image on
both of the zones.
[0023] The method is adaptable to existing X-ray machines and can
be applied to the radiography of any organ at all.
[0024] Whatever the extent and centering of the range of gray
levels chosen by the user for visualization, a choice made to favor
certain types of organs or tissues in the organ studied, the image
is processed in a manner adapted to that choice.
BRIEF DESCRIPTION OF THE INVENTION
[0025] The present invention will be better understood by studying
the detailed description of an embodiment taken by way of
non-limitative example and illustrated by the attached drawings, in
which:
[0026] FIG. 1 is a schematic representation of an organ composed of
zones of different thicknesses;
[0027] FIG. 2 is a schematic representation of an organ composed of
zones of different compensated thicknesses;
[0028] FIG. 3 is a schematic view of a breast compressed for a
mammogram; and
[0029] FIG. 4 is a schematic view of a system according to an
embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0030] As can be seen in FIG. 1, the dynamics of an image of an
organ can exceed the dynamics of thicknesses representable by the
display screen and represented by the two horizontal dotted lines.
Thus, blocks 1 to 3 are at too high a gray level, while blocks 8
and 9 are at too low a level. The physical phenomena or parts of
the organ found in such zones are therefore not satisfactorily
displayed. By maintaining the same screen dynamics, it is
conceivable, first of all, to display blocks 1 to 4, then, secondly
and after modification of the width and center of the range of gray
levels of the screen, blocks 5 to 9. By taking the example of the
mammogram, a first image would be obtained, on which only the
contour of the breast would be seen, without viewing its internal
tissues. The second image would show solely its internal tissues,
adipose zone or glandular zone, without its contour being
perceived, which is hardly practical and complicates the work of
the radiologist.
[0031] In an embodiment of the invention, the digital compensation
of the crude image is based on the following principle: the image
compensation process simulates the addition or removal of an
appropriate quantity of material in boundary regions, for example,
between zones of high absorption and zones of low absorption of
X-rays, so that the dynamics are reduced, while taking into account
the constraint of preservation of the difference and real relations
between the anatomical structures. Thus, it can be seen that in
FIG. 2, blocks 1 to 9 are contained within the limits of the screen
dynamics. Blocks 1 to 3, instead of being white-saturated, are
brought back just below the upper limit of saturation and preserve
their original relation, namely: block 1 lighter than the block 2
lighter than the block 3, etc. The same is true for blocks 8 and 9
which, instead of being black-saturated, are situated in proximity
to the lower limit of the dynamics, so as to be visible and, while
preserving their original difference, block 7 lighter than the
block 8 lighter than the block 9.
[0032] In FIG. 3, a breast 10 can be seen, compressed between a
plate 1 1 and a table 12 which form part of an X-ray apparatus, not
represented. The breast is subjected to an X-ray beam 13 emitted by
a source not represented. A digital detector, not represented, is
placed on the path of the X-ray beam 13 after crossing the breast
10. It can be observed that some X-rays, referenced 14, cross only
a part of the thickness of the breast owing to its rounded shape at
the tip. Consequently, these X-rays are less attenuated than those
having crossed the total thickness of the breast, which risks
leading to a saturation of the image portion corresponding to rays
14 and to the display of a black screen portion. In fact, a white
portion corresponds to a thick zone and a black portion corresponds
to a thin zone by convention.
[0033] As can be seen in FIG. 4, an application of the system
according to the invention consists of a mathematical processing
module 15 capable of applying to an image received in input from a
digital detector, not represented, a logarithm function. In fact,
the level of each pixel of the input image is representative of the
intensity of the X-radiation received. The logarithm function makes
it possible to pass from radiological intensities to
thicknesses.
[0034] For a given point of the image, the number I of photons
received by a given point of the means of detection is determined
by the following equation: I=Imax.times.e.sup.-.mu..sup..sub.t, t
being the thickness of the organ crossed by the X-rays and .mu.
being the density of the organ crossed. Applying the logarithm
function leads to .mu..times.t=1n (Imax)-1n (I), Imax being known
and corresponding to the number of photons which would be received
on a given point of the means of detection if the organ was not
present in the field of view. The product .mu..times.t is therefore
known, being called densitometric or radiological thickness.
[0035] In practice, it may be preferred to apply a slightly
different equation, based on quantity G, which is the gray level of
a given point of the means of detection: .mu.t=K.times.1n
(Gmax+1)=K.times.1n (G+1). Gain K is applied to protect the
variation of gray level higher than 1% of maximum intensity, such
as K.times.1n (Gmax+1)-K.times.1n (0.99 Gmax+1).gtoreq.1. The
application of that logarithm function makes it possible to obtain
a histogram of thickness.
[0036] The processing system further includes a filter 16 of
low-pass type for receiving the pixels X of an input thickness
image and supplying on output pixels filtered or of input mask Mx,
an operator 17 placed downstream from the filter 16 and performing
a function noted .PHI., and a summator-subtractor 18 receiving the
pixels of the input thickness image on the output of the operator
15 and also receiving the pixels filtered on the output of the
filter 16 in order to subtract the filtered pixels from the pixels
of the input thickness image and to supply a contrast image of
pixels C=X-Mx. The filter 16 may preserve the structures of size
greater than a nominal value, being associable with the size of
structures of interest present in the organ, for example, equal to
2 cm.
[0037] The processing system includes a summator-adder 19 placed
downstream from the operator 17 and from the summator-subtractor 18
receiving the contrast pixels C and the pixels My. The output
pixels of the operator 17 are noted My with My=.PHI.(Mx). The
summator-adder 19 performs the operation of addition of the
contrast pixels C and output pixels My of the operator 17 and
supplies on output the output pixels noted Y, with Y=C+My or even
Y=X-Mx+.PHI.(Mx).
[0038] An image transformation is thus made. The function .PHI. of
the operator 17 is a monotone function with adjustable slope in
order to simulate the addition or removal of an appropriate
quantity of material in different regions, so that the dynamics of
the image will be reduced, while preserving the real relations
between the anatomical structures. The preservation of the
relations is associated with the monotony of the function .PHI..
Preferably, .PHI. is a linear function of slope .alpha., the slope
.alpha. being adapted to the contrast chosen by the user. If
.alpha.=1, then the dynamics are preserved and My=Mx. The operator
17 can contain a prerecorded table which takes into account the
contrast .chi. of the image having to be displayed, as chosen by a
user. As a variant, the operator 17 can follow an analytical
law.
[0039] The contrast .chi. of the image having to be displayed may
be determined by a user by choosing a range of gray levels Ww of
the image and a center of the range of gray levels, for example, by
keyboard command or by means of a mouse, not represented. The slope
.alpha. established by a table or by an analytical law evolves in a
manner inversely proportional to the contrast .chi. and, more
precisely, to the width Ww of the range of gray levels. In other
words, when the user changes the width Ww of the range of gray
levels from Ww to W'w, the slope evolves from .alpha. to .alpha.',
with Ww/W'w=.alpha.'/.alpha.. The general appearance of the image
is thus preserved and so is that of the tissues observed, whatever
their type, for example, pectoral, glandular, adipose, subcutaneous
or cutaneous in mammography.
[0040] On such change of contrast, the different stages can be
repeated from the thickness image stored in a memory, not
represented, of the system which is directly accessed or by means
of a data bus.
[0041] The filtered image can also be stored in a memory and repeat
only the stages subsequent to filtering, which reduces the times of
calculation, but slightly complicates the management of memory
access.
[0042] If a particular pixel Xn is of gray level representative of
a thickness greater than a pixel Xm and less than a pixel Xp, the
same relation will exist between the output pixels: Ym<Yn<Yp.
The dynamics of an image is thus reduced in a proportion which will
depend on the dynamics of an input image sent by a digital detector
and the dynamics of an output image, so that the display means are
capable of making it available to a user. The different processings
are carried out on so-called "thickness" images, that is, in which
the value of each pixel is representative of the thickness of the
tissues crossed by the X-rays.
[0043] The output pixels Y are sent to a display table 20 from to
which a user can choose the contrast .chi. of the image
visualized.
[0044] The processing method is available for visualizing on the
same image tissues of very different radiological characteristics,
by breaking free, at least in part, of the constraints of external
luminosity associated with illumination of the site on which
visualization takes place.
[0045] The image obtained according to disclosed embodiments
presents no artifact and is of normal appearance. The method can be
used in various fields of radiology, while being adapted to digital
detection and processing, without changing the users' practices. An
increase of contrast is prevented from being manifested by a
reduction of dynamics of the image displayed, as commonly occurs.
An increase of contrast is also prevented from being manifested by
a saturation of certain zones of the image representing tissues
that it is desirable to observe at the same time as other tissues
represented by nonsaturated zones, as, for example, in mammography,
the saturation of adipose zones becoming too dark if the contrast
is increased on the glandular zones or, conversely, the saturation
of the adipose zones becoming too dark if the contrast is increased
on the glandular zones or, conversely, saturation of the glandular
zones becoming too light, if the contrast on the adipose zones is
increased. This is due to the fact that contrast is taken into
account in image processing for display.
[0046] Various modifications in structure and/or steps and/or
function may be made by one skilled in the art without departing
from the scope of the invention.
* * * * *