Adjustable dual-detector image data acquisition system

Abstract: An imaging system for acquiring imaging data generated by an object positioned about a lateral axis (48) is described. The system comprises first and second detectors (4) which can be rotated about the object in a circular path, with the angular displacement between the two detectors around the circular path being adjustable to a selected magnitude. Preferably the detectors can also be moved toward or away from the lateral axis to increase the resolution of the system. A method for acquiring image data utilizing such a system is also described.

Description: The present invention relates generally to imaging systems and more particularly to imaging systems for use in nuclear medicine. Gamma ray cameras are used in nuclear medicine to generate high quality images for brain, SPECT ( S ingle P hoton E mission C omputer T omograph), 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 high patient throughput for both economic and therapeutic reasons. The cost for diagnosing each patient is reduced if more 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. Patient throughput is increased if imaging time is reduced. Other factors, such a 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 for 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 are considered. The acquisition of data for a total body scan requires movement of the detector along the length of the patient'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 1/2 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 1/3 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'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 2/3 the time of a single-head system for a 180° scan because two detectors are within the 180° arc about 1/3 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. GB-A-2120060 describes a prior art radiological examination system. The preambles to the independent claims are based on that document. 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, there is provided an imaging system for acquiring imaging data generated by an object positioned about a lateral axis, said system comprising: first and second detectors, said first detector pointing in a first direction and said second detector pointing in a second direction; means, coupled to said first and second detectors, for rotating said first and second detectors in a predetermined circular path having said lateral axis approximately at its centre and in a plane oriented substantially perpendicular to the lateral axis, with said first and second directions lying in said plane; and means, coupled to said means for rotating and to said first and second detectors, for adjusting the angular displacement, relative to the lateral axis and along the predetermined circular path, between said first and second detectors to a selected magnitude while maintaining said first and second directions in said plane, characterised in that the imaging system further comprises: means, responsive to said means for rotating, for changing the vertical and horizontal position of the object, relative to the detectors and the lateral axis and without displacing the object from said plane, when the detectors are rotated to decrease the distance between the object and the detectors to improve resolution. Thus the first and second detectors can be positioned with a relative angle of 90° to reduce the image time for a 180° scan by a factor of two over a two detector system having the detectors positioned at a fixed relative angle of 180°. Furthermore, the angular displacement between two detectors may be adjusted to any angle between 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 a further embodiment of the invention, each detector may be independently moved toward or away from the lateral axis whereby they can be independently rotated along different circular paths centered on the lateral axis. According to another embodiment 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 embodiment 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 another embodiment 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. Viewed from another aspect, the invention provides a method for acquiring image data that utilizes a camera including first and second detectors, the first detector pointing in a first direction and the second detector pointing in a second direction, with the detectors rotatably mounted on a mechanism for supporting the detectors where the mechanism rotates about a lateral axis and in a plane substantially perpendicular to the lateral axis, said method comprising the steps of: orienting said first and second directions to a predetermined angular separation; positioning the oriented detectors at a first angular stop relative to the lateral axis; and repositioning the oriented detectors at a plurality of angular stops to acquire additional image data; characterised by further comprising the steps of: positioning a patient substantially parallel to the lateral axis and substantially touching the oriented detectors positioned at the first angular stop to determine the size of the patient; and changing the vertical and horizontal position of the patient relative to the repositioned detectors and the lateral axis, according to the previously determined size of the patient, to maintain an object to be imaged in said plane and to decrease the distance between the body of the patient and the oriented detectors as the detectors are repositioned to acquire image data to facilitate creating a high-resolution image. A number of preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: 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-2B 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. 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 10 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 1/2 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 30L and 30R, 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 45L and idler gear 46L 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 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 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 B-B 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 130R and idler gear 131R. 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 130L and 131L (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 30L and 30R 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 45R and idler gear 46R (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 30L and 30R of the gantry 30. Fig. 6 is a cross-sectional view, taken along C-C 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 30L and 30R 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 44R 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 engaged. In operation, both detectors rotate together when both clutches 177 and 178 are engaged 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 disengaged. As described above, high patient throughput requires that detectors having a wide field of view be utilized. However, when the detector image direction arrows 2 are oriented at 90°, to efficiently perform a 180° scan, the physical size of the detectors 4 limits their radial motion. Referring to Fig. 8, the detector edges will touch when the radius Rmin is reached. Thus the detectors 4 are not able to touch the body 10 of the patient which is necessary to achieve high resolution. Also, each detector 4 has a lateral shielding section 182 to prevent external gamma rays from reaching the scintillation medium. 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 4 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 2/3 of the imaging time of a single-head system because both detectors 4 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 the place the collimator 184 closer to the body 10 of the patient. In Fig. 10A, a configuration where the centers of the detectors 4 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 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 4. 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. 13b, 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.

Claims: An imaging system for acquiring imaging data generated by an object (10) positioned about a lateral axis (48), said system comprising: first and second detectors (4), said first detector pointing in a first direction and said second detector pointing in a second direction; means (32,34,36,38), coupled to said first and second detectors, for rotating said first and second detectors in a predetermined circular path having said lateral axis at its centre and in a plane oriented substantially perpendicular to the lateral axis, with said first and second directions lying in said plane; and means (45,46), coupled to said means for rotating and to said first and second detectors, for adjusting the angular displacement, relative to the lateral axis and along the predetermined circular path, between said first and second detectors to a selected magnitude while maintaining said first and second directions in said plane, characterised in that the imaging system further comprises: means (40,42,200), responsive to said means for rotating, for changing the vertical and horizontal position of the object (10), relative to the detectors (4) and the lateral axis (48) and without displacing the object from said plane, when the detectors are rotated to decrease the distance between the object and the detectors to improve resolution. The system of claim 1 further comprising: means (40), coupled to said means for rotating said first detector and coupled to said first detector, for displacing said first detector toward or away from the lateral axis; and means (42), coupled to said means for rotating said second detector and coupled to said second detector, for displacing said second detector toward or away from the lateral axis. The imaging system of claim 1 or 2 further comprising: extended collimators (184) coupled to said first and second detectors and oriented along said first and second directions for increasing the resolution of the system. The imaging system of claim 1, 2 or 3 wherein said second detector is narrower than the first detector to facilitate closer positioning of the detectors to a patient thereby increasing resolution. A method for acquiring image data that utilizes a camera including first and second detectors (4), the first detector pointing in a first direction and the second detector pointing in a second direction, with the detectors rotatably mounted on a mechanism for supporting the detectors where the mechanism rotates about a lateral axis (48) and in a plane substantially perpendicular to the lateral axis, said method comprising the steps of: orienting said first and second directions to a predetermined angular separation; positioning the oriented detectors at a first angular stop relative to the lateral axis; and repositioning the oriented detectors at a plurality of angular stops to acquire additional image data; characterised by further comprising the steps of: positioning a patient (10) substantially parallel to the lateral axis and substantially touching the oriented detectors positioned at the first angular stop to determine the size of the patient; and changing the vertical and horizontal position of the patient relative to the repositioned detectors and the lateral axis, according to the previously determined size of the patient, to maintain an object to be imaged in said plane and to decrease the distance between the body of the patient and the oriented detectors as the detectors are repositioned to acquire image data to facilitate creating a high-resolution image. A method as claimed in claim 5 wherein the predetermined angular separation is substantially 90°.

Inventors: HINES HORACE, HUG PAUL, LAMP MARK L, HINES, HORACE, HUG, PAUL, LAMP, MARK L.
Applicants: ADAC LAB, ADAC LABORATORIES
Application year: 1992
Main classification: G01T1
IPC codes: G01T1, A61B6