Method for processing a radiation image

A method for processing a radiographic image to obtain a processed image signal Sproc based on an original image signal Sorg. The method including the steps of: obtaining the original image signal Sorg representing the radiographic image based on radiographic image information transmitted through an object; obtaining an unsharp mask signal Sus corresponding to the original image signal Sorg; and conducting an operation presented by the formula: Sproc=A(Sorg)+B(Sus). In the formula, A(Sorg) is the function which does not include the unsharp mask signal Sus, B(Sus) is the function which does not include the original image signal Sorg.

FIELD OF THE INVENTION 
The present invention relates to a method for processing a radiation image, 
and more particularly relates to a method for processing a radiation image 
in a radiographic system used for medical diagnosis. 
BACKGROUND 
Conventionally, the following radiographic system is known: 
After radioactive rays have been transmitted through a photographic object, 
they are absorbed by a fluorescent substance, so that the radioactive ray 
image information is recorded in the fluorescent substance. Then the 
fluorescent substance is scanned and excited with laser rays, and light is 
emitted from the fluorescent substance. This emitted light is detected by 
a photo detector. Light beams are modulated by this detected radioactive 
ray image information, and the radioactive ray image is recorded on a 
recording medium such as a photographic film. 
Compared with a conventional silver salt type radioactive ray photographic 
system, the above radioactive ray photographic system in which the 
fluorescent substance is used has a wide latitude of radioactive ray 
exposure so as to record an image. From this point of view, the above 
radioactive ray photographic system has a very high utility value. The 
above radioactive ray photographic system is effectively applied to an 
X-ray photographic system for photographing human bodies. 
X-rays are harmful to human bodies when the exposure dosage is increased. 
Therefore, it is desirable to make an amount of information provided by 
one photographing operation as large as possible. However, X-ray 
photographic films to be used at present must have not only an aptitude 
for photographing but also an aptitude for observation. Therefore, the 
X-ray photographic films to be used at present have both aptitudes to some 
extent. For this reason, concerning the aptitude for photographing, the 
X-ray exposure latitude is not sufficiently wide, and concerning the 
aptitude for observation, the image quality is not sufficiently high for 
medical diagnosis. 
In order to solve the above problems, the following method for processing 
radioactive ray images are disclosed: 
(1) Japanese Patent Publication No. 62373/1987 (Conventional Example 1) 
This method is described below: 
A fluorescent substance is scanned with rays of exciting light, so that 
radioactive ray image information recorded in the fluorescent substance is 
read out. After the image information has been converted into electric 
signals, it is reproduced on a recording medium in the following manner. 
An unsharp mask signal Sus corresponding to an extremely low frequency is 
found at each scanning point. Operation is carried out in accordance with 
the following expression. 
EQU S'=Sorg+.beta.(Sorg-Sus) (1) 
where Sorg is an original image signal that has been read from the 
fluorescent substance, .beta. is an emphasis coefficient, and S' is a 
reproduction image signal. In this way, frequency components not lower 
than the above extremely low frequency are emphasized. 
In the above expression, the unsharp mask signal Sus corresponding to the 
extremely low frequency is defined as a signal in which the original image 
is blurred so that the original image only contains low frequency 
components lower than the extremely low frequency component. In this 
method, the emphasis coefficient .beta. is simply increased in accordance 
with an increase of the original image signal Sorg or the unsharp mask 
signal Sus. Further, in this method, the unsharp mask signal Sus is found 
when the original image signals Sorg are simply averaged in the mask. 
According to this method, the frequency components not lower than the 
extremely low frequency which are effective for medical diagnosis are 
emphasized so that the contrast can be enhanced. In this way, the medical 
diagnosis performance can be improved. 
(2) Japanese Patent Publication No. 62383/1987 (Conventional Example 2) 
This method is described below: 
An accumulation type fluorescent substance is scanned with rays, so that 
radioactive ray image information recorded in the fluorescent substance is 
read out. After the image information has been converted into electric 
signals, it is reproduced as a visual image on a recording medium in the 
following manner. An unsharp mask signal Sus corresponding to an extremely 
low frequency is found at each scanning point. Operation is carried out in 
accordance with the following expression. 
EQU S'=Sorg+F(X) (2) 
(In this case, X and F(X) are expressed as follows. 
X=Sorg-Sus. 
F(X) is described below. 
When .vertline.X1.vertline.&lt;.vertline.X2.vertline., the inequalities 
F'(X1).gtoreq.F' and (X2).gtoreq.0 are satisfied. With respect to X0 that 
satisfies 
.vertline.X1.vertline.&lt;.vertline.X0.vertline.&lt;.vertline.X2.vertline., F(X) 
is a monotone increasing function that satisfies the inequality F'(X1) 
&gt;F'(X2). 
where Sorg is an original image signal that has been read from the 
fluorescent substance, and S' is a reproduction image signal. In this way, 
frequency components not lower than the above extremely low frequency are 
emphasized. 
In this case, values of F(X) can be stored in the form of a table. 
There is a tendency that an artifact is generated in a region where the 
difference signal .vertline.Sorg-Sus.vertline. is high. From the viewpoint 
described above, according to this method, the frequency is more 
emphasized in a portion where the difference signal 
.vertline.Sorg-Sus.vertline. is high. Accordingly, it is possible to carry 
out image processing in which the generation of an artifact is suppressed. 
According to the former conventional method described above, it requires a 
long period of time to carry out an operation. In order to reduce the 
operation time, it is suggested to employ a table-looking system in which 
the operation values are previously stored in the form of a table. 
However, when a ratio of emphasis of frequency, which is the emphasis 
coefficient .beta. in the conventional example 1, is changed in accordance 
with an increase of Sorg or Sus, the following problems may be 
encountered. 
When the emphasis coefficient .beta. is a function which changes in 
accordance with a change in Sorg (in this case, the function is expressed 
by .beta. (Sorg)), the expression of the conventional example 1 can be 
transformed as follows. 
##EQU1## 
In this case, 
F(Sorg)=(1+.beta.(Sorg)).multidot.Sorg 
G(Sorg, Sus)=-.beta.(Sorg).multidot.Sus 
(F is a function of Sorg, and G is a function of Sorg and Sus.) 
When the emphasis coefficient .beta. is a function which changes in 
accordance with a change in Sus (in this case, the function is expressed 
by .beta.(Sus)), the expression of the conventional example 1 can be 
transformed as follows. 
##EQU2## 
In this case, 
P(Sorg, Sus)=(1+.beta.(Sus)).multidot.Sorg 
Q(Sus)=-.beta.(Sus).multidot.Sus 
(F is a function of Sorg and Sus, and G is a function of Sus.) 
In any cases, terms of both Sorg and Sus are included. Accordingly, when 
the values are stored in the table-looking system, it is necessary to 
provide a two-dimensional arrangement. Therefore, a large capacity of 
memory is required for storing the table. 
In some cases, it is preferable that the table is calculated or transformed 
for each image. In this case, however, it takes a long period of time for 
table calculation. Therefore, it is not possible to reduce the operation 
time. Further, when .beta. is sharply changed, an artifact is generated, 
which may cause a wrong diagnosis. Further, when Sus is found by 
calculating a simple average of the signals in the mask, it is necessary 
to conduct a division, so that it takes a long period of time for 
operation. 
In the latter conventional system described above, a table is required, the 
dimensions of which are (a range of values of Sorg+a range of values of 
Sus). Not only the addition and subtraction of each term but also the 
operation of the argument X (=Sorg-Sus) is required for each pixel. 
Therefore, it is difficult to shorten the operation time sufficiently. In 
order to omit the calculation of the argument X, X may be a function of 
Sorg and Sus like f (Sorg, Sus). In this case, however, a large capacity 
of table memory is required. Further, there is a description that f(X) may 
include a function of Sorg and/or Sus. In this case, a large capacity of 
table memory is also required in the same manner as that of the 
conventional technique (1) described before. In the description, there is 
provided no concept that the operation speed is increased when the 
function of only one of Sorg and Sus is added or subtracted. 
SUMMARY OF THE INVENTION 
In view of the problems described above, the present invention has been 
accomplished. It is an object of the present invention to provide a method 
for processing a radiation image in which the operation speed is increased 
and the memory capacity is reduced. 
The present invention provides a method for processing a radiation image by 
which an original image signal "Sorg" to express an original image based 
on a radiation image transmitted through a photographic object is 
processed and a processed image signal "Sproc" having a frequency 
characteristic different from that of the original image is found. 
One example of the present invention is characterized in that the 
processing is carried out in accordance with the following expression. 
EQU Sproc=A(Sorg)+B(Sus) (5) 
where Sus: Unsharp signal 
A(Sorg): Function of Sorg excluding Sus 
B(Sus): Function of Sus excluding Sorg 
The other example of the present invention is described as follows. The 
present invention is to provide a method for processing a radiation image 
by which an original image signal "Sorg" based on the radiation image 
information transmitted through a photographic object is processed using 
an unsharp signal Sus and a processed image signal "Sproc" having a 
frequency characteristic different from that of the original image is 
found. In this case, the aforementioned Sus is found when the total Stotal 
of the original image signals Sorg of pixels, the number of which is N, in 
a predetermined mask including the objective pixels is shifted to the 
right by z bits (z is a positive integer), that is, when the total Stotal 
of the original image signals Sorg of pixels is shifted in the direction 
of LSB (Least Significant Bit). 
For example, as the value of Stotal is expressed as "1010" in a binary 
code, the value of Stotal becomes "0101" when it is shifted one bit to the 
direction of LSB. 
In the present invention, the processed image signal Sproc is expressed in 
the form of addition of A(Sorg) and B(Sus). According to this expression, 
A(Sorg) and B(Sus) are respectively independent functions of Sorg 
(original image) and Sus (unsharp image). The expression does not include 
a term composed of both Sorg and Sus. Therefore, the operation speed is 
increased. Even when A(Sorg) and B(Sus) are respectively stored in the 
form of a table, the memory capacity is not so large. 
Also, the sharp signal Sus is found when the Stotal, which is the total of 
the original image signals Sorg of N pixels in a predetermined mask area 
including the target pixel, is shifted to the right by z bits (z is a 
positive integer). Due to the foregoing, the processing is not carried out 
by the operation of Stotal/N, but the processing is carried out by the 
right shift operation. Accordingly, the processing speed can be greatly 
increased.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to the accompanying drawings, an example of the present 
invention will be explained in detail as follows. 
FIG. 1 is a block diagram showing an example of system structure for 
realizing the method of the present invention. When X-rays are applied to 
human's body, X-rays are transmitted through the body and incident upon a 
fluorescent material board. On this fluorescent material board, energy of 
the X-ray image is accumulated to the trap level of the fluorescent 
material. Due to the X-ray photography described above, the radiation 
image is accumulated and recorded on an accumulation type fluorescent 
sheet 1. The accumulation type fluorescent sheet 1 is conveyedby a roller 
2. The sheet 1 is conveyed by the roller 2 in the direction of arrow A, 
and the subsidiary scanning is conducted on the sheet 1 so as to read the 
image on the sheet. 
The primary scanning is conducted when the laser beams emitted by a laser 
beam source 3, the wavelength of excitation light of which is 500 to 800 
nm, are subjected to scanning operation by a scanning mirror 3a in the 
direction of arrow B. By the scanning of the excitation light, photo 
stimulated luminescence light is generated, the wavelength range of which 
is 300 to 500 nm. This photo stimulated luminescence light is detected and 
converted into an electric signal by a photo detector 4 such as a photo 
multiplier arranged at the output end of the light collecting body 4a 
composed of light-conductive sheets. This electric signal is amplified by 
an amplifier 5 and converted into a digital signal by an A/D converter 6. 
Then the digital signal is sent to an operation section 7. 
In the operation section 7, unsharp mask signal Sus is found by a 
calculating element 7a, function B(Sus) is found by a calculating element 
7b, function A(Sorg) is found by a calculating element 7c, and 
Sproc=A(Sorg)+B(Sus) is found by a calculating element 7d. The thus 
provided digital signal Sproc is converted into an analog signal by the 
D/A converter 9 and amplified by the amplifier 10. Then the amplified 
signal is inputted into a light source 11 for recording. 
Light is emitted from the light source 11 for recording. Then light passes 
through the lens. After that, light is irradiated on a recording material 
such as a photographic film attached onto the printing drum 14. The 
radiation image is reproduced on this photographic film, and the 
reproduced image is used for q medical diagnosis. 
The present invention is to provide an image processing method employed in 
the calculating section 7. Therefore, it should be noted that the present 
invention is not limited to the specific image input method and image 
display method described above. For example, the following image input 
method may be adopted: 
A photographic film is irradiated with a beam of light. The reflected beam 
of light or a transmitted beam of light is optically read so as to obtain 
a digital image signal. Concerning image display method, instead of 
printing an image on a photographic film, it may be displayed on a CRT. 
Operation of the calculating section 7 will be explained as follows. In the 
present invention, Sproc is found by the following expression. 
EQU Sproc=A(Sorg)+B(Sus) (5) 
where Sus: Unsharp signal 
A(Sorg): Function of Sorg excluding Sus 
B(Sus): Function of Sus excluding Sorg 
When the unsharp signal Sus is a simple average of the original image 
signals in a predetermined mask including the objective pixels, the 
calculating time can be preferably shortened. However, the weighted 
average, median and mode may be used instead of the simple average. 
Configuration of the mask may be rectangular, circular, cross, X-shaped 
and annular. Further, these various configurations may be combined. When 
data is processed by a computer, it is preferable that the mask 
configuration is rectangular, and it is most preferable that the mask 
configuration is square. 
In the case where the unsharp signal Sus is found, signals of all pixels in 
the mask may be used. Alternatively, sampling may be carried out at 
regular intervals and signals of some of the pixels may be used. When at 
least one of functions A(Sorg) and B(Sus) is previously stored in the form 
of a table, it is not necessary to carry out the operation for each pixel 
each time. Accordingly, the calculating time is preferably reduced. The 
composition of the photographic object is different for each part, and 
even when the part is the same, the composition is different for each 
photographic object. Therefore, when a table is made for each image in 
accordance with the characteristic amount of an image such as a maximum 
value, minimum value, average and frequency characteristics in the most 
concerned region, the image can be most appropriately processed, and the 
image quality can be enhanced. 
When the reference table is rotated and moved in parallel in accordance 
with the characteristic amount of an image, the reference table is varied. 
When the varied table is used, a complicated function, which is not 
expressed by a simple expression, can be simply made for each image. 
Therefore, it is preferable that the reference table is rotated and moved 
in parallel in accordance with the characteristic amount of an image to 
find the varied table. Alternatively, a different reference table may be 
stored for each part, and the reference table is selected in accordance 
with the photographed part information. Further, the reference table may 
be varied for each image in accordance with the characteristic amount and 
the radioactive ray condition. 
It is preferable that A(Sorg) is at least the product of function a(Sorg), 
which changes in accordance with a change in Sorg, and Sorg. It is also 
preferable that B(Sus) is at least the product of function b(Sus), which 
changes in accordance with a change in Sus, and Sus. 
It is preferable that a(Sorg) and b(Sus) are expressed by the following 
expressions. 
EQU a(Sorg)=(1+k(Sorg)) 
EQU b(Sus)=-k(Sus) (6) 
As a result, the processed image signal Sproc is expressed by the following 
expression. 
##EQU3## 
In the case where A(Sorg) and B(Sus) are sharply changed, a mock image 
appears. At this time, when A(Sorg) is the product of function a(Sorg), 
which changes in accordance with a change in Sorg, and Sorg, and also when 
B(Sus) is the product of function b(Sus), which changes in accordance with 
a change in Sus, and Sus, the conversion table is made using the following 
expressions. 
##EQU4## 
In the above calculating expressions, a(S) or b(S) is accumulated from 0 to 
Sorg, or from 0 to Sus. Due to the foregoing, a change in the value on the 
correction table becomes gentle, so that the occurrence of a mock image 
can be prevented and the image quality can be improved. 
This point will be explained in further detail. 
The calculating expression of the frequency emphasizing processing of the 
conventional example described above can be deformed as follows. 
##EQU5## 
In the above expression, the first term includes a mutual operation of Qus 
and Qorg. Therefore, it takes time for operation. For this reason, it is 
desirable to realize a calculating expression in which the first and 
second terms do not include a mutual operation of Qus and Qorg and further 
the image quality is not deteriorated. For example, when the first term of 
the calculating expression is a function of only Qorg and the second term 
of the calculating expression is a function of only Qus, LUT processing 
can be independently carried out in parallel, so that the calculating time 
can be shortened. That is, k(Qus) in the above expression may be replaced 
with k(Qorg). In other words, k(Qus) in the above expression may be 
approximated to k(Qorg). Then the following operation may be carried out. 
EQU Qproc={1+k(Qorg)}.multidot.Qorg-k(Qus).multidot.Qus (9) 
When the emphasizing coefficient k is gradually changed, no problems are 
caused. However, when the emphasizing coefficient k is changed stepwise as 
shown in FIG. 2(a), there is a possibility of generation of a mock image. 
The signal shown in FIG. 3 is taken as an example and explained below. As 
illustrated in FIG. 3, Qorg changes like triangular waves, however, Qus is 
an average value, so that it is constant. The values of Qorg at the points 
x1, x2 and x3 are respectively Q1, Q2 and Q3, and Qus is Q2. FIG. 2(b) 
shows the first term {1+k(Qorg)}.multidot.Qorg and the second term 
k(Qus).multidot.Qus. The second term is always (2)' at the points x1, x2 
and x3. In the case of the expression (8), the first terms are 
respectively (1), (2) and (3) with respect to Q1, Q2 and Q3. In the case 
of the expression (9), the first term with respect to Q3 is (4), which is 
larger than (2) with respect to Q2. Accordingly, when the operation 
processing is carried out onlyby the expression (9), there is a 
possibility of generation of a mock image. 
In order to prevent the occurrence of a mock image, a method may be 
employed in which the following expression is deformed. This method is 
based on the concept described below: 
Expression (6) is a linear expression of Qorg and Qus, and {1+k(Qorg)} and 
Q(Qus) are differential coefficients of the linear expression. When the 
differential coefficient is integrated by Qorg (or Qus), the original 
function is obtained. 
EQU Qorg=A(Qorg)-B(Qus) (10) 
where 
A(Qorg): Accumulation function of 1+k(Q) 
B(Qus): Accumulation function of k(Q) 
According to this method, both A(Q) and B(Q) are continuous functions, so 
that a difference between (3) and (4) is small, and the first term is not 
reversed (shown in FIG. 2(c)). Accordingly, it is possible to prevent the 
generation of a mock image. 
Next, the calculation method of the unsharp signal Sus will be explained as 
follows. When Sus is found by a simple average of Sorg, the correct method 
is described as follows: The total of Stotal of the original image signals 
Sorg of pixels, the number of which is N, in a predetermined mask 
including the objective pixel is divided by N so as to find Stotal/N. 
However, this method is disadvantageous in that the capacity of hardware 
used for operation is increased, and further an amount of software 
processing is increased. Accordingly, instead of dividing Stotal by N, 
Stotal is divided by 2.sup.z. That is, the operation is made in such a 
manner Stotal/2.sup.z This division is simple. The division is made in 
such a manner that Stotal is shifted to the right by z bits, wherein z is 
a positive integer. However, an error of 2.sup.z /N is generated between N 
and 2.sup.z. Therefore, Sus is found in such a manner that the total 
Stotal of the original image signals Sorg of pixels, the number of which 
is N, in a predetermined mask including the objective pixel is shifted to 
the right by z bits. When .alpha.=2.sup.z /N, operation is made in 
accordance with the following expression. 
EQU Sproc=A(Sorg, .alpha..multidot.Sus) (11) 
In this case, the operation is made when the function of Sproc is corrected 
by .alpha. times in the direction of Sus axis. In this way, it is not 
necessary to conduct a division, so that the operation time can be 
reduced. Especially when the calculating expression is stored in the form 
of a table, the operation time can be greatly reduced. Therefore, it is 
preferable to store the calculating expression in the form of a table. 
The matter described above will be explained in detail referring to FIGS. 
4(a) and 4(b). When A'(Sorg, Sus)=A(Sorg, .alpha.Sus) is used instead of 
A(Sorg, Sus), a function to find Sproc is corrected by .alpha. times on 
the Sus axis. In order to simplify the explanation, a specific explanation 
will be made on a plane of Sproc-Sus. For example, in the case of 
Sproc=A(Sorg, Sus), as illustrated in FIG. 4(a), Sproc is expressed by a 
linear function which increases in accordance with an increase of Sus, and 
an inclination of the straight line is changed by Sus=1024,2048. In the 
case of Sproc=a(Sorg, .alpha..multidot.Sus), the inclination is changed 
when s=1024,2048. Consequently, the function of A'(Sorg, Sus) is expressed 
in the form of a graph in such a manner that the function is reduced by 
1/.alpha. in the direction of Sus axis. In the case of .alpha.&lt;1, the 
function is extendedby 1/.alpha. in the direction of Sus axis. 
Further, when Sus, obtainedby shifting the value of Stotal for z bits to 
the direction of LSB as described above, is in the range that Sus is used 
without multiplying by the correction value .alpha., the operation can be 
carried out according to the function expressed by Sproc=A(Sorg, Sus). 
Next, an improvement of operation accuracy to find Sus will be explained as 
follows. Sus described above is found in the following manner: 
The total Stotal of the original image signals Sorg of pixels, the number 
of which is N, in a predetermined mask including the objective pixel is 
shifted to the right by z bits. In this case, x is a minimum integer 
satisfying the inequality N .ltoreq.2.sup.z+P-q the unsharp mask signal 
Sus is expressed by p bit of the gray scale level, and the original image 
signal Sorg is expressed by q bit of the gray scale level. When 
.alpha.=2.sup.z /N, operation is made in accordance with the expression of 
Sporc=A(Sorg, .alpha..multidot.Sus). In this way, Sus can be expressed in 
a range of the bit number p in a desired gray scale level. When p is made 
to be larger than q, the occurrence of an error can be suppressed when the 
data is arranged in the form of a table. The larger the value of p-q is, 
the higher the operation accuracy is improved. However, when the operation 
accuracy is enhanced, the memory capacity of the table is increased. 
Therefore, from the viewpoint of practical use, the value of p-q 
preferably satisfies the inequality of 2.ltoreq.p-q.ltoreq.6. 
Next, the effect of the present invention will be explained in detail. FIG. 
5 is a table for explaining the effect of the present invention. In the 
table, the effect of the present invention is compared with the effect of 
comparative examples. In FIG. 5, the operation time of the conventional 
example 1 and the operation time of the example of the present invention 
are compared. FIGS. 6(a) to 6(f) are graphs showing various functions used 
in the operation. 
[COMATIVE EXAMPLE 1] 
Image data was subjected to software processing in a general-use work 
station. With respect to Sorg, Sus and Sproc, image data was provided as 
follows: 
2048 pixels.times.2048 pixels.times.density resolution 12 bits (4096 
gradation) 
Sus was found when the total Stotal of Sorg in the mask of 31.times.31 
pixels was divided by the number 31.times.31 of pixels. The expression 
used for the operation was the same as that of the example of the prior 
art, which will be described below. 
##EQU6## 
Operation was made for each pixel in accordance with the expression (1). 
Concerning the emphasis coefficient k, the following two values were 
adopted. One is k2 shown in FIG. 6(a), and the other is kl shown in FIG. 
6(b). 
[COMATIVE EXAMPLE 2] 
The following expression was used. 
EQU A(Sorg, Sus)=(1+k(Sus)).times.Sorg-k(Sus).times.Sus 
In this case, A(Sorg, Sus) was previously found in the form of a table in 
which the input was determined to be 12 bits and the output was determined 
to be 12 bits. The processed image signal Sproc was found for each pixel 
using the following expression. 
EQU Sproc=A(Sorg, Sus) 
Other points were the same as those of Comparative Example 1. fin this 
case, the table A was in a two-dimensional arrangement, the number of 
elements of which was 4096.times.4096. 
[Example 1] 
Image data was subjected to software processing in a general-use work 
station. With respect to Sorg, Sus and Sproc, image data was provided as 
follows: 
2048 pixels.times.2048 pixels.times.density resolution 12 bits (4096 
gradation) 
Sus was found when the Sorg in the mask of 31.times.31 pixels was simply 
averaged. The following expression was used in the operation. 
EQU Sporc=(1+k(Sorg)).times.Sorg-k(Sus).times.Sus 
where 
A(Sorg) = (1+k(Sorg)).times.Sorg 
B(Sus) = -k(Sus).times.Sus 
In this case, A(Sorg) and B(Sus) were previously found in the form of a 
table in which the input was determined to be 12 bits and the output was 
determined to be 12 bits. The processed image signal Sproc was found for 
each pixel using the following expression. 
EQU Sporc=A(Sorg)+B(Sus) 
In this case, the tables A and B were in a one-dimensional arrangement, the 
number of elements of which was 4096. In this case, the function k(Sorg) 
was established in the following two manners. One is a function in which 
k1 is used, and the other is a function in which k2 is used. 
[Example 2] 
##EQU7## 
where .DELTA.s=1 
A(Sorg) and B(Sus) were found by the above expressions, and other points 
were the same as those of Example 1. 
[Example 3] 
In this example, Sus was found in such a manner that the Stotal, which is 
the total of the Sorg in the mask of 31.times.31 pixels, was shifted to 
the right for 10 bits. Instead of B(Sus) , the table B'(Sus) was used, the 
input of which was composed of 12 bits and the output of which was 
composed of 12 bits. Other points were the same as those of Example 2. 
EQU B'(Sus)=B(.alpha..multidot.Sus) (16) 
where .alpha.=2.sup.10 /31 .times.31, and the configuration of B' is the 
same as that illustrated in FIG. 6(e). 
[Example 4] 
In this example, the number of bits of Sus in the dynamic range was 16, and 
Sus was found in such a manner that the Stotal, which is the total of Sorg 
in the mask of 31.times.31 pixels, was shifted to the right for 6 bits. 
Instead of B(Sus) , the table B'(Sus) was used, the input of which was 
composed of 12 bits and the output of which was composed of 12 bits. Other 
points were the same as those of Example 2. 
EQU B'(Sus)=B(.alpha..multidot.Sus) (17) 
where .alpha.=2.sup.6 /31.times.31, and the configuration of B' is the same 
as that illustrated in FIG. 6(f). 
In FIG. 5, the following are shown: 
Number of addition and subtraction in the operation of Sproc, number of 
multiplication, capacity of memory necessary for the table, operation 
time, operation time of Sus, and existence of artifact. In Comparative 
Example 1, 30 seconds were required for the operation of Sus, and 25 
seconds were required for the operation of Sproc, so that 55 seconds were 
required in total. In the case of k1 in which the function of k(Sus) is 
gently changed, the occurrence of artifact was avoided. In the case of k2 
which was sharply changed, the occurrence of artifact was observed when 
the signal value was about 1024. 
In Comparative Example 2, it took 72 seconds to calculate the table. 
Accordingly, the operation time of Comparative Example 2 was longer than 
that of Comparative Example 1. Further, the memory capacity of 32 M bytes 
was required for the table, which is not suitable for practical use. 
In Example 1, the amounts of addition, subtraction and multiplication were 
greatly reduced. Therefore, the operation time of Sproc was reduced from 
25 seconds to 10 seconds. The table operation time was not more than 0.1 
second, which was negligibly small. Memory capacity necessary for the 
table was 16 K bytes, which caused no practical problems. 
The operation time of Example 2 was the same as that of Example 1. Even 
when the function of k(Sus) was k2 which changed sharply, the occurrence 
of artifact was not observed and high image quality was provided. 
Although the operation time of Sproc of Example 3 was the same as that of 
Example 1, the Sus operation time was reduced by 15 seconds. Therefore, 
the total operation time was 25 seconds, that is, the total operation time 
was reduced to a half of that of Comparative Example 1. 
The operation time of Example 4 was the same as that of Example 3. Although 
it was not checked by the visual inspection, according to the analysis of 
signal values, it was found that the amount of quantization noise of 
Example 4 was larger than that of Example 1. The amount of .quantization 
noise of Example 4 was smaller than that of Example 3. 
As described above, according to the present invention, a calculating 
expression is used, in which the processed image signal Sproc is expressed 
in the form of a summation of the functions of one of the original image 
signal Sorg and the unsharp signal Sus. Accordingly, it is possible to 
provide a method for processing a radiation image by which the operation 
speed is increased and the memory capacity is reduced. Therefore, the 
present invention can provide a great practical effect. 
When the rate of emphasis of frequency is changed in accordance with the 
increase of Sorg and/or Sus, the diagnosis ability can be enhanced and the 
operation time can be shortened. In the example of the present invention, 
the rate of emphasis of frequency is monotonously reduced in accordance 
with the increase of Sus or Sorg, that is, k(Sus) and k(Sorg) are 
monotonously reduced in accordance with the increase of Sus or Sorg. 
However, it should be noted that the rate of emphasis of frequency is not 
limited to the monotonous reduction. It is preferable that the monotone 
decreasing function, monotone increasing function and more complicated 
function are appropriately used in accordance with the part of an original 
image. 
Even when the image processing method of the present invention is combined 
with other image processing methods such as gradation processing, the same 
effect can be provided. Therefore, it is preferable to combine the image 
processing method of the present invention with other image processing 
such as gradation processing. In this case, other image processing may be 
carried out before or after the image processing of the present invention.