An emission computed tomograph provided with a collimator ring rotatable about an object being examined and having a plurality of slits for defining the direction of incidence of the radiation emitted from the objection the radiation detectors. The slits are arranged circumferentially of the ring and directed at successively different angles relative to the radial direction of the collimator ring, so that as the ring is rotated, the object is tomographically scanned. The collimator ring may be divided into a plurality of equal arcuate portions, in each of which the slits are directed toward the object in parallel with each other, so that as the collimator ring is rotated to scan the object, only the parallel profile data thereof can be obtained.

Background of the Invention 
This invention relates to an emission computed tomograph and a collimator 
for use therein. 
Emission computed tomography commonly referred to as ECT is a technique for 
obtaining an image of the distribution of radioactivity of radioisotope in 
a desired plane perpendicular to the axis of the body of a patient within 
a particular organ thereof by administering to the patient a 
pharmaceutical compound labelled with the radioisotope, detecting from 
outside the body the gamma (.gamma.) radiation emitted by the isotope that 
has been accumulated in the particular organ and processing the detected 
data by an electronic computer to obtain the image. 
In one known type of emission computed tomograph especially for single 
photons, the radiation detector is directed to the body of a patient to be 
examined at different angles within 360.degree. within a plane 
perpendicular to the axis of the body so that for each of the angles the 
body is scanned to obtain profile data and the various profile data thus 
obtained for all the scanning angles are processed by an electronic 
computer to reconstruct the tomographic image of the body in that plane. 
To make scanning with a radiation detector easy, it has been proposed to 
arrange a plurality of radiation detectors circumferentially about the 
axis of a human body to be scanned and provide each of the detectors with 
a collimator connected through a suitable transmission to a common drive, 
so that all the collimators are simultaneously operated to scan the body 
in a particular plane perpendicular to the axis of the body. 
It has also been proposed to construct such a collimator by a pair of 
swingable plates of a material capable of blocking radiation provided in 
front of each of the radiation detectors. A report on the mechanical 
structure of this type of emission computed tomograph is expected to be 
published in IEEE Transactions on Plasma Nuclear Sciences in February 
1981. 
The primary object of this invention is to provide a collimator for use in 
an emission computed tomograph, which is capable of defining the direction 
or angle of incidence of radiation on the detector with high accuracy and 
precision so as to eliminate any artifact in the quality of the 
tomographic image reconstructed thereby, and which is simple in 
construction, easy to manufacture and exchange for a different collimator 
and can be driven by a simple driving mechanism. 
Another object of the invention is to provide such a collimator as 
aforesaid, which makes it possible to obtain data that can be processed by 
a computer more simply and easily than the data obtained by using the 
prior art collimators. 
Another object of the invention is to provide such a collimator as 
aforesaid, which makes it possible to obtain only parallel profile data 
thereby to shorten the time required for processing the data and simplify 
the process. 
Another object of the invention is to provide such a collimator as 
aforesaid, which is capable of providing selectively high resolution (with 
low sensitivity) and high sensitivity (with low resolution). 
Another object of the invention is to provide an emission computed 
tomograph provided with such a collimator as aforesaid. 
Other objects and advantages of the invention will become apparent from the 
following description with reference to the accompanying drawings.

SUMMARY OF THE INVENTION 
In one embodiment of the invention, the collimator is in the form of an 
annular body or ring rotatable about an object to be examined and provided 
with a plurality of slits for defining the direction of incidence of the 
radiation emitted from the object being examined on the radiation 
detectors. The slits are arranged circumferentially of the ring and 
directed at successively different angles relative to the radial direction 
of the ring, so that as the ring is rotated, the object is tomographically 
scanned. 
In another embodiment of the invention, the collimator is divided into a 
plurality of equal arcuate portions, in each of which the slits are 
directed toward the object being examined in parallel with each other, so 
that as the collimator is rotated to scan the object, only the parallel 
profile data thereof are obtained. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to the drawings, first to FIG. 1, which schematically shows a 
part of a human body, e.g., the head 1 to be examined supported on a 
suitable holder or table T. A plurality of radiation detectors 2.sub.1 to 
2.sub.n are arranged side by side within a plane perpendicular to the axis 
of the body and along a circle C whose center coincides with the axis 0 of 
the body. The circular arrangement of radiation detectors will be referred 
to collectively as the detector ring 2. 
A collimator 3 in the form of a ring is provided concentrically within the 
circle C or the detector ring 2 and is rotatable about the center 0 in a 
manner to be described later in detail. The collimator ring 3 comprises a 
plurality of slits 4.sub.l to 4.sub.n schematically shown in FIG. 1 
arranged circumferentially of the collimator ring 3 so as to regulate the 
direction of incidence of radiation on each of the detectors 2.sub.l to 
2.sub.n. The slits 4.sub.l to 4.sub.n are directed at successively 
different angles within the sector-shaped range from +.theta..degree. to 
-.theta..degree. relative to the radial direction of the collimator ring 
3. 
As fragmentarily shown by way of example in FIG. 2, the collimator 3 
comprises a plurality of annular plates 3.sub.p of the same dimension 
coaxially arranged and axially spaced a suitable distance from each other, 
with a plurality of divider plates 3.sub.q disposed between each adjacent 
two of the annular plates 3.sub.p perpendicularly thereto and 
circumferentially spaced apart from each other so as to divide the space 
between the adjacent two annular plates into the slits 4.sub.l to 4.sub.n 
arranged circumferentially of the collimator ring. In FIG. 2 there are 
three circumferential arrays of slits axially arranged side by side, each 
of the circumferential arrays including slits 4.sub.l to 4.sub.n. The 
arrangement is not essential but preferable since it improves the 
resolution in the direction of thickness of the slice of the object. 
In FIG. 1, suppose that one of the detectors, say, 2.sub.i has one of the 
slits, say, 4.sub.n positioned in front of the detecting surface thereof. 
The direction of incidence of radiation on the detector 2.sub.i is as 
shown by an arrow a, making an angle +.theta..degree. with the radial 
direction of the collimator ring 3. As the collimator 3 is rotated 
counterclockwise as shown by an arrow X, the succeeding slits directed at 
successively decreasing angles relative to the radial direction come to be 
positioned before the detector 2.sub.i so that the angular direction of 
incidence of radiation on the detector 2.sub.i is successively changed 
from the direction a through b and thence the radial direction toward the 
center 0 until the direction becomes c when the collimator 3 has been 
rotated through approximately 360.degree.. This means that the angular 
direction of incidence of radiation on the detector 2.sub.i has been 
deflected 2.theta..degree. from the direction a to the direction c. 
The same is true with all the other detectors. Therefore upon one rotation 
of the collimator ring 3 each of the detectors 2.sub.l to 2.sub.n has 
scanned the sector-shaped area of the angle 2.theta.. 
FIG. 3 shows a collimator ring 3 constructed in accordance with another 
embodiment of the invention. The collimator is so designed that upon one 
rotation thereof the direction of incidence of radiation on each of the 
detectors 2.sub.l to 2.sub.n changes from one extremity of the 
sector-shaped area of the angle 2.theta. between +.theta..degree. and 
-.theta..degree. to the opposite extremity and back to the one extremity. 
To this end the collimator is provided with a first plurality of slits 
4A.sub.l to 4A.sub.n in the semicircle ABC of the collimator ring and a 
second plurality of slits 4B.sub.l to 4B.sub.n in the other semicircle CDA 
thereof. The first group of slits 4A.sub.l to 4A.sub.n in the semicircle 
ABC are directed at successively different angles within the range of the 
angle .theta. between 0.degree. and -.theta..degree. relative to the 
radial direction of the collimator ring 3 so that as the collimator 3 is 
rotated counterclockwise as shown by the arrow X through 180.degree., the 
angular direction of incidence of radiation on the detectors is deflected 
from 0.degree., that is, the radial direction toward the center 0 leftward 
(as the center 0 is viewed radially inwardly from the detectors) as far as 
-.theta..degree., that is, the direction making an angle .theta. to the 
left with the radial direction and then back to 0.degree. (the radial 
direction) again. 
The second group of slits 4B.sub.l to 4B.sub.n in the semicircle CDA are 
directed at successively different angles within the range of the angle 
.theta. between 0.degree. and .theta..degree. relative to the radial 
direction of the collimator ring 3 so that as the collimator is rotated 
counterclockwise through 180.degree., the angular direction of incidence 
of radiation on the detectors is deflected from 0.degree., that is, the 
radial direction toward the center 0 rightward (as the center 0 is viewed 
radially inwardly from the detectors) as far as +.theta..degree., that is, 
the direction making an angle .theta. to the right with the radial 
direction and then back to 0.degree. (the radial direction) again. 
As can be easily seen, upon one counterclockwise rotation of the collimator 
each of the detectors scans the sector-shaped area of the angle 2.theta. 
between the +.theta..degree. and -.theta..degree. at the opposite sides of 
the radial direction. 
As previously mentioned, the slits of the collimator are directed at 
successively different angles relative to the radial direction of the 
collimator ring. If the angular direction of each of the slits is so set 
as to differ by an equal from the adjacent slits, the following problem 
will result. 
As is well known in the art, in order to reconstruct a tomographic image of 
an object being examined, profile data in various directions in the 
tomographic plane are required. In order to obtain such profile data the 
data in parallel directions only must be collected. However, in the data 
of any profile the interval or distance between adjacent two sampling 
positions is greater in the central portion of the scanned area than in 
the peripheral portion thereof due to the following reasons. 
Referring to FIG. 4, let the line OB be the initial line which lies at 
angle 0.degree. and the position P of each of the slits of the collimator 
be expressed by the angle .theta. that the line OP makes with the initial 
line OB. 
If, as previously mentioned, the angular direction of each of the slits 
changes by an equal angle from that of the adjacent slit, the direction of 
the slit or the angle .PSI. that the direction or line PQ makes with the 
radial direction or the line OP can be expressed as: 
EQU .PSI.=f(.theta.)=k.theta. (1) 
wherein k is a constant. 
Let the line or radius OB be of a length r and the length L is given as 
EQU L=r sin .PSI. (2) 
The space or distance .DELTA.L between adjacent two data can be expressed 
as: 
EQU .DELTA.L=r sin (.PSI.+.DELTA..PSI.)-r sin .PSI. (3) 
Since the collimator ring is rotated at a constant angular speed and the 
sampling cycle of the detectors is constant, from the above equations (2) 
and (3) we obtain 
##EQU1## 
The distance .DELTA.L is (dL/d.theta.) if expressed in the form of 
differential. When the collimator is made as expressed by equation (1), we 
obtain 
##EQU2## 
Therefore, as the angle .theta. approaches (.pi./n) or -(.pi./n), the 
distance (dL/d.theta.) becomes smaller and smaller, so that the density of 
the data obtained is not uniform, with the data being relatively sparse in 
the central portion of the scanned area, where supplementation of data is 
required to ensure the accuracy of the reconstructed image. 
To avoid the above defect and inconvenience, it is better to keep the 
distance (dL/d.theta.) constant. In other words, the following relation is 
very preferable. 
##EQU3## 
Solving the above equation, we obtain 
EQU sin .PSI.=K.theta.+C (7) 
wherein K and C are both constants. Since .PSI.=0 when .theta.=0, C=0, and 
if the angle .PSI. is the maximum (.PSI.=.PSI..sub.max) when 
.theta.=(.pi./n), we obtain 
##EQU4## 
where n is an integer which expresses the number of scanning in one 
rotation of the collimator ring. 
Therefore, if the direction or angle .PSI. of each of the slits is so 
determined for the position of the slit (as expressed in terms of .theta.) 
as to meet the above equation (8), the parallel data having a uniform data 
density can be obtained. 
FIG. 5 shows a third embodiment of the invention, wherein the collimator 3 
is provided with a plurality, say, four groups of slits 4A, 4B, 4C and 4D. 
The collimator 3 comprises four sections 3A, 3B, 3C and 3D, in each of 
which one of the four groups of slits are formed in such a manner that 
they are directed toward the object 1 to be examined and extend in 
parallel with each other. 
The slits may be formed in a manner similar to that in the previous 
embodiments, with axially spaced annular plates and dividing plates being 
so arranged as to define the parallel slits between the annular plates. 
The collimator ring 3 is rotatable about the center 0 as in the previous 
embodiments. 
With the collimator ring 3 being positioned relative to the detector ring 2 
as shown in FIG. 5, of all the radiations emitted from the body 1 in all 
directions only those radiations which advance vertically up and down and 
horizontally right and left can pass through the slits 4A-4D to enter the 
detectors behind the slits. Therefore, when the collimator ring 3 has been 
rotated for a quarter of one rotation, all of the radiations that have 
been emitted by the body in all directions have entered the detectors so 
that the necessary data have been obtained. Any radiation emitted in any 
direction from any point within the area 5 defined by a circular dotted 
line never falls to enter any of the detectors. In this sense the area 5 
may be referred to as the image reconstructing area or the field of view. 
In FIG. 5, the collimator is divided into four arcuate sections. It may 
also comprise as many sections as is desired, for example, two, three, 
five, six or more sections. With a greater number of sections provided, 
the number of the detectors which cannot be used (that is, those detectors 
which are not situated behind the parallel slits and therefore cannot 
receive radiation) is reduced, but the image reconstructing area 5 becomes 
smaller. 
In any of the previous embodiments, the smaller is the width of the slits 
in the collimator ring 3, that is, the distance between each adjacent two 
of the dividing plates 3.sub.q in FIG. 2, the higher the resolution 
becomes, whereas the greater the slit width is, the higher the sensitivity 
becomes. In accordance with the invention as many collimators of different 
characteristics as is desired may be provided for selective use in a 
specific application. 
The radiation detectors the number of which is 64 or 72 for example are 
circumferentially arranged side by side with a gap or a radiation shield 
between adjacent detectors. 
When the collimator of FIG. 1 or 3 is used, the distributions of the data 
are just like many sectors which have their top at the center of the 
detectors and completely cover the field of view. Finer data density is 
required for better reconstructed images. When the detector ring is 
rotated half the angle between adjacent two of the detectors, the number 
of the sectors is doubled so that better images can be expected. 
An effective way of obtaining a finer data density with the collimator of 
FIG. 5 is to wobble the entire detector assembly in a short radius, for 
example approximately 75% of the detector interval in the ring. A concrete 
arrangement for effecting such rotation or wobbling will be described 
presently. 
FIG. 6 shows by way of example a concrete arrangement of an emission 
computed tomograph constructed in accordance with the invention. There is 
shown an annular support plate 10 having a central opening 10'. The 
detector ring 2 comprising a plurality of detectors and schematically 
shown for simplicity of illustration is fixed to one end face of the inner 
periphery of the annular plate 10 by means of appropriate mechanical 
means. Each of the detectors usually comprises a scintillation crystal and 
a photomultiplier tube though they are shown schematically. The plate 10 
is rotatable about a horizontal axis 0 and also capable of wobbling as 
will be described later in detail. 
A bowl 11 is provided with a spindle 12 by which the bowl is rotatable 
about the horizontal axis 0 and a flange 13 which is disposed 
concentrically within the central opening 10' of the plate 10. The 
collimator ring 3 schematically shown for simplicity of illustration is 
fixed to the flange 13 by adhesive or any appropriate mechanical means, so 
that the collimator ring 3 is disposed concentrically within the detector 
ring 2 in radial alignment therewith. 
A support arm 14 has its one end fixed to the end face of the annular 
support plate 10 opposite to the end face to which the detector ring 2 is 
fixed, and extends horizontally a suitable distance. The arm 14 is 
provided on the underside thereof with a pair of guide rails only one of 
which is shown at 15 for guiding a carriage 16 having rollers 17 engaging 
the rails 15. A vertical rod 18 depends from the carriage 16 and supports 
a rotatable sleeve 19 through a pair of bearings 20. 
Four horizontal support sleeves 21 are fixed to and extend radially from 
the lower end of the vertical sleeve 19 spaced 90.degree. apart from each 
other. In FIG. 6 only two of the four horizontal sleeves 21 are shown 
extending oppositely to the right and left. The spindle 12 of the bowl 11 
is rotatably supported by the horizontal sleeve 21 through a pair of 
bearings 23. The horizontal sleeve also supports a motor M.sub.1, which 
drives a pinion gear 24 meshing with a gear 25 fixed to the bowl 11. 
The other three horizontal sleeves, only one of which is shown oppositely 
to the one described just above, have a bowl 11 of the same construction 
as the one above-mentioned but with a collimator of different 
characteristics as shown at 3'. In other words, there are four bowls 11 
provided about the vertical sleeve 19 spaced 90.degree. apart from each 
other so that four different types of collimators are available for 
selective use in the manner to be described presently. Two of the 
collimators may be designed for positron annihilation gamma rays. 
To exchange the collimator 3 shown in the operative position in FIG. 6 for 
the other collimator 3', a locking means 26 is released and the carriage 
16 together with the various parts and members carried thereby is moved 
leftward as shown by an arrow X as far as the collimator ring 3 is out of 
radial alignment with the detector ring 2 as shown by the dash-and-dot 
line. Then, the vertical sleeve 19 is rotated to bring one of the three 
bowls 11 which has the desired type of collimator to the position where 
the previous bowl has been. A disc 22 fixed to the upper end of the 
vertical sleeve 19 cooperates with an appropriate member not shown but 
carried by the carriage 16 to maintain the vertical sleeve 19 in the 
rotated position. Then the carriage 16 with everything thereon is moved 
rightward to bring the selected collimator ring into radial alignment with 
the detector ring with the locking means 26 operated to fix the carriage 
in position. Thus the apparatus can be operated selectively with a high 
resolution collimator, a high sensitivity collimator, or a collimator for 
position annihilation gamma rays. 
An annular plate 27 capable of wobbling motion has a central opening 28 in 
which the support plate 10 with everything carried thereby is positioned. 
The wobble plate 27 is also provided with a plurality, say, four rollers 
29 circumferentially spaced 90.degree. apart from each other and engaging 
the peripheral edge of the support plate 10 so as to support the plate 10 
rotatably about the axis 0. 
A ring gear 30 having external teeth 31 is fixed to the end face of the 
annular support plate 10 opposite to the end face to which the collimator 
ring 2 is fixed. A motor M.sub.2 is mounted on the wobble plate 27 and 
rotates a pinion 32 which in turn rotates the ring gear 30 and 
consequently the detector ring 2 through a reduction gear 33 for the 
purpose to be described later. 
A stationary annular plate 34 having a central opening 35 is provided with 
a plurality, say, three crank shafts 36 spaced 120.degree. apart from each 
other, only one of which is shown in FIG. 6 for simplicity of 
illustration. The stationary annular plate 34 is held upright, that is, 
with its plane extending vertically and fixed by means of bolts 37 to an 
annular frame 38 which is in turn fixed to a base member 39. The detector 
ring 2 and everything inside the ring are positioned inside the opening 35 
of the stationary annular plate 34. 
Each of the crank shaft 36 is rotatably supported by the stationary annular 
plate 34 through a bearing 40 and supports the wobble plate 27 through a 
bearing 41. A gear 42 is fixed to each of the crank shaft 36 and meshes 
with a drive gear 43 rotated by a motor M.sub.3 mounted on the stationary 
support plate 34. 
As the motor M.sub.3 is rotated, the crank shafts 36 cause the support 
plate 27 with everything carried thereon to wobble about the axis 0 as 
shown in FIG. 7. 
A gantry 44 encloses all of the above-mentioned parts and members and is 
provided at one end face thereof with a recess 45 inwardly projecting 
through the collimator ring 3 into the bowl 11. In the recess 45 one end 
of the table T is positioned removably therefrom. The gantry 44 is also 
provided at the opposite end face thereof with a door 46 for access to the 
interior mechanism for operation, adjustment, repair or any other 
necessary work. 
FIGS. 8 and 9 show another arrangement that enables selective use of a 
higher resolution (but with a lower sensitivity) or a lower resolution 
(but with a higher sensitivity). 
The collimator provided inside the detector ring 2 comprises an outer 
collimator ring 3a and an inner collimator ring 3b (both schematically 
shown for simplicity of illustration) concentric with the outer ring. The 
two component rings 3a and 3b are provided with slits 4a and 4b formed and 
arranged in a manner similar to that in the previous embodiment of FIG. 1 
or 3. The slits 4a and 4b are related to each other in a manner to be 
described in detail presently. 
The outer collimator ring 3a is fixed axially to an outer hollow 
cylindrical support member 11a provided with a spindle 12 by which the 
member 11a is rotatable about the axis 0 and a gear 25 meshing with a 
pinion gear 24 driven by a motor M.sub.1. The spindle 12 is rotatably 
supported by a sleeve 21 through a pair of bearings 23. The sleeve is 
fixed to the lower end of an L-shaped arm 50 fixed to an annular support 
plate 10. Those component parts which are not shown in FIG. 9 are 
substantially the same as in FIG. 6 except that in FIG. 9 there is no such 
means for exchanging the bowls 11 as shown in FIG. 6. 
The inner collimator ring 3b is also fixed axially to an inner support ring 
11b. The inner collimator ring 3b and the inner support member 11b are 
concentrically disposed within the outer collimator ring 3a and the outer 
support member 11a in such a manner that the outer and inner support 
members (and consequently the outer and inner colimator rings) are rotated 
simultaneously at the same angular speed, but that the inner support 
member 11b with the inner collimator ring 3b can be axially slid relative 
to the outer support member 11a as shown by an arrow X by means of, say, a 
spline connection 51 thereby to displace the inner collimator ring 3b out 
of radial alignment with the outer collimator ring 3a. 
The inner support ring 11b is provided with an inner circumferential rib 
52. A slide 53 engages the rib 52 through a bearing 54 so that as the 
support member 11a and 11b are rotated, the slide 53 slides 
circumferentially along the rib 52. A handle lever 55 is fixed to the 
slide 53 and extends through an axial slot 56 formed in the wall W of the 
recess 45 of the gantry 44. With the end portion of the table T removed 
from inside the recess 45, by moving the handle lever 55 axially in either 
direction as shown by a double-headed arrow it is possible to move the 
inner collimator ring 3b selectively out of or into radial alignment with 
the outer collimator ring 3a. 
With the two collimator rings radially aligned as shown in FIG. 9, each 
corresponding two of the slits 4a and 4b in the outer and inner collimator 
rings 3a and 3b are directed in the same direction and linearly aligned so 
that as the two collimator rings are rotated as a single whole, the 
radiation emitted from the object being examined may pass through the 
aligned slits in the two rings to enter the detector therebehind. 
When the slits 4a and 4b in the outer and inner collimator rings 3a and 3b 
are radially aligned in the above manner, the length of the slit for 
defining the direction of incidence of radiation on each of the detectors 
becomes longer than otherwise so that the direction of incidence of 
radiation can be defined with a higher accuracy and consequently a higher 
resolution can be obtained. 
When the inner collimator ring 3b is disposed axially out of radial 
alignment with the outer collimator ring 3a, the latter ring alone remains 
to define the direction of incidence of radiation, so that the accuracy 
with which the direction of incidence of radiation on the detector is 
defined is reduced and the resolution is lowered. However, the amount of 
the radiation incident on the detectors increases with resulting increase 
in the sensitivity. 
In the illustrated embodiment, the collimator comprises a pair of 
concentric rings. The collimator may also comprise three or more adjacent 
concentric rings, the inner ones of which are between the outermost and 
innermost rings and are so arranged as to be axially displaceable from the 
outermost ring successively from the innermost ring first. This 
arrangement enables more delicate adjustment of the resolution and 
sensitivity of the apparatus. 
In operation, as the collimator ring is rotated with a position detector 60 
detecting the angular position of the collimator ring, the head 1 of a 
patient P being examined is tomographically scanned with the radiation 
emerging from inside the head passing through the slits to enter the 
radiation detectors. The position detector 60 may be connected to the 
motor M.sub.1 by means of, e.g., a gear 61 meshing with the gear 24 so 
that the rotational angle of the collimator can be detected by decoding 
the output of the position detector 60. 
Referring to FIG. 10, the data collected from the radiation detectors as 
well as from the position detector 60 are transmitted through an interface 
61 to an electronic computer 62 where they are processed so as to 
reconstruct a tomographic image of the object, which is displayed on a 
suitable display unit 63 and/or recorded in a data storage device 64. 
The motors M.sub.1 to M.sub.3 may be controlled by the computer 62 in 
accordance with a predetermined program. 
Since the collimator of the invention has no movable parts but is formed as 
a mechanically integral whole, the direction of incidence of radiation 
onto the detectors can be defined with a higher accuracy and precision 
than in the prior art machines in which the collimators are separately 
operated. 
The construction and the driving mechanism of the apparatus of the 
invention is simple and easy to manufacture, and has various other 
advantages obvious to those skilled in the art.