Abstract:
A subject ( 10 ) is disposed adjacent a detector array ( 18 ) for the purposes of nuclear imaging. The subject ( 10 ) is injected with a radioactive isotope ( 14 ) and γ-ray emissions indicative of nuclear decay are detected at the detector array ( 18 ). P-ASIC ( 60 ) preamplifier circuits are complex low-noise integrated circuits which dissipate a considerable amount of power (300-500 mW each). These components account for most of the dissipated power on the daughter cards ( 62 ). In order to facilitate the cooling of these electrical components, they are mounted on circuit boards ( 62 ) that are arranged parallel to each other extending perpendicularly away from the detector array ( 18 ). This provides channels between the boards through which cooling air is drawn by an array of fans ( 84 ).

Description:
BACKGROUND OF THE INVENTION 
     The present invention deals with the diagnostic imaging arts. It finds particular application in conjunction with electronics used in nuclear cameras and will be described with particular reference thereto. However, it is to be appreciated that the present invention has application in other devices that have electronics that require cooling and is not limited to the aforementioned application. 
     Nuclear imaging employs a source of radioactivity togs image the anatomy of a subject. Typically, a radiopharmaceutical is injected into the patient. This radiopharmaceutical contains atoms that decay at a predictable rate. Each time an atom decays, it releases a γ-ray. These γ-rays are detected, and from information such as their detected position and energy, a representation of the interior of the subject is reconstructed. 
     Typically, a nuclear camera has one, two, or three detector heads. Each head has a large scintillator sheet, such as doped sodium iodide, which converts incident radiation into flashes of light. An array of photomultiplier tubes is disposed in back of the scintillator to monitor for light flashes. The output of the photomultiplier tubes and associated circuitry indicates the coordinates of each scintillation on the sodium iodide crystal and its energy. Unfortunately, there are numerous non-uniformities and inaccuracies when using a large scintillator crystal and an array of photomultiplier tubes. 
     Rather than using a single, large scintillator and photomultiplier tubes, others have proposed using an array of small scintillators, each associated with a photodiode or other photoelectrical device which senses a scintillation in each individual scintillation crystal. Other types of individual solid-state detectors have also been suggested. 
     For resolution on the order of a millimeter, each scintillator/photodiode or other detector element is typically on the order of a millimeter square. Each of the detector elements needs to be powered and to have its output electrical signals processed. Typically, the powering and at least a portion of the processing circuitry is mounted in close association with the individual detectors. This leads to a high density of electrical components, many of which generate significant heat. Cooling the electronics becomes a significant problem. 
     The present invention provides a new and improved method and apparatus that overcomes the above referenced problems and others. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a nuclear imaging apparatus is given. An array of detectors detects γ-rays, information about the γ-rays are processed by electronics, a cooling system cools the electronics, and a reconstruction processor converts the γ-ray information into an image representation. 
     According to a more limited aspect of the present invention, the detector is an array of cadmium-zinc-telluride crystals. 
     According to a more limited aspect of the present invention, the detectors are collimated in one dimension. 
     According to a more limited aspect of the present invention, the cooling system includes fans and a cooling region. 
     According to a more limited aspect of the present invention, L-shaped connectors allow heat producing circuit boards to be disposed such that their disposition facilitates cooling. 
     According to a more limited aspect of the present invention, event analyzers detect events, their strength, and their locations, and sends this information to be stored in an event archive. 
     According to another aspect of the present invention, a nuclear camera is given. A detector head includes a housing within which are an array of detectors, circuit boards defining air channels, and heat generating P-ASICs mounted on the circuit boards. 
     According to a more limited aspect of the present invention, the circuit boards are mounted back to back in pairs. 
     According to another aspect of the present invention, a method of nuclear imaging is given. A subject is injected with a radiopharmaceutical that emits γ-rays. The γ-rays are detected by electronics and reconstructed into an image representation. The electronics are arranged to facilitate their cooling and they are cooled. 
     According to another aspect of the present invention, a method of nuclear imaging is given. γ-rays are detected by an array of detector arrays. The array is mounted on heat generating circuitry and air is passed along the circuitry to cool it. Signals are processed from the detector array and converted into an image representation. 
     According to a more limited aspect of the present invention, the detector array is biased by high-voltage circuitry on the board. The signals from the detector array are pre-amplified by P-ASICs on the circuit boards. 
     One advantage of the present invention is that it keeps electrical components at safe temperature levels. 
     Another advantage of the present invention is that it allows for a large number of detectors in a small area. 
     Another advantage of the present invention resides in high sensitivity and detailed spatial sampling resolution. 
     Yet another advantage of the present invention is that it avoids the use of liquid coolant or cryogenic cooling. 
     Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a nuclear imaging device in accordance with the present invention; 
     FIG. 2 is a perspective view of a detector array and associated circuit boards; 
     FIG. 3 is a perspective view of one of the circuit boards and detector array; 
     FIG. 4 is a flowchart illustrating the activity of an event analyzer, in accordance with the present invention; and, 
     FIG. 5 is a perspective view of the detector array, circuit boards, and cooling fans. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a subject  10  defines an imaging region  12 . In the preferred embodiment, a radioactive isotope  14  is injected into the subject, near a region to be imaged. For example, if a physician wanted to view a blockage in the aorta, the isotope would be injected into the bloodstream upstream from the blockage. As another example, the radiopharmaceutical is injected into the circulatory system and its absorption by tissue of interest is monitored. 
     As quantum physics predicts, atomic nuclei of the radioactive isotope decay over time. Energy is released at the time of decay in the form of a photon, more specifically, a γ-ray of characteristic energy. 
     Many of the γ-rays produced during an imaging process will be lost, propagating in useless directions. However, some of the γ-rays pass through collimators  16 , thin tungsten vanes in the preferred embodiment, and strike a detector array  18 . In the preferred embodiment and with reference to FIG. 2, the detector array  18  includes a 4×24 array of cadmium zinc telluride (CZT) crystal arrays  20 , each having 4×8 individual detectors  22 . In the preferred CZT embodiment, a potential difference of −600 V applied across the detector arrays by high voltage filter circuits powered by a high voltage power supply  24 . 
     In the preferred embodiments, the detector array  18  and collimators  16  are mounted on a mechanized drive  30  that moves the detector array. Preferably, the array moves with lateral rotational components of motion, although various trajectories are contemplated. In some applications, the detector array is stationarily mounted within a movable gantry that is indexed around the region of interest. 
     In the preferred embodiment, the support is mounted on a rotatable gantry  32  which extends fully around the subject  10 . A motor control  34  selects a range of motion of the detector array  18 , if any, within the rotatable gantry and rotation of the gantry  32  stepwise or continuously around the image region. 
     In SPECT imaging, the collimator  16  limits access to the detector array  18  to radiation following prescribed paths or trajectories, e.g., trajectories perpendicular to the plane of the detector array  18 . In this manner, each radiation event defines a trajectory along which a radioisotope decayed. If the movable gantry  32  remains stationary, the detectors define a projection image of the radioisotope distribution in the region of interest. An event analyzer  42  determines the location at which each event strikes the detector array, i.e., which detector receives it and the amount of energy of the radiation event. The radiation events collected at each stationary position of the detector array are stored in an archive  44 . When the rotatable gantry  32  is rotated to different angular positions around the subject, a plurality of projection images from different angular orientations are collected. A reconstruction processor  46  backprojects or otherwise reconstructs the data from the archive memory  44  into a volumetric image representation for storage in a volumetric image memory  48 . A video processor  50  under operator control selectively withdraws portions of the volumetric image representation and converts them into appropriate form for display on a video or other human-readable monitor  52 . 
     With reference to FIG.  3  and continuing reference to FIGS. 1 and 2, received γ-rays are detected and their energy measured by electronics attached to the detector array  18 . Four P-ASIC  60  are mounted on each side of a circuit board or pair of back-to-back circuit boards  62  that support four of the detector arrays. More specifically to the preferred embodiment, each P-ASIC preamplifies half of one of the arrays  20 . 
     Each time a γ-ray strikes one of the detectors, an avalanche effect releases electrons producing an output electrical pulse. Associated P-ASICs  60  powered by voltage regulator  66  amplify and condition the pulses from the detectors. Subsequent electronic components  64  select and multiplex a signal of interest to circuits that digitize and archive the series of outputs to portions of the event analyzer  42  mounted remotely. 
     With reference to FIG. 4, in the preferred embodiment, the circuitry  64  or the remote portion of event analyzer  42  is normally idle  70 , awaiting an electrical signal. When the event analyzer  42  receives an electrical signal, it compares it to a threshold  72 . If the signal is less than the threshold, it is ignored as noise, and the analyzer  42  goes back to idle. If the signal is above the threshold, the analyzer classifies it as an event  74 , and records  76  the energy of the signal, and the position of the detector  22  that sent the signal. The analyzer  42  then communicates  78  this information to the event archive  44  where it is stored for future use. After this communication, the analyzer  42  returns to idle awaiting the next event. In the preferred embodiment, once the analyzer  42  comes out of idle, in the next clock cycle, (preferably within 20 ns) other events are locked out until the analyzer  42  returns to idle. In the case of a true event, the process is completed, and the analyzer is back in idle in less than 2 μs. In the case of a false event, the analyzer  42  includes a timeout feature that only allows a set amount of time to determine the truth of an event. In the case of noise, the analyzer waits to see if the threshold is reached, but will return to idle within 2 μs of detecting the noise. The circuit  64  includes plural parallel channels, in the preferred embodiment a channel for each half of an array  20 . In the unlikely instance of two or more simultaneous (within 20 ns of each other) events incident upon the same half of a detector array, both events are discarded as they are difficult to isolate. 
     In the preferred embodiment, with reference to FIG. 5 a set of fans  84  draw outside air through the channels  80  across the circuit boards  62  cooling the components located thereon. More specifically, the fans are mounted in apertures in one side wall of a housing opposite an air inlet  86 . When the top and bottom of the housing are closed, air flow is constrained to flow along the boards, over the components. 
     As shown in FIG. 3, the circuit boards  62  have vertical connector sockets  82 . More specifically, each of the detector arrays  20  has two rows of pins extending from its lower side. Each back-to-back circuit board pair or two-sided circuit board  62  includes four sockets  82  mounted facing toward one of the edges. Placing sockets on opposite sides of the circuit board provides a stable mechanical mounting for each array as well as a reliable electrical interconnection. 
     In the preferred embodiment, with reference to FIG. 5 a set of fans  84  draw outside air through the channels  80  across the circuit boards  62  cooling the components located thereon. More specifically, the fans are mounted in apertures in one side wall of a housing  84  opposite an air inlet  86 . When the top and bottom of the housing are closed, air flow is constrained to flow along the boards, over the components. 
     Optionally, a coolant or liquid cryogen, additional fans, or the like are incorporated in the housing to enhance cooling. 
     In an alternate embodiment, the radioactive source is mounted and fixed on the opposite side of the subject across from the detector array. In this manner, the γ-rays which originate outside the subject either from a point or line source of radioactive material or a low power x-ray tube pass through the subject. 
     The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.