Method of automatically determining imaged body posture in medical image display

An accumulative histogram of an image signal bearing a transmitted image of a human body is generated. The rate of change of the accumulative histogram is determined in a predetermined region of the image, and the imaged posture of the image is determined from the rate of change. Alternatively, to determine the imaged posture, the separation of a histogram is determined, or the pattern of the histogram is compared with a plurality of reference histogram patterns. The derivative of second order of a function approximated by the histogram pattern may be checked for its sign, or the histogram may be checked to determine whether it is of a single-peak form or a double-hump form. The dispersion of the histogram may also be determined.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of automatically determining the 
posture of an imaged human body in a medical imaging system such as a 
radiation imaging system. 
2. Description of the Prior Art 
A certain phosphor, when exposed to a radiation such as X-rays, 
.alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet 
rays, stores a part of the energy of the radiation. When the phosphor 
exposed to the radiation is exposed to stimulating rays such as visible 
light, the phosphor emits light (stimulated emission) in proportion to the 
stored energy of the radiation. Such a phosphor is called a stimulable 
phosphor. 
There has been proposed a radiation image recording and reproducing system 
employing such a stimulable phosphor. In the proposed radiation image 
recording and reproducing system, the radiation image information of an 
object such as a human body is recorded on a sheet having a layer of 
stimulable phosphor, and then the stimulable phosphor sheet is scanned 
with stimulating rays such as a laser beam to cause the stimulable 
phosphor sheet to emit light representative of the radiation image. The 
emitted light is then photoelectrically detected to produce an image 
signal that will be recorded as a visible image of the object on a 
recording medium such as a photographic material or displayed as a visible 
image on a CRT or the like (see Japanese Laid-Open Patent Publications 
Nos. 55-12429 and 56-11395, for example). 
The radiation image recording and reproducing system is highly advantageous 
over conventional radiographic systems employing silver-salt photographs 
in that images can be recorded in a very wide range of radiation exposure. 
More specifically, it is known that the amount of light emitted from 
stimulable phosphor upon exposure to stimulating rays is proportional to 
the amount of radiation to which the stimulable phosphor has been exposed, 
in a highly wide range. Even if the amount of radiation exposure varies 
greatly under various imaging conditions, a read-out gain is set to a 
suitable level, and the amount of light emitted from a stimulable phosphor 
sheet is read and coverted into an electric signal by a photoelectric 
transducer. The electric signal is processed to produce a visible 
radiation image which is recorded on a recording medium such as 
photographic material or displayed as a visible image on a CRT or the 
like. By selecting a suitable read-out gain setting, the radiation image 
can be obtained which is not affected by variations in the amount of 
radiation exposure. 
In the radiation image recording and reproducing system, in order to 
eliminate influences due to varying imaging conditions or obtain a 
radiation image which can well be observed, "recording information" 
indicating either a recording condition in which the radiation image 
information is recorded on a stimulable phosphor sheet or a recording 
pattern determined by the area of an object to be imaged such as a chest 
or a stomach, and an imaging process such as a simple imaging process or a 
contrast radiographic process, is reviewed prior to the output of a 
visible image to be observed. Then, the read-out gain is adjusted to a 
suitable level based on the recording information, and a recording scale 
factor is determined in order to optimize the resolution according to the 
contrast of the recording pattern. Where the image signal which is read 
out is processed such as for gradation processing, image processing 
conditions should be optimized. 
One known process of reviewing recording information of a radiation image 
prior to the output of a visible image is disclosed in Japanese Laid-Open 
Patent Publication No. 58-67240. According to this known process, 
stimulating light having a level lower than the level of stimulating light 
to be applied in a "main reading mode" for obtaining a visible image to be 
observed is used to read, in a "preliminary reading mode", the recording 
information of a radiation image stored on a stimulable phosphor sheet 
prior to the main reading mode. In the preliminary reading mode, the 
condition in which the radiation image is recorded can roughly be 
understood. For effecting the main reading mode, the read-out gain is 
suitably adjusted, and the recording scale factor is determined, or image 
processing conditions are selected, on the basis of the information 
obtained in the preliminary reading mode. 
According to the above conventional method, the recording condition and 
recording pattern of the radiation image information recorded on the 
stimulable phosphor sheet can be known prior to the main reading mode. 
Therefore, even if a reading system having a very wide dynamic range is 
not relied upon, a radiation image that can well be observed can be 
produced by adjusting the read-out gain to a suitable level and 
determining the recording scale factor based on the recording information, 
and processing an electric signal generated in the main reading mode 
according to the recording pattern. 
Once the reading conditions and/or the image processing conditions of the 
radiation image information are thus determined, the densities of areas of 
interest in reproduced images of an object may vary from each other when 
the object is imaged at different postures. More specifically, in order to 
diagnose the thoracic vertebra of a patient, the chest is imaged from its 
front side as shown in FIG. 2A of the accompanying drawings and from a 
lateral side thereof as shown in FIG. 2B. When the chest is imaged from 
its front side as shown in FIG. 2A, the area of interest or the thoracic 
vertebra K overlaps the mediastinum which is less permeable to radiation. 
Therefore, the amount of radiation stored in the area of the stimulable 
phosphor sheet corresponding to the thoracic vertebra is low, and the 
amount of light which will later be emitted from this area is also low. 
When the chest is laterally imaged as shown in FIG. 2B, the thoracic 
vertebra K lies over the lung P which is more permeable to radiation. 
Consequently, the amount of radiation stored in the area of the stimulable 
phosphor sheet corresponding to the thoracic vertebra is high, and the 
amount of light which will later be emitted from this area is also high. 
Since the maximum value Smax and the minimum value Smin of an image signal 
read from the stimulable phosphor sheet remain substantially unchanged 
regardless of whether the chest is imaged from its front side or lateral 
side, the reading conditions and/or the image processing conditions which 
are determined on the maximum value Smax and the minimum value Smin are 
substantially the same when the chest is imaged from its front side and 
lateral side. When a radiation image is reconstructed in the main reading 
mode under these reading and/or image processing conditions, therefore, 
the imaged thoracic vertebra is of a relatively low density when it is 
imaged front its front side, and of a relatively high density when it is 
imaged from its lateral side. 
The same problems may be caused for other reasons than those described 
above. For example, in order to diagnose a joint J in a foot, the foot is 
imaged from its front side as shown in FIG. 11A and from a lateral side 
thereof as shown in FIG. 11B. When the foot is imaged from its front side 
as shown in FIG. 11A, the heel Y overlaps the long bones from the instep 
to the toes which are less permeable to radiation. Therefore, the amount 
of radiation stored in the area of the stimulable phosphor sheet 
corresponding to the heel is low. The amount of light which will later be 
emitted from this area is lower than the amount of light which will be 
emitted from the area of interest or the joint J. When the foot is 
laterally imaged as shown in FIG. 11B, the heel Y does not lie over the 
instep and the toes of the foot. Consequently, the amount of radiation 
stored in the area of the stimulable phosphor sheet corresponding to the 
heel as imaged from the side is higher than the amount of radiation stored 
in the area of the heel as imaged from the front. The amount of light 
which will later be emitted from the area of the heel as imaged from the 
side is substantially the same as the amount of light that will be emitted 
from the area of the joint J as imaged from the side. Since the minimum 
value Smin of an image signal read from the stimulable phosphor sheet for 
the front foot image is lower than the minimum value of an image signal 
read for the side foot image, when images are read and reproduced under 
the reading conditions and/or the image processing conditions which are 
determined on the maximum value Smax and the minimum value Smin, the 
imaged foot joint is of a relatively high density when it is imaged from 
its front side, and of a relatively low density when it is imaged from its 
lateral side. 
It may be possible to dispense with the preliminary reading mode, and 
establish image processing conditions appropriately based on an image 
signal read in the main reading mode. However, the above drawbacks are 
also experienced with such an alternative. 
To solve the aforesaid problems, it has been customary to enter information 
indicating what posture the object is taking while it is being imaged, 
into an image reading device or an image processing device, when radiation 
image information is read from a stimulable phosphor sheet, and to 
establish reading conditions and/or image processing conditions based on 
the entered posture information. 
However, it has been highly tedious and timeconsuming to enter posture 
information each time radiation image information is to be read from a 
stimulable phosphor sheet. In addition, the operator may enter wrong 
posture information in error. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method of 
automatically determining the imaged posture of an object image which is 
recorded on a stimulable phosphor sheet, for example, for medical 
purposes. 
According to the present invention, there is provided a first method of 
determining the imaged posture of a medical image, comprising the steps of 
generating an accumulative histogram of an image signal bearing a 
transmitted image of a human body, determining a rate of change of the 
accumulative histogram in a predetermined region, and determining the 
imaged posture of the image based on the rate of change. 
According to the present invention, there is provided a second method of 
determining the imaged posture of a medical image, comprising the steps of 
generating a histogram of an image signal bearing a transmitted image of a 
human body, determining the separation of the histogram, and determining 
the imaged posture of the image based on the separation. 
According to the present invention, a third method of determining the 
imaged posture of a medical image, comprises the steps of generating a 
histogram of an image signal bearing a transmitted image of a human body, 
determining the degree of matching between a pattern of the histogram and 
a plurality of reference image signal histogram patterns predefined of 
imaged postures of the image, and determining the imaged posture of the 
image based on the degree of matching. 
The degree of matching between the signal distribution pattern and the 
reference signal distribution pattern can be checked by any of various 
known pattern matching processes. 
According to the present invention, a fourth method of determining the 
imaged posture of a medical image, comprises the steps of generating a 
histogram of an image signal bearing a transmitted image of a human body, 
determining a function approximating a pattern of the histogram, 
thereafter, determining a derivative of second order of the function, 
checking whether the derivative of second order is of a positive or 
negative value in a predetermined region in the image, and determining the 
imaged posture of the image based on whether the derivative of second 
order is of a positive or negative value. 
According to the present invention, there is provided a fifth method of 
determining the imaged posture of a medical image, comprising the steps of 
generating a histogram of an image signal bearing a transmitted image of a 
human body, detecting whether the histogram is basically of a single-peak 
form or a double-hump form, and determining the imaged posture of the 
image based on whether a pattern of the distribution is basically of a 
single-peak form or a double-hump form. 
According to the present invention, a sixth method of determining the 
imaged posture of a medical image, comprises the steps of generating an a 
histogram of an image signal bearing a transmitted image of a human body, 
determining the dispersion of the histogram, wholly or partly, and 
determining the imaged posture of the image based on the dispersion. 
A front chest image which is taken by imaging the chest from its front side 
has an image signal histogram substantially as shown in FIG. 3A, and a 
side chest image which is taken by imaging the chest from its lateral side 
has an image signal histogram substantially as shown in FIG. 3B. 
Accumulative histograms, which are used in the first method of the present 
invention, are as shown in FIGS. 4A and 4B for the front and side chest 
images. The rate of change of the accmulative histogram in the vicinity of 
the middle signal value is considerably small for the accumulative 
histogram of FIG. 4A, and is considerably large for the accumulative 
histogram of FIG. 4B. Therefore, if the rate of change of the accumulative 
histogram is relatively small, the chest image can be judged as the front 
chest image, and if the rate of change of the accumulative histogram is 
relatively large, the chest image can be judged as the side chest image. 
The pattern of the histogram shown in FIG. 3A has high frequency values 
relatively close to minimum and maximum signal values Smin, Smax, whereas 
pattern of the histogram shown in FIG. 3B has high frequency values near 
the middle signal value Smid.. The separation, used in the second method 
of the invention, of the histogram of FIG. 3A is larger than the 
separation of the histogram of FIG. 3B. If, therefore, the separation of 
the histogram is relatively large, the image can be determined as the 
front chest image, and if the separation of the histogram is relatively 
small, the image can be determined as the side chest image. 
The histogram patterns as shown in FIGS. 3A and 3B, which are used in the 
third method of invention, are stored in advance in a memory means 
respectively as a reference histogram pattern for front chest images and a 
reference histogram pattern for side chest images. The degree of matching 
bewteen a histogram pattern of an image signal bearing a front chest image 
or a side chest image and the above two reference histogram patterns. If 
the histogram of the image signal better matches the reference histogram 
pattern shown in FIG. 3A than the reference histogram pattern shown in 
FIG. 3B, then the image borne by the image signal is judged as the front 
chest image. If image signal histogram pattern matches the reference 
histogram pattern of FIG. 3B better than the reference histogram pattern 
of FIG. 3A, then the image borne by the image signal is judged as the side 
chest image. 
When the histogram patterns as shown in FIGS. 3A and 3B are approximated by 
a function used in the fourth method of the invention, the derivative of 
second order of the function is of a positive value in a region in which 
the function or the histogram pattern is downwardly convex, and the 
derivative of second order of the function is of a negative value in a 
region in which the function or the histogram pattern is upwardly convex. 
The histogram pattern shown in FIG. 3A is downwardly convex in the 
vicinity of the middle signal value Smid, and the histogram pattern shown 
in FIG. 3B is upwardly convex in the vicinity of the middle signal value 
Smid. The derivative of second order is checked for its sign in the region 
near the middle signal value Smid. If the derivative of second order is 
positive, then the chest image is determined as the front chest image, and 
if the derivative of second order is negative, then the chest image is 
determined as the side chest image. 
Moreover, the histogram as shown in FIG. 3A is basically of a double-hump 
form having higher frequency values relatively close to the minimum and 
maximum signal values Smin, Smax, whereas the histogram as shown in FIG. 
3B is basically of a single-peak form having higher frequency values close 
to the middle signal value Smid. Therefore, if the histogram is basically 
of a double-hump form, the chest image is determined as the front chest 
image, and if the histogram is basically of a single-peak form, the chest 
image is determined as the side chest image. 
The radiation image of a foot, for example, of a human being which is taken 
from its front side has an image signal histogram substantially as shown 
in FIG. 12A, and the radiation image of the foot which is taken from its 
lateral side has an image signal histogram substantially as shown in FIG. 
12B. The front foot image includes an area less permeable to the radiation 
than the side foot image. Therefore, the histogram of the front foot image 
has a wide distribution of low image signal values. The dispersion, used 
in the sixth method of the invention, is larger in the histogram of the 
front foot image than in histogram of the side foot image. Accordingly, if 
the dispersion is relatively large, the image is determined as the front 
foot image, and if the dispersion is relatively small, the image is judged 
as the side foot image. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which preferred 
embodiments of the present invention are shown by way of illustrative 
example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows, by way of example, a radiation image recording and 
reproducing system for determining an imaged posture of a human body 
according to a method of a first embodiment of the present invention. The 
radiation image recording and reproducing system basically comprises a 
radiation image generating unit 20, a preliminary reading unit 30, a main 
reading unit 40, and an image reproducing unit 50. In the radiation image 
generating unit 20, a radiation 102 is emitted from a radiation source 100 
such as an X-ray tube toward an object 101 such as a human body under 
examination. The radiation 102 passes through the object 101 and is then 
applied to a stimulable phosphor sheet 103 capable of storing radiation 
energy for thereby recording the transmitted-radiation image information 
of the object 101 on the stimulable phosphor sheet 103. 
The stimulable phosphor sheet 103 with the transmitted-radiation image 
information of the object 101 being recorded thereon is then fed to the 
preliminary reading unit 30 by a sheet feed means 110 such as a feed 
roller. In the preliminary reading unit 30, a laser beam 202 emitted from 
a preliminary reading laser source 201 passes through a filter 203 which 
cuts off the wavelength range of light which will be emitted from the 
stimulable phosphor sheet 103 upon exposure to the laser beam 202, and 
then is linearly deflected in a main scanning direction by a light 
deflector 204 such as a galvanometer mirror, after which the laser beam 
202 is reflected by a plane mirror 205 onto the stimulable phosphor sheet 
103. The laser beam 202 emitted as stimulating light from the laser source 
201 has a waveform range selected so as not to overlap the waveform range 
of light emitted from the stimulable phosphor sheet 103. The stimulable 
phosphor sheet 103 is simultaneously fed for auxiliary scanning in the 
direction of the arrow 206 by a sheet feed means 210 such as feed rollers. 
As a result, the laser beam 202 is applied two-dimensionally to the entire 
surface of the stimulable phosphor sheet 103. The intensity of the laser 
beam 202 emitted from the laser source 201, the diameter of the laser beam 
202, the scanning speed of the laser beam 202, and the speed of travel of 
the stimulable phosphor sheet 103 in the auxiliary scanning direction are 
selected so that the energy of the stimulating light (laser beam 202) in 
the preliminary reading mode will be lower than the energy of stimulating 
light emitted in a main reading mode (described later). 
When the laser beam 202 is applied to the stimulable phosphor sheet 103 in 
the preliminary reading unit 30, the stimulable phosphor sheet 103 gives 
off light in an amount proportional to the amount of radiation energy 
stored therein, and the emitted light enters a preliminary reading light 
guide 207. The light is guided by the light guide 207 to reach a light 
detector 208 such as a photomultiplier. The light detector 207 has a light 
detecting surface to which there is attached a filter that passes only the 
wavelength range of the emitted light from the stimulable phosphor sheet 
103 but cuts off the wavelength range of the stimulating light from the 
laser source 201. Therefore, the light detector 207 can detect only the 
light emitted from the stimulable phosphor sheet 103. The detected light 
is then converted to an electric signal bearing recording information of 
the stored radiation image, and the electric signal is amplified by an 
amplifier 209. The amplified output signal from the amplifier 209 is 
converted by an A/D converter 211 to a digital signal, which is then 
applied as a preliminary reading image signal Sp to a main reading control 
circuit 314 of the main reading unit 40. The main reading control circuit 
314 determines a read-out gain setting a, a recording scale factor setting 
b, and a reproduced image processing condition setting c through histogram 
analysis, for example, based on the recording information represented by 
the preliminary reading image signal Sp. 
When the preliminary reading mode is completed, the stimulable phosphor 
sheet 103 is fed into the main reading unit 40. In the main reading unit 
40, a laser beam 302 emitted from a main reading laser source 301 passes 
through a filter 303 which cuts off the wavelength range of light which 
will be emitted from the stimulable phosphor sheet 103 upon exposure to 
the laser beam 302, and then is accurately adjusted in its diameter by a 
beam expander 304. The laser beam 302 is then linearly deflected in a main 
scanning direction by a light deflector 305 such as a galvanometer mirror, 
after which the laser beam 302 is reflected by a plane mirror 306 onto the 
stimulable phosphor sheet 103. Between the light deflector 305 and the 
plane mirror 306, there is disposed an f.theta. lens 307 for uniformizing 
the beam diameter of the laser beam 302 which scans the stimulable 
phosphor sheet 103. The stimulable phosphor sheet 103 is simultaneously 
fed for auxiliary scanning in the direction of the arrow 308 by a sheet 
feed means 320 such as feed rollers. As a result, the laser beam 302 is 
applied two-dimensionally to the entire surface of the stimulable phosphor 
sheet 103. When the laser beam 302 is applied to the stimulable phosphor 
sheet 103 in the main reading unit 40, the stimulable phosphor sheet 103 
gives off light in an amount proportional to the amount of radiation 
energy stored therein, and the emitted light enters a main reading light 
guide 309. The light is guided by the light guide 309 while repeating 
total reflection therein to reach a light detector 310 such as a 
photomultiplier. The light detector 310 has a light detecting surface to 
which there is attached a filter that selectively passes only the 
wavelength range of the emitted light from the stimulable phosphor sheet 
103. Therefore, the light detector 310 can detect only the light emitted 
from the stimulable phosphor sheet 103. 
The light detector 310 photoelectrically detects the emitted light which 
represents the radiation image recorded on the stimulable phosphor sheet 
103. The output signal of the light detector 310 is then amplified to an 
electric signal of a suitable level by an amplifier 311 with its read-out 
gain being set by the read-out gain setting a determined by the control 
circuit 314. The amplified electric signal is applied to an A/D converter 
312 which coverts the signal to a digital signal with a recording scale 
factor suitable for a signal variation width based on the recording scale 
factor setting b. The digital signal is applied to a signal processing 
circuit 313. The signal processing circuit 313 processes the digital 
signal for image processing such as gradation processing, for example, 
based on the reproduced image processing condition setting c so that a 
well-observable radiation image can be reproduced. 
A read-out image signal (main reading image signal) So issued from the 
signal processing circuit 313 is applied to a light modulator 401 of the 
image reproducing unit 50. In the image reproducing unit 50, a laser beam 
403 emitted from a recording laser source 402 is modulated by the main 
reading image signal So from the signal processing circuit 313, and the 
modulated laser beam 403 is deflected by a scanning mirror 404 to scan a 
photosensitive sheet 405 such as a photographic film. In synchronism with 
the scanning by the modulated laser beam 403, the photosensitive sheet 405 
is fed in a direction (indicated by the arrow 406) normal to the direction 
in which the phostosensitive sheet 405 is scanned by the modulated laser 
beam 403. Therefore, a radiation image based on the main reading image 
signal So is recorded on the photosensitive sheet 405. The radiation image 
may otherwise be reproduced by display on a CRT or in any of various other 
known ways. 
A method according to a first embodiment of the present invention for 
automatically determining the imaged posture of the object will now be 
described below. The preliminary reading image signal Sp issued from the 
A/D converter 211 is applied to an imaged posture determining circuit 500 
as well as the main reading control circuit 314. FIG. 5 shows the imaged 
posture determining circuit 500 in detail. The imaged posture determining 
circuit 500 has a histogram generator 511 which, in response to the 
preliminary reading image signal Sp supplied thereto, generates a 
histogram of the image signal Sp. If the image recorded on the stimulable 
phosphor sheet 103 is the image of a chest, the histogram thus generated 
is as shown in FIG. 3A when the chest is imaged from its front side and as 
shown in FIG. 3B when the chest is imaged from its lateral side. The 
following description is directed to analysis of such chest images by way 
of example. Information H indicating the histogram is fed to an 
accumulative histogram generator 512 which generates an accumulative 
histogram of the preliminary reading image signal Sp based on the 
information H. The accumulative histogram thus produced is as shown in 
FIGS. 4A and 4B for the imaging of the chest from its front and lateral 
sides, respectively. InformaTion Ha representing the accumulative 
histogram is fed to a histogram analyzer 513 which determines the rate of 
change r of the accumulative histogram represented by the information Ha, 
in a region I (see FIGS. 4A and 4B) near a middle signal value Smid. The 
rate of change r may be defined by the value of (.beta.-.alpha.) between 
the accumulative frequencies .alpha.,.beta. at the ends of the region I. 
The region I may be defined between positions indicated by 40 % and 60 % 
of the image width from the minimum value Smin to the maximum value Smax 
of the preliminary reading image signal Sp, but may appropriately be 
defined otherwise dependent on the image to be analyzed, or may be 
confined to a point in the image. When generating the histogram, a 
histogram of the entire preliminary reading image signal Sp may be 
produced, or a histogram of other image signal than an image signal which 
is not directly related to the object, i.e., which comes from an area to 
which the radiation is directly applied (a blank area) may be produced, or 
a histogram area corresponding to the image signal coming from the blank 
area may be removed from a histogram of the entire preliminary reading 
image signal Sp. Stated otherwise, the histogram produced by in the method 
of the invention may be representative of the entire image signal, or a 
portion of the image signal. This is also the case with methods of other 
embodiments described later on. 
Information indicating the rate of change r is then sent to a determining 
unit 514 which compares a reference value Th supplied from a reference 
setting unit 515 and the rate of change r. If r&gt;Th, then the image borne 
by the preliminary reading image signal Sp is determined as a side chest 
image, and the determining unit 514 issues a corrective signal T. If 
r.ltoreq.Th, then the image borne by the preliminary reading image signal 
Sp is determined as a front chest image, and the determining unit 514 
issues no corrective signal. The corrective signal T is fed to a gain 
correcting circuit 507 shown in FIG. 1. In response to the corrective 
signal T, the gain correcting circuit 507 corrects the read-out gain 
setting a so that the read-out gain will be lowered. As described above, 
if the image reading conditions and the image processing conditions remain 
constant, the density at the area of the thoracic vertebra K of the side 
chest image is higher than that of the front chest image. When the rate of 
change r is relatively large, i.e., when the side chest image is read, the 
read-out gain is lowered to reduce the overall level of the main reading 
image signal So. Therefore, the entire density of the reproduced radiation 
image recorded on the photosensitive sheet 405 is lowered. As a result, 
the density at the area of the thoracic vertebra K in the reproduced side 
chest image is substantially equalized to the density at the area of the 
thoracic vertebra K in the reproduced front chest image. The degree to 
which the read-out gain may be corrected may be determined experimentally 
or empirically. 
In the aforsaid embodiment, the front chest image is read at the read-out 
gain determined by the main reading control circuit 314, and the side 
chest image is read at the read-out gain which is corrected so as to be 
lower. Conversely, the side chest image may be read at the read-out gain 
determined by the main reading control circuit 314, and the front chest 
image may be read at the read-out gain which is corrected so as to be 
higher. (This also holds true for methods of other embodiments.) The 
density of the reproduced image may be adjusted by varying the read-out 
gain, as described above, and also by varying the conditions for the 
recording scale factor in the A/D converter 312, or varying the conditions 
for the gradation processing in the signal processing circuit 313. These 
alternative adjusting processes may be combined with each other. (This 
also holds true for methods according to other embodiments described 
below.) 
The method of the first embodiment of the present invention may be used to 
determine other imaged postures than the imaged postures of front and side 
chest images, and also imaged postures of other body parts. More 
specifically, when a certain body part is imaged at different postures, it 
is highly likely for the rate of change of the accumulative histogram of 
the image signal of one of the body part images to vary greatly from the 
rate of change of the accumulative histogram of the image signal of the 
other body part image in a certain area in the image. The imaged postures 
can therefore be determined by detecting the magnitude of the rate of 
change. 
A method according to a second embodiment will be described below. FIG. 6 
shows in detail an imaged posture determining circuit 500A which can be 
used in the system of FIG. 1 in place of the imaged posture determining 
circuit 500 shown in FIG. 5. The imaged posture determining circuit 500A 
has a histogram generator 521 which, in response to the preliminary 
reading image signal Sp supplied thereto, generates a histogram of the 
image signal Sp. If the image recorded on the stimulable phosphor sheet 
103 is the image of a chest, the histogram thus generated is as shown in 
FIG. 3A when the chest is imaged from its front side and as shown in FIG. 
3B when the chest is imaged from its lateral side, as with the method of 
the first embodiment. The following description is directed to analysis of 
such chest images by way of example. Information H indicating the 
histogram is fed to a histogram analyzer 523 which determines the maximum 
value Rmax of class separation R used in determination analysis as the 
separation of the histogram represented by the information H. There is 
established a point Sm by which the entire values of the signal in the 
histogram are divided into two lower and higher groups or areas A1, A2, 
and it is assumed that the ratios of the accumulated frequency values in 
the areas A1, A2 are indicated by w1, w2, (w1+w2=1), the average weighting 
values for the signal in the areas A1, A2 are indicated by .mu.1, .mu.2, 
and the dispersion of the entire histogram is expressed by .sigma.2. The 
maximum value Rmax of class separation R can be defined by the maximum 
value of: 
EQU R=w1w2(.parallel.1-.mu.2).sup.2 /.sigma..sup.2 
which varies according to m. The average signal values .mu.1, .mu.2 are 
given by: 
##EQU1## 
where Si is the signal value and fi is the frequency. 
Information Sr indicating the maximum value Rmax of class separation R is 
then sent to a determining unit 524 which compares a reference value Th 
supplied from a reference setting unit 525 and the maximum value Rmax. If 
Rmax.ltoreq.Th, then the image borne by the preliminary reading image 
signal Sp is determined as a side chest image, and the determining unit 
524 issues a corrective signal T. If Rmax&gt;Th, then the image borne by the 
preliminary reading image signal Sp is determined as a front chest image, 
and the determining unit 524 issues no corrective signal. The corrective 
signal T is fed to the gain correcting circuit 507 shown in FIG. 1. As 
with the first embodiment, in response to the corrective signal T, the 
gain correcting circuit 507 corrects the read-out gain setting a so that 
the read-out gain will be lowered. When the class separation R is 
relatively small, i.e., when the side chest image is read, the read-out 
gain is lowered to lower the overall level of the main reading image 
signal So. Therefore, the entire density of the reproduced radiation image 
recorded on the photosensitive sheet 405 is lowered. As a result, the 
density at the area of the thoracic vertebra K in the reproduced side 
chest image is substantially equalized to the density at the area of the 
thoracic vertebra K in the reproduced front chest image. 
The separation of the histogram may be defined by a correlative standard or 
a least square standard rather than the class separation. 
The method of the second embodiment of the present invention may be used to 
determine other imaged postures than the imaged postures of front and side 
chest images, and also imaged postures of other body parts. More 
specifically, when a certain body part is imaged at different postures, it 
is highly likely for the separation of the histogram of the image signal 
of one body part image to vary greatly from the separation of the 
histogram of the image signal of the other body part image. The imaged 
postures can therefore be determined by determining the magnitude of the 
separation. 
A method according to a third embodiment will be described below. FIG. 9 
shows an imaged posture determining circuit 500B which can be used in the 
system of FIG. 1 in place of the imaged posture determining circuit 500 
shown in FIG. 5. The imaged posture determining circuit 500B has a 
histogram generator 531 which, in response to the preliminary reading 
image signal Sp supplied thereto, generates a histogram of the image 
signal Sp. If the image recorded on the stimulable phosphor sheet 103 is 
the image of a chest, the histogram thus generated is generally of a 
pattern as shown in FIG. 3A when the chest is imaged from its front side 
and as shown in FIG. 3B when the chest is imaged from its lateral side, as 
with the method of the first embodiment. The following description is 
directed to analysis of such chest images by way of example. 
The general histogram patterns shown in FIGS. 3A and 3B can be determined 
by producing histograms of preliminary reading image signals Sp relative 
to several typical front and side chest images with the histogram 
generator 531, and sending the histograms to a function generator 532 by 
which the histograms are averaged and smoothed. The function generator 532 
produces functions g1(i), g2(i) which approximate the typical histograms 
thus produced (see FIGS. 8A and 8B). (i indicates an image signal value.) 
The functions can be generated as being composed of a multinomial of 
higher degree by using, for example, the regression analysis process. The 
functions g1(i), g2(i) are stored in a memory means 535 as indicating 
reference histogram patterns of the front and side chest images. 
For determining the imaged posture of each of radiation images, the 
histograms of the preliminary reading image signals Sp of the radiation 
images are sequentially produced in the histogram generator 531. The 
actual histogram of each image is as shown in FIG. 8C, for example, and 
can be defined as a function f(i) of the image signal i. Information F 
indicating the function f(i) is then delivered to a mismatch calculating 
unit 533, which finds the degree of a mismatch between a histogram of a 
preliminary reading image signal Sp which is produced for each image and 
the above two reference histograms. More specifically, when supplied with 
the information F, the mismatch calculating unit 533 receives items of 
information G1, G2 indicating the functions g1(i), g2(i) from the memory 
means 535, and determines the degrees of mismatches according the 
following equations: 
##EQU2## 
The information thus determined as indicating the mismatch degrees S1, S2 
is then sent to a determining unit 534. 
If S1&gt;S2, then the determining unit 544 determines that the image borne by 
the preliminary reading image signal Sp is a side chest image, and issues 
a corrective signal T. If S1&lt;S2, then the image borne by the preliminary 
reading image signal is judged as a front chest image, and no corrective 
signal is issued. More specifically, if S1&lt;S2, the degree of a mismatch 
between the functions f(i) and the function g1(i) is smaller than the 
degree of a mismatch between the functions f(i) and the function g2(i). 
Stated otherwise, the function f(i) matches the function g1(1) better than 
the function g2(i). Therefore, the pattern of the histogram produced by 
the preliminary reading image signal Sp can be judged as better matching 
the reference histogram pattern of FIG. 3A than the reference histogram 
pattern of FIG. 3B. 
The corrective signal T is fed to the gain correcting circuit 507 shown in 
FIG. 1. In response to the corrective signal T, the gain correcting 
circuit 507 corrects the read-out gain setting a so that the read-out gain 
will be lowered, as with the first embodiment. When S1&gt;S2, i.e., when the 
side chest image is read, the read-out gain is lowered to reduce the 
overall level of the main reading image signal So. Therefore, the entire 
density of the reproduced radiation image recorded on the photosensitive 
sheet 405 is lowered. As a result, the density at the area of the thoracic 
vertebra K in the reproduced side chest image is substantially equalized 
to the density at the area of the thoracic vertebra K in the reproduced 
front chest image. 
The mismatch degree may also be defined as 
##EQU3## 
the maximum value from the group of f(1)-g(1), f(2)-g(2), . . . f(N)-g(N). 
The method of the third embodiment of the present invention may be used to 
determine other imaged postures than the imaged postures of front and side 
chest images, and also imaged postures of other body parts. More 
specifically, when a certain body part is imaged at different postures, it 
is highly likely for the histogram patterns of the image signals of the 
body part images to have mutually different basic patterns. Therefore, the 
matching degree between those basic histogram patterns and the histogram 
pattern of an actual image signal is determined, and the imaged postures 
can be determined according to the determined matching degree. 
A method according to a fourth embodiment will be described below. FIG. 10 
shows an imaged posture determining circuit 500C which can be used in the 
system of FIG. 1 in place of the imaged posture determining circuit 500 
shown in FIG. 5. The imaged posture determining circuit 500C has a 
histogram generator 541 which, in response to the preliminary reading 
image signal Sp supplied thereto, generates a histogram of the image 
signal Sp. If the image recorded on the stimulable phosphor sheet 103 is 
the image of a chest, the histogram thus generated is as shown in FIG. 3A 
when the chest is imaged from its front side and as shown in FIG. 3B when 
the chest is imaged from its lateral side, as with the method of the first 
embodiment. The following description is directed to analysis of such 
chest images by way of example. Information H representing the histogram 
is sent to a function generator 542 which produces a function f(S) that 
approximates the pattern of the histogram indicated by the information H. 
The function f(S) can be generated as being composed of a multinomial of 
higher degree by using, for example, the regression analysis process. 
Information F representing the function f(S) is then supplied to a 
quadratic differential calculating unit 543, which quadratically 
differentiates the function f(S) to find a derivative of second order 
f"(S). Information F" indicative of the derivative of second order f"(S) 
is sent to a determining unit 544. 
The determining unit 544 is connected to a region indicator 545 which is 
supplied with the histogram information H. The region indicator 545 finds 
a middle signal value Smid (see FIGS. 3A and 3B) of the histogram based on 
the information H. Information m indicating the middle signal value Smid 
is then delivered to the determining unit 544. The determining unit 544 
determines a value f"(Smid) which the derivative of second order f"(S) 
takes at the middle signal value Sm. If the value of f"(Smid) is negative, 
then the determining unit 544 determines that the image borne by the 
preliminary reading image signal Sp is a side chest image, and issues a 
corrective signal T. If the value of f"(Smid) is positive, then the image 
borne by the preliminary reading image signal is judged as a front chest 
image, and no corrective signal is issued. The corrective signal T is fed 
to the gain correcting circuit 507 shown in FIG. 1. In response to the 
corrective signal T, the gain correcting circuit 507 corrects the read-out 
gain setting a so that the read-out gain will be lowered, as with the 
first embodiment. When the value of f"(Smid) is negative, i.e., when the 
side chest image is read, the read-out gain is lowered to reduce the 
overall level of the main reading image signal So. Therefore, the entire 
density of the reproduced radiation image recorded on the photosensitive 
sheet 405 is lowered. As a result, the density at the area of the thoracic 
vertebra K in the reproduced side chest image is substantially equalized 
to the density at the area of the thoracic vertebra K in the reproduced 
front chest image. 
The value to be put in the derivative of second order f"(S) is not limited 
to the middle signal value Smid, but may appropriately be selected 
according to a typical histogram pattern of an image to be analyzed. 
The method of the fourth embodiment of the present invention may be used to 
determine other imaged postures than the imaged postures of front and side 
chest images, and also imaged postures of other body parts. More 
specifically, when a certain body part is imaged at different postures, it 
is highly likely for the histogram pattern of the image signal of one of 
the body part images to be downwardly convex and for the histogram pattern 
of the image signal of the other body part image to be upwardly convex in 
a certain region. Therefore, the imaged postures can be determined 
according to the sign of the derivative of second order in this region. 
A method according to a fifth embodiment of the present invention will be 
described below. The method of the fifth embodiment determines an imaged 
posture by checking whether a histogram of the preliminary reading image 
signal Sp in the system of FIG. 1 is of a double-hump form as shown in 
FIG. 3A or of a single-peak form as shown in FIG. 3B. Whether a generated 
histogram is of a double-hump form or a single-peak form can be 
ascertained by, for example, the circuit 500 shown in FIG. 5. More 
specifically, in the circuit 500, information representative of the rate 
of change r (FIGS. 4A and 4B) of the accumulative histogram indicated by 
the information Ha in the region I near the middle signal value Smid is 
fed to the determining unit 514. The determining unit 512 then compares 
the rate-of-change r and the reference value Th from the reference setting 
unit 515. If r&gt;Th, then the histogram is judged as being of a single-peak 
form, i.e., the image borne by the preliminary reading image signal Sp is 
judged as a side chest image, and a corrective signal T is issued. If 
r&lt;Th, then the histogram is judged as being of a double-hump form, i.e., 
the image borne by the preliminary reading image signal Sp is judged as a 
front chest image, and no corrective signal is issued. If the histogram of 
the preliminary reading image signal Sp is of a double-hump shape as shown 
in FIG. 3A and the signal frequency in the vinicity of the middle signal 
value Smid is low, the rate of change r near the middle signal value of 
the accumulative histogram is relatively small. On the other hand, if the 
histogram of the preliminary reading image signal Sp is of a single-peak 
shape as shown in FIG. 3B and the signal frequency in the vicinity of the 
middle signal value Smid is high, the rate of change r near the middle 
signal value of the accumulative histogram is relatively large. Therefore, 
imaged postures can be determined on the basis of the value of the rate of 
change r. 
The corrective signal T is fed to the gain correcting circuit 507 shown in 
FIG. 1. In response to the corrective signal T, the gain correcting 
circuit 507 corrects the read-out gain setting a so that the read-out gain 
will be lowered, as with the first embodiment. 
In the above fifth embodiment, whether a signal distribution pattern is of 
a single-peak form or a double-hump form is detected from the rate of 
change of the accumulative histogram in a certain region. However, the 
histogram pattern can be judged in another way. For example, where a 
histogram is of a double-hump form as illustrated in FIG. 3A, the pattern 
of the histogram is downwardly convex near the middle signal value Smid. 
Where a histogram is of a single-peak form as illustrated in FIG. 3B, the 
histogram pattern is upwardly convex near the middle signal value Smid. 
Therefore, whether a historam pattern is of a double-hump form or a 
signle-peak form can be detected by checking whether the histogram pattern 
is downwardly convex or upwardly convex in the vicinity of the middle 
signal value Smid. The direction in which the histogram pattern is convex 
can be determined by having the histogram pattern approximate a certain 
function, and checking whether the derivative of second order of such a 
function is positive or negative. 
Whether a histogram pattern is of a single-peak form or a double-hump form 
can be detected by the circuit 500C shown in FIG. 10. In the circuit 500C, 
the determining unit 544 determines a value f"(Smid) which the derivative 
of second order f"(S) takes at the middle signal value Smid. If the value 
of f"(Smid) is negative, then the determining unit 544 determines that the 
histogram is judged as being of a single-peak form, i.e., the image borne 
by the preliminary reading image signal Sp is a side chest image. If the 
value of f"(Smid) is positive, then the histogram is judged as being of a 
double-hump form, i.e., the image borne by the preliminary reading image 
signal is judged as a front chest image. 
Whether the histogram pattern is of a single-peak form or a double-hump 
form may also be detected by determining the dispersion or separation of 
the histogram or by a known pattern matching process. 
FIGS. 3A and 3B show smoothed forms of actual signal distribution patterns. 
Generally, an actual signal distribution pattern has small changes or 
fluctuations which are not illustrated in FIGS. 3A and 3B. The terms 
"single-peak form" and "double-hump form" used above are indicative of 
basic histogram patterns irrespective of such small changes or 
fluctuations which the actual patterns have. 
The method of the fifth embodiment may be used to determine other imaged 
postures than the imaged postures of front and side chest images, and also 
imaged postures of other body parts. More specifically, when a certain 
body part is imaged at different postures, it is highly likely for the 
histogram pattern of the image signal of one body part image to be of a 
single-peak form and for the histogram pattern of the image signal of the 
other body part image to be of a double-hump form. Therefore, the imaged 
postures can be determined by detecting whether the histogram patterns are 
of a single-peak form or a double-hump form. 
A method according to a sixth embodiment will be described below. FIG. 13 
shows in detail an imaged posture determining circuit 500D which can be 
used in the system of FIG. 1 in place of the imaged posture determining 
circuit 500 shown in FIG. 5. The imaged posture determining circuit 500D 
has a histogram generator 561 which, in response to the preliminary 
reading image signal Sp supplied thereto, generates a histogram of the 
image signal Sp. If the image recorded on the stimulable phosphor sheet 
103 is the image of a foot, the histogram thus generated is as shown in 
FIG. 12A when the foot is imaged from its front side and as shown in FIG. 
12B when the foot is imaged from its lateral side. The following 
description is directed to analysis of such foot images by way of example. 
Information H indicating the histogram is fed to a histogram analyzer 563 
which determines the dispersion .sigma..sup.2 of the histogram represented 
by the information H. As is well known, the dispersion .sigma..sup.2 is 
expressed by: 
##EQU4## 
where Si is the value of the signal, fi is the frequency, and S is the 
average signal value. The dispersion .sigma..sup.2 may be determined from 
the entire area of the histogram, or determined with respect to an image 
signal in a region I (see FIGS. 12A and 12B) except the image signal of an 
area where the radiation is directly applied (blank area). 
Information Sm indicating the dispersion .sigma..sup.2 is then sent to a 
determining unit 564 which compares a reference value Th supplied from a 
reference setting unit 565 and the dispersion .sigma..sup.2. If 
.sigma..sup.2 &lt;Th, then the image borne by the preliminary reading image 
signal Sp is determined as a side foot image, and the determining unit 564 
issues a corrective signal T. If .sigma..sup.2 .gtoreq.Th, then the image 
borne by the preliminary reading image signal Sp is determined as a front 
foot image, and the determining unit 564 issues no corrective signal. The 
corrective signal T is fed to the gain correcting circuit 507 shown in 
FIG. 1. As with the first embodiment, in response to the corrective signal 
T, the gain correcting circuit 507 corrects the read-out gain setting a so 
that the read-out gain will be lowered. As described above, provided the 
image reading conditions and the image reading conditions are constant, 
the density of the joint J in the reproduced side foot image is lower than 
the density of the joint in the reproduced front foot image. When the 
dispersion .sigma..sup.2 is relatively small, i.e., when the side foot 
image is read, the read-out gain is increased to lower the overall level 
of the main reading image signal So. Therefore, the entire density of the 
reproduced radiation image recorded on the photosensitive sheet 405 is 
increased. As a result, the density at the area of the joint J in the 
reproduced side foot image is substantially equalized to the density at 
the area of the joint J in the reproduced front foot image. 
The method of the sixth embodiment of the present invention may be used to 
determine other imaged postures than the imaged postures of front and side 
foot images, and also imaged postures of other body parts. More 
specifically, when a certain body part is imaged at different postures, it 
is highly likely for the dispersion .sigma..sup.2 of the histogram of the 
image signal of one body part image to vary greatly from the dispersion 
.sigma..sup.2 of the histogram of the image signal of the other body part 
image. The imaged postures can therefore be determined by determining the 
magnitude of the dispersion .sigma..sup.2. 
In each of the above embodiments, an imaged posture is determined by 
employing the preliminary reading image signal. In the event that no 
preliminary reading mode is carried out and image processing conditions 
are set in the signal processing circuit 313 based on the main reading 
image signal So, an imaged posture may be determined using the main 
reading image signal So. While the density of a reproduced image is 
corrected dependent on the determined imaged posture, an imaged posture 
may be determined to serve other purposes. 
In each of the aforesaid embodiments, the imaged posture of an image 
recorded on the stimulable phosphor sheet 103. However, the principles of 
the present invention are not limited to the determination of the imaged 
posture of a radiation image recorded on the stimulable phosphor sheet 
103, but are also applicable to the determination of the imaged postures 
of other images taken in medical imaging systems. 
According to the method of the present invention, the imaged posture of an 
image taken for medical diagnosis can automatically and accurately be 
determined. By carrying out the method of the invention in a radiation 
image information recording and reproducing system, as described above, 
the densities of areas of interest in reproduced images of an object can 
be uniformized even if the object is imaged at different postures, with 
the result that the radiation images can smoothly and accurately be 
diagnosed. 
Although certain preferred embodiments have been shown and described, it 
should be understood that many changes and modifications may be made 
therein without departing from the scope of the appended claims.