Radiation image read-out apparatus

A radiation image read-out apparatus is constituted for scanning a stimulable phosphor sheet carrying a radiation image stored thereon with a beam of stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy, and photoelectrically detecting the light emitted by a stimulable phosphor sheet portion scanned with the beam of stimulating rays by use of a photodetector to obtain an image signal. The radiation image read-out apparatus is constituted so that the beam diameter of the beam of stimulating rays is adjusted to be smaller than a picture element size and each of picture elements is scanned by a plurality of scanning passes with the beam of stimulating rays. An addition device is provided for adding a plurality of image signals per picture element obtained by the plurality of scanning passes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a radiation image read-out apparatus for reading 
out a radiation image stored on a stimulable phosphor sheet by exposing 
the stimulable phosphor sheet to stimulating rays which cause it to emit 
light in proportion to the stored radiation energy, and photoelectrically 
detecting the emitted light. 
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 ultraviolet 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 stimulable phosphor sheet) is first exposed 
to a radiation passing through an object to have a radiation image of the 
object stored thereon, and is then scanned with stimulating rays such as a 
laser beam which cause the stimulable phosphor sheet to emit light in 
proportion t the stored radiation energy. The light emitted by the 
stimulable phosphor sheet when it is exposed to stimulating rays is 
photoelectrically detected and converted into an electric image signal, 
which is processed 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). 
The radiation image recording and reproducing system using a stimulable 
phosphor sheet is advantageous over conventional radiography using a 
silver halide photographic material in that the image can be recorded over 
a very wide range (latitude) of radiation exposure. More specifically, 
since the amount of light emitted upon stimulation after the radiation 
energy is stored on the stimulable phosphor sheet varies over a wide range 
in proportion to the amount of said stored energy, it is possible to 
obtain an image having desirable density regardless of the amount of 
exposure of the stimulable phosphor sheet to the radiation, by reading out 
the emitted light with an appropriate read-out gain and converting it into 
an electric signal by use of a photoelectric conversion means to reproduce 
a visible image on a recording medium such as a photographic film or a 
display device such as a CRT. 
In the aforesaid radiation image recording and reproducing system, read-out 
of the radiation image is generally conducted by use of a read-out 
apparatus constituted so that a beam of stimulating rays deflected by a 
light deflector is made to scan on the stimulable phosphor sheet in a main 
scanning direction, and at the same time the stimulable phosphor sheet is 
conveyed in a sub-scanning direction approximately normal to the main 
scanning direction. 
As the light deflector, a multi-face rotating mirror (i.e. polygon mirror) 
rotating at a high speed may be used. The multi-face rotating mirror is 
advantageous in scanning stability over other light deflectors such as a 
galvanometer mirror. However, for achieving high scanning stability, it is 
necessary to rotate the multi-face rotating mirror at a high speed. On the 
other hand, in order to stimulate the stimulable phosphor sheet carrying a 
radiation image stored thereon, it is necessary to expose the sheet to 
stimulating rays of comparatively high energy. However, when the 
multi-face rotating mirror is rotated at a high speed, the scanning speed 
of the beam of stimulating rays in the main scanning direction becomes 
high, and the level of stimulation energy which the stimulable phosphor 
sheet receives decreases. As a result, the level of light emitted by the 
stimulable phosphor sheet decreases, and the S/N ratio of a read-out image 
signal becomes low. 
SUMMARY OF THE INVENTION 
The primary object of the present invention is to provide a radiation image 
read-out apparatus which provides a read-out image signal with a high S/N 
ratio even though a beam of stimulating rays is scanned at a high speed on 
a stimulable phosphor sheet. 
Another object of the present invention is to provide a radiation image 
read-out apparatus for obtaining a read-out image signal with a high S/N 
ratio in a simple manner even though a beam of stimulating rays is scanned 
at a high speed on a stimulable phosphor sheet. 
The present invention provides a radiation image read-out apparatus for 
scanning a stimulable phosphor sheet carrying a radiation image stored 
thereon with a beam of stimulating rays which cause the stimulable 
phosphor sheet to emit light in proportion to the stored radiation energy, 
and photoelectrically detecting the light emitted by a stimulable phosphor 
sheet portion scanned with the beam of stimulating rays by use of a 
photodetector to obtain an image signal, 
wherein the improvement comprises: 
(i) constituting said radiation image read-out apparatus so that the beam 
diameter of said beam of stimulating rays is adjusted to be smaller than a 
picture element size and each of the picture elements is scanned by a 
plurality of scanning passes with said beam of stimulating rays, and 
(ii) providing an addition means for adding a plurality of image signals 
per picture element obtained by said plurality of scanning passes. 
When the beam diameter of the beam of stimulating rays is adjusted to be 
substantially small, the energy density of the beam becomes high. Also, 
when each of the picture elements is scanned by a plurality of scanning 
passes with the beam of stimulating rays having a small beam diameter 
(i.e. different portions of a single picture element are respectively 
scanned by different scanning passes), the total amount of light emitted 
per picture element becomes substantially large. Therefore, a read-out 
image signal obtained by adding a plurality of image signals per picture 
element represents the total amount of light emitted by said picture 
element. Thus it is possible to obtain a visible reproduced image having a 
high image quality with a high S/N ratio by use of the read-out image 
signal which is of a high level.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will hereinbelow be described in further detail with 
reference to the accompanying drawings. 
Referring to FIG. 1, a stimulable phosphor sheet 10 carrying a radiation 
image of an object stored thereon by being exposed to a radiation such as 
X-rays passing through the object such as the human body is conveyed by a 
sheet conveyance means 11 constituted by an endless belt or the like in a 
sub-scanning direction as indicated by the arrow Y. A laser beam 13 
emitted as stimulating rays by a laser beam source 12 is deflected by a 
multi-face rotating mirror 14 which rotates at a high speed. The laser 
beam 13 is then converged by a converging lens 18 usually constituted by 
an f.crclbar. lens, and is reflected by a mirror 19 onto the stimulable 
phosphor sheet 10. Thus the laser beam 13 scans the stimulable phosphor 
sheet 10 in a main scanning direction as indicated by the arrow X 
approximately normal to the sub-scanning direction as indicated by the 
arrow Y. When the stimulable phosphor sheet 10 is exposed to the laser 
beam 13, the exposed portion of the sheet 10 emits light 15 in an amount 
proportional to the stored radiation energy. The emitted light 15 is 
guided by a light guide member 16 and photoelectrically detected by a 
photomultiplier 17 acting as a photodetector. The light guide member is 
fabricated by forming a light guiding material such as an acrylic plate, 
and has a linear light input face 16a disposed to extend along the beam 
scanning line on the stimulable phosphor sheet 10, and a ring-like light 
output face 16b closely contacted with a light receiving face of the 
photomultiplier 17. The light 15 emitted by the stimulable phosphor sheet 
10 and entering the light guide member 16 from its light input face 16a is 
guided inside of the light guide member 16 through total reflection up to 
the light output face 16b, and received by the photomultiplier 17. In this 
manner, the amount of the light 15 representing the radiation image stored 
on the stimulable phosphor sheet 10 is detected by the photomultiplier 17, 
which converts the detected light amount into an analog output signal 
(image signal) S. 
The analog output signal S generated by the photomultiplier 17 is amplified 
by a logarithmic amplifier 20, and digitized by an A/D converter 21 with a 
predetermined scale factor into a digital image signal Sd. The digital 
image signal Sd thus obtained is sent to an addition section 22 which 
conducts addition processing. The addition processing will hereinbelow be 
described in detail. As shown in FIG. 2, the laser beam 13 is converged to 
a beam diameter substantially smaller than the size of a picture element P 
on the stimulable phosphor sheet 10. The main scanning speed and the 
sub-scanning speed of the laser beam 13 are adjusted to appropriate 
values, so that the laser beam 13 scans the single picture element P by 
three scanning passes in the main scanning direction (i.e. an 
approximately 1/3 portion of the single picture element P is scanned by 
each scanning pass in the main scanning direction). Therefore, as shown in 
FIG. 3, the digital image signal Sd comprises signals of three sets such 
as, for example, (a1, b1, c1, . . . , n1), (a2, b2, c2, . . . , n2), and 
(a3, b3, c3, . . . , n3) for a single picture element string Pa, Pb, Pc, . 
. . , Pn in the main scanning direction. The addition section 22 adds the 
signals common to each picture element as expressed by (a1+a2+a3), 
(b1+b2+b3), (c1+c2+c3), . . . , (n1+n2+n3). The addition in the addition 
section 22 is conducted by creating addresses of the picture elements by 
an address creating section 24 which receives an X-clock pulse Cx and a 
Y-clock pulse Cy respectively in synchronization with the main scanning 
and sub-scanning of the laser beam 13, making the respective signals 
entered later correspond to the respective signals which have been stored 
at the respective addresses in a line memory 25, at the corresponding 
addresses, and adding the signals corresponding to each other. The 
addition may also be conducted by adding the signals (a2, b2, c2, . . . , 
n2) obtained by the second main scanning pass to the signals (a1, b1, c1, 
. . . , n1) stored in the line memory 25 to obtain sum signals (a1+a2), 
(b1+b2), (c1+c2), . . . , (n1+n2), storing the sum signals in the line 
memory 25, and then adding the signals (a3, b3, c3, . . . , n3) obtained 
by the third main scanning pass to the stored sum signals. Or, the signals 
(a1, b1, c1, . . . , n1), (a2, b2, c2, n2), and (a3, b3, c3, . . . , n3) 
obtained by three main scanning passes may be stored in the line memory 
25, and the addition may be conducted after the three main scanning passes 
are finished. Instead of the line memory 25, a frame memory capable of 
storing the image signals over the whole surface of the stimulable 
phosphor sheet 10 may be used. However, when the signals are added each 
time one main scanning pass is conducted as mentioned above, it is 
possible to employ a line memory having a small capacity for signals of 
one or two lines. 
The sum signals (a1+a2+a3), (b1+b2+b3), (c1+c2+c3), . . . , (n1+n2+n3) are 
stored as read-out image signals Sp respectively at the picture elements 
Pa, Pb, Pc, . . . , Pn in a large-capacity memory 26 constituted by an 
optical disk, a magnetic disk, or the like. In the same manner, sum 
signals at picture element strings following the picture element string 
Pa, Pb, Pc, . . . , Pn are stored in the memory 26, and thus the read-out 
image signals Sp detected over the whole surface of the stimulable 
phosphor sheet 10 are stored in the memory 26. 
In the above example, the signal obtained after logconverted by the 
logarithmic amplifier 20 is added. However, it is possible to 
antilog-convert the log-converted signal to an anti-logarithm and add the 
signal of the anti-logarithm so that the amount of the emitted light 
itself may be added, and log-convert the signal again after the addition. 
That is, in case of signals a1, a2, a3 of the picture element Pa, for 
instance, a signal of 
EQU log (10.sup.a1 +10.sup.a2 +10.sup.a3) 
may be made the image read-out signal for the picture element Pa. 
In order to reproduce the radiation image stored on the stimulable phosphor 
sheet 10, the read-out image signals Sp are read from the large-capacity 
memory 26 and sent to an image reproducing apparatus 28 constituted by a 
CRT, a light beam scanning recording apparatus or the like, via an image 
processing device 27. At the image reproducing apparatus 28, the radiation 
image stored on the stimulable phosphor sheet 10 is reproduced as a 
visible image. 
With the aforesaid embodiment, since the beam diameter of the laser beam 13 
as stimulating rays is adjusted to be substantially smaller than the size 
of the picture element P, the energy density of the laser beam 13 becomes 
substantially high. Also, since different portions of each picture element 
P are scanned by different scanning passes with the laser beam 13 having 
substantially high energy density, the total amount of light emitted per 
picture element becomes substantially high. Therefore, the read-out image 
signals Sp obtained by conducting the addition as mentioned above are of a 
high level corresponding to the total amount of light emitted per picture 
element. When the read-out image signals Sp of a high level are used, it 
is possible to reproduce a visible image of a high image quality with a 
high S/N ratio at the image reproducing apparatus 28. 
The number of scanning passes per picture element with the beam of 
stimulating rays is not limited to three, and may be two or four or more. 
In this case, the beam diameter of the beam of stimulating rays may be 
adjusted to an appropriate value in accordance with the number of scanning 
passes per picture element and the size of the picture element. Also, the 
shape of the beam of stimulating rays (i.e. the laser beam 13) is not 
limited to the one shown in FIG. 2, and may be comparatively flat in the 
main scanning direction as shown in FIG. 4. As shown in FIG. 5, the 
diameter of the beam of stimulating rays may be adjusted so that spots 
scanned by adjacent main scanning passes overlap partially. 
Also, as shown in FIG. 6, the image signals may be sampled at a plurality 
of (three, in this example) sampling points per picture element in the 
main scanning direction, and the image signals detected in the main 
scanning direction and in the sub-scanning direction may be added as 
expressed by