Patent Number: 052280691
Section: description

GENERAL DESCRIPTION In FIG. 1 a front end including the gantry of a rotate-rotate computerized tomographic scanner is shown at 11. The scanner comprises the gantry 12 mounted to a base 13. The gantry has an opening 14 for receiving the patient therein. An X-ray source means 16 is rotatably mounted on the gantry and is at a fixed distance from the detector array 17. Both the source means and the detector array 17 rotate together under the control of angular displacement means 21 about the patient 18, shown resting on a bed or cot 19. The rotation is about the isocenter 22 shown at a distance Y1 from the source means and a distance Y2 from the detector array. Means such as processor 23 process the data from the detector array 17 utilizing memory means 24 to provide a display 26 on display means 27. Means for shifting the position of the detector relative to the source means, such as detector shift means 28 are shown for selectively shifting the detector in order to increase the effective spatial resolution of the system in a manner well known to those skilled in the art. It should be understood that the source means could be shifted instead of the detector means. The shift is relative to the source means 16. To further increase the resolution in a preferred embodiment, the source means may be a dual focal spot source used in a manner described in U.S. Pat. No. 4,637,040 which issued on Jan. 13, 1987, and is assigned to the assignee of this invention. In addition the processing means includes means for minimizing non-coplanarity caused artifacts according to the system and methods taught by U.S. Pat. No. 4,578,753 which issued on Mar. 25, 1984, and is assigned to the assignee of this invention. Non-coplanarity caused artifacts due to beam divergence are generally reduced to insignificance by scanning through 360 degrees. Alternatively, a source shifter 29 may be provided which shifts the source in the Z direction. It should be understood that the shifting of the source in the Z direction is relative to the detector array. Thus, the detector array can also be shifted in the Z direction. The shifting of the source in the Z direction is to locate the center of the source means at the junction point of the dual detectors in the detector array. Note that the source means is preferably centered over the center of the detector array in the X direction. The source means can be shifted so that its center in the Z direction is either over the center of the detector of the basic detector array or over the line of abuttment of the dual detectors. The source means is over the center of detectors in the X direction without any shift. The means for shifting the source is indicated at block 29. Arrows indicating X and Y directions are shown at 31 and 32 respectively. FIG. 2 shows a prior art single row detector array at 36. The detector array is made up of a plurality of detectors, one of which is shown at 37. The array extends in the X direction while the length of the individual detectors extend in the Z direction. The prior art array is made up of single detectors in the Z direction. A dual slice double row detector array is shown in FIG. 3 at 38. It is made up of a plurality of rows of detectors containing detectors such as detector 39 in a basic row abutting detector 41 in a second row. A plurality of such dual detectors are mounted in the array 38 to form the dual detector array. Care must be taken to avoid or minimize non-sensitive areas such as 42 of the abutting detectors that cannot acquire data because of light shielding. There must, however, be light shielding between detectors 39 and 41 to prevent scintillations in detector 39, for example, from affecting detector 41. The shielding can be accomplished in a collimator or by an actual shielding between the detectors 39 and 41. However, space between detectors such as space 42 between detectors 39 and 41 has to be kept to a minimum to avoid a loss of imaging areas and a consequent loss of image information between slices. In FIGS. 4 the detectors 39 and 41 are shown. In FIG. 4a, there is a front view which particularly shows detector 39. FIG. 4b is the side view which shows both detectors 39 and 41. As shown in FIGS. 4a and 4b detectors 39 and 41 both comprise a crystal 46 which reacts to the impingement thereon of X-rays by expelling a quantum of light. The quantum of light strikes the photo-diode layer 47a, 47b respectively (FIG. 4b) which converts the light into electrical charges. It is important that quantums of light from the crystal 46a above one photo-diode 47a does not impinge the photo-diode 47b that is below crystal 46b. Therefore, shielding means 40 is provided between the crystals. The shielding means prevents quantum of light from crystals not directly above the photo-diodes from affecting these photo-diodes. The shielding may be such things as aluminum foil attached to the crystals at the abutment area or paint administered to the transparent crystals at the abutment area. The shield should be in the order of no more than 0.05 to 0.1 mm thick. The electrical charges are received and transmitted by the electronic circuitry not shown but connected to support blocks 48(a) and 48(b). The support blocks 48a and 48b are connected to electronic circuitry over leads 51, 52 and 53 and from the electronic circuitry to the processor 23 which includes an analog to digital converter. Ideally leads 51 and 53 carry the electrons while lead 52 is connected to ground. In a preferred embodiment the photo-diode substrate 47 is also divided into parts and optically separated at 54 to assure that there is no inter-action between the scintillations caused by X-rays striking either crystal 39 or 41. Thus, X-rays striking crystal 39 have almost no effect on photo-diode 47b. Similarly, X-rays striking crystal 41 have almost no effect on photo-diode 47a. The front end electronics provides analog signals which are converted into digital signals in the processor for processing into image data to provide the image 26 in display unit 27. As shown in the YZ plane of FIG. 5, ideally the source means shown at 16 has its center 61 aligned with the junction 40 of detectors 39i and 41i. The distance between the source means at 16 and the detector array 17 extends in the Y direction. The isocenter 22 shown as a dot-dash line is indicated along with the patient 18. Notice that there is a crossover area 55 shown as cross-hatch section in the patient wherein data is obtained both by detector 39i and detector 41i. In the preferred embodiment, the dual focal spots mentioned earlier are located aligned with point 61 and extending in the X direction. A preferred embodiment of the detector array is shown in the plan view of FIG. 6. Therein the dual detectors are used only in a small portion of the array detector, sufficient for example, to cover the head of the patient. Thus, the array 17 is comprised of a basic or major detector array 17a which is the complete detector array and a minor detector array 17b comprising a reduced number of detectors which helps reduce the probability of partial volume artifacts. The whole body fits between the dashed lines 61a and 61b. The head, for example, fits between the full lines 62a and 62b. The space between lines 62a and 62b is where partial volume artifacts can be significant. The thickness of the dual slices is substantially the dimension of the detectors in the Z direction. The X and Z directions in FIG. 6 are shown at 63. The detector array of the type shown in FIG. 6 can also be used during the acquisition of multiple slices by moving either the patient or the source detector array assembly in a well known manner so as to obtain even contiguous slices. FIG. 7 shows the shifting of the source relative to the detectors, or of the detectors relative to the source for example, detectors 67a and 67b. When the detector array 17a is used exclusively then the source is shifted or the detectors are shifted so that the center 61 of the source lies over the center 68 of detector 67a. When both detectors 67a and 67b are used then the source 16 is shifted or the detectors are shifted as shown in FIG. 7b so that the center 61 of the source is aligned with the junction area 40 of detectors 67a and 67b. The X, Y and Z axes are shown at 69. FIG. 8 is a plan view of the source detector arrangement showing the relationship between the source and the detectors in the different scanning modes. Thus, prior to scanning while utilizing detector array 17a then the source 16 is shifted so that its center point 61 is located in the middle of detector array 17a in the Z direction. The X and Z axes are shown at 70 in FIG. 8. The source shifter 29 shifts the source or the detector shifter 28 shifts the detector array so that the center point 61 is over the junction 40 between array 17a and 17b at the approximate center point in the X-direction of the detector array 17a. In the preferred whole body scanning procedure the body appears between the lines 61a and 61b whereas the head or the spine, for example, appears between lines 62a and 62b in FIG. 8. In practice, the patient rests on the cot and is moved into the scanner for obtaining a computerized tomographic scan. Dual slices are simultaneously obtained utilizing the rotate-rotate source detector arrangement. To minimize the number of detector needs utilization of a second detector array abutting the first detector array and having fewer detectors therein can be used. The second detector array is preferably, but not necessarily, aligned with the center of the source means in the X and Z directions. The invention has been described in relation to specific procedures and embodiments, however, it should be understood that this description is made by way of example only and is not meant as a limitation on the scope of the invention.