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
  • G01T1/1648—Ancillary equipment for scintillation cameras, e.g. reference markers, devices for removing motion artifacts, calibration devices
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
  • A61B6/42—Arrangements for detecting radiation specially adapted for radiation diagnosis
  • A61B6/4291—Arrangements for detecting radiation specially adapted for radiation diagnosis the detector being combined with a grid or grating
• • 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
  • G01T1/1615—Applications in the field of nuclear medicine, e.g. in vivo counting using both transmission and emission sources simultaneously
• • 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
  • G01T1/1644—Static instruments for imaging the distribution of radioactivity in one or two dimensions using one or several scintillating elements; Radio-isotope cameras using an array of optically separate scintillation elements permitting direct location of scintillations
• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21K—HANDLING OF PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
  • G21K1/00—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating
  • G21K1/02—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diaphragms, collimators
  • G21K1/025—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diaphragms, collimators using multiple collimators, e.g. Bucky screens; other devices for eliminating undesired or dispersed radiation
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
  • A61B6/02—Arrangements for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
  • A61B6/03—Computed tomography [CT]
  • A61B6/037—Emission tomography

Definitions

• the present application relates to the production of images with a nuclear camera. It finds particular application in conjunction with a fan beam collimator having a slit, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
• Nuclear imaging techniques generally obtain images in one of two different ways.
• Emission images are generated by introducing a radioactive isotope to the subject and collecting radiation coming from within the subject with a detector sensitive to such radiation (camera).
• Such emission images include Single Photon Emission Computed Tomography (SPECT) images and are often used to provide functional information regarding the subject, such as a tumor within a patient.
• SPECT Single Photon Emission Computed Tomography
• Transmission images are generated by positioning the subject intermediate a radiation source, such as a source of gamma radiation, and the detector so that the radiation that passes through the subject is detected with the camera. Transmission images provide information about the distribution of radiation-attenuating or radiation-absorbing matter in the subject.
• a radiation source such as a source of gamma radiation
• Transmission images and emission images are often performed simultaneously, for example, for correction of the emission image for attenuation of the radiation in the object.
• a SPECT imaging device is sometimes used to generate both emission and transmission images.
• Photons generated inside and transmitted through the subject are detected by the detector, such as an NaI crystal and an array of photomultipliers.
• a collimator is placed before the camera.
• a predetermined spatial geometry of the radiation source also determines the trajectory of the transmission radiation events.
• the collimator which includes a grid or honeycomb-like array of radiation absorbent material, is located between the detector and the subject being examined to limit the angle of acceptance of radiation which impinges on the detector. Resolution and efficiency are defined by the shape of the collimator and the height and thickness of septa defining grid, with thicker septa generally being used for higher energy ranges.
• Static detectors i.e., those which maintain the same general orientation towards the radiation source
• the parallel hole detector includes a collimator with parallel slats in a first direction which intersect parallel slats in a second direction.
• the collimator is used in combination with a planar source for transmission measurements.
• the efficiency of such a collimator is relatively low because the planar radiation source radiates radiation in all directions within a particular solid angle, but only a very limited portion is utilized in making the transmission image, namely, only the portion that is directed in the direction of the passages of the collimator. As a consequence, a relatively strong source is used for making a transmission image with a predetermined brightness. Improvements in efficiency over the parallel hole detector may be achieved by focused collimators in which the slats are oriented towards a focus which is at the same side of the collimator as the object to be measured.
• a fan beam collimator includes slats which are focused in one direction and which are parallel in the other direction.
• the fan beam collimator is used with a line radiation source for transmission measurements which is arranged along the focal line.
• a cone beam collimator includes slats which are focused in both directions and may be used with a point radiation source at the focal point for transmission measurement.
• the use of a line source together with a fan beam collimator or a point source with a cone beam collimator provides an advantage over the combination of a planar source and a parallel collimator in that the radiation produced is better utilized, and hence the amount of radioactive matter of the radiation source can be less.
• the emission and transmission recordings are improved because converging collimators, due to the magnifying effect, count more photons than do parallel collimators.
• the improvements which can be achieved by such focused collimators are limited, especially for higher energy photons which require thick septa and cause partial shadowing of the pixels.
• a radiation imaging device includes a detector.
• a collimator is positioned adjacent a radiation receiving face of the detector.
• the collimator includes a plurality of slats having a common focus.
• a body, adjacent the slats, defines an elongate slit.
• the slit is arranged such that radiation passes through the slit and between the slats to the detector.
• the body is at least substantially impermeable to the radiation.
• an imaging method includes directing radiation from a source towards a detector and interposing a slit and a plurality of spaced slats having a common focus between the radiation source and the detector, whereby radiation passes through the slit and between the slats onto the detector.
• a detection system in accordance with another aspect, includes a detector which defines a plurality of pixels.
• a plurality of spaced slats have a common focus and define passages therebetween, each of the passages extending adjacent a row of the pixels.
• a plate covers the slats and defines a slit.
• the slit has a width and a length perpendicular to the width. The length is greater than the width and greater than a spacing between the slats, whereby radiation entering the slit travels through the passages to the detector pixels.
• the invention may take form in various components and arrangements of components, and in various steps and arrangements of steps.
• the drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention.
• FIGURE 1 is a perspective view of a nuclear imaging system according to the present embodiment
• FIGURE 2 is a perspective view of a first embodiment of a detection system for the nuclear imaging system of FIGURE 1;
• FIGURE 3 is a side perspective view of the detection system of FIGURE 2;
• FIGURE 4 is a schematic perspective view of a detector pixel demonstrating the efficiency of a collimator;
• FIGURE 5 is a is a schematic perspective view of a detector pixel demonstrating the spatial resolution of a collimator
• FIGURE 6 is a top plan view of a second embodiment of a detection system for the nuclear imaging system of FIGURE 1;
• FIGURE 7 is a top plan view of a third embodiment of a detection system for the nuclear imaging system of FIGURE 1.
• the imaging system includes an imaging region 10 in which a subject, such as a patient 12 lies or is transported therethrough on a moveable support 14.
• a subject such as a patient 12 lies or is transported therethrough on a moveable support 14.
• One or more radiation detection systems or heads 16 are positioned adjacent to the patient to monitor and record transmitted and/or emitted radiation events.
• a source of transmission radiation 18, such as a gamma radiation source, is positioned such that radiation emitted by the source enters the subject and received in the form of photons by an opposing detection system 16.
• a radiopharmaceutical introduced to the subject 12 serves as a source of radiation for SPECT imaging.
• the emission and transmission radiation have different energies to facilitate differentiating them.
• the detection system includes a collimator 20 and a detector 22.
• the collimator is positioned intermediate the subject 12 and the detector 22 to limit the angle of acceptance of radiation which impinges on the detector.
• the detector 22 is linked to a processing system 24, which may be embodied in an operator work station, computer network, or other suitable hardware/software.
• the processing system 24 reconstructs an image of the subject, based on signals from the detector, which is displayed by a display 26, such as a screen or printout.
• the detector 22 includes a sensor, which detects radiation (typically photons) generated as a result of the interaction of the radiation with the subject.
• One suitable sensor includes a scintillator, such as a single crystal, such as a sodium iodide crystal, or a matrix of smaller crystals.
• the crystal is positioned adjacent a matrix of photomultiplier tubes ("PMTs").
• PMTs photomultiplier tubes
• Each radiation event impinging on the scintillator generates a corresponding flash of light (scintillation) that is seen by the PMTs.
• radiation events are mapped, which include the energy and position of radiation rays impinging the scintillator.
• Other detectors include a matrix of scintillation crystals, i.e. a pixelated detector, which are mated with photodiodes or avalanche detectors in place of photomultiplier tubes.
• a cadmium zinc telluride (CZT) or other direct conversion detector which converts radiation photons directly to electrons (current) without a scintillator.
• CZT cadmium zinc telluride
• the image quality of the SPECT images is typically determined by a count sensitivity of the detector and the geometry of the collimator.
• the collimator 20 is formed of a dense radiation absorbing material, such as tungsten.
• the collimator 20 can be regarded as a transmission means with a direction-selective transmission characteristic, which ensures that a detection segment of the detection surface of the detector can only be irradiated by radiation with a predetermined limited range of directions.
• the presently illustrated collimator can be described as a fan beam-slit collimator.
• the fan beam portion of the collimator is provided by a plurality of closely spaced slats or septa 30 (five are illustrated but the number is typically much larger).
• the slats 30 are planar.
• a generally planar radiation receiving face or detection surface 32 of the detector 22 lies in a plane defined by y and x axes and the collimator slats 30 extend away from the plane generally in a z direction.
• the slats 30 extend from the planar surface 32 of the detector towards the radiation source 18 and converge towards a focal line 34.
• the source 18 comprises a line source which is collinear with the focal line 34 of the slats for optimal efficiency, although it is also contemplated that the source may be spaced further from, or closer to the detector 22.
• the outer slats are oriented at a shallower angle ⁇ to the plane of the detector than the inner slats, the angle ⁇ increasing towards the center, where the center slat is oriented at, 90° to the plane x, y.
• the slats 30 define a plurality of parallel passages 36 therebetween. As a result of the focused slats, a centerline of each passage intersects the line-shaped radiation source 18.
• the passages 36 extend the full width B of the detector plane in the x direction, i.e., there are no intersecting slats as in a conventional fan beam or cone-shaped collimator.
• Ends 40 of the slats closest to the detector 22 are equally spaced, adjacent the detector in the y direction, with a pitch P.
• the ends 40 of the slats are spaced to coincide with the interface between adjacent rows of elements of a pixelated detector. Due to the thickness of the slats t, the distance between adjacent slats D —P-t.
• the slats have a height L in the z direction and a width B in the x direction, which is greater than the height L.
• the pitch P y is about 1-3 mm
• the distance between adjacent slats D is about 0.80 to about 2.95 mm
• the thickness of the slats t is about 0.05 mm to about 0.5 mm, e.g., about 0.15 mm
• the height L is from about 10 mm to about 100mm, and in one specific embodiment, L is less than about 60 mm.
• the slit portion of the collimator is defined by a body generally in the form of a plate 44 which lies in a plane parallel to the plane 36 of the detector 22.
• the plate is formed of a radiation opaque material, such as a radiation impermeable, or substantially impermeable material.
• the plate defines an elongate slit 46 which extends a length G of the detector in the y direction, which is greater than the distance P y between slats.
• the slit has a width w in the x direction. Length G is substantially greater than the width w. In one embodiment, w is from about 0.3 mm to about 3 mm, the narrower the slit, the higher the resolution.
• the plate 44 has a thickness h in the z direction, which is the height of the slit 46, of from about 0.5mm to about 5mm, the higher thickness being more suited to higher energies.
• the plate 44 is located at height L above the plane 32, i.e., in contact with upper ends 48 of the slats 30 although it is also contemplated that the plate may be spaced from the plane by a distance somewhat greater than L, but generally less than IL.
• the focus 34 is located a distance /from the plane 44.
• Side walls 50 of the slit may be aligned with the z axis, as shown, or angled to the z axis, for example in a V shape or an inverted V shape.
• the illustrated detector 22 is pixellated. As shown in FIGURE 2, a row of pixels 52 extends between each pair of adjacent slats 30. The pixels are configured for independently sending signals to the processing system 24.
• the pixels each have a pixel width or pitch P x in the x direction and a length D in the y direction.
• P x is from about 0.5 mm to about 2.5 mm. Seven pixels 50 are illustrated in each row/passage, although a much greater number of pixels is typically employed.
• the pixel pitch P x is greater than or equal to half the slit width w but less than the interval between slits.
• the field of view (FOV) 62 is the area within the plane.
• the area of the projected image on the detector, which in the embodiment of FIGURE 2 is defined by the area of the plane 32, is smaller in the x direction than the object plane.
• each pixel 52 in the detector has an area P x x D, which is smaller in the x direction and larger in the y direction than the area of a corresponding pixel 64 in the object plane, resulting in a magnifying effect in the y direction.
• the detector plane 32 is typically larger than the projected image, for example, where multiple slits and/or multiple modules are employed.
• each slit serves a different portion, or partially overlapping portion of the detector plane 32, to increase the area of detection.
• the slits are oriented parallel to one another, as illustrated in FIGURES 6 and 7, each slit and its associated slats comprising a module.
• the slats of one module may be continuations of the slats of an adjacent module. Or, more than one module may share the same slit.
• the spacing S between adjacent slits is from about 5 to 100 mm, and in one specific embodiment, from 5 to 50 mm (FIGURE 7).
• the detector 22 is a static detector, i.e., does not rotate but remains fixed, relative to the slats and detector plane.
• the entire detector system 16 may, of course, rotate around the subject, for example, by means of a rotating gantry.
• the fan beam-slit collimator finds particular application in planar or SPECT imaging. Planar imaging is performed without rotation of the detector about the patient.
• the detector is also suitable for use in a rolec-type camera in which the detector rotates about an axis that is perpendicular to the detector plane.
• the fan beam-slit arrangement is beneficial for optimizing efficiency while maintaining resolution.
• simultaneous optimization of slat length (for the fan beam) and collimator-detector distance (for the slit) in combination with parallel readout of neighboring detector areas which is possible when slit collimators are used with solid state detectors in de-magnification mode, provides a performance which exceeds that of conventional detection systems.
• thick slats are used for high energy photons, the optimization is not completely possible, but efficiency values are still high.
• An analytical model for spatial resolution and geometric efficiency of a collimator in combination with a pixellated detector such as a pixellated CZT- based detector, can be derived.
• Descriptions for static and rotating detector concepts can be derived, for example, using National Electrical Manufacturers Association (NEMA) performance criteria for detection efficiency and measures adapted for spatial resolution of pixellated detectors, based on the sampling of the single pixel response function.
• NEMA National Electrical Manufacturers Association
• Geometric efficiency E is calculated as part of the radiation from a point source that irradiates the open detector area of one pixel (normalized solid angle), multiplied by the average area in the object plane that is seen by any point on the detector pixel (region seen) and normalized by the area of the object plane equivalent to one pixel
• FWHM FWHM of the single pixel detector response for a point source in the object plane, integrated in one dimension. This definition is equivalent to the standard measurement method used in solid-state x-ray imaging where a line phantom is slightly tilted with respect to one of the detector axes.
• Focused collimators provide higher efficiency at the expense of a smaller field-of-view (FOV) or, equivalently, larger detector area.
• the solid angle is reduced by a factor cos 3 ⁇ for all pixels outside the center part of the detector, explained by the oblique angle of incidence ⁇ and the larger object-detector distance.
• the region seen is reduced by a factor ///' or (///') 2 , and the voxel area is given by the pixel area divided by m for a fan beam (for a cone beam the voxel area is given by the pixel area divided by p 2 /m 2 ).
• the linear magnification factor m is defined by the slit geometry in the x direction and the pixel pitch p y defined by the slats in the y direction. Specifically:
• Table 1 shows performance parameters of pixellated detector concepts, six of conventional type and two with fan beam-slit collimators of the type disclosed herein.
• collimator geometry is defined by pixel size D, septa length or collimator-detector distance L.
• Performance is given in terms of spatial resolution R, efficiency E, space-bandwidth product, SBP, which is essentially the number of detector pixels when the Nyquist criterion is fulfilled.
• Another method of expressing efficiency is as the efficiency-space-bandwidth product, ESBP, a measure of the amount of information received by different collimators. All detector values are calculated for 0.152 mm septa thickness (VXGP) and 5 mm resolution at 100 mm source-collimator distance.
• Two configurations of a fan beam-slit collimator are considered. Both employ more than one module, i.e., a plurality of slits. In both cases, five slits were used. The focus of the slats, /(and the location of the source) was 10-20 cm.
• configuration 1 shown in FIGURE 6 (55 cm x 73.3 cm in Table 1), the field of view 62 (FOV) is 55 cm in the x direction and 40 cm in the y direction.
• the slits 46 are oriented in the y direction, the slats (not shown) are oriented in the x direction.
• Several modules sharing a long slit overlap the 40 cm FOV 62 in the y direction. A large number of modules cover the 55 cm FOV in the x direction. Due to the large opening angle of the fan beams in the y direction, the average efficiency is reduced (the correction factor a con e given above is much smaller than 1).
• FIG. 7 In the second configuration (FIGURE 7) two detector module arrangements (called 'rings' if they form part of a ring around the object) with 55 cm in the x direction and a much smaller size in y are shown. A third detector module arrangement is shown under a different SPECT angle. Modules are smaller in the y direction, therefore the opening angle is smaller and the efficiency is higher (the correction factor given above is nearly equal to 1). Each of the module 'rings' sees only the corresponding region 62 A, 62B. The missing region 70 in between the rectangles 62A, 62B is imaged by a shift of the detector rings in y or by additional detectors 74 under different SPECT angles, as shown in FIGURE 7. The efficiency per detector area is much higher than in configuration 1. Optimizing for maximum efficiency of static detector concepts, it has been found that a combination of fan beam and slit, employing a detector with focused septa in one dimension and a pinhole detector in the other dimension, gives a higher efficiency than other known static detector arrangements.
• the first value of D for the Fan Beam Slit configurations 1 and 2 corresponds to the distance between the slats in the y direction and the second value of D corresponds to the slit width w.
• the ESBP of a fan-beam slit collimator is more than

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• 2005
  • 2005-09-30 US US11/577,093 patent/US7683333B2/en active Active
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