Apparatus for examining a body by means of penetrating radiation

Apparatus for examining a body by means of penetrating radiation such as X- or .gamma.- rays has a radiation beam assembly comprising a member with a radiation source for producing a collimated beam of penetrating radiation, and a member with detector means for receiving radiation from the source. Guide means slidably mount the members on a support structure containing a circle of reconstruction within which the body is located. The members are located on opposite sides of the center of the circle of reconstruction and are translatable as a unit in parallel paths to cause the beam to sweep through and define the plane of the circle of reconstruction. A flexible connection links the members together for maintaining a fixed spatial relationship therebetween during translation at least while the beam sweeps through the circle of reconstruction. In one form of the invention, the flexible connection is constituted by a pair of belts to which the members are respectively attached, the belts being connected together at a common point. In another form of the invention, the flexible connection is constituted by a single belt.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus for examining a body by means of 
penetrating radiation, such as X- or .gamma.- rays, to obtain the 
distribution of absorption coefficients with respect to the radiation in a 
plane passing through the body. 
Such apparatus, hereinafter termed apparatus of the type described, is 
disclosed in U.S. Pat. Nos. 3,867,634 and 3,946,234. These patents 
disclose, respectively, a rectangular yoke and a C-shaped yoke which is 
slidably mounted on a support structure for bidirectional translation in a 
plane parallel to a circle of reconstruction within which a body to be 
examined is located. Rigidly attached to the yoke is a radiation beam 
assembly including a radiation source located on an arm of the yoke on one 
side of the center of the circle of reconstruction for producing a 
collimated beam of penetrating radiation, and detector means located on 
the opposite arm of the yoke for receiving radiation from the source 
passing through the circle of reconstruction. The structure is mounted for 
rotation about the center of the circle of reconstruction so that a body 
therein is scanned linearly by the collimated beam during translation of 
the yoke and at many different angles as the structure is rotated. The 
data produced by the detector means are processed in a computer in a known 
manner to provide the distribution of absorption coefficients over the 
plane of the circle of reconstruction. 
The rectangular or C-shaped yoke provides a rigid mechanical coupling 
between the source and the detector means which maintains a fixed spatial 
relationship therebetween during translation of the radiation beam 
assembly on the support structure. Such relationship is critically 
important because relative displacement between the source and the 
detector means degrades the computed distribution of absorption 
coefficients. As a consequence of the geometrical considerations requiring 
a large radial dimension of the yoke, special designs must be resorted to 
in order to accomodate the resultant dynamic loadings due to the high 
accelerations experienced during the linear portion of the scanning 
operation as efforts are made to reduce scanning time. 
Until the present, it has not been possible to reduce the physical size and 
mass of the coupling between the source and the detector means while 
maintaining the required rigidity therebetween during linear scans. It is 
therefore an object of the present invention to provide, in apparatus of 
the type described, new and improved coupling means which are more compact 
than the prior art, and which involves less mass and provides more 
favorable static moments than the coupling means of the prior art. 
SUMMARY OF THE INVENTION 
According to the present invention, a member with a radiation source for 
producing a collimated beam of penetrating radiation, and a member with 
detector means for receiving radiation from the source are slidably 
mounted on respective guides attached to the support so that the members 
are on opposite sides of the center of the circle of reconstruction and 
are translatable in parallel paths. A flexible connection links the 
members for maintaining a fixed spatial relationship therebetween during 
their translation on the support at least while the beam sweeps through 
the circle of reconstruction. 
In one embodiment of the invention, the flexible connection includes a pair 
of belts to which the members are respectively attached, and the belts are 
connected together at a common point. In another embodiment of the 
invention, the flexible connection includes a single belt. Preferably, a 
counterweight is also attached to the flexible connection for translation 
in a direction opposite to the direction in which the members move. 
The flexible connection of the present invention results in the reduction 
of static moments with respect to the two movable members, a reduction in 
rotational inertia of the members as well as a reduction in the over-all 
dimensions of the apparatus. The latter reduction is important when the 
source produces a fan beam with an apical angle of 30 degrees.

DETAILED DESCRIPTION 
Referring now to FIG. 1, reference numeral 10 designates apparatus of the 
type described comprising support structure 12, radiation beam assembly 
14, guide means 16 and coupling means 18. Support structure 12 is a 
circular disc supported on peripheral bearings (not shown) for rotation 
about center 20 of the circle of reconstruction 22 within which body 24 is 
located. The periphery of structure 12 is toothed (not shown) for 
operative engagement with pinion 26 which selectively rotates the 
structure in a known manner. 
Radiation beam assembly 14 includes member 28 with a radiation source 30 
for producing a collimated beam of penetrating radiation such as x- or 
gamma- rays. The radiation is in the form of fan beam 32 which is 
essentially planar and defines the plane of the circle of reconstruction 
22. In addition to member 28, assembly 14 includes member 34 with detector 
means 36 for receiving radiation from source 30. 
Guide means 16 slideably mounts members 28 and 34 on support 12 for 
bi-directional translation in a plane parallel to the plane of the circle 
of reconstruction. Means 16 thus includes a pair of parallel rails 38, 40 
lying in a plane parallel to the plane of reconstruction and rigidly 
attached to structure 12. These rails are located on one side of center 20 
and are operatively associated with member 28 which is provided with 
bearings slideably receiving the rails. Means 16 also includes a pair of 
parallel rails 41, 42 rigidly attached to structure 12 and also lying in a 
plane parallel to the plane of the circle of reconstruction. These rails 
are located on the side of center 20 opposite to the side on which rails 
38, 40 are located. Rails 41, 42 are operatively associated with member 34 
which is provided with bearings slideably receiving the rails. As a 
consequence, members 28 and 34 are constrained for movement in planes 
parallel to the plane of reconstruction which is defined by the plane 
swept out by beam 32 as member 28 moves between terminal positions on 
rails 38, 40. 
Coupling means 18 is a flexible connection (44 in FIG. 4) that links 
members 28 and 34 together for maintaining a fixed spatial relationship 
therebetween during translation of the radiation beam assembly on 
structure 12. In the embodiment of the invention shown in FIGS. 1 and 4, 
coupling means 18 is in the form of a pair of belts to which the members 
are respectively attached. Specifically, flexible connection 44 includes a 
first belt 46 operatively engaging a pair of spaced pulleys 48, 50 
rotatably mounted on structure 12. The engagement is such that no slippage 
occurs. Belt 46 thus defines a pair of parallel runs 52, 54 each of which 
is located on one side of center 20. Connection 44 also includes a second 
belt 56 operatively engaged with pulleys 58, 60 rotatably mounted on 
structure 12, and operatively engaged with belt 46 where the latter is 
wrapped around pulleys 48, 50. The engagement of belt 56 with the portions 
of belt 46 wrapped around pulleys 48, 50 is such that no slippage occurs. 
Belt 56 defines a pair of runs 62, 64 parallel to runs 52, 54 of belt 46. 
The runs of belt 56, however, are located on opposite sides of center 20. 
Consequently, runs 62 and 54 of the belts overlie each other and no 
relative movement between the runs takes place. 
Belt 46 is attached to member 28 by clamp means 66, 68 and to belt 56 by 
clamp means 70, 72 on counter-weight 74 which is slideably mounted on 
parallel rails 76, 78 rigidly connected to structure 12 and lying in a 
plane parallel to the plane of the circle of reconstruction. Rails 76, 78 
are also parallel to rails 38, 40 and to runs 52 and 54 of belt 46, and to 
run 62 of belt 56. Finally, clamp means 70, 72 also attach belt 56 to the 
counter-weight which thus acts to connect the two belts together at a 
common point. 
Belts 46 and 56 are preferably formed by stacking together a plurality of 
individual, very thin, flat steel bands. However, other types of flexible 
connections can be used. For example, chain or cable can be used since the 
moving masses are relatively small and elongation of the flexible 
connection is minimized by the utilization of counter-weight 74. 
In operation, a reciprocating force is applied to member 34, preferably the 
heavier of the two members. Belts 46 and 56 transmit motion to members 28 
and 34 so that both move in the same direction while counter-weight 74 
moves in an opposite direction. As a result, a bi-directional translation 
of assembly 14 takes place causing beam 32 to sweep through the circle of 
reconstruction and thus linearly scan body 24. Linear scans are repeated 
at many different angular positions of the structure in a known manner. 
During all of these scans, the output of the detector means is supplied to 
a storage device (not sown) which stores the output of the detector means 
as a function of the linear displacement of the assembly with respect to 
the center of the circle of reconstruction, and as a function of the 
angular position of structure 12. In a known manner, the data acquired 
during scanning are processed to provide the distribution of absorption 
coefficients across the circle of reconstruction. 
Counter-weight 74 moves in a direction opposite to the direction of 
movement of assembly 14 and serves to reduce the tensile stress in belt 56 
when the angular position of structure 12 is such that the direction of 
linear scan is inclined to the horizontal. In this way, when 
counter-weight 74 has a mass equal to the mass of member 28, only belt 46 
is subject to gravitational forces, while belt 56 transmits acceleration 
forces only. Thus, an any angular position of the structure 12, belt 56 is 
under minimal tensile stress during a constant velocity traverse of the 
circle of reconstruction. Thus, elastic deformation in the belts is 
minimized. Alternatively, one of the pulleys, e.g., pulley 48, is 
selectively powered in one direction or the other by a motor (not shown). 
Alternative to the flexible connection 44, connection 44A in the second 
embodiment shown in FIG. 5 can be used. In such case, a single belt 80 is 
employed together with four pulleys 82, 84, 86, 88 rotatably mounted on 
the support structure. Belt 80 is wrapped twice around two of the pulleys, 
i.e., pulleys 82, 84, located on one side of the center of the circle of 
reconstruction. The belt also passes around pulleys 86, 88 located on the 
other side of the center. Belt 86 thus defines a number of parallel runs 
associated with pulleys 82, 84 (specifically, runs 90 and 92) and run 94 
associated with pulleys 82, 86. Runs 90, 92, and 94 of belt 80 correspond 
respectively to runs 46, 54/62 and 61 of belts 46, 56 of connection 44. 
Members 28 and 34 are connected to the runs of belt 80 in the same manner 
as the members are connected to corresponding runs of belts 46 and 56. 
It is believed that the advantages and improved results furnished by the 
apparatus of the present invention are apparent from the foregoing 
description of the several embodiments of the invention. Various changes 
and modifications may be made without departing from the spirit and scope 
of scope of the invention as sought to be defined in the claims that 
follow.