Image gradation processing method and apparatus for radiographic image copying system

A method and apparatus for processing a frontal chest radiograph in a radiographic image copying system in which an original radiograph is scanned with a scanning light beam and the light transmitted through the radiograph is detected by a photodetector which gives an output to be processed and used for recording a visible image on a recording medium. The frontal chest radiograph is gradation processed. The gradation processing is characterized in that the density at the boundary of the heart and the lungs of the image on the recording medium is lowered to lower the contrast of the heart and raise the contrast of the lungs.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of processing a radiographic image in a 
radiographic image copying system used for medical diagnosis and apparatus 
therefor. This invention particularly relates to an image gradation 
processing method and apparatus in a radiographic image copying system in 
which an original frontal chest radiograph is read out and recorded on a 
recording medium. 
2. Description of the Prior Art 
In the chest radiography, an X-ray film is used for recording the X-ray 
transmission image of radiography is observed with the naked eyes for 
diagnosis. In the chest radiography, there are recorded lungs, a heart and 
a spine. The spine has the lowest density since the transmittance thereof 
to the X-rays is low. The heart has the second lowest density since the 
transmittance thereof to the X-rays is comparatively low. The lungs has 
high density since the transmittance thereof to the X-rays is high. 
Further, since the lungs have complex trachea bronchus and blood vessels, 
the image of the lungs is very complicated. The part outside the 
substantial image of the human body has the upmost density since this part 
of the X-ray film is exposed to X-rays directly coming from the X-ray 
source. 
As mentioned above, the chest radiography has various information of 
various parts of the human body which is recorded in the density having a 
wide range of levels. Sometimes, the density ranges from 0 to 3.5 in terms 
of optical density. Further, since the various parts are not recorded in 
the desirable contrast respectively, it is very difficult and necessary to 
have a great skill to make proper diagnosis from the radiograph in which 
the disease must be found out from a very slight variation in density in 
the image. 
It is generally known in the art that the image properties can be changed 
by processing the image by use of an electronic signal or information 
processing method. For instance, even in radiography, it is possible to 
read the image recorded on the X-ray film by an optical scanning means and 
process the read out signal by a signal processing means to change the 
various image properties such as contrast and the density level and then 
record a visible image on a recording film or the like based on the 
processed signal. 
In the radiography, however, the recorded image is used for the purpose of 
"diagnosis" and the diagnostic efficiency and accuracy (the level of 
easiness for diagnosis or adaptability to diagnosis) are not simply 
enhanced by simply making so-called "good" image from the point of the 
ordinary image quality factors such as sharpness, granularity and 
contrast. Rather than these factors, the diagnosis efficiency and accuracy 
are influenced by other complex factors such as reference with the normal 
pattern, reference with the anatomical structure and utilization of other 
diagnostic view or records. 
On the other hand, it has been known in the art as one example of image 
processing to record the radiograph on a microfilm in a reduced size. For 
instance, as shown in Japanese Patent Laid Open No. 48(1973)-25523, it is 
known to use a photographic film having a modified gradient contrast in 
which the contrast (gamma) is lowered in the high density area to compress 
the density range at the time of copying, and further conduct an unsharp 
masking process to compensate the lowering of sharpness caused by image 
size reduction and copying. This process, however, is effective only for 
preventing the lowering of the image quality in the image size reduction 
and copying steps and is not made for the purpose of enhancing the 
diagnostic efficiency and accuracy. 
SUMMARY OF THE INVENTION 
The primary object of the present invention is to provide a method of and 
apparatus for gradation processing of the chest radiograph. 
Another object of the present invention is to provide a method of and 
apparatus for processing a radiographic image in a radiographic image 
copying system which is capable of obtaining a radiographic image having 
high diagnostic efficiency and accuracy. 
The method of and apparatus for gradation processing the radiographic image 
of the chest of this invention are characterized in that the density at 
the boundary of the heart and the lungs of the image on the recording 
material is lowered to lower the contrast of the heart and thereby raising 
the contrast of the lungs in the radiographic image finally recorded on a 
recording medium. 
In accordance with the gradation process as mentioned above, the density 
range of the heart becomes narrow and the lower density of lungs is more 
lowered and the contrast of the lungs is enhanced, and consequently the 
image thus gradation processed is improved of its diagnostic efficiency 
and accuracy. It should be noted that, though the contrast of the heart is 
lowered, it does not affect the diagnostic efficiency and accuracy since 
this part is the brightest part and human eyes have high gradational 
response to such a part. Since the spine has substantially the same 
density as the heart, this part does not affect the diagnostic efficiency 
and accuracy either. 
In general, most of the chest radiographs are made for the purpose of 
mainly observing the lungs. Therefore, in accordance with the present 
invention in which the contrast and the density of the lungs are 
particularly improved, the diagnostic efficiency and accuracy of the lungs 
are markedly enhanced without substantially lowering the diagnostic 
efficiency and accuracy of the spine and the heart.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Now the present invention will be described in detail with reference to the 
drawings. 
Referring to FIG. 1, a frontal chest radiograph has images of lungs 1, 
heart 2, and spine 3. Since the conventional chest radiograph does not 
have desirable density and contrast, the accurate diagnosis is not easy 
from the radiograph as it is. In accordance with the present invention, 
therefore, the X-ray image information is read out from the image of the 
original radiograph (hereinafter referred to as "original image") and the 
read out information is processed by a signal conversion system for 
varying the gradation when the image is finally recorded on a recording 
medium like a photographic film. 
Among the densities of the original image, the density of to the lungs 1 
has the highest level excluding the area outside the object, that is, 
human body, and the densities of the spine 3 and the heart 2 have the 
lowest level. The difference between the density of the spine 3 and the 
density of the heart 2, the latter being a little higher than the former, 
is very samll and accordingly both densities can be represented by one of 
the signals practically. These densities will hereinbelow be represented 
by the density of the heart 2. Therefore, by obtaining the minimum value 
Dmin of the density of the spine and the heart and the maximum value Dmax 
of the density of the lungs and performing the density conversion only on 
the densities having a level between these values, it is possible to 
conduct the gradation processing on the necessary image to obtain a 
radiographic image having high diagnostic efficiency and accuracy. 
In the present invention, the density at the boundary of the lungs 1 and 
the heart 2 is lowered to lower the contrast of the heart 2 and raise the 
contrast of the lungs 1. By lowering the density at the boundary of the 
lungs 1 and the heart 2, the diagnostic efficiency and accuracy are 
enhanced. 
FIG. 2 is a graph showing the gradation processing, in which the chain line 
4 represents a linear signal conversion shown by a straight line extending 
between the maximum density point at the maximum value of the original 
density Dmax and the minimum density point at the minimum value of the 
original density Dmin. It should be noted that processing shown by the 
line 4 means no processing. That is, processed image has the same 
gradation as the original image. 
It should be understood here that the photographic film used for recording 
the final radiographic image does not usually have such a linear 
characteristics, and accordingly, in order to effect such a linear signal 
conversion on the photographic film it is necessary to perform a well 
known "gamma correction". Therefore, it should be noted that the gamma 
correction should be performed together with the gradation processing. In 
the description hereinbelow, the description will be made only with 
respect to the gradation processing on the assumption that the signals are 
to be subjected, if necessary, to the gamma correction. 
Referring now back to FIG. 2, the solid line 5 shows an example of the 
signal processing conducted in accordance with the present invention. The 
signal level corresponding to the density at the boundary between the 
lungs 1 and the heart 2 is indicated with the reference character Do and 
the lowered density from the original level at the boundary level Do is 
indicated with .DELTA.D. In the embodiment as shown in FIG. 2, the degree 
of lowering the density is made maximum at the boundary level Do. 
Therefore, the contrast between the heart 3 and the spine 2 within the 
density range between Dmin and Do is lowered, and the contrast of the 
lungs 1 within the density range between Do and Dmax is raised as shown by 
the gradient of the curve 5 in FIG. 3 
Further, when the maximum density Dmax of the lungs 1 is not so high, for 
instance 1, 5 or less, the density may be converted so that maximum 
density on the final radiographic image on the film may become higher than 
that of the original radiograph to further raise the contrast in the lung 
1. The broken line 6 in FIG. 2 shows an example of such a gradation 
processing. 
The minimum density Dmin at the heart 2 and the spine 3 is often at the 
same level as the fog density of the film. In such a case, it is often 
preferred to raise the minimum density Dmin by 0.01 to 0.1. 
The preferred degree of lowering of the density at the boundary level Do 
depends upon the observer who conducts the diagnosis based on the 
radiographic image recorded on the film and also upon the characteristics 
of the radiograph itself such as the difference in density between the 
heart and the lungs. In general, when the degree of lowering of the 
density .DELTA.D is small, the gradation change is small, and, therefore, 
improvement of diagnostic efficiency and accuracy is too small. When 
.DELTA.D is too large, the contrast of the heart is too much lowered and 
the diagnostic efficiency and accuracy in the heart are lowered. As the 
result of tests, it was proved that the diagnostic efficiency and accuracy 
were improved when .DELTA.D was within the range of 0.1 to 0.5. 
In the embodiment shown in FIG. 2, since the gradation variation is made 
discontinuous at the boundary level Do. Therefore, the variation in 
density is also made discontinuous at the boundary and the image appears 
unnatural because of the discontinuity. Accordingly, it is desirable that 
the variation of the density be made continuous or smooth at the boundary 
level Do as shown in FIGS. 3 and 4. In FIG. 3, the angled point at the 
boundary level Do is rounded. In FIG. 4, the whole curve is changed 
continuously. With these embodiments as shown in FIGS. 3 and 4, the 
gradation variation is made smooth and natural. 
The results were evaluated by four radiologists since it was impossible to 
evaluate the diagnostic efficiency and accuracy by the objective physical 
evaluation by use of sharpness, contrast and granularity. 
The standard evaluation was as follows. 
+2: The diagnostic efficiency and accuracy were greatly enhanced and 
improved. For instance, the diseased portions which were hardly recognized 
in the original radiograph have become clearly recognizable. 
+1: The diagnostic efficiency and accuracy were improved. For instance, the 
diseased portions which were difficult to recognize have become 
recognizable. 
0: The diagnostic efficiency and accuracy were not so improved, though the 
image has become somewhat clearer. 
-1: The diagnostic efficiency and accuracy were lowered in some parts even 
though they were somewhat improved in other parts. 
-2: The diagnostic efficiency and accuracy were lowered with no parts where 
they were improved. 
Under the above standard, original images of ten samples of chest 
radiograph including the normal pattern, cancer pattern, pneumonia pattern 
and so forth which were subjected to the gradation processing of various 
types as shown in FIG. 5 were presented to four radiologists and the 
diagnostic efficiency and accuracy were evaluated for these samples. In 
FIG. 5, the curves 7 and 10 are examples of the gradation processing in 
which the density at the boundary between the heart and the lungs were 
lowered by 0.3. The curve 8 represents variation without any gradation 
processing, and curve 9, 11 and 12 show example of the gradation 
processing not based on the present invention. 
The results of the evaluation are shown in Table 1. 
TABLE 1 
______________________________________ 
Gradation Evaluation 
Processing 
(average value) 
General Evaluation 
______________________________________ 
Curve 7 +1.5 Improved 
Curve 8 +0.1 No change 
Curve 9 -2.0 Degraded 
Curve 10 +1.9 Improved 
Curve 11 -0.5 No change or Slightly 
degraded 
Curve 12 -1.2 Degraded 
______________________________________ 
Then, in order to find out the effective range of the degree of density 
lowering .DELTA.D at the boundary level Do, ten samples were eveluated by 
four radiologists. 
The gradation processes conducted are shown in FIG. 6. The degree of 
density lowering .DELTA.D was made 0.05, 0.2 and 0.6 in the examples shown 
by curves 13, 14 and 15. The curve 12 shows an example without any 
gradation processing. 
The results of the evaluation are shown in Table 2. 
TABLE 2 
______________________________________ 
Gradation Evaluation 
Processing 
(average value) 
General Evaluation 
______________________________________ 
Curve 12 +0.1 No change 
Curve 13 +0.3 No particular change 
Curve 14 +1.6 Improved 
Curve 15 -2.0 Degraded, 
Heart disappeared 
______________________________________ 
According to the tests conducted by the present inventors, the degree of 
lowering .DELTA.D of the density at the boundary level Do was within the 
range of 0.1 to 0.5 in order to effectively enhance the diagnostic 
efficiency and accuracy. 
FIG. 7 is a block diagram showing the outline of the radiographic image 
copying system in which the method and apparatus of this invention are 
embodied. An original radiograph 20 on which a chest image is recorded is 
mounted on a drum transparent 21. The drum transparent 21 is moved in the 
axial direction as well as rotated about its axis so that the radiograph 
20 is exposed to a light beam from a read-out light source 22 which is 
located inside the drum transparent 21. Thus, the light beam scans the 
radiograph 20 in the two dimensional scanning mode. As for the light beam 
scanning means may be used a CRT or a flying spot scanner. 
The light passing through the radiograph is received by a photodetector 23 
and converted to an electric signal, which is amplified by an amplifier 24 
and converted to a digital signal through an A/D converter 25. The digital 
signal thus obtained is memorized in a magnetic memory tape 26. The date 
memorized in the magnetic memory tape 26 is input into arithmetic and 
logical units or processer 27 like a computer, wherein the Dmax, Do and 
Dmin of the original image signal are analyzed. 
In the normal chest radiograph, a histogram as shown in FIG. 8 can be 
obtained for the whole chest image. In the histogram shown in FIG. 8, the 
density for the area outside the object is not included. This histogram 
has two peaks the lower one thereof representing the frequency 
distribution for the heart and the higher one thereof representing that of 
the lungs. The height of the two peaks and the width thereof are different 
upon the range of the part of the human body for which the histogram was 
made, the shape of the chest and so on. The minimum and maximum value Dmin 
and Dmax are calculated as the points where the frequency of the histogram 
falls to zero or a predetermined small value, e.g. 5% of the maximum 
frequency. The boundary level Do of the density at the boundary of the 
heart and the lungs can be determined as the level of the density at the 
bottom of the valley of the histogram between the two peaks as shown in 
FIG. 8 or as the average value of the two densities Dh and Df 
corresponding to the two peaks, i.e. (Dh+Df/2). 
When two histograms 28 and 29 are separately made for the heart and lungs 
as shown in FIG. 9, the boundary level Do can be determined as the average 
value of the two peaks, i.e. (Dh+Df/2), or as the level of the signal at 
the crossing point of the two histograms. It will be mentioned hereinafter 
how to designate the heart and the lungs in the image. 
In view of the simplicity of calculation, the above mentioned methods of 
determining the boundary level Do are advantageous in practice, though 
there are various other methods possible for determining the boundary 
level Do. Though it is practically very difficult to obtain the true 
boundary level Do from the histogram, it has been confirmed that favorable 
results were obtained even with the above-mentioned approximate value of 
the boundary level Do. 
The designation of the heart and the lungs can be conducted as follows. 
The first method is a statistical method in which the boundary level is 
statistically presumed from a number of radiographs. This method is able 
to obtain a sufficiently accurate boundary level for practical use by 
making a proper compensation for the characteristics of the human body to 
be the object of the radiographic image as desired. 
The second method is a direct searching method in which the density 
observed at the time of scanning the original radiograph is utilized for 
searching the position of the boundary between the heart and the lungs. In 
other words, the density of the radiograph is detected to confirm the 
position of the boundary by making the histogram as shown in FIG. 9. By 
making the histogram as shown in FIG. 9, it is possible to know the 
boundary level Do as the level at the crossing point of the two histograms 
28 and 29 or to calculate the boundary level Do as the average value of 
the two peaks of the histograms (Dh+Df/2) as mentioned hereinbefore. 
Further, it is also possible to determine the boundary level Do by use of a 
density signal obtained by scanning the original image as shown in FIGS. 
10A to 10C. Referring to FIG. 10A, the central part of the chest 
radiograph is scanned by a read-out light beam and the density signal 
waveform obtained thereby along the scanning line is used in combination 
with a differentiated waveform of the density signal as shown in FIGS. 10B 
and 10C. Referring to FIGS. 10A to 10C, when the chest radiograph is 
horizontally scanned at the center thereof, four pulses are obtained as 
shown in FIG. 10C at the boundaries between the lungs 1 and other 
portions. The area between the first pulse and the fourth pulse 
corresponds to the chest itself. Therefore, from the maximum value and the 
minimum value within this area, Dmax and Dmin can be obtained. The second 
pulse and the third pulse correspond to the boundaries between the lungs 1 
and the heart 2. Therefore, the value of the original image density Do at 
these points is the boundary level. When the values at these points are 
different from each other, the average value of these two values can be 
used as the boundary level Do. 
After these values, Dmin, Do and Dmax, have been obtained, the data 
recorded in the magnetic memory tape 26 are processed to perform the 
density conversion of gradation processing as shown in FIGS. 2 to 6 so 
that the radiograph may be reproduced on the final recording medium in the 
desirable gradation. The processed data are returned to the magnetic 
memory tape 26 for memorization of the date after processing. The 
operation of the density can also be performed in the form of analog 
signals. Further, the signal processing may include the process for 
compensation for the gradation of the final recording medium such as a 
photographic film. Furthermore, an unsharp masking process and/or a 
frequency filtering can be conducted to control the sharpness of the 
image. 
The gradation processed date are read out from the magnetic memory tape 26, 
converted to an analog signal by a D/A converter 30 and input into a 
recording light source 32 after amplified by an amplifier 31. 
The light emitted from the light source 32 is focused on a copy film 34 by 
means of a lens 33 to record an image thereon. The copy film 34 is mounted 
on a recording drum 35 which is rotated and axially moved for causing the 
copy film mounted thereon to be exposed to the light from the light source 
32 in a two dimensional scanning mode so that a radiographic image is 
recorded on the copy film 34 in the gradation processed form. 
As for the copy film 35 can be used a photosensitive material like a silver 
halide photographic film, diazo film, electrophotographic material and so 
forth. Further, it is possible to display the image on a monitor like a 
CRT instead of recording the image on a photosensitive material.