Abstract:
A system, for the nuclear imaging of the breast, comprising: 
     at least one gamma camera, of a size appropriate for the scanning of a breast, having a radiation detecting surface, which detects gamma radiation and provides data signals responsive to radiation from the breast; 
     a collimator, positioned over the reception surface; 
     a gantry on which the radiation detector is mounted and which provides rotational movement of the radiation detection surface around the axis of the breast; and 
     a computer which receives and analyzes the data signals and constructs an image of radiation sources therefrom.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to SPECT instruments and in particular to a small SPECT device, dedicated to breast imaging. 
     BACKGROUND OF THE INVENTION 
     SPECT is one of several nuclear imaging techniques. Generally, in nuclear imaging, a radioactive isotope is injected to, inhaled by or ingested by a patient. The isotope, provided as a radioactive-labeled pharmaceutical (radio-pharmaceutical) is chosen based on bio-kinetic properties that cause preferential uptake by different tissues. The gamma photons emitted by the radio-pharmaceutical are detected by radiation detectors outside the body, giving its spatial and uptake distribution within the body, with little trauma to the patient. 
     FIG. 1 illustrates a general nuclear-imaging detector  10  comprising a NaI(T 1 ) scintillation crystal  12 . Generally, scintillation crystal  12 , of a diameter D 1 , is large enough to image a significant part of the human body (typically 40 cm). An array of photo-multiplier tubes (PMTs)  14  view scintillation crystal  12 , to give positional sensitivity. Each PMT  14  has an x and a y coordinate. When a photon is absorbed by scintillation crystal  12 , light is generated. A number of PMTs  14  receive the light and produce signals. The X and Y coordinates of the event are determined by the strength of the signals generated by each PMT. The energy of the event is proportional to the sum of the signals, called the Z signal. Only Z signals within a given range are counted. 
     Semiconductors with high atomic numbers and relatively high densities such as CdZnTe, CdTe, HgI 2 , InSb, Ge, GaAs, Si, PbCs, PbCs, PbS, or GaAlAs, have a high stopping power and can be used as gamma ray detectors with good photon detection efficiencies, good spatial resolution, and a relatively high photon-energy resolution. Solid state semiconductor gamma cameras generally comprise arrays of pixelated detector, hereinafter referred to as “pixelated detectors”. One type of pixelated detector is described in PCT publication WO 98/23974, the disclosure of which is incorporated herein by reference. FIG. 2 shows a typical construction of a pixelated detector  20  comprising a crystal  22  formed from a semiconductor material such as one of those noted above. A face  24  of crystal  22  has a large single cathode electrode  26 . An opposite face  28  of crystal  22  has an anode  30  comprising a rectangular array of identical small square anode pixels  32 . Typically, sizes of anode pixels  32  vary between 1 and 4 mm 2 , and the thickness of crystal  22 , between anode  30  and cathode  26  is on the order of millimeters to a centimeter. In operation, a voltage difference is applied between anode and cathode so that an electric field, hereinafter referred to as a “detector field”, is generated in crystal  22 . This field is typically on the order of a few kilovolts per centimeter. 
     When a photon, having an energy typical of the energies of photons used in gamma cameras, is incident on crystal  22 , it generally loses all its energy in crystal  22  by ionization and produces pairs of mobile electrons and holes in a localized region of crystal  22 . As a result of the detector field, the holes drift to cathode  26  and the electrons drift to anode  30 , thereby inducing charges on anode pixels  32  and cathode  26 . The induced charges on anode pixels  32  are sensed and generally partially processed by appropriate electronic circuits located in a detector base  34  to which detector  20  is mounted. Signals from the induced charges on pixels  32  are used to determine the time at which a photon is detected, how much energy the detected photon deposited in the crystal and where in the crystal the photon interaction took place. 
     Generally, a collimator  16  is placed between scintillation crystal  12  or  22  and the tissue. Commonly, collimator  16  is honeycomb shaped, comprising a large number of holes separated by parallel lead septa. The purpose of collimator  16  is to intercept and eliminate gamma-ray photons that are not traveling in an accepted direction, parallel to the lead septa. The geometric configuration of collimator  16  (e.g., the size and shape of the holes and the length of lead walls) determine the geometric response function of collimator  16 . In general, there is a tradeoff in which the collection efficiency of detector  16  increases as the geometric response function widens and the spatial resolution decreases. 
     SPECT (Single-Photon-Emission Computed Tomography) is based on conventional nuclear imaging technique and tomographic reconstruction methods, wherein projection (or planar) data acquired from different views around the patient are reconstructed, using image reconstruction methods, to generate cross-sectional images of the internally distributed radio-pharmaceuticals. SPECT images provide enhanced contrast, when compared with planer images obtained with conventional nuclear imaging methods. 
     A typical SPECT system consists of a single or multiple units of radiation detectors arranged in a specific geometric configuration, a mechanism for moving the radiation detectors and/or a specially designed collimator to acquire data from different projection views. 
     A typical system is based on a single or multiple scintillation cameras, mounted on a rotating gantry. This may consist of a single-camera system, a dual-camera system, a triple-camera system or a quadruple-camera system. Generally, camera-based SPECT systems use Anger scintillation cameras such as those used in conventional planar nuclear medicine. These cameras allow for truly three-dimensional imaging, by providing a large set of contiguous trans-axial images that cover the entire organ of interest. They are easily adaptable for SPECT imaging of the brain or body, by simply changing the radius of rotation of the camera. Because of the relatively low counting rate capability, it is desirous to use two or more cameras. The increase in sensitivity per slice is proportional to the number of cameras. 
     The most common collimator systems for SPECT systems are parallel-hole collimators, wherein the septa are perpendicular to the scintillation crystal. As noted above, there is an unavoidable tradeoff between detection efficiency and spatial resolution of parallel-hole collimators. An alternative design is a converging-hole collimator that increases the angle of acceptance of incoming photons. Examples of these special collimator designs and their points of convergence are given in FIGS. 3A-3D. FIG. 3A illustrates a parallel-hole collimator. 
     FIG. 3B illustrates a fan-beam collimator, where the collimator holes are converged to a line that is parallel to the axis of rotation. FIG. 3C illustrates a cone-beam collimator, where the collimator holes are converged to a point. FIG. 3D illustrates a varifocal collimator, where the collimator holes are converged to various focal points. Converged collimators have smaller fields of view when compared with parallel-hole collimators. 
     In general, camera-based SPECT systems may be comprised of detectors that rotate about the region to be imaged or arrays of detectors that completely surround the region to be imaged, for example with a ring of detectors. In either event, the object of the imaging system is to acquire data from directions including at least 180 degrees of view. 
     SPECT breast imaging is especially challenging, since there is no way to acquire cross-sectional data suitable for analytic tomographic reconstruction, especially from slices near the chest wall, from the front of the patient, since there is no way to acquire views of a slice over 180 degrees of view. Furthermore, rotating around the axis of the patient is not desirable due to the distance of the back of the body from the breast and the amount of radiation from the body which may overwhelm the radiation emitted from the breast. 
     SUMMARY OF THE INVENTION 
     An aspect of some embodiments of the present invention relates to providing a small SPECT system, dedicated to the nuclear imaging of the breast. In exemplary embodiments, the SPECT system is positioned on a gantry, so as to be substantially parallel with the axis of the breast and perpendicular to the body. The axis of the breast is a vertical line passing through the nipple when the test subject is lying face downward and prone, with the breast passing through a hole in the surface supporting the patient or a similar line. In an embodiment of the invention, the SPECT system rotates substantially around the axis of the breast. The patient may lay prone on a table, facing down, with the breast protruding through a hole in the table and thus pulled down by gravity. Additionally or alternatively, the breast may be pulled out by a vacuum pump. Alternatively, other means for extending the breast may be used. 
     In some embodiments that incorporate this aspect, the SPECT system comprises a single scintillation camera that rotates about the breast. Alternatively, the SPECT system comprises a plurality of scintillation cameras, arranged around the axis of rotation, in order to enhance the system sensitivity. In other embodiments of the invention, a plurality of ring-shaped detectors arranged about the breast may be used. In still other embodiments of the invention, a plurality of detector bars, arranged about the axis of the breast to form a ring of detectors, may be used. 
     In some embodiments that incorporate this aspect, the total scintillation-crystal length is longer than the breast and covers the length of the breast in one rotational pass. Alternatively, several rotational passes are required, at different breast “heights”to cover the length of the breast. 
     In some embodiments that incorporate this aspect, the face of the camera or detectors are parallel to the axis of the breast. Alternatively, the camera is mounted on a pivot or a hinge and can be tilted at various angles with respect to the axis of the breast. In some embodiments, one or more rotational passes are made with the detectors at a acute angle with the axis of the breast, for imaging of the breast near the chest wall, and another rotational pass is made with the detectors parallel with the axis of the breast, for imaging of the main portion of the breast. 
     In some embodiments that incorporate this aspect, the camera-based SPECT system comprises at least one NaI(T 1 ) crystal and an array of PMTs for determining position. Alternatively, a single position sensitive PMT views the crystal. Alternatively, the camera-based SPECT system comprises at least one pixelated solid-state detector, which is smaller and more suitable for use as a small camera. Due to the constricted space between the breast and the importance of imaging the region near the chest wall, small cameras are especially useful in carrying out the invention. 
     An aspect of some embodiments of the present invention relates to providing a small multidetector SPECT system, dedicated to the nuclear imaging of the breast. In an embodiment of the invention, an arrangement of four cameras, arranged in a square around the breast, is used. The cameras may be rotated around the breast. Alternatively, a stationary circular ring of detectors is used. 
     An aspect of some embodiments of the present invention relates to providing a special collimator for improved imaging of the chest wall. Preferably, the collimator comprises septa having a diverging field of view on the edge of the collimator adjacent to the chest wall (diverging-hole collimator). In an embodiment of the invention, the direction of the septa changes away from the chest wall edge to provide a view parallel to the rotation axis (approximately equal to the axis of the breast). In this embodiment, reconstruction may be done by an iterative method at least for slices at least near the chest wall. For portions of the breast away from the chest wall, either iterative or analytic (back-projection) methods may be used. 
     There is thus provided, in accordance with an embodiment of the invention, a system for the nuclear imaging of the breast, comprising: 
     at least one gamma camera, of a size appropriate for the scanning of a breast, having a radiation detecting surface, which detects gamma radiation and provides data signals responsive to radiation from the breast; 
     a collimator, positioned over the reception surface; 
     a gantry on which the radiation detector is mounted and which provides rotational movement of the radiation detection surface around the axis of the breast; and 
     a computer which receives and analyzes the data signals and constructs an image of radiation sources therefrom. 
     Optionally, the at least one gamma camera comprises a plurality of gamma cameras placed about and facing the axis of rotation. Optionally, the plurality of gamma cameras are symmetrically placed around the axis of rotation. 
     In an embodiment of the invention, the radiation detection surface of the at least one gamma camera is tilted with respect to the axis during rotation of the at least one gamma camera about the axis, such that it is partially facing the chest of a study subject. 
     There is further provided, in accordance with an embodiment of the invention a system, for the nuclear imaging of the breast, comprising: 
     at least one gamma camera, of a size appropriate for the scanning of a breast, having a radiation detecting surface, which detects gamma radiation and provides data signals responsive to radiation from the breast; 
     a collimator, positioned over the reception surface; 
     a gantry on which the radiation detector is mounted and which provides rotational movement of the radiation detection surface around the axis of the breast; and 
     a computer which receives and analyzes the data signals and constructs an image of radiation sources therefrom, 
     wherein the radiation detection surface of the at least one gamma camera is tilted with respect to the axis during rotation of the at least one gamma camera about the axis, such that it is partially facing the chest of a study subject. 
     Optionally, the system includes means for selectively tilting the at least one gamma camera such that the radiation detection surface is selectively tilted or parallel to the axis of rotation, during rotation of the at least one gamma camera about the axis. 
     Optionally, the collimator comprises septa near the center of the collimator which accept radiation from a direction perpendicular to the radiation detection surface and septa near an edge of the collimator which accept radiation from an outward facing acute angle to the perpendicular direction. 
     There is further provided, in accordance with an embodiment of the invention a system, for the nuclear imaging of the breast, comprising: 
     at least one gamma camera, of a size appropriate for the scanning of a breast, having a radiation detecting surface, which detects gamma radiation and provides data signals responsive to radiation from the breast; 
     a collimator, positioned over the reception surface; 
     a gantry on which the radiation detector is mounted and which provides rotational movement of the radiation detection surface around the axis of the breast; and 
     a computer which receives and analyzes the data signals and constructs an image of radiation sources therefrom, 
     wherein the collimator comprises septa near the center of the collimator which accept radiation from a direction perpendicular to the radiation detection surface and septa near an edge of the collimator which accept radiation from an outward facing acute angle to the perpendicular direction. 
     Optionally, the septa which accept radiation near one edge accept such radiation only near one edge of the collimator. Optionally, the one edge is an edge near the chest wall of the study subject. 
     In an embodiment of the invention, the computer constructs the image of radiation sources utilizing an iterative approach for at least part of a reconstructed volume. 
     In an embodiment of the invention, the computer constructs the image of the radiation source utilizing an iterative approach for at least a portion of the volume for which data is acquired with the radiation detection surface at the angle. An analytic approach is used for at least a part of the volume for which data is acquired with the radiation detection surface parallel to the angle or rotation. 
     In an embodiment of the invention, the detector is a detector having an extent between 10 and 20 cm. More preferably, the extent is between 10 and 15 cm. A suitable extent is about 12 cm. 
     There is further provided, in accordance with an embodiment of the invention, a gamma ray detector system comprising: 
     at least one gamma ray detector having a detector surface that has a normal thereto at each point thereof; and 
     at least one collimator placed on the detector surface, the collimator having a first portion that selectively passes gamma rays which impinge it at the normal angle and at angles near said angle and a second portion that selectively passes gamma rays which impinge on it from an outward looking direction with respect to the normal; 
     wherein the first portion comprises a region near one edge of the collimator and a central region of the collimator and wherein the second portion comprises a region near a second edge of the collimator opposite the one edge. 
     Optionally, the at least one detector surface is a planar surface and wherein the first portion comprises the entire planar surface except for a region near the second edge. 
     In an embodiment of the invention, the detector is a detector having an extent of less than 20 cm. Optionally, the extent is less than 15 cm. 
     In an embodiment of the invention, the at least one detector comprises first and second detectors the first detector and second detectors having first and second detector surfaces that are parallel and face each other, each said detector having a collimator placed thereon, as aforesaid. Optionally, the second portions of the collimators associated with the first and second detectors are situated opposite each other. 
     In an embodiment of the invention, the detector surface comprises a cylindrical surface and wherein the first portion comprises the detector surface except for a region near one end of the cylindrical surface. The cylindrical surface may have an inner diameter of between 10 and 20 cm. The collimator has an inner diameter of less than 20 cm. Optionally, the collimator has an inner diameter of less than about 15 cm. 
     There is further provided, in accordance with an embodiment of the invention, a gamma camera comprising a gamma ray detector system according to the invention and including a rotator that rotates the at least one detector surface about an axis parallel to and within the field of view of the detector surface, wherein the outward looking direction is at an acute angle with the axis. Optionally, the outward looking angle varies over the second portion, with the angle being 90 degrees near the first portion and a minimum value near at the second edge. Optionally, the minimum value is about 60 degrees. 
     In an embodiment of the invention, the gamma ray detector system is sized and configured to rotate about a breast of a human patient, wherein the axis of rotation coincides generally with the axis of the breast. 
     In an embodiment of the invention, the at least one detector generates nuclear medicine imaging signals responsive to gamma rays detected by the at least one detector and including image processing circuitry that receives the signals and produces a three dimensional image therefrom. Optionally, the image processing circuitry produces the image utilizing an iterative approach for a portion of the image, the portion including at least the portion including information acquired from the outward looking direction. 
     In an embodiment of the invention, the outward looking angle varies over the second portion, with the angle being 90 degrees near the first portion and having a minimum value near at the second edge. Optionally, the minimum value is about 60 degrees. 
     In an embodiment of the invention, the gamma ray detector system is sized and configured to fit around the breast of a human patient, wherein the axis of the cylinder coincides generally with the axis of the breast. 
     In an embodiment of the invention, the cylindrical detector generates nuclear medicine imaging signals responsive to gamma rays detected by the detector and including image processing circuitry that receives the signals and produces a three dimensional image therefrom. 
     Optionally, the image processing circuitry produces the image utilizing an iterative approach of a portion thereof, the portion including at least the portion from which imaging information is acquired from the outward looking direction. 
     There is further provided, in accordance with an embodiment of the invention, a method of imaging of the breast using gamma rays, comprising: 
     acquiring gamma ray data from the breast of a subject utilizing a gamma ray detector head generally parallel to the axis of the breast; and 
     reconstructing a three dimensional image of a radiopharmaceutical in the breast. 
     Optionally, acquiring gamma ray data comprises acquiring date from gamma rays emanating from tissue near the chest wall of the subject. In a preferred embodiment of the invention, the chest wall data is incomplete in the sense that it does not allow for analytic reconstruction of the distribution. 
     Optionally, reconstructing a three dimensional distribution comprises utilizing an iterative algorithm at least for regions of the breast for which an analytic solution is not possible. 
     Optionally, an iterative algorithm is used for reconstructing the distribution in portions of the breast and an analytic algorithm is used for reconstruction of the distribution of portions of the breast. 
     There is further provided, in accordance with an embodiment of the invention,  43 . A method of imaging using gamma rays, comprising: 
     acquiring gamma ray data from a subject utilizing at least one gamma ray detector head; and 
     reconstructing a three dimensional image of the sources of the gamma rays, utilizing an iterative approach for a first portion of the volume for which data is acquired and utilizing an analytic approach for a second portion of the volume for which data is acquired. 
     In an exemplary embodiment the first portion includes a volume for which said gamma ray data does not include a full set of tomographic data. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are described in the following detailed description of the preferred embodiments of the invention together with the attached drawings, in which same number designations are maintained throughout the figures for each element and in which: 
     FIG. 1 is a schematic illustration of a nuclear-imaging detector, comprising an NaI(T 1 ) scintillation crystal, as known in the art; 
     FIG. 2 is a schematic illustration of a pixelated, solid-state scintillation detector, as known in the art; 
     FIGS. 3A-3D are schematic illustrations of special collimator designs, as known in the art; 
     FIG. 4 is a schematic illustration of a small, camera-based SPECT system dedicated to the nuclear imaging of the breast, wherein the camera is rotated about an axis parallel with the axis of the breast, in accordance with an embodiment of the present invention; 
     FIG. 5A is a schematic illustration of a diverging-hole collimator design for a camera-based SPECT system, dedicated to the nuclear imaging of the breast and chest wall, in accordance with an embodiment of the present invention; 
     FIG. 5B is a schematic illustration of the field of view of a camera-based SPECT system with a diverging-hole collimator when imaging the breast, in accordance with an embodiment of the present invention; 
     FIGS. 6A and 6B are schematic illustrations of a tiltable camera-based SPECT system dedicated to the nuclear imaging of the breast and chest wall, in accordance with embodiments of the present invention; and 
     FIG. 7 is a schematic illustration of a small multidetector SPECT system dedicated to the nuclear imaging of the breast, comprising one or more stationary circular rings of detectors, in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference is now made to FIG. 4 which is a schematic illustration of a small, SPECT system  100 , dedicated to the nuclear imaging of the breast. Preferably, SPECT system  100  is positioned on a shaft  102  that rotates around breast  110 , on an axis  104  perpendicular to the body. Rotation may be provided by a motor (not shown) or other device. In the embodiment illustrated, a patient lies prone on a table  106 , facing down, and breast  110  protrudes through a hole  108  in the table, and is thus pulled down by gravity. Additionally or alternatively, the breast may be pulled out by a vacuum pump (not shown) or a radiolucent cup or other construction for extending the breast. Alternatively, the patient stands “on four”on the examination table. A chest-wall area within a diameter D is of interest. Breast  110  a height H. Optionally, a radiolucent, protective cap  116  is put on breast  110 , to protect it from contact with the moving detectors. 
     In an embodiment of the invention, SPECT system  100  is a dual-camera SPECT system, comprising two gamma cameras  120 , 180° apart, and perpendicular to the table. Preferably, each of cameras  120  is mounted on a leg  111 . Optionally, a sliding mechanism  109  on shaft  102  allows the distance, W, between legs  111  to be adjusted, depending on the diameter of the breast. 
     In some embodiments of the invention, each camera  120  comprises a collimator  36 , preferably of special design, as described below, and a detector  22 . Preferably, collimator  36  and detector  22  are substantially equal in size, and have a length L, that is at least equal to breast height H. Optionally, collimator  36  has a design described below, with respect to FIGS. 5A and 5B. However, other collimator types may be used, for some aspects of the invention. 
     Optionally, output from each camera is sent to a computer  114 , which analyzes the results and produces cross-sectional views of the breast, including the region adjacent the chest wall. 
     Reference is now made to FIG. 5A, which is a schematic illustration of collimator  36  for nuclear imaging of the breast and chest wall, in accordance with an embodiment of the present invention. Optionally, collimator  36  comprises septa  37  whose field of view diverge near the chest wall edge of the collimator. That is to say that septa  37  view toward the chest wall rather than perpendicular to the axis of the breast. Optionally, the direction of view changes gradually away from the chest wall such that the septa  38  farther from the wall have a direction of view perpendicular to the axis and to the face of the camera. In this manner, it is possible to obtain information from the region of the breast close to the chest wall. This information cannot be obtained with a perpendicular parallel-hole collimator, with rotation around the breast. In these embodiments, reconstruction is preferably by iterative (ART) methods, as known in the art, at least for regions near the chest wall. 
     Reference is now made to FIG. 5B, which is a schematic illustration of the field of view with special diverging-hole collimator  36  for a SPECT system for nuclear imaging of the breast. SPECT analysis can be obtained for the area bounded by lines g, h, and i, wherein with conventional, parallel-hole collimator, only the rectangle bounded by the two parallel cameras could have been analyzed. However, the additional information is acquired in exchanged for a certain loss in spatial resolution and an increase in artifacts, for the portion of the collimator having diverging holes. In addition, data should be acquired over 360° rotation. 
     Reference is now made to FIGS. 6A and 6B which are schematic illustrations of a camera-based SPECT system, dedicated to the nuclear imaging of the breast and chest wall, wherein the camera may be tilted, in accordance with embodiments of the present invention. Optionally, camera-based SPECT system  200  comprises two cameras  220 , 180° apart. Optionally, each camera  220  comprises a parallel-hole collimator  16 , with septa at a right angle to the collimator length, and a detector  22 . 
     Optionally, cameras  220  are mounted on pivots, or hinges  202 , on legs  111 , so that cameras  220  can be tilted so as to scan at various angles with respect to axis of the breast  104 . Optionally, a sliding mechanism  109  on shaft  102  allows cameras  220  to be adjusted closer together or further apart. In some embodiments, a slot  113  on each of legs  111  allows cameras  220  to be moved up and down along axis  104 . 
     FIG. 6A illustrates the collimator field of view when parallel-hole cameras  220  are at 30° with respect to axis  104 . Data acquired using this configuration includes gamma events occurring within the breast, near the chest wall. Optionally, length L of cameras  220  is sufficiently long so that at a 30° tilt, its projection covers the complete diameter of interest, D, at the base of breast  110 . However, small or larger tilt angles may be used if required to cover the breast near the wall. Preferably, as small an angle as possible is used. In this manner, chest wall information is acquired with no loss in spatial resolution. However, a second rotational pass should be made for SPECT analysis of the breast itself, as shown in FIG.  6 B. Alternatively, the gamma ray activity is calculated using only the angulated camera configuration. 
     In some embodiments, length L of camera  220  is shorter than breast height H. When this happens, several rotational passes are made with cameras  220  parallel to breast axis  104 , at different breast “heights”, as mounting hinges  202  of cameras  220  are moved down slots  113  of legs  111 . In exemplary embodiments of the invention, the length L of the camera is between 10 and 15 cm square and it is about 10-15 cm apart. Preferably, a size about 12 cm square is suitable. 
     Although the embodiments that have been described with reference to a dual-camera SPECT systems, other camera-based SPECT systems are possible. For example, the SPECT systems may comprise single scintillation camera. Alternatively, three or four scintillation cameras, arranged around the axis of rotation, may be used in order to enhance the system sensitivity. 
     In some embodiments of the invention, the camera-based SPECT system comprises at least one NaI(T 1 ) crystal and an array of PMTs for position sensitivity. Alternatively, the camera-based SPECT system comprises at least one pixelated solid-state scintillation crystal, which is smaller and more suitable to the small and cramped circumstances. Alternatively, it may comprise a single position sensitive PMT. 
     Reference is now made to FIG. 7 which is a schematic illustration of a small multidetector SPECT system  400 , dedicated to the nuclear imaging of the breast, comprising one or more stationary circular rings of detectors  420 , in accordance with an embodiment of the present invention. Optionally, multidetector SPECT detector system  400  has a diameter which is slightly larger than the diameter of interest at the chest wall, D. Optionally, the rings are mounted on a shaft  402  that allows travel up and down along axis  104 , guided by a slot  413 , and powered by a motor (not shown). Optionally, a diverging-hole collimator design is used to obtain information regarding the chest wall and the breast. Alternatively, an interchangeable collimator system is used, with a slanted, parallel-hole collimator and a right-angle, parallel-hole collimator, to obtain information regarding the chest wall and the breast. Preferably, solid state pixelated detectors, are used for the detectors in the rings. In an embodiment of the invention, the ring may be rotated by a small amount to increase resolution or may be dithered to improve image quality as described in the above referenced PCT application. Alternatively or additionally, sub-element axial motion or dithering may be provided. 
     Alternatively, enough rings are provided to cover the entire length of the breast. A collimator having an inward diverging view is preferably used for the rings near the chest wall. 
     The present invention has been described using non-limiting detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. Variations of embodiments described will occur to persons of the art. In addition, while preferred embodiments of the invention have been described as having certain groups of features, some preferred embodiments of the invention may include fewer of more of the features or other combinations of features. Furthermore, the terms “comprise,” include,” and “have” or their conjugates shall mean: “including but not necessarily limited to.” The scope of the invention is limited only by the following claims.