Multiple electron beam target for use in X-ray scanner

Disclosed is a target structure for use in an X-ray transmission scanner in which X-rays are generated by directing an electron beam at an X-ray generating target. The structure includes a support bracket having a recessed portion and a support surface formed in the recessed portion. A bolt extends through the bracket and maintains a target member in abuttment with the support surface which positions the target in proper alignment with an electron beam.

This invention relates generally to a high speed multiple section 
computed-tomographic (CT) medical scanning system, and more particularly 
the invention relates to an electron beam target structure for use 
therein. 
Disclosed in U.S. Pat. No. 4,352,021 is a high speed X-ray scanning system 
in which the X-ray source and the X-ray detectors are stationary and a 
plurality of fan beams of radiation is generated by sweeping an electron 
beam across a plurality of targets arcuately arranged whereby each target 
generates radiation fan beams. 
The electronic scanning system incorporates a single electron beam tube. 
The electron beam is deflected by suitable magnetic and/or electric fields 
to produce a movable X-ray source on one of four adjacent semi-circular 
target rings to provide scanning fan beams that can be used to image an 
entire volume of tissue in multiple sections. Such an electronic scanning 
system is vastly superior in speed to the mechanical scanning systems in 
the prior art referenced in U.S. Pat. No. 4,352,021. Fraction-of-a-second 
scan time of a volume can be achieved as compared to one or more seconds 
required for the mechanical scan of a single section. The system 
eliminates the need for moving parts that require high precision and 
alignment. In addition, elaborate systems of sliding electrical contacts 
are eliminated. The scanner is an improvement over that shown and 
described in U.S. Pat. No. 4,158,142, in that it permits nearly 
simultaneous viewing multiple sections of the body which may encompass a 
region as large as the heart. The scanner can provide as many as eight 
sections. 
The system employs a plurality of detectors mounted opposite the target 
rings. The detectors are arranged in two adjacent partial-circular ring 
arrays. Each of the arrays contains a multiplicity of detectors as, for 
example, 444 detectors each, providing a total of 888 detectors. The 
angular separation of two adjacent detectors is in the order of 0.5 
degrees resulting in very high resolution. The scanning system is provided 
with collimators both for the X-ray source and for the detectors. The 
source collimator provides a fan-shaped beam 30.degree. of which covers 
the body with a 30.degree. opening angle. The detector collimators provide 
interchangeable options: dual section detector arrays, single section 
detector arrays and high resolution single section detector arrays. A 
variety of scanning modes can be selected with up to eight sections being 
scanned at a rate of at least one scan per second. 
A problem encountered in the described system results from the high 
temperatures of the electron beam target which can cause burnout or 
degraded X-ray sources. Typically, the electron beam current is one ampere 
at 120 Kv, and should the electron beam scan be stopped, a target on which 
the beam impinges would be melted. Thus, a need exists for an improved 
target structure which can be readily replaced. 
Accordingly, an object of the present invention is an improved high speed 
X-ray scanning system. 
Another object of the present invention is an improved target for use in a 
scanned electron beam X-ray system. 
Still another object of the invention is a multiple target structure in 
which individual targets can be readily and accurately replaced. 
A further object of the invention is an inexpensive target structure which 
is easily manufactured, assembled, and maintained. 
A feature of the invention is a clamp arrangement for planar target members 
which facilitates accurate and rigid positioning of the targets. 
Briefly, the target structure in accordance with the invention includes an 
arcuate frame and a plurality of support brackets extending therefrom. 
Each support bracket has at least one recessed portion including one 
inclined surface at an angle required by the surface of an electron beam 
target. The electron beam target comprises a member having a planar 
surface. The member is supported in the recessed portion with the planar 
surface engaging the inclined recessed wall. Fastening means engages the 
target and maintains the planar surface of the target in forced engagement 
with the inclined recessed surface. The fastening means accommodates 
targets of various thicknesses and allows for thermal expansion of the 
targets. Further, each target can be readily replaced in the field. 
Advantageously, in a multiple target arrangement each target shields the 
support bracket from the electron beam thereby reducing heat and 
minimizing damage to the bracket.

Referring to FIG. 1, the system of U.S. Pat. No. 4,352,021 is seen to 
include three major components: a scan tube 11 including a cylindrical 
portion 12, and a semi-circular conical portion 13; a detectory array 14; 
and, a computer system 16. The scan tube projects an electron beam to 
target rings which generate X-rays. The X-rays are intercepted by the 
detector array 14. The output of the detector array is applied to the 
computer system 16. The computer system includes a plurality of storage 
discs 18 for recording the data for later processing. The computer system 
also includes an output which controls the scan tube. A video display 19 
presents the data. 
Referring more particularly to FIGS. 2 and 3, the scanning system and 
detection system is shown in more detail. The electron beam tube 11 
includes a vacuum envelope 21 which houses an electron gun 22 at the 
cylindrical end 12. The electron gun projects an axial electron beam 23 
along the cylindrical portion. The focus coils 24 focus the beam onto 
targets 26. Bending coils 27 bend the beam so that it fans out along the 
partial-circular conical portion of the tube to impinge upon the 
partial-circular target rings. The target assembly 26 includes a plurality 
of partial-circular target rings 28, 29, 30 and 31. Suitable cooling coils 
32, 33, 34 and 35 are associated with each of the target rings 28, 29, 30 
and 31 respectively and serve to cool the target rings. 
The bending magnets not only deflect the beam but rapidly sweep it along 
the partial-circular targets shown in FIGS. 2 and 3. The target rings are 
scanned serially to obtain a multiple section examination as will be 
presently described. Ring collimators 37, 38, 39 and 40 are disposed to 
intercept X-rays emitted by the target rings and define an X-ray beam 
projected as a one or two centimeter thick planar beam. A fan-shaped 
sector of this beam is passed through a detector collimator 46 and is 
detected by the curved detector array and the measured values are utilized 
to reconstruct a tomographic image of the region 44. 
As described above, the electron beam which impinges on the targets is 
approximately one ampere of current at 120 Kv voltage and generates 
substantial heat in the target. The present invention is directed to an 
inexpensive target structure which is readily assembled in the system for 
ease in maintenance and replacement. FIG. 4 is a perspective view of the 
targets 28-31 each of which is arcuate in configuration (e.g. 210.degree.) 
and comprises a plurality of segments such as the six segments 28-1 
through 28-6 for the target 28. Each segment may be solid tungsten sheet 
or a mounting substrate such as OFHC copper on which a tungsten layer is 
deposited by plasma spray or to which a tungsten sheet is brazed. Other 
X-ray emitting surface material, such as tantalum and molybdenum, may be 
used. 
FIG. 5 is a section view illustrating the support structure for the targets 
28-31. Each target is positioned in a recessed portion of a support 
bracket 50 with each recessed portion having an inclined surface (e.g. 
12.degree.) for receiving a planar surface of each of the targets in 
proper alignment for reception of the electron beam along the nominal beam 
lines 61 and optimum direction of the emitted X-ray beams from the 
targets. The support bracket 50 is fastened to the system frame 51 by 
means of bolt 52. Each target segment is maintained against the inclined 
surface of the recessed portion by a threaded bolt 55. Each bolt 55 is 
threadably received in a bore through the support bracket 50 with a lock 
nut 57 provided to maintain the bolt in the desired pressure engagement 
with the spacer 53. The targets can be cooled by heat conduction to the 
support bracket. Coolant lines can be provided in the support structure to 
facilitate heat removal. 
Advantageously, the targets 28-31 are oriented in the support bracket 50 so 
that the support bracket between targets is protected from the electron 
beam as shown by the shaded portion 58. Thus, heating of the support 
bracket is minimized. 
FIG. 6 is a section view of a portion of the support bracket 50 
illustrating an alternative embodiment of fastening means for the target 
28. In this embodiment the spacer 73 has an inclined surface to engage a 
surface of the target segment 28, and another surface engages an opposing 
wall of the recessed portion. 
The target segments are readily replaced in the support bracket as required 
for maintenance thereby permitting exchange of targets in the field. 
Further, the inclined surface of the recessed portion readily receives the 
target segments in proper alignment with the electron beam for X-ray 
generation. Target segments of varying thickness are readily accommodated 
using the spacer in the clamping arrangement in accordance with the 
invention. The entire target structure can be electrically insulated from 
the rest of the system by using an insulative layer such as mica between 
the support bracket and the mounting surface. Further, the clamping method 
allows for thermal expansion of the target strips as they heat during 
operation. 
While the invention has been described with reference to specific 
embodiments, the description is illustrative of the invention and is not 
to be construed as limiting the invention. Various modifications and 
applications may occur to those skilled in the art without departing from 
the true spirit and scope of the invention as defined by the appended 
claims.