Abstract:
The imaging system has two detectors which can be rotated in a circular path about an object with the angular displacement between the detectors and their radial position with respect to the axis being adjustable. Preferably, the distance of the detectors from the lateral axis is adjustable to increase resolution of the system. A gantry has supports for drive gear rings for the detectors with radial motion mechanisms connecting one detector to the interior surface of a drive gear ring and the other to the exterior of its drive gear ring via a support arm. A drive gear and idler gear move one detector along the circular path and a radial drive motor moves the detectors radially with respect to the axis.

Description:
This is a continuation of U.S. patent application Ser. No. 07/704,759 of Hines et al., filed on May 23, 1991, now U.S. Pat. No. 6,184,530, issued Feb. 6, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to imaging systems and more particularly to imaging systems for use in nuclear medicine. 
     2. Description of the Relevant Art 
     Gamma ray cameras are used in nuclear medicine to generate high quality images for brain, SPECT (Single Photon Emission Computer Tomograph), and total body bone studies. These cameras are most frequently used for cardiac and total body bone studies. 
     It is very important that the gamma ray camera be designed for hign patient throughput for both economic and terapeutic reasons. The cost for diagnosing each patient is reduced if mire patients can be diagnosed per unit time. For very sick patients or patients in intensive care it is important to minimize the time required to acquire image data. Other factors, such as patient set-up time also affect patient throughput. 
     Modern gamma ray cameras utilize detectors, such as Anger cameras, having a wide field of view so that it is possible to image the full width of the body of a patient at each angular stop without the requirement of rectilinear scanning. These detectors use thick lead collimators to focus images and are thus very heavy. The collimators must be positioned as close to the patient as possible to acquire image data required to generate high resolution images. The image data acquired by the detectors is processed by a computer to generate an image. Techniques for processing image data are well-know in the art and described in “Principles of Instrumentation in SPECT” by Robert Eisner,  Journal of Nuclear Medicine , Vol. 13, #1, March 1985, pp. 23-31; Computed Tomography in Nuclear Medicine” by John Keyes, (chapter in)  Computer Methods , C. V. Mosley, St. Louis, 1977, pp. 130-138; and “Single Photon Emission Computed Tomography,” by Bernard Oppenheim and Robert Appledown, (chapter in)  Effective Use of Computers in Nuclear Medicine , Michael Gelfand and Stephen Thomas, McGraw-Hill Book Co., New York 1988, pp. 31-74. 
     Recent technological innovations have produced dual-head systems, with two detectors having their detector image direction arrows oriented at a fixed angle of 180°, and triple-head systems, with three detectors having their image direction arrows oriented at fixed angles of 120°, SPECT gamma ray cameras capable of rapid, high quality SPECT imaging. FIGS. 1A and 1B are schematic diagrams depicting the fixed orientation of the detector image direction arrows  2  of the detectors  4  in a dual-head and triple-head system. 
     When the detectors rotate about the patient, a lateral axis is defined as the mechanical axis of rotation aligned with the computer matrix for reconstructing the SPECT images. 
     The single, dual, and triple head cameras each have certain features which are advantageous for a particular type of application. To determine which system is best for a particular application factors such as 1) the ability of the camera to perform required imaging tasks; 2) the quality of the images generated; and 3) patient throughput should be considered. 
     The acquisition of data for a total body scan requires movement of the detector along the length of the patient&#39;s body. The dual-head system is very efficient because image data for anterior/posterior images can be acquired simultaneously. The time required to complete a scan can be reduced from 45 to 60 minutes, for a single-head camera, to 30 minutes. The triple-head system is no more efficient than the single-head system because the detectors cannot be aligned to acquire simultaneous anterior/posterior or left/right lateral data. 
     To generate high-quality SPECT for brain, bone, or liver studies views taken along a complete 360° circle (360° scan) around the body of the patient are required. Typically, about 64 to 128 angular stops are required to acquire the image data. The above-described dual-head camera reduces the imaging time to ½ the imaging time of a single-head system because data is acquired from two stops simultaneously. The triple-head camera reduces the imaging time to about ⅓ the imaging time of a single-head system because data is acquired from three stops simultaneously. 
     For cardiac SPECT studies, the analysis of complex imaging considerations has led to the use of at least 32 stops over a 180° arc about the patient&#39;s body (180° scan). For a 180° scan the imaging time of a single-head and dual-head system are the same because only one detector of the dual-head system is within the 180° arc at any given time. A triple-head system reduces the image time to about ⅔ the time of a single-head system for a 180° scan because two detectors are within the 180° arc about ⅓ of the time. 
     In view of the above it is apparent that the mechanical system for orienting the detectors must be designed to provide a mechanism for accurately orienting the detectors at various angular stops relative to the patient and to position the collimator as close to the patient as possible. Additionally, the system must be stable so that the heavy detectors are held still at each stop to facilitate the acquisition of accurate imaging data. Other attributes that are required of the mechanical system are ease of patient positioning, size of footprint, and overall size. 
     Further, as described above, the existing systems each have advantages for particular applications but generally lack the flexibility for optimal performance over a range of applications. Additionally, although cardiac SPECT imaging accounts for about 33% of the use of gamma ray cameras, none of the systems significantly reduce the imaging time for the 180° scan used in forming cardiac SPECT images. 
     SUMMARY OF THE INVENTION 
     The present invention is a unique system for reducing the imaging time required to generate a 180° SPECT image. According to one aspect of the invention, first and second detectors are positioned with a relative angle of 90° to reduce the imaging time for a 180° scan by a factor of two over a two detector systems having the detectors positioned at a fixed relative angle of 180°. 
     According to another aspect of the invention, the angular displacement between two detectors may adjusted to any angle between about 90° and 180° and the detectors can be rotated to any desired angular position along a circular path centered on a lateral axis. Thus, the system can be optimally configured for total body scans and 360° SPECT (relative angular displacement of 180°) and 180° SPECT (relative angular displacement of 90°) to provide a very flexible system. 
     According to further aspect of the invention, each detector can be independently rotated along different circular paths centered on the lateral axis. 
     According to a still further aspect of the invention, each detector may be independently moved toward or away from the lateral axis. 
     According to a still further aspect of the invention, extended collimators are used to decrease the distance between the collimator and the body of a patient when the relative angular displacement of the detectors is less than 180° to improve resolution. 
     According to a still further aspect of the invention, a detector used to form a lateral image of a patient is narrower than the detector used to form a horizontal image so that the detectors can be positioned nearer to the body of a patient to improve resolution. 
     According to a still further aspect of the invention, a table, oriented parallel to the lateral axis for supporting a patient, is displaced vertically and horizontally from the lateral axis to move the body of the patient close to the detectors to improve resolution. 
     Other features and advantages of the invention will be apparent in view of the appended figures and following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are schematic views depicting the fixed orientation of the detectors for existing dual-head and triple head imaging systems; 
     FIGS. 1C-1E are schematic views showing  3  of the multiple angular stops required for a 360° scan with the angular displacement of the detectors at 180°; 
     FIGS. 2A-2C are schematic views showing 3 of the multiple angular stops required for a 180° scan with the angular displacement of the detectors at 90°; 
     FIG. 3 is a perspective view of a preferred embodiment of the invention; 
     FIG. 4 is a view taken along A—A of FIG. 3; 
     FIG. 5 is a view taken along B—B of FIG. 3; 
     FIG. 6 is a view taken along C—C of FIG. 3; 
     FIG. 7 is a top view of the embodiment depicted in FIG. 3; 
     FIGS. 7A-7C are a schematic views of an alternative rotational drive mechanism; 
     FIG. 8 is a schematic view of two detectors oriented at 90°; 
     FIG. 9 is a schematic view of two detectors oriented at 120°; 
     FIG. 10 is a schematic view of two detectors having extended collimators and oriented at 90°; 
     FIG. 10A is a schematic view of two detectors having their centers displaced from the lateral axis; 
     FIG. 11 is a schematic view of two detectors oriented at 90° with a reduced lateral detector; 
     FIG. 12 is a schematic view depicting a patient table that can be horizontally and vertically displaced relative to the lateral axis; 
     FIGS. 13A and 13B are cut away views of mechanisms for displacing the table from the lateral axis; and 
     FIG. 14 is a schematic view of a positional feedback mechanism. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1C-1E depict the required angular stops for two detectors  4  displaced by 180° to accomplish a 360° scan. In the 180° configuration the size of the detectors does not limit their radial motion and the detectors  4  can be positioned to touch the body  10  of the patient at each stop. However, the detectors cannot be moved in circular path while maintaining close proximity to the body of the patient  10  because the body of the patient is not circular. 
     FIGS. 2A-2C depict a preferred embodiment of the invention. The detectors  4  have their image direction arrows oriented at 90° to reduce the imaging time of a 180° scan to ½ the imaging time of a single-head system because data is acquired from two stops simultaneously. 
     FIG. 3 is a perspective view of a preferred embodiment of the invention that allows the adjustment of the relative angular displacement of the detectors to have any magnitude from less than 90° upto 180°. Further, each detector may be independently moved toward or away from the lateral axis  48 . 
     In FIG. 3, a gantry  30 , having left and right upright sections  30 L and  30 R, supports first and second detector I drive gear rings  32  and  34  and first and second detector II drive gear rings  36  and  38 . A detector I radial motion mechanism  40  connects detector I to the interior surface of the second detector I drive gear ring  34  and a detector II radial motion mechanism  42  connects detector II, via a first detector II support arm  44 , to the exterior surface of the first detector II drive gear ring  36 . 
     A left drive gear  45 L and.idler gear  46 L controllably engages the first detector drive gear ring  36  to move detector II in a circular path about a lateral axis  48 . 
     FIG. 4 is a perspective view of the detector I radial motion mechanism  40  taken along A—A of FIG.  3 . In FIG. 4, base plates  60  and  62  having slotted guide bars  64  and  66  fixedly mounted thereon, are attached to the interior surface of the second detector I ring gear  34 . Lead screws  68  and  70  are rotatably mounted in bearing blocks  72 ,  74 ,  76 , and  78  which are fixedly attached to the base plates  60  and  62 . Arm supports  80  and  82  are engaged with the grooves of the guide bars  64  and  66  by guide rollers  84  and  86 . Swivel nuts (only one  90  is shown) couple the lead screws to the arm supports  80  and  82  via brackets (only one  94  is shown). A detector support arm  88  is fixedly mounted to the arm supports  80  and  82 . 
     A drive motor has a lead drive gear  98  coupled to a trailer gear  100  mounted on the second lead screw  70  by a drive chain  102 . First and second lead screw coupling gears  104  and  106  are coupled by a coupling chain  108 . 
     FIG. 5 is an end view, taken along  5 — 5  of FIG. 3, of the rotary drive mechanisms for detectors I and II. In FIG. 5, a first rotary drive motor  120  has a lead drive pulley  122  coupled to a transmission shaft drive pulley  124  by a first drive belt  126 . A first transmission shaft  128  is coupled to the second detector I ring gear  34  by a right drive gear  130 R and idler gear  131 R. The first transmission shaft extends through the gantry  30  parallel to the lateral axis  48  and is also coupled to the first detector I ring gear  32  by left drive and idler gears  130 L and  131 L (not shown). The drive and idler gears  130  and  131  for driving the detector I ring gears  32  and  34  are located on the interior sides of the upright sections  30 L and  30 R of the gantry  30 . 
     Similarly, a second rotary drive motor  132  has a lead drive pulley  134  coupled to a transmission shaft drive pulley  136  by a second drive belt  138 . A second transmission shaft  140  is coupled to the second detector II ring gear  38  by a right drive gear  45 R and idler gear  46 R (depicted in phantom). The second transmission shaft extends through the gantry  30  parallel to the lateral axis  48  and is also coupled to the second detector II ring gear  36  by drive and idler gears. The drive and idler gears  45  and  46  for driving the detector II ring gears  36  and  38  are located on the exterior sides of the upright sections  30 L and  30 R of the gantry  30 . 
     FIG. 6 is a cross-sectional view, taken along  6 — 6  of FIG. 3, depicting the drive and detector support mechanisms. The detector ring gears  32 ,  34 ,  36 , and  38  have support grooves which are engaged with gear support bearings  150 ,  152 ,  154 ,  156 ,  158 ,  160 ,  162 , and  164  mounted on the upright sections  30 L and  30 R of the gantry  30 . Detector I and the detector I radial drive mechanism are mounted on the interior surfaces of the first and second detector I ring gears  32  and  34 . The radial drive mechanism for detector II is mounted on the exterior surface of the detector II ring gears  36  and  38 . The detector II support arms  44 R and L are coupled to the exterior surfaces of the detector II ring gears and extend through the annular space created by the ring gears and supports detector II. 
     FIG. 7 is a top view of the embodiment depicted in FIG.  3  and further depicts the details of the rotary drive mechanism. The first transmission shaft  128  transmits the rotary motion of the first rotary drive motor  122  to both the first and second detector I ring gears  32  and  34  and the second transmission shaft  140  transmits the rotatory motion of the second rotary drive motor  132  to the first and second detector II ring gears  36  and  38 . 
     The operation of the embodiment depicted in FIGS. 3-7 will now be described. Detectors I and II may be independently rotated about the lateral axis  48  by activating either the first or second rotary drive motors  132  or  122 . If the first rotary motor is activated rotary motion is transmitted to the first detector ring gears  32  and  34  which in turn impart rotary motion to detector I through the support arms  88 . 
     Additionally, each detector may be independently moved radially toward or away from the lateral axis  48  by activating the radial drive motor  96  in the radial drive mechanism for the detector. 
     FIGS. 7A and 7B depict an alternative rotary drive mechanism utilizing a single rotary drive motor  122  coupled to the first and second transmission shafts  128  and  140 . In FIG. 7A a lead drive gear  166  is directly coupled to the shaft drive gears  167  and  168  to move to rotate both transmission shafts  128  and  140  in the same direction. 
     The rotational motion of shaft drive gear  166  is transmitted to the first transmission shaft  128  when a first electromagnetic clutch  169  is engaged and rotation of the first transmission shaft  128  is stopped when a first electromagnetic brake  170  is engaged. Similarly, the rotational motion of shaft drive gear  166  is transmitted to the second transmission shaft  140  when a second electromagnetic clutch  171  is engaged and rotation of the second transmission shaft  140  is stopped when a second electromagnetic brake  172  is, engaged. 
     FIG. 7B is a view, taken along A—A of FIG. 7A, depicting the rotation of the lead gear  166  and shaft drive gears  167  and  168 . 
     In operation, both detectors I and II are rotated when both clutches  169  and  171  are engaged and both brakes  170  and  172  are disengaged. Detector I is moved independently if the first clutch  169  is engaged and the first brake  170  is disengaged and detector II is moved independently if the second clutch  171  is engaged and the second brake  172  is disengaged. The brakes are used for safety reasons and to counteract the system in balance. 
     FIG. 7C is a schematic view of an alternative drive system that uses a single drive motor  122  and drive shaft  128 . Drive gears  48  are fixed on the end of the shaft  128  and engaged with the first and second detector II ring gears  36  and  38 . First and second shaft gears  175  and  176  couple the rotational motion of the shaft  128  to the first and second detector I ring gears  32  and  34  when electromagnetic clutches  177  and  178  are engaged and the motion of the first and second detector I ring gears  32  and  34  is stopped when the electromagnetic brakes  179  and  180  are disengaged. 
     In operation, both detectors rotate together when both clutches  177  and  178  are released and the brakes  179  and  180  are released and the rotational drive motor  122  is activated. Detector II is independently rotated to adjust the angular displacement relative to detector I when the brakes  179  and  180  are engaged and the clutches  177  and  178  are engaged. 
     In another embodiment, depicted in FIG. 10, extended collimators  184  are utilized to decrease Rmin and to place the collimator  184  closer to the body  10  of the patient. 
     In one embodiment of the invention the detector image direction arrows  2  are oriented at 120° when a 180° scan is to be performed. As depicted in FIG. 9, this orientation allows greater radial motion to allow the detectors I and II to be positioned closer to the body  10  of the patient than in the 90° configuration. However, the imaging time is reduced to only about ⅔ of the imaging time of a single-head system because both detectors I and II are within the 180° arc only a fraction of the time. 
     In another embodiment, depicted in FIG. 10, extended collimators  184  are utilized to decrease Rmin and to place the collimator  184  closer to the body  10  of the patient. 
     Additionally, as depicted in FIG. 10, the detectors I and II have beveled edges that allow the detectors to be moved closer together when oriented at 90°, thereby reducing R MIN . 
     In FIG. 10A, a configuration where the centers of the detectors I and II are displaced from the lateral axis  48  so that the image arrows  2  do not point toward the lateral axis is depicted. SPECT algorithms for correcting for such displacements are known in the art. 
     Alternatively, as depicted in FIG. 11, detector II is oriented laterally to the body  10  of the patient and has a narrower cross-section and field of view. The smaller cross-section of detector II facilitates closer positioning of the collimator to the body of the patient. 
     In another embodiment of the invention, depicted in FIG. 12, a table  200  holding the patient is displaced vertically and horizontally from the lateral axis  48  so that the body  10  of the patient touches the detectors I and II. 
     FIGS. 13A and B depict mechanisms for imparting horizontal motion and vertical motion of the table  200  relative to the lateral axis  48 . In FIG. 13A, a view taken parallel to the lateral axis  48 , a horizontal drive motor  202  imparts rotary motion to an axle  204 , supported by bearings  205 , through bevel gear  206 . Horizontal motion of the table  200  is effected by movement along gear racks  208 , oriented perpendicularly to the lateral axis  48 , through rotational motion imparted to gears  210  engaged to gear racks  208  by axle  204 . 
     In FIG. 13 b , a view taken perpendicular to the lateral axis  48 , a vertical drive motor  212  imparts rotational motion to a lead screw  214  through a drive mechanism  216 . The threads of the lead screw  214  are engaged to threads of a telescope tube  219  to impart vertical motion to the telescope tube and table  200  when the vertical drive motor  212  is activated. 
     FIG. 14 depicts a positional feedback device for indicating the positions of the detectors. In FIG. 14, a sensor gear  250  engages a ring gear  32  and has a sprocket  252  coupled to a chain  254 . The chain engages sprockets  256  and  258  coupled to a potentiometer  260  and an encoder  262 . 
     In operation, the potentiometer  260  is used for coarsely indicating position and the encoder  262  for finely indicating position. For example, the sprockets can be sized so that for each revolution of the ring gear  32  the potentiometer  260  makes 10 turns varying the resistance from 0 to 1,000 ohms. If power is lost the potentiometer  260  will not loose its position. 
     Similar devices are utilized to indicate the radial position of the detectors and the vertical and horizontal displacement of the table  200 . 
     An improved method for imaging that utilizes the movable table  200  will now be described. The table is moved up and down or left and right using microprocessor control and the positional feedback device enables the microprocessor to calculate the position of the table. 
     First, the motion limits of the detectors and table are defined. The operator moves the detectors to have the desired relative angular displacement (e.g., 90°). The table holding the patient is positioned on the lateral axis. The operator then moves the detectors into the desired position relative to the patient (e.g. anterior and lateral). The operator then moves the table so that the body of the patient touches the lateral detector and the microprocessor stores the x-location. The operator the moves the table so that the body of the patient touches the anterior detector and the microprocessor stores the y-location. The microprocessor then calculates the required table motion based on the size of the detectors, the number of angular stops required, and x and y locations determined above. 
     Once the motion limits are defined image data is acquired. The table is moved to a location to allow motion of the detectors and the detectors are moved to the first angular stop. The table is then moved to the starting position for the first angular stop and data is acquired. The positions of the table and the detectors are stored. The procedure is repeated until data is acquired for all the required angular stops. The stored location data is utilized to generate an image from the acquired data. 
     The invention has now been described with reference to the preferred embodiments. Alternatives and substitutions will now be apparent to persons of ordinary skill in the art. For example, if detectors I and II were to be maintained at a fixed angle, e.g., 120° or 90°, then both detectors and their radial drive mechanisms could be attached to the detector I ring gears  32  and  34 . Accordingly, it is not intended to limit the invention except as provided by the appended claims.