X-Ray Tomography System for and Method of Improving the Quality of a Scanned Image

This invention relates generally to computer assisted tomography (CAT), and more particularly to improvements in the quality of a CAT scan image.

CAT scanners of the third generation type include an X-ray source and X-ray detector system secured respectively on diametrically opposite sides of an annular-shaped disk. The latter is rotatably mounted within a gantry support so that during a scan the disk continuously rotates about a rotation axis while X-rays pass from the source through an object positioned within the opening of the disk to the detector system.

The detector system typically includes an array of detectors disposed as a single row in the shape of an arc of a circle having a center of curvature at the point, referred to as the "focal spot," where the radiation emanates from the X-ray source. The X-ray source and array of detectors are all positioned so that the X-ray paths between the source and each detector all lie in the same plane (hereinafter the "slice plane" or "scanning plane") normal to the rotation axis of the disk. Because the ray paths originate from substantially a point source and extend at different angles to the detectors, the ray paths resemble a fan, and thus the term "fan" beam is frequently used to describe all of the ray paths at any one instant of time. The X-rays that are detected by a single detector at a measuring instant during a scan is considered a "ray." The ray is partially attenuated by all the mass in its path so as to generate a single intensity measurement as a function of the attenuation, and thus the density of the mass in that path. Projection views, i.e., the X-ray intensity measurements, are typically done at each of a plurality of angular positions of the disk.

An image reconstructed from data acquired at all of the projection angles during the scan will be a slice along the scanning plane through the object being scanned. In order to "reconstruct" or "back project" a density image of the section or "slice" of the object in the defined scanning plane, the image is typically reconstructed in a pixel array, wherein each pixel in the array is attributed a value representative of the attenuation of all of the rays that pass through its corresponding position in the scanning plane during a scan. As the source and detectors rotate around the object, rays penetrate the object from different directions, or projection angles, passing through different combinations of pixel locations. The density distribution of the object in the slice plane is mathematically generated from these measurements, and the brightness value of each pixel is set to represent that distribution. The result is an array of pixels of differing values which represents a density image of the slice plane.

In order for the image reconstruction process to work, however, the ray sums must be assigned to the proper pixels, i.e., each pixel that lies in the path traversed by the individual respective ray. When ray sums are assigned to improper pixels, not only are the density values of the affected pixels in error, but, because of the Radon algorithm, artifacts in the form of rings and other geometric patterns can appear in the image and tend to obscure it.

Since a typical scan involves over a hundred million ray sums, in order to accurately map the ray paths through the pixel field without an unmanageable amount of calibration work, it is very useful to be able to assume very uniform and accurately located detectors and measurements timed to occur at precise rotation intervals, e.g., exactly equivalent to the spacing between detectors in the array. Co-pending patent application, U.S. Ser. No. 08/190,945 entitled "Modular Detector Arrangement for X-ray Tomographic Scanner," filed simultaneously herewith in the name of John Dobbs and David Banks and assigned to the present assignee (Attorney's Docket No. ANA-23), describes and claims a modular arrangement for very accurately locating and aligning detectors on a support spine. In order for the detectors to be accurately positioned and aligned with respect to the X-ray source and the patient object, however, there is a need for a readily adjustable mounting system to mount the spine to the rotating gantry.

In addition to inaccurate detector placement, incorrect assignment of rays to pixels can be caused by even slight patient object movement during the scan. It is very advantageous therefore, to be able to complete the scan within a few seconds, so that the patient can remain still during a scan, and if necessary hold his or her breath.

While such faster scans can help reduce motion artifacts, certain scans require a minimum exposure (for example, a typical brain scan requires a typical X-ray source to receive a total of 300-400 milliamp-seconds to provide sufficient X-ray exposure), and with faster scans at the same X-ray exposure level, the number of photons received by each detector will necessarily decrease because of the shorter exposure time. Thus, shortening the time of the scan necessarily means that the tube must be operated at a higher level in order to provide a sufficient number of photons to complete the scan. Where the power source available is insufficient to deliver high rates of power, the conventional wisdom is that the scan must be slowed down sufficiently so that an adequate number of photons are provided during the 360.degree. rotation to provide sufficient image data for the scan. This, of course, increases the errors attributed to motion artifacts. If the patient moves during the scan the data taken during the latter part of the scan will be inconsistent with that taken earlier. The magnitude of the artifact will be related to the magnitude of the motion.

One solution is to provide a high speed, multiple scan, without substantially increasing the overall scanning time nor the X-ray exposure level. Thus, for example, by halving the time of a typical 360.degree. scan, e.g., a 4-second scan, and repeating the scan to duplicate the data, i.e., performing two successive 2-second, 360.degree. scans, and summing the duplicated data from the two scans so as to superimpose two scan images, errors due to motion artifacts can be reduced. Thus, if a patient only moves during the first 2-second scan, the second scan will free of motion artifacts. Summing the two scans results in the summation of a 2-second scan with an artifact with an 2-second artifact free scan. If the patient moves continuously during the entire 4 seconds, then the magnitude of the artifact will be smaller in each of the 2-second scans than it would be in the slower 4-second scan so that the summed picture will contain two small artifacts instead of one larger artifact. Since artifact creation is very non-linear, small motions have a disproportionately smaller effect on the image quality than large motions.

Another parameter affecting image quality is related to the total number of unique ray paths of a scan. In general, the larger the number of unique ray paths of a scan that can be assigned to the proper image pixels, the better the resulting image. To obtain the benefit of increasing the number of unique ray paths that can be assigned to the image pixels of the reconstructed image, it is well known to mount the detector array relative to the source and rotation center of the disk, i.e., the "isocenter", so that ray placement is offset by a fixed one-quarter of the spacing of the detector. The offset is referenced to the center line passing from the focal spot of the source through the isocenter to a detector of the array. Thus, without an offset the center ray is aligned with the center of the detector. With a fixed quarter of a detector offset the center ray is offset a fixed quarter of the spacing of the detector from the center of the detector. With such a fixed offset, during a second 180.degree. scan all of the detectors will be displaced by one-half of the detector-spacing from the detector placement during the first 180.degree. of the scan. This, therefore, doubles the number of unique ray paths for a 360.degree. rotation scan from the centered arrangement, without generating redundant information.

For an illustrative explanation of this prior art scheme reference is made to FIGS. 1-3. FIGS. 1 and 2 depict the focal spot of the X-ray source 12, a particular detector 14 within the array of detectors 16 of a scanning tomography system and a portion 18 of the patient object disposed between the source and detectors, portion 18 being referred to as the measurement field. FIG. 1 shows an instant when the X-ray source 12 is at the top and detector 14 is at the bottom. FIG. 2 represents an instant 180 degrees later in the same scan, when detector 14 is at the top and source 12 is at the bottom. Dot-dash line 20, which passes between source 12 and detector 14, also passes through isocenter or rotation center 22. The latter is the center of rotation, or where the rotation axis of the rotating disk supporting the source 12 and detector array 16 intersects the scanning plane of the system. Line 20 is broken in two places to indicate that, while detector 14 and object portion 18 are drawn to the same approximate scale, the distances between them and the source have had to be greatly shortened for ease of illustration.

A ray has already been defined as that portion of the X-ray beam that is detected by a single detector. Since the rays detected by detector 14 always pass through the rotation center 22, they may be termed "central rays". Cross-hatched area 26 in each figure shows the area in the object portion 18 which has been exposed by the central ray at each of the two positions shown. Since the detectors are offset by an amount l.sub.1 equal to one-quarter the spacing of a detector (w/4) the center 28 of the central ray exposing the area 24 and detected by detector 14 (the center of the ray is drawn from source 12 to the center of detector 14) does not extend through the center 22 but is displaced to the left in FIG. 1 by an amount proportional to one-quarter the spacing of the detector, and is displaced to the right in FIG. 2 by an amount proportional to one-quarter the spacing of the detector. Thus, referring to FIG. 3, the center line 28 of the ray 20 as seen in FIGS. 1 and 2 is offset in the two figures by an amount l.sub.2 equal to one-half of the ray spacing (w/2) so that ray placement for the detectors at the position shown in FIG. 2 which occurs during the second 180.degree. (shown as dashed arrows pointing up) will be precisely between the ray placement for the detectors at the diametrically opposite position shown in FIG. 1 which occurs during the first 180.degree. scan (shown as dashed arrows pointing down). As a result, the area of object portion 18 penetrated by the central ray at the measuring instant of FIG. 2, as indicated by cross-hatched area 26, is displaced by an amount proportional to one-half the spacing of the detector as indicated at 30 in FIG. 2 from that penetrated during the measuring instant of FIG. 1. The same is true of all the rays for the 360.degree. of rotation. As a result, the measurement data from the second 180 degrees of the scan represents samples taken at intervals half-way between those of the first 180 degrees. This doubles the number of unique ray paths and greatly contributes to clarity of the reconstructed image. In a modern practical system where there may be, for example, eight detectors to a degree of rotation, and detectors are centered approximately 1.8 millimeters apart, this offset represents only 1/32 of a degree and less than about 0.05 mm.

Accordingly, one object of the present invention is to reduce the effects of motion artifacts on tomography imaging without increasing the X-ray exposure to a patient during a scan.

Another object of the present invention is to provide an improved X-ray tomographic scan having the benefit of a longer scan from the point of view of quantum statistics without actually increasing the scanning time, nor the X-ray exposure level.

And another object of the present invention is to provide an improved tomographic scan which can be accomplished where the power source available is insufficient to deliver high rates of power.

Yet another object of the present invention to generally improve the image quality of a reconstructed image provided by a CAT scan system of the type having an array of detectors offset by a fixed quarter of the spacing of each detector.

Still another object of the present invention is to provide an improved tomography system providing additional unique ray paths from those provided by the prior art system using a standard one-quarter detector-spacing offset.

And yet another object of the present invention is to provide an improved mounting system for supporting an array of detectors in a tomography system so as to provide precise detector placement relative to the X-ray source and isocenter and sufficiently rugged to sustain high angular acceleration forces, and yet adapted to rotate the detector array at least through a small angle about the focal spot between at least two precisely defined positions.

And still another object of the present invention is to provide such an improved mounting system for supporting an array of detectors in a tomography system wherein the precise detector placement is easily adjustable through an input device, such as a keyboard, making it easier to ship the tomography system, set the system up during installation and calibrate the detector placement as necessary on an on-going basis.

And yet another object of the present invention is to provide an improved tomography system capable of operation in either of at least two modes for defining scans for two different quality images.

The foregoing objects are achieved by an improved detector support system which includes means for moving the detector array relative to a line passing through the focal spot and isocenter, between at least two positions. The detectors can be moved between scans and/or during a scan. Detector movement preferably is provided so as to increase the number of unique ray paths during a scan. For example, the preferred embodiment described hereinafter in connection with the drawings is designed for a multiple scan comprising two 360.degree. scans. The two positions are defined to provide a one-eighth detector-spacing placement for the first 360.degree. scan and a three-eighths detector-spacing placement for the second 360.degree. scan, although the detector-spacing placements selected for each position can vary, as described in greater detail hereinafter. The image data from both scans are combined so as to provide improved image quality of the reconstructed image from that produced by the one-quarter detector-spacing offset during a single 360.degree. scan. In the preferred system, the detector array can also be moved to a one-quarter detector-spacing offset to provide a second operating mode. The mounting system for securing the detector array to the tomography system is preferably a suspension system, such as a four bar suspension system, for allowing the detector array to move relative to the radiation source to approximate rotation about the focal spot over a small angle equal to the one-quarter detector-spacing movement, or other angles depending on the design of the CAT scan system.

These and other objects of the invention will in part be apparent and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts, and the processes involving the several steps and the relation and order of one or more of such steps with respect to the others, all of which are exemplified in the following detailed disclosure and the scope of the application all of which will be indicated in the claims.

FIGS. 1 and 2 are diagrams illustrating the effect of a detector array offset according to the prior art;

FIG. 3 is a schematic diagram illustrating ray placement of the rays for the two projection views shown in FIGS. 1 and 2 which are diametrically opposite one another;

FIG. 4 is an axial view of a CAT scanner embodying the preferred arrangement of the present invention;

FIG. 5 is an axial view of the post support for attaching the spine of the mounting system to the rotating disk;

FIG. 6 is a tangential view of the post support shown in FIG. 5;

FIG. 7 is a radial view of the post support shown in FIGS. 5 and 6;

FIG. 8 is a partial, tangential view (opposite to that shown in FIG. 6) of the post support shown in FIGS. 5-7;

FIG. 9 is an exploded perspective view of the preferred suspension system of the present invention;

FIGS. 10 and 11 are a diagram and a pictorial view, respectively, illustrating the spatial adjustment of the detector array in accordance with the present invention;

FIG. 12 is a schematic diagram illustrating ray placement as a result of the preferred method of the present invention; and

FIG. 13 is a block diagram of the detector array motion control system.

For a more complete understanding of the invention reference is made first to FIG. 4, which represents a tomography scanner 40 incorporating the principles of the present invention. To provide the data for a CAT scan, scanner 40 includes a source 42 of X-rays and detector assembly 44 comprising an array of detectors and mounted to a disk 46. Source 42 and detector assembly 44 are rotated about rotation axis 48 (extending normal to the view shown in FIG. 4) so as to rotate around the object 50 that extends through the central opening of the disk during the CAT scan. Object 50 may be a part of a live human patient, such as the head or torso. Source 42 emits within the scanning plane (normal to rotation axis 48) a continuous fan-shaped beam 52 of X-rays, which are sensed by the detectors of assembly 44 after passing through object 50. An array of anti-scatter plates 54 is located between object 50 and the detectors of assembly 44 to substantially prevent scattered rays from being sensed by the detectors. In the preferred embodiment the detectors number 384 and cover an arc of 48.degree., although the number and angle can vary. Disk 46, which may advantageously be of a light weight material, such as aluminum, is caused to rotate rapidly and smoothly around axis 48. The disk 46 is of an open frame construction so that object 50 can be positioned through the opening of the disk. Object 50 may be supported, for example, on a table 56, which of course, should be as transparent as practical to X-rays. As disk 46 rotates, the detectors of assembly 44 are periodically sampled to provide discrete measurements of X-rays passing in the scanning plane through object 50 from many projection angles. The measurements, representing image data, are then processed electronically with appropriate signal processing equipment (not shown), in accordance with well-known mathematical techniques, so as to produce the final image information. The image information may then be placed in memory, analyzed in a computer, or suitably displayed.

If a typical scan of one 360.degree. revolution is performed at a predetermined time interval at a preselected X-ray exposure level, better results can be achieved by performing the scan so that it comprises two or more consecutive 360.degree. scan revolutions, described herein as a "multiple scan", during substantially the same time interval and at the same X-ray exposure level. The image data from each of the 360.degree. scan revolutions is summed with the corresponding image data from the other 360.degree. scan revolutions so that the effect of motion artifacts occurring during the multiple scan on the final image is reduced. With the detector array mounted relative to the source so that the array is fixed throughout the entire scan, e.g., the center line or ray passing through the focal spot and the isocenter of the rotatable disk is aligned with the center of a detector of the array with all of the detectors subtending the same angle relative to the focal spot of the source, the scan will essentially repeat itself every 360.degree. and the data acquired during each 360.degree. scan should duplicate the image data acquired. Thus, by shortening the time period of each revolution of the multiple scan, there is a greater chance that the patient will move less within each shorter period of one 360.degree. of rotation, and by providing at least a second set of data from the second 360.degree. of rotation, and adding the second set of data to the first set of data, will reduce the effect of the errors due to the artifacts. Yet the total X-ray flux passing through the patient during the measurement period will not appreciably change. Thus, by summing the data from the all of the revolutions of the multiple scan, one will reduce the effects of the patient movement, without appreciably increasing the patient's exposure to X-rays.

As an example, if a typical single 360.degree. revolution scan is performed during a 4 second interval, the data taken during the final projection view at the end of the scan must be reconciled with the very first projection view at the beginning of the scan. This reconciliation will fail to the extent the patient moves during the 4 second interval. By taking a two revolution multiple scan, i.e., taking data during two 360.degree. consecutive revolutions at the same exposure level and over the same 4 second interval (at twice the rotational speed), the final projection view at the end of the first revolution must be reconciled with the first projection view at the beginning of the revolution but, will fail to the extent that the patient moves during only a 2 second interval. A similar reconciliation must be made during the second revolution, but again will only suffer from the patient's movement during a 2 second interval. The data from the first revolution is summed with the data from the second revolution so as to provide data from a 4 second multiple scan at the same exposure level as the single revolution 4 second scan, thus accomplishing the multiple scan without increasing the level of X-rays to which the patient is exposed.

In accordance with the present invention, in order to improve image quality, the detector assembly 44 is mounted on the disk so as to be precisely positioned relative to the source 42 and the rotation axis 48, but movable through a small rotation angle about a central point coincident with the focal spot of the source 42 between at least two positions so that the number of unique ray paths provided during a multiple scan can be increased from that provided with a fixed one-quarter detector-spacing offset. In this case the data collected during the second revolution does not duplicate the data collected during the first revolution of the scan because of the different ray paths during the second revolution. The ray paths of the second revolution are interleaved with the ray paths of the first revolution so as to provide an improved quality reconstructed image.

An embodiment of the suspension system of the invention which facilitates the desired precision adjustment of the detector array while possessing the desired ruggedness is further illustrated in FIGS. 5-11. The illustrated mounting system 58 preferably includes a detector and antiscatter plate support spine 60 which supports the detector assembly 44 and antiscatter plates 54 along an arcuate line 62 (seen in FIG. 4, and schematically illustrated in FIGS. 10 and 11) which has its center of curvature at the focal spot (indicated at point 124 of FIG. 10) of source 42 such that each detector subtends an equal angle about that center. Mounting system also includes two post supports 64, each of which attaches to disk 46 and to a respective end of spine 60. A reversible stepping motor 66, which may be attached to one of supports 64. The stepping motor can provide very small precise movements with each step and therefore can provide an automatic detector array angular position adjustment, as described in greater detail hereinafter. By keeping track of the number of steps one can determine at any one instant of time the precise position of the detector array and therefore provide the precise positioning of the detector array in accordance with the principles of the present invention.

A suitable arrangement for attaching the post supports 64 to respective ends of spine 60 and to disk 46 is shown in FIGS. 5-8, and the partially exploded assembly drawing of FIG. 9. The end of support 64 nearest the X-ray source 42 is shown as the top in these drawings. The major components of a support 64 are a support body 70, which as seen in FIG. 5 bolts to disk 46, a thin, flexible mounting plate 72, which respectively bolts at opposite ends to body 70 and spine 60. The plate 72 is preferably made of a material having high flexibility and strength, such as stainless steel.

For reasons which will be more clearly described hereinafter, flexible plate 72 is sandwiched between three pairs of rigid surfaces, five of which are on separate stiffeners 76, 78, 80, 82 and 84. As best seen in FIG. 6, a first stiffener 76 holds the upper section of plate 72 against body 70 by bolts 75 or the like. A pair of stiffeners 78 and 80 may be applied to opposite sides of the middle section of plate 72 by bolts 79 through stiffener 78 and plate 72 into stiffener 80. Similarly, a pair of stiffeners 82 and 84 may be secured to opposite sides of a lower section of plate 72 by bolts 83. As best seen in FIGS. 5 and 6 additional bolts 85 through the center of stiffeners 82 and 84 and plate 72 into threaded holes in the end of spine 60 securely fasten the spine 60 to stiffening member 84, and thus to post support 64.

As best seen in FIGS. 5 and 9, fine adjustment of the position of spine 60 in the radial direction, toward and away from the source 42, to assure that all detectors are the same distance from the focal spot, may be provided by a lever 90, a pivot ball 92, a screw 94, located at the top of each support 70. One end of lever 90 protrudes through a rectangular hole 96 (see FIG. 9) in plate 72. Screw 94 extends through lever 90 at the other end and is threaded into the top of body 70. Ball 92, partly recessed into body 70 and into lever 90, serves as a fulcrum so that as screw 94 is driven into body 70, plate 72 is raised, and vice versa. The elongated holes 98 in plate 72 through which bolts 75 extend accommodate this adjustment, which is made before stiffener 76 is firmly secured. A pair of dowel pins 97, in body 70, and associated slots 99 in plate 72 and stiffener 76, assure radial alignment wherein the width of each slot is dimensioned to tightly receive the corresponding pin.

Plate 72 may be kept under constant tension to avoid movement when subjected to operating forces by a pair of tensioning levers 100, which protrude through slots 101 provided in body 70, and into holes 102 in plate 72. Both levers 100 pivot up and down in slots 101 on a common pivot rod 104, that passes through body 70 and levers 100. As best seen in FIG. 8, driving each of tensioning levers 100 may be a push rod 106, an appropriate stack of compression (belleville) washers 108 and a tensioning screw 110. The compression washers maintain a constant pressure on push rods 106 and therefore constant tension on plate 72 over a limited range of movement. The size washer is chosen according to the amount of tension desired, and the number of washers in the stack is chosen to accommodate the range of movement desired for final adjustment.

In order to effect movement of the spine the motor 66 rotates a screw 112 (seen in FIGS. 5 and 9) against stiffener member 84. Thus, operating the motor in one direction moves the screw against the stiffener and thus against the tension provided by the flexible plates 72 so as to effect movement of the spine in one direction. Operating the motor in the opposite direction allows the screw to move away from the stiffening member 84 so that the spine moves in the opposite direction due to the tension provided by the flexible plates.

The fine precision movement for moving the detector array will now be described in greater detail. While it is possible to practice the invention without them, stiffeners 76, 78, 80, 82 and 84 serve a very useful purpose in this context. They limit the bending of flexible plate 72 to two narrow, unsupported areas, which span the width of the plate, closely simulating two hinges. Since the detector array is hung from two such restrained plates, it is essentially supported on four hinges, in what is commonly termed a four-bar suspension system. An understanding of the operation of this arrangement and its advantages may be aided by reference to FIGS. 10 and 11.

FIG. 10 diagrammatically illustrates a four-bar suspension system, similar to that provided by the two supports 64 and spine 60. Two pivot points, P.sub.1 and P.sub.2, are fixed; the other two, P.sub.3 and P.sub.4, are movable. Rigid beams L.sub.1, L.sub.2 and L.sub.3 connect P.sub.1 and P.sub.3, P.sub.2 and P.sub.4 and P.sub.3 and P.sub.4, respectively. P.sub.3 is restricted to move along a portion of a circle with P.sub.1 as its center, and P.sub.4 is restricted to move along a portion of a circle with P.sub.2 as its center. Since beam L.sub.3 joins them, P.sub.3 and P.sub.4 must move together in the same sense, clockwise or counterclockwise. When P.sub.3 goes up to the left therefore, as shown by arrow 120, P.sub.4 goes down to the left, as shown by arrow 122. Arrow 120 is tangent not only to a circle about point P.sub.1, but to every circle whose center is on the straight line defined by P.sub.1 and P.sub.3. Similarly, arrow 120, which defines the movement of the right side of the four-bar system, is tangent to every circle whose center is on the line defined by P.sub.2 and P.sub.4. For small distances, therefore, the movement of beam L.sub.3 is a very close approximation to rotation about the point 124, which is common to both lines, i.e., where L.sub.1 extended intersects L.sub.2 extended, shown by dotted lines.

Moving from the diagram of FIG. 10 to the sketch of FIG. 11, points P.sub.1 and P.sub.2 correspond approximately to those points between adjacent stiffeners. L.sub.1 and L.sub.2 correspond to the mid-sections stiffened by stiffeners 78 and 80. Point 124 corresponds to the position of the focal spot of the X-ray source 42. When the position of spine 60 is changed by adjustment screw 112 (as determined by stepping motor 66), therefore, the whole detector array moves the small distance along the arc that has the focal spot as its center. Since the detectors are mounted on spine 60 along arcuate line 62 with its center at the focal spot, the adjustment merely rotates the array without changing the distance between the source and any detector and thus each detector will still subtend an equal angle. It should be noted here, that while stiffeners 76, 78, 80 82 and 84 provide some advantage, the system still approximates a four-bar suspension without them. Similarly, it will be understood that a pair of suitable hinges could be substituted for each plate 72, where the latter attaches to the disk and to the spine to practice the invention.

Preferably, the detector array can be shifted between a position where the detector array is offset n/8 of the spacing of a detector, where n is an odd integer 1, 3, 5 or 7 during a 360.degree. rotation of a two revolution scan, and offset n/8.+-.1/4 of the spacing of a detector during the second 360.degree. rotation of the two revolution scan. For example, the array can be shifted between a position where the detector array is offset 1/8th of the spacing of a detector (w/8), and offset 3/8ths of the spacing of a detector during the second 360.degree. rotation of the two revolution scan. In the example, the sequence can be either providing the one-eighth detector-spacing offset set during the first 360.degree. or during the second 360.degree. rotation with the same results. The shift occurs precisely when the first 360.degree. of the scan has occurred. Because of the time necessary to effect the shift, the disk 46 will actually rotate a predetermined amount before the detectors are set to provide image data for the second 360.degree. revolution of the two revolution scan. The actual amount of rotation of the disk between the two revolutions of the disk when image data is acquired, is a function of the rotational speed of the disk and the response time of the detector support and shifting mechanisms. If, for example, the detectors provide data for a first 360.degree. rotation of a multiple scan, and during the shift of the detectors (when no data is acquired) the disk rotates 40.degree., the detectors will begin providing data for the next 360.degree. rotation so that the disk actually rotates 760.degree. for a two revolution multiple scan.

For ease of illustration assuming the disk rotates 760.degree. during a two revolution scan, FIG. 12 shows the ray placement of four positions of the disk 46: 180.degree. and 360.degree. of the first revolution when data is acquired, and those same positions of the disk (540.degree. and 720.degree.) during the second revolution of the two revolution scan. Assuming the detector offset is one-eighth of a detector-spacing (w/8) for the first 360.degree. rotation, and three-eighths of a detector-spacing (3w/8) for the second 360.degree. rotation, the arrows 28a, 28b, 28c and 28d indicate the center line of the ray corresponding to the 0.degree. (720.degree.), 180.degree., 360.degree. and 540.degree. positions of disk when projection views are taken, so that the rays provided during the second 360.degree. revolution are interleaved with the rays provided during the first 360.degree. revolution. The ray placements from the two revolutions are thus spaced apart by one-quarter of a detector-spacing (w/4), doubling the number of unique ray paths provided by the one-quarter of a detector offset of the prior art. Such a shift can be readily made without significantly slowing the rotation cycle if, for example, adjusting screw 112 is driven by stepping motor 66 as shown in FIG. 2. Such an arrangement, for example, can provide the shift in about 0.2 sec, so that where one revolution takes approximately 2 seconds, the disk 46 rotates approximately 1/10th of a revolution (36.degree.) during the shift before data is again acquired during a second revolution. Alternatively, spine 60 can be driven between the two positions by an electrical solenoid.

It should be appreciated that since the detector system is movable between a one-eighth detector (w/8) offset and three-eighths detector (3w/8) offset by operating the stepping motor 66 in one mode of operation to achieve spacings between ray paths equal and opposite to one another of one-quarter of a detector-spacing (w/4), the system can be operated in a second mode of operation wherein the detector array is preset with a fixed one-quarter of a detector-spacing offset (w/4) prior to scanning by operating the stepping motor 66 to move the detector to that position.

The mounting system of the invention, therefore, provides the very fine adjustment needed for simplified setup and movement between scans and during a scan, as well as the ruggedness needed for rapid acceleration and deceleration. Because the mounting system of the invention meets these needs so well, it may be used in a novel manner according to the method of the invention to greatly improve the quality of the image by creating additional unique ray paths during a scan. Specifically, if the detector array is rapidly and accurately shifted a judicious amount at a precise position of the disk, as the disk rotates during a scan, a completely new set of ray paths can be provided so as to greatly enhance the quality of the image. It should also be appreciated that the system can work in other modes, and/or be constructed to provide detector shifts with other spacings relative to the center ray which might require additional whole or partial revolutions of a multiple scan, or may be accomplished all within a single 360.degree. revolution of the disk. For example, shifts of w/8 and 7w/8 of a detector-spacing, or 3w/8 and 5w/8 of a detector-spacing would work equally well with a two revolution multiple scan. In a similar manner a four revolution multiple scan could employ detector positions w/16, 3w/16, 5w/16 and 7w/16, 9w/16, 11w/16, 13w/16 and 15w/16 of a detector-spacing relative to the center ray during the respective four revolutions of the four revolution multiple scan, wherein a shift occurs after each 360.degree. rotation of the disk during the scan.

FIG. 13 shows a block diagram of the control system for accomplishing the offset shift in accordance with the principles of the present invention. As shown, position measurement system 140 is provided for sensing the angular position of the source 42 and detector array 44 so that the shifters of the detectors can be synchronized with the rotation of the disk 46. Position measurement system 140 is used to time the detector array shift at the prescribed 360.degree. rotation of the two revolution scan and can be any device for determining angular position of a rotating object at the prescribed positions where shifts are to occur. For example, a cam secured to rotate with disk 46 can operate a micro-switch, or a magnet mounted on the disk can induce a trigger signal in a stationary pickup, at a precise angle of the source and detector array. In a preferred method, a shaft encoder of the type including indicia on the circumference of the disk is detected by a sensor arrangement and serves to time both the periodic sampling of the detectors and the shifting of the detector array. The preferred position measurement system is described in co-pending U.S. patent application Ser. No. 08/162,653; entitled Apparatus for and Method of Measuring Geometric, Positional and Kinematic Parameters of a Rotating Device; filed Dec. 6, 1993, in the names of Bernard M. Gordon, David Winston, Paul Wagoner and Douglas Abraham; and assigned to the present assignee, the application disclosure being herein incorporated. The output of position measurement means is connected to the control system 142 which is adapted to provide an output shift signal to control the stepping motor 66. The output is capable of controlling the reversible stepping motor one step at a time in either direction so that the number of steps required to move the detector array among the three positions of interest, e.g., one-eighth and three-eighths detector shift for the detector shift mode of operation, and one-quarter detector shift for the one-quarter detector shift offset mode of operation. A user input 146 can be provided to control 142 for selecting the mode of operation. Thus, the control will initially set the position of the detector array during each mode of operation; and, during the mode of operation requiring a shift of the detectors following the first 360.degree. rotation of the two revolution scan, the control will provide a signal to the stepping motor so as to step the motor a sufficient amount so that the detector array shifts between the one-eighth and three-eighths of a detector-spacing. The control system has the added advantage that the user input indicated at 146 can be an easily accessible device, such as a keyboard, so that the detector array positions can be set and changed at any time without the necessity of taking the CAT scan system apart. Thus, precise detector placement is easily adjustable, making it easier to ship the tomography system, set the system up during installation and calibrate the detector placement as necessary on an on-going basis.

We have thus described a system and method of making tomographic images that provide better quality images by creating additional ray paths during a tomographic scan so that the additional data can be interleaved with other data. Combining fast multiple scans with ray path shifting reduces the effects of motion artifacts on tomography imaging without increasing the X-ray exposure to a patient during a scan. Such a combination provides an improved X-ray tomographic scan having the benefit of a longer scan from the point of view of quantum statistics without actually increasing the scanning time, nor the X-ray exposure level, as well as allowing the scan to be accomplished where the power source available is insufficient to deliver high rates of power. Increasing the number of additional ray paths by moving the detectors during the scan improves the image quality of a reconstructed image provided by a CAT scan system of the type having an array of detectors offset by a fixed quarter of the spacing of each detector. The improved mounting system for supporting the array of detectors provides precise detector placement relative to the X-ray source and isocenter and is sufficiently rugged to sustain high angular acceleration forces, and yet is adapted to rotate the detector array at least through a small angle about the focal spot between at least two precisely defined positions. The mounting system also facilitates and allows for easy adjustment of detector placement through an input device, such as a keyboard, making it easier to ship the tomography system, set the system up during installation and calibrate the detector placement as necessary on an on-going basis. The detector shift mounting system moves the detector array so that the tomographic system is capable of operation in either of at least two modes for defining scans for two different quality images.

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted in an illustrative and not in a limiting sense.