Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/16—Measuring radiation intensity
  • G01T1/161—Applications in the field of nuclear medicine, e.g. in vivo counting
  • G01T1/164—Scintigraphy
  • G01T1/1641—Static instruments for imaging the distribution of radioactivity in one or two dimensions using one or several scintillating elements; Radio-isotope cameras
  • G01T1/1642—Static instruments for imaging the distribution of radioactivity in one or two dimensions using one or several scintillating elements; Radio-isotope cameras using a scintillation crystal and position sensing photodetector arrays, e.g. ANGER cameras
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/16—Measuring radiation intensity
  • G01T1/1603—Measuring radiation intensity with a combination of at least two different types of detector
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
  • A61B6/02—Devices for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
  • A61B6/03—Computerised tomographs
  • A61B6/037—Emission tomography

Definitions

• the present invention relates to the diagnostic imaging systems and methods. It finds particular application in conjunction with the Positron Emission Tomography (PET) and Single Photon Emission Tomography (SPECT) systems and will be described with particular reference thereto. It will be appreciated that the invention is also applicable to other like applications and diagnostic imaging modes.
• Nuclear imaging employs a source of radioactivity to image the anatomy of a patient. Typically, a radiopharmaceutical is injected into the patient. Radiopharmaceutical compounds contain a radioisotope that undergoes gamma-ray decay at a predictable rate and characteristic energy. A radiation detector is placed adjacent to the patient to monitor and record emitted radiation.
• the detector is rotated or indexed around the patient to monitor the emitted radiation from a plurality of directions. Based on information such as detected position and energy, the radiopharmaceutical distribution in the body is determined and an image of the distribution is reconstructed to study the circulatory system, radiopharmaceutical uptake in selected organs or tissue, and the like.
• the detector In a traditional scintillation detector, the detector has a scintillator made up of a large scintillation crystal or matrix of smaller scintillation crystals. In either case, the scintillator is viewed by a matrix of sensors. A commonly employed sensor is a photomultiplier tube ("PMT").
• a collimator which includes a grid- or honeycomb- like array of radiation absorbent material may be located between the scintillator and the subject being examined to limit the angle of acceptance of radiation which impinges on the scintillator.
• Each radiation event impinging on the scintillator generates a corresponding flash of light (scintillation) that is seen by the PMTs.
• the event is primarily seen by the closest PMT and the six PMTs that surround it.
• An individual PMT's proximity to the flash's origin affects the degree to which the light is seen by that PMT.
• Each PMT that sees an event generates a corresponding electrical pulse.
• the respective amplitudes of the electrical pulses are generally proportional to the distance of each PMT from the flash.
• the gamma camera maps radiation events, i.e., it determines the energy and position of radiation rays impinging the scintillator.
• the event position is determined using a conventional Anger method for event positioning, which sums and weights signals output by PMTs after the occurrence of an event.
• the Anger method for event positioning is based on a simple first moment calculation. The energy is typically measured as the sum of all the PMT signals, and the position is typically measured as the "center of mass" or centroid of all the PMT signals.
• the current real-time positioning algorithms are not as accurate as known iterative methods.
• the iterative methods are computationally intensive to support real-time image generation and processing. There is a need for the method and apparatus that would permit to use the iterative techniques to determine a more accurate position of the event.
• the present invention provides a new and improved imaging apparatus and method which overcomes the above-referenced problems and others.
• a diagnostic imaging system is disclosed.
• a matrix of sensors is situated to view an event, the sensors having respective outputs that are responsive to the event.
• Each of the sensors is connected to an individual analog-to-digital converter for converting output analog values of associated sensors to digital numbers.
• a means identifies a high sensor in the matrix which, in response to the event, has a highest output value relative to the other sensors.
• a means identifies a number of outer sensors in the matrix that are closest neighbors to the high sensor.
• a means compresses outputs of the outer sensors to reduce a number of bits of the respective outputs.
• a lookup table is addressed by the compressed outputs to retrieve a corresponding event location.
• a method of diagnostic imaging is disclosed.
• Radiation is detected with a matrix of sensors of gamma cameras, which sensors are situated to view an event and having respective outputs that are responsive to the event.
• Output analog values of associated sensors are converted to digital numbers with analog-to-digital converters, each of the sensors being connected to an individual analog-to-digital converter.
• a high sensor in the matrix is identified, which high sensor, in response to the event, has a highest output value relative to the other sensors.
• a number of outer sensors in the matrix that are closest neighbors to the high sensor is identified.
• Outputs of the outer sensors are compressed to reduce a number of bits of the respective outputs.
• a lookup table is addressed by the compressed outputs to retrieve a corresponding event location.
• One advantage of the present invention resides in more accurate estimate of the event position through the use of iterative algorithms. Another advantage resides in performing event discrimination / elimination for events that cannot be properly positioned. Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.
• FIGURE 1 is a diagrammatic illustration of a diagnostic imaging system
• FIGURE 2 is a diagrammatic illustration of a portion of a diagnostic imaging system
• FIGURE 3 is a portion of a software program in accordance with the present invention
• FIGURE 4 is a diagrammatic illustration of one encoding scheme in accordance with the present invention
• FIGURE 5 is a diagrammatic illustration of a 4-tube cluster employing horizontal symmetry
• FIGURE 6 is a diagrammatic illustration of another encoding scheme in accordance with the present invention
• FIGURE 7 is a diagrammatic illustration of a 7-fube cluster, in which an event occurred close to the outer tubes
• FIGURE 8 is a diagrammatic illustration of another encoding scheme in accordance with the present invention
• FIGURE 9 is a diagrammatic illustration of a 7-tube cluster in which an event occurred close to the center of the high
• a PET scanner 10 includes a plurality of detectors heads or gamma cameras or detectors 12 facing and, preferably, mounted for movement around a subject 14 (preferably containing a radionuclide distribution) located in an examination region 16.
• Each of the detectors 12 includes a scintillator 20 that converts a radiation event (e.g., a ray of radiation from the radionuclide distribution that impinges on the scintillator 20) into a flash of light or scintillation.
• a matrix of sensors such as photomultiplier tubes (PMT) 22 is situated to view or receive the light flashes from the scintillator 20.
• the matrix of sensors 22 is a close hexagonal packed arrangement of PMTs.
• the radiation produces gamma quanta that arise in the disintegration of radio isotopes.
• the disintegration quanta strike the scintillator 20, which preferably includes doped sodium iodide (Nal), causing a scintillation.
• Light from the scintillation is distributed over a number of the sensors 22. The amount of light from a particular scintillation that a given sensor 22 sees or receives tends to progressively diminish with the sensor's distance from the event.
• Each of the sensors 22 generates a respective output signal, e.g., an analog electrical pulse, in response to a received light flash, the output signal being proportional to that of the received light flash.
• each of the sensors 22 is electrically connected to an analog-to-digital (A/D) converter 24 that converts the respective analog outputs to digital signals.
• A/D analog-to-digital
• the information is conveyed to a processor 28 which measures or otherwise determines the location (x, y) and/or energy (z) of respective scintillation events that occur relative to each detector head.
• the output value of each sensor 22 is optionally determined by: integration of the sensor's output signal response to the event (i.e., finding the area or some portion thereof under a curve plotting amplitude or intensity vs.
• the scanner 10 is selectively operable as desired in either a SPECT mode or a PET mode.
• the cameras 12 have collimators attached thereto (not shown) which limit acceptance of radiation to particular directions, i.e., along known rays.
• the determined location on the scintillator 20 at which radiation is detected and the angular position of the camera 12 define the ray along which each radiation event occurred.
• These ray trajectories and camera angular positions (e.g., obtained from an angular position resolver 30) are conveyed to a reconstruction processor 32 which backprojects or otherwise reconstructs the rays into a volumetric image representation which is stored in an image memory 34.
• the collimators are removed.
• the location of a single scintillation event does not define a ray.
• the radionuclides used in PET scanning undergo an annihilation event in which two photons of radiation are emitted simultaneously in diametrically opposed directions, i.e., 180 degrees apart.
• a coincidence detector 36 detects when scintillations on two cameras 12 occur simultaneously. The locations of the two simultaneous scintillations define the end points of a ray through the annihilation event.
• a ray or trajectory calculator 38 calculates the corresponding ray through the subject 14 from each pair of simultaneously received scintillation events. The ray trajectories from the ray calculator 38 are conveyed to the reconstruction processor 32 for reconstruction into a volumetric image representation. The resultant image representation is stored in the volumetric image memory 34.
• a video processor 40 processes and/or formats the image representation data for display on a monitor 42.
• the processor 28 includes a lookup table 44 which is generated to perform real-time event positioning using iterative algorithms.
• the PMTs 22 are calibrated by one of known isotopes.
• the location of the center of the high tube 50 is known precisely from a priori information, e.g., physical measurement.
• the calibration information for each tube 22 is stored in the lookup table 44.
• the lookup table 44 is used to identify a location of an event relative to the center of the high tube.
• the processor 28 includes a PMT value determining means 46 which determines initial values of the PMTs 22.
• a highest output determining means or algorithm 48 identifies a high tube 50 having the highest output value relative to all the other PMTs 22, and a number of closest neighboring tubes or outer tubes 52 adjacent the high tube 50, e.g. a PMT subset or a PMT cluster 54 as seen in FIGURE 5.
• the maximum number of closest neighboring tubes 52 is six, defining a 7-PMT hexagonal, close-packed cluster, with the center sensor being the one having the highest output value.
• the cluster 54 may include a different number of tubes, e.g. three, or five.
• a fractions determining means 56 divides outputs of the tubes 50, 52 of the cluster 54 by the value of the high tube 50, accordingly eliminating one of the variables, e.g. the value of the high tube 50.
• the operation results in three to six 10 bit fractions F1-F6 depending on the number of the outer tubes 52 in the cluster 54.
• a fractions order determining means 58 identifies a descending order of the fractions F1-F6 of the tubes 52, e.g. the highest fraction Fl, the second highest fraction F2, and lower fractions F3-F6.
• the fractions FIFO are stored in a PMT values memory 60.
• the fractions F1-F6 identify the location of the event relative to the high tube center and could be used to address the lookup table 44.
• each analog-to-digital converter 24 outputs a 10 bit intensity value.
• the lookup table 44 would have 10 21 memory addresses.
• a compression means 70 dynamically applies a compression algorithm to reduce the number of bits for addressing the lookup table 44, e.g. the 10 bit fractions Fl- F6 are compressed to the space available in the lookup table 44 based on various criteria as discussed in a greater detail below.
• a dynamic compression is preferably applied, which compresses low fractions to a greater degree than high fractions.
• symmetry is used to eliminate the need to encode the event spatial location(s) of the next highest tube(s) relative to the highest tube 50.
• an encoding selection means 72 dynamically selects an encoding scheme for each event based upon the number of address bits available for the lookup table 44, the number of tubes 50, 52 in the cluster 54, the magnitude of the highest fraction Fl, and other criteria.
• the tubes 52 carrying the most information are compressed the least, while those carrying the least information are compressed the most.
• a non-linear compression e.g., a weighted square-root function
• the tube fractions F1-F6 is used to reduce the number of bits required for encoding each fraction.
• an address bit is used to indicate when biasing of a signal has been performed (e.g., a fixed value may be subtracted from the highest fraction) to reduce the dynamic range that the values must be compressed to minimize quantization errors.
• an address generating means 74 generates an address of an event. The resulting event address accesses the lookup table 44 to retrieve the event location relative to the highest tube center.
• a position determining means 80 retrieves the 16 bit address that contains the event's x, y position and breaks it into distinct 8 bit x- and y- values.
• the x, y position of the center of the high tube location is adjusted with the lookup table output by a position adjusting means 82 to derive the event's final position.
• the high tube 50 is disposed within a 4- tube cluster 54, i.e. the high tube 50 has three neighboring or outer tubes 52.
• a horizontal symmetry along a horizontal axis is used to reduce the number of cases by a factor of 2.
• the 4-tube cluster 54 is interpreted as if the outer tubes 52 are positioned on the top of the detector.
• a flag is set to indicate that the highest fraction Fl always exists in the top quadrant.
• a flipping means 84 flips the event location from the lookup about an axis of symmetry in accordance with the quadrant in which the highest fraction Fl is located. For example, the sign of the x-location adjustment is selected based on the location of the highest fraction Fl relative to the y-axis and the sign of the y-correction is adjusted based on location relative to the x-axis.
• the position determining means 80 performs the lookup at the encoded address and retrieves x, y position relative to the center of the high tube 50. If a flip is not performed, the position adjusting means 82 calculates the final x, y position as equal to the high tube 50 center location plus the relative x, y position.
• the position adjusting means 82 calculates the final x, y position as equal to the high tube 50 center location minus the relative x, y position.
• the encoding scheme selecting means 72 dynamically selects the encoding scheme 6 to create the lookup table 44.
• three address bits e.g. bits 25-27, are used to indicate how the tube fractions F1-F3 have been encoded and compressed.
• each fraction Fl, F2, F3 is converted to 8 bits (256 codes) using the square-root function (A):
• F comp (int) ( sqrt( k * F ) + 0.5 ), where F comp is the weighted square root compression applied to the corresponding tube fraction F1-F6;
• each fraction F1-F6 encoded with the encoding scheme 6 is achieved by using the function:
• the high tube 50 is disposed within a 7- tube cluster 54, i.e. the high tube 50 has six neighboring or outer tubes 52. Similar to the embodiment of FIGURE 5, the horizontal symmetry is used such that only three-highest fractions positions are needed.
• the flipping means 84 flips the tube signals if the highest- fraction Fl is one of the bottom tubes.
• the encoding scheme selecting means 72 selects encoding scheme 0, 1, or 2 when the cluster 54 has seven tubes and the highest fraction Fl is more than 255, e.g. the event occurs away from the high tube 50 center, approaching one or more of the neighboring tubes 52 which indicates a PMT double point d or triple point t.
• step 94 the encoding scheme selecting means 72 discards the event when it is determined that the highest fraction Fl and the second highest F2 are not adjacent each other.
• step 96 the encoding scheme selecting means 72 discards the event when it is determined that one of the lower fractions F3-F6 is greater than 255, which indicates that the event is contaminated.
• bits 25-27 are used to indicate how the tube fractions have been encoded and compressed.
• the encoding ID is set using the tube diagram of FIGURE 7 that indicates the position of the tube having the highest fraction Fl .
• bit 24 is ordinarily set to CCW, e.g. the second highest fraction F2 is counter-clockwise with respect to the highest fraction Fl.
• Bit 24 is cleared if the second highest fraction F2 is clockwise with respect to the highest fraction Fl. With continuing reference to FIGURE 6, bits 17-22 are designated for the highest fraction Fl. A compression is performed by subtracting 255 (OxFF) from the highest fraction Fl and converting the result to 6 bits using the square-root function (B)
• bits 12-16 are designated for the second highest fraction F2.
• the encoding scheme selecting means 72 selects the encoding scheme based on the value of the second highest fraction F2. If the second highest fraction F2 is greater than 200 (0xC8), the compression is performed by subtracting 201 (0xC9) from the value of the second highest fraction F2 and compressing the resultant value to 5 bits (32 codes) using the square-root function (C) (input range is 0 to 822 [0x336]). Bit 23 is set to 1 to indicate that the subtraction has been performed.
• F2 comp is the weighted square root compression applied to the second highest fraction F2;
• ft is a constant scalar used to compress the tube fraction,
• k (Code max ) 2 / F2 max ;
• the decoding or uncompression of fraction F2 encoded with the encoding function (D) is achieved by using the function:
• bits 0-11 are designated for the lower fractions F3-F6, e.g. 3 rd through 6 th fractions.
• the lower fractions F3-F6 are converted each to 3 bits (8 codes) using the square-root function E. It is assumed that a maximum value of fraction F3-F6 is equal to 255 (OxFF):
• the encoding scheme selecting means 72 selects encoding scheme 3 when the cluster 54 has seven tubes 50, 52 and the highest fraction Fl is more than 200 and is equal to or less than 255.
• Fractions F1-F6 are converted each to 4 bits (16 codes) using the square-root function G. It is assumed that a maximum fraction Fl is equal to 255 (OxFF):
• the encoding scheme selecting means 72 selects encoding scheme 4 when the cluster 54 has seven tubes 50, 52 and the highest fraction Fl is more than 160 and is equal to or less than 200, e.g. the event occurred near the center of the high tube 50.
• Fractions F1-F6 are converted to 4 bits (16 codes) using the square-root function H. It is assumed that a maximum fraction Fl is equal to 200 (0xC8):
• step 102 the encoding scheme selecting means 72 selects encoding scheme 5 when the cluster 54 has seven tubes 50, 52 and the highest fraction Fl is equal to or less than 160.
• Fractions F1-F6 are converted each to 4 bits (16 codes) using the square-root function J. It is assumed that a maximum value of fractions F1-F6 be equal to 160 (OxAO):
• F COmP is the weighted square root compression applied to the corresponding tube fraction F1-F6;
• ft is a constant scalar used to compress the tube fraction,
• k (Code max ) 2 / F max ;

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• 2005
  • 2005-01-05 WO PCT/IB2005/050059 patent/WO2005071438A1/en active Application Filing
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  • 2005-01-05 JP JP2006548515A patent/JP2007518096A/ja active Pending
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