Radiation image read-out method

In a method of reading out a radiation image stored in a stimulable phosphor sheet by scanning the stimulable phosphor sheet by stimulating rays, preliminary read-out and final read-out are conducted. The read-out line density of the stimulating rays in the final read-out is detected in advance. Image read-out conditions and/or image processing conditions adjusted on the basis of image information obtained by the preliminary read-out are corrected in accordance with the read-out line density in the final read-out so that the same final read-out image information and/or the same image signal after image processing is obtained from the stimulable phosphor sheet carrying the same image information stored therein.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a radiation image read-out method used in a 
radiation image recording and reproducing system. This invention 
particularly relates to a radiation image read-out method for correcting 
final read-out conditions and/or image processing conditions adjusted on 
the basis of image information obtained by preliminary read-out in a 
radiation image recording and reproducing system wherein preliminary 
read-out and final read-out are carried out. 
2. Description of the Prior Art 
When certain kinds of phosphors are exposed to a radiation such as X-rays, 
.alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultra-violet 
rays, they store a part of the energy of the radiation. Then, when the 
phosphor which has been exposed to the radiation is exposed to stimulating 
rays such as visible light, light is emitted by the phosphor in proportion 
to the stored energy of the radiation. A phosphor exhibiting such 
properties is referred to as a stimulable phosphor. 
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent 
Publication No. 56(1981)-11395, it has been proposed to use a stimulable 
phosphor in a radiation image recording and reproducing system. 
Specifically, a sheet provided with a layer of the stimulable phosphor 
(hereinafter referred to as a stimulale phosphor sheet or simply as a 
sheet) is first exposed to a radiation passing through an object to have a 
radiation image stored therein, and is then scanned with stimulating rays 
such as a laser beam which cause it to emit light in the pattern of the 
stored image. The light emitted by the stimulable phosphor sheet upon 
stimulation thereof is photoelectrically detected and converted to an 
electric image signal, which is processed as desired to reproduce a 
visible image on a recording medium such as a photographic film or on a 
display device such as a cathode ray tube (CRT). 
One embodiment of the aforesaid radiation image recording and reproducing 
system is disclosed, for example, in Japanese Unexamined Patent 
Publication No. 58(1983)-67240. In the embodiment, before final read-out 
for scanning the stimulable phosphor sheet carrying a radiation image of 
an object stored therein by stimulating rays which cause the stimulable 
phosphor sheet to emit light in proportion to the radiation energy stored, 
detecting the emitted light by a photoelectric read-out means and 
converting it into an electric image signal is conducted, preliminary 
read-out for approximately detecting the image information stored in the 
stimulable phosphor sheet is conducted by use of stimulating rays of a 
level lower than the level of the stimulating rays used in the final 
read-out. Read-out conditions for the final read-out (hereinafter referred 
to as the final read-out conditions) and/or image processing conditions 
used in image processing of the electric image signal obtained by the 
final read-out (hereinafter simply referred to as the image processing 
conditions) are adjusted on the basis of the image information obtained by 
the preliminary read-out (hereinafter referred to as the preliminary 
read-out image information). 
As described above, the level of the stimulating rays used in the 
preliminary read-out should be lower than the level of the stimulating 
rays used in the final read-out. That is, the effective energy of the 
stimulating rays which the stimulable phosphor sheet receives per unit 
area in the preliminary read-out should be lower than the effective energy 
of the stimulating rays used in the final read-out. In order to make the 
level of the stimulating rays used in the preliminary read-out lower than 
the level of the stimulating rays in the final read-out, the output of the 
stimulating rays source such as a laser beam source may be decreased in 
the preliminary read-out, or the stimulating rays emitted by the 
stimulating ray source may be attenuated by an ND filter, an AOM, or the 
like positioned on the optical path. Alternatively, a stimulating ray 
source for the preliminary read-out may be positioned independently of the 
stimulating ray source for the final read-out, and the output of the 
former may be made lower than the output of the latter. Or, the beam 
diameter of the stimulating rays may be increased, the scanning speed of 
the stimulating rays may be increased, or the moving speed of the 
stimulable phosphor sheet may be increased in the preliminary read-out. 
In image read-out by scanning a stimulable phosphor sheet carrying a 
radiation image stored therein by stimulating rays, it has theretofore 
been considered that the amount of light emitted per unit area of the 
stimulable phosphor sheet is proportional to the stimulating ray energy 
per unit area thereof. 
This conventional assumption will be explained with reference to FIGS. 1A 
and 1B which are schematic views showing the conditions of scanning a unit 
area 2 at the same position of the same stimulable phosphor sheet 1 
carring the same radiation image stored therein by stimulating rays at a 
low read-out line density and at a high read-out line density of the 
stimulating rays, FIG. 2 which is a graph showing the relationship between 
the read-out line density and the amount of light emitted per picture 
element, and FIG. 3 which is a graph showing the relationship between the 
stimulating ray energy per unit area of the stimulable phosphor sheet and 
the amount of light emitted per unit area of the stimualble phosphor 
sheet. Specifically, it has heretofore been considered that, if the 
stimulating ray energy per picture element 4 is maintained the same when 
the density of read-out lines 3 of the stimulating rays is doubled as 
shown in FIGS. 1A and 1B, the amount of light emitted per picture element 
4 does not change as indicated by the chain line in FIG. 2 but only the 
number of picture elements per unit area changes in proportion to the 
square of the read-out line density. Since 
the stimulating ray energy per unit area=stimulating ray energy per picture 
element.times.number of picture elements per unit area, 
and 
the amount of light emitted per unit area=amount of light emitted per 
picture element.times.number of picture elements per unit area, 
the stimulating ray energy per unit area and the amount of light emitted 
per unit area change in proportion to the square of the read-out line 
density. Therefore, as indicated by the chain line in FIG. 3, if the 
stimulating ray energy per picture element is maintained the same when the 
read-out line density is changed, the amount of light emitted per unit 
area changes in proportion to the stimulating ray energy per unit area 
with the ratio of the amount of light emitted per picture element to the 
stimulating ray energy per picture element being the proportionality 
factor. 
Therefore, in the radiation image recording and reproducing system wherein 
the preliminary read-out and the final read-out are carried out, even when 
the read-out line density in the final read-out (hereinafter referred to 
as the final read-out line density) is changed, the change in the final 
read-out line density has not heretofore been considered, and the final 
read-out conditions and/or image processing conditions have been adjusted 
on the basis of the preliminary read-out image information by presuming 
that the amount of light emitted per picture element is always the same if 
the stimulating ray energy per picture element is the same. 
However, experiments conducted by the inventors revealed that, if the 
read-out line density of stimulating rays is changed when image read-out 
is carried out by scanning the unit area 2 present at the same position of 
the stimulable phosphor sheet 1 carrying the same radiation image by 
stimulating rays as described above, the amount of light emitted per 
picture element changes even when the stimulating ray energy per picture 
element is the same. For example, when the read-out line density is 
increased from that shown in FIG. 1A to that shown in FIG. 1B, the amount 
of light emitted by one picture element 4 in FIG. 1B becomes smaller than 
the amount of light emitted by one picture element 4 in FIG. 1A as 
indicated by the solid line in FIG. 2. Therefore, as indicated by the 
solid line in FIG. 3, the amount of light emitted per unit area 2 is not 
proportional to the stimulating ray energy per unit area 2. 
Accordingly, in the radiation image recording and reproducing system 
wherein preliminary read-out and final read-out are carried out, even when 
the radiation image stored in the stimulable phosphor sheet is the same 
and the stimulating ray energy per picture element is the same, the amount 
of light emitted per picture element becomes different if the final 
read-out line density is different. As a result, the image information 
obtained by the final read-out (hereinafter referred to as the final 
read-out image information), i.e. the final read-out electric image signal 
becomes different. Therefore, when the final read-out conditions and/or 
image processing conditions are adjusted on the basis of the preliminary 
read-out image information by ignoring the change in final read-out line 
density, the final read-out image information and/or the image signal 
after the image processing is caused to change undesirably by the change 
in final read-out line density. 
The picture element 4 is a division of a predetermined length present on 
the read-out line 3 of the stimulating rays. The predetermined length 
changes in accordance with the read-out line density and is, in general, 
inversely proportional to the read-out line density. Therefore, in 
general, the number of picture elements per unit area is proportional to 
the square of the read-out line density. 
SUMMARY OF THE INVENTION 
The primary object of the present invention is to provide a radiation image 
read-out method wherein the final read-out conditions and/or image 
processing conditions are adjusted so that the final read-out image 
information and/or image signal after image processing is consistently 
obtained from a stimulable phosphor sheet carrying the same radiation 
image stored therein even when the final read-out line density is 
different. 
Another object of the present invention is to provide a radiation image 
read-out method wherein the final read-out conditions and/or image 
processing conditions are adjusted as described above, whereby a visible 
image having an improved image quality, particularly a high diagnostic 
efficiency and accuracy can be reproduced consistently regardless of a 
change in final read-out line density. 
The above objects are accomplished by correcting the final read-out 
conditions and/or image processing conditions, which are adjusted on the 
basis of the preliminary read-out image information, in accordance with 
the final read-out line density. 
The final read-out conditions and/or image processing conditions based on 
the preliminary read-out image information are corrected so that the final 
read-out image information and/or the image signal after image processing 
is consistently obtained regardless of a change in final read-out line 
density. Stated differently, the final read-out conditions and/or image 
processing conditions adjusted on the basis of the preliminary read-out 
image information are corrected by a value corresponding to the value of 
the change in amount of light emitted per picture element caused by the 
change in final read-out line density. 
In the present invention, as described above, the final read-out conditions 
and/or image processing conditions are adjusted on the basis of the 
preliminary read-out image information so that the final read-out image 
information and/or the image signal after image processing becomes 
constant regardless of the final read-out line density. In other words, 
when the final read-out line density is changed, the aforesaid conditions 
are corrected by the value of the change in the amount of light emitted 
per picture element, which is caused by the change in final read-out line 
density, so that the same condition as when the amount of light emitted 
per picture element is the same is obtained. 
Therefore, in the present invention, it is possible to adjust the final 
read-out conditions and/or image processing conditions so that the same 
final read-out image information and/or the same image signal after image 
processing are obtained even when the final read-out line density is 
different. 
Also, since the final read-out conditions and/or image processing 
conditions can be adjusted as described above, it is possible to obtain 
the same final read-out image information and/or the same image signal 
after image processing even when the final read-out line density changes, 
thereby consistently reproducing a visible image having an improved image 
quality, particularly a high diagnostic efficiency and accuracy. 
Further, when the final read-out conditions are adjusted as described 
above, it becomes possible to decrease the signal resolution at the time 
of signal recording and to constitute the components of the system such as 
an A/D converter, an image processor and a memory at a low cost.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will hereinbelow be described in further detail with 
reference to the accompanying drawings. 
In the case where, for example the amount of light emitted per picture 
element decreases as indicated by the solid line in FIG. 2 as the final 
read-out line density is increased, the final read-out conditions and/or 
image processing conditions are corrected so that the level of the final 
read-out image information and/or the level of the image signal after 
image processing increase by the value corresponding to the decrease 
amount A when the final read-out line density is 5 lines/mm, the decrease 
amount B when it is 6.7 lines/mm, or the decrease amount C when it is 10 
lines/mm. The correction amounts A, B and C correspond to A', B' and C' in 
FIG. 3. 
Specifically, the final read-out conditions are corrected as described 
below. For example, when the final read-out is carried out at a read-out 
line density of 5 lines/mm, the actual amount of light emitted per picture 
element is Ao. The final read-out gain is not adjusted so that an electric 
image signal of a predetermined level corresponding to Ao is obtained. 
Instead, the final read-out gain is adjusted so as to obtain an electric 
image signal of a predetermined level corresponding to the sum (Ao+A) of 
the actual light amount Ao and the decrease amount A. When the final 
read-out is conducted at a read-out line density of 10 lines/mm, the final 
read-out gain is adjusted so as to obtain an electric image signal of a 
predetermined level corresponding to the sum (Co+C) of the actual light 
amount Co and the decrease amount C. 
The image processing conditions are corrected as described below. For 
example, when the final read-out is carried out at a read-out line density 
of 5 lines/mm, the final read-out is conducted by use of the read-out 
conditions determined by the preliminary read-out, and a constant fixed by 
the read-out scale factor is added to the electric image signal which 
corresponds to the emitted light amount Ao and which is obtained by the 
final read-out so that the level of the image signal becomes equal to the 
level of the electric image signal which will be obtained when the light 
amount (Ao+A), i.e. the sum of the actual light amount Ao and the decrease 
amount A, is detected by the final read-out carried out by using the 
read-out conditions determined by the preliminary read-out. 
As described above, in the present invention, the final read-out conditions 
and/or image processing conditions adjusted on the basis of the 
preliminary read-out image information are corrected in accordance with 
the final read-out line density. The adjustment includes not only the case 
where both the final read-out conditions and image processing conditions 
are adjusted on the basis of the preliminary read-out image information 
but also the case where the conditions are adjusted on the basis of the 
preliminary read-out image information, the object, the image recording 
portion of the object and the image recording conditions. By "read-out 
conditions" are meant the conditions governing the relationship between 
the light emitted by the stimulable phosphor sheet at the final read-out 
step and the electric image signal generated from the emitted light by a 
photoelectric read-out means. By "image processing conditions" are means 
the conditions governing the relationship between the electric image 
signal sent to the image processing means and the electric image signal 
generated thereby. 
In order to correct the final read-out conditions, it is also possible to 
change the stimulating ray energy and/or the read-out time in accordance 
with the read-out line density so that the amount of light emitted per 
picture element becomes constant even when the final read-out line density 
is changed. When the stimulating ray energy is changed, a stimulating ray 
source of a large capacity is required as the read-out line density 
becomes high. When the read-out time is changed, the signal recording time 
becomes long as the read-out line density becomes high. 
The method of the present invention is based on the precondition that the 
final read-out is carried out by changing the final read-out line density. 
For example, when a desired read-out line density is determined in advance 
based on the size of the stimulable phosphor sheet, the sheet size is 
detected and the final read-out line density is adjusted on the basis of 
the detected sheet size. When a desired read-out line density in the final 
read-out is determined in advance based on the object, the image recording 
portion of the object or the image recording conditions, a bar code label 
or a magnetic recording medium carrying such information recorded thereon 
may be secured to the stimulable phosphor sheet. In this case, the 
information on the final read-out line density is detected from the label 
at the preliminary read-out step, and the final read-out line density is 
adjusted on the basis of the detected information.