Abstract:
Detector crystals in a positron emission tomography (PET) apparatus gantry are cooled by directing cooling gas flow into a cooling duct bounded by the crystals and a cover defining the patient scanning field within the gantry. The cooling gas cools the crystals. Cooling gas may also be directed radially outwardly from the cooling duct into spatial gaps defined between detector enclosures that include the crystals, further isolating heat generated by other components within gantry from the detector crystals. Cooling gas is provided by a cooling system that may be incorporated within the gantry, external the gantry or a combination of both. Cooling gas can be provided by directing air within the gantry in contact with internal gantry cooling tubes and routing cooled air directly into the cooling duct with a powered fan.

Description:
BACKGROUND OF THE DISCLOSURE 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to positron emission tomography (PET) scanners used to image areas of interest in patients, and particularly apparatus and methods for cooling PET scanner detector scintillation crystals, so that they are maintained at a stabilized temperature selected for desired detector performance. 
         [0003]    2. Description of the Prior Art 
         [0004]    PET scanners image areas of interest in patients who have ingested radioactive imaging solutions. The scanner utilizes an array of scintillation crystals in a generally annular scanner gantry to detect radioactive particle emissions from the patient; and then correlates those emissions with patient tissue structure. One known scintillation crystal material is lutetinium oxy-orthosilicate (LSO). In PET scanner operation, a radioactive particle emitted from the patient striking a detector crystal within a detector element causes a light emission. That light emission is in turn detected by a photomultiplier tube (PMT), charge coupled device or the like. The PMT in cooperation with a detector electronics assembly (DEA) converts the scintillation crystal&#39;s detected light emission to an electrical signal that is used by scanner to generate an image of patient tissue in the area of interest. 
         [0005]    Light output of scanner scintillation crystals can be temperature dependent. As shown in  FIG. 1 , a lutetinium oxy-orthosilicate (LSO) crystal reduces effective light output (hence detector sensitivity) unless stabilized detector crystal temperature is maintained below 300° Kelvin (81° F. or 27° C.). Known PET scanners passively maintain detector crystal temperature. Components within the PET scanner gantry, for example the DEAs, generate and emit heat during their operation that is trapped within enclosed, sealed gantries. Ambient air ventilation is not commonly utilized in PET scanner gantries. Known PET scanners utilize water-cooled heat exchanger cooling rings in the sealed gantry structure to absorb and transfer heat away from the scanner. Cooling rings incorporated in the gantry structure rely primarily on convective heat transfer, assisted somewhat by forced exhaust fans incorporated within the DEA structures. Detector crystals are indirectly cooled by attempting to maintain the overall operating temperature within the gantry sufficiently low to meet the crystal operational temperature needs. However, localized temperature zones within the scanner gantry are not controlled with sufficient precision to assure constant operational temperature needed to maximize scintillation crystal detector sensitivity. Thus, the cooling ring heat transfer system is operated at a higher heat transfer output level than necessary to cool all gantry components generally, without assurance that the indirectly cooled detector crystals are being maintained at their optimum stabilized temperature threshold. 
       SUMMARY OF THE INVENTION 
       [0006]    Accordingly, an object of the invention is to maintain desired stabilized temperature of PET scanner detector scintillation crystals during scanner operation, so that detector sensitivity is increased and stabilized. 
         [0007]    Another object of the present invention is to reduce cooling energy in PET scanner gantries that is needed to maintain desired stabilized temperature of detector scintillation crystals. 
         [0008]    An additional object of the present invention is maintain desired stabilized temperature of PET scanner detector scintillation crystals during scanner operation in a manner that does not reduce scanner sensitivity. 
         [0009]    These and other objects are achieved in accordance with the present invention by directly cooling detector crystals in a positron emission tomography (PET) apparatus gantry. The detector crystals are cooled by directing cooling gas flow into a cooling duct bounded by the crystals and a cover defining the patient scanning field within the gantry. The cooling gas cools the crystals. Cooling gas may also be directed radially outwardly from the cooling duct into spatial gaps defined between detector enclosures that include the crystals, further isolating heat generated by other components within gantry from the detector crystals. Cooling gas is provided by a cooling system that may be incorporated within the gantry, external the gantry or a combination of both. Cooling gas can be provided by directing air within the gantry in contact with internal gantry cooling tubes and forcing/routing the routing cooled air directly into the cooling duct with a powered fan. 
         [0010]    One aspect of the present invention features a positron emission tomography (PET) scanner apparatus comprising a gantry having therein a cover defining a patient scanning field within the gantry. The gantry includes a plurality of detector enclosures, each having a detector crystal facing the patient scanning field. A cooling duct is formed in spatial volume bounded by the cover and the detector crystals. A cooling system is coupled to the cooling duct, and provides a source of forced cooling gas flow, such as cooled air, into the cooling duct. 
         [0011]    Another aspect of the present invention features a positron emission tomography (PET) scanner apparatus comprising an annular gantry having therein a cover defining a patient scanning field within an inner radial circumference of the gantry. The gantry also has an array of a plurality of detector enclosures, each respective detector enclosure having a detector crystal facing the patient scanning field. The gantry includes a pair of opposed spaced axial shields axially bounding the detector enclosure array. An annular cooling duct bounded by the cover, the pair of axial shields and the detector crystals forms a spatial volume that is cooled by a cooling system that is directly coupled to the cooling duct. The cooling system provides a source of forced cooling gas flow into the cooling duct in order to cool the crystals. 
         [0012]    Yet another aspect of the present invention features a method for cooling PET scanner detector crystals, the scanner having a gantry including a cover defining a patient scanning field within the gantry; a plurality of detector enclosures, each having a detector crystal facing the patient scanning field; and a cooling duct bounded by the cover and the detector crystals. The detector crystal cooling method is performed by coupling a cooling system capable of generating a forced cooling gas flow to the cooling duct and directing forced cooling gas flow into the cooling duct with the cooling system. In the case of LSO detector crystals an additional feature and aspect of this cooling method is maintaining crystal temperature below  81  degrees Fahrenheit (27° C. or 300° K), for higher light output (hence, greater detector sensitivity) than LSO crystals operated above that temperature. 
         [0013]    The objects and features of the present invention may be applied jointly or severally in any combination or sub combination by those skilled in the art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a graph of relative light output as a function temperature in an LSO crystal; 
           [0016]      FIG. 2  is a perspective view of a positron emission tomography (PET) scanner imaging apparatus directed to an embodiment of the present invention; 
           [0017]      FIG. 3  is a perspective view of the embodiment of  FIG. 1  without external covers and cooling ducts; 
           [0018]      FIG. 4  is an elevational schematic view of a detector crystal cooling embodiment of the present invention; 
           [0019]      FIG. 5  is a detailed elevational schematic view of the embodiment of  FIG. 4 , showing cooling gas circumferential flow across detector crystals and radial flow through gaps formed between detector enclosures within the detector array; 
           [0020]      FIG. 6  is an elevational schematic view of another detector crystal cooling embodiment of the present invention; and 
           [0021]      FIG. 7  is a cross-sectional elevational view taken along  7 - 7  of  FIG. 6 . 
       
    
    
       [0022]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
       DETAILED DESCRIPTION 
       [0023]    After considering the following description, those skilled in the art will clearly realize that the teachings of the present invention can be readily utilized in positron emission tomography (PET) scanner apparatus to cool detector crystals. Further, cooling the detector crystals is achieved without adding materials between the detector crystal face and the patient—thus not sacrificing scanner sensitivity. In the case of LSO material detector crystals, maintaining an operating temperature below 81° F. (27° C.) enhances their light output, and hence detector sensitivity. 
         [0024]      FIGS. 2-7  show embodiments of the detector crystal cooling apparatus and methods of the present invention.  FIG. 2  shows a PET scanner  10  having a patient bed  12  that translates relative to a scanning field. General structure and operation of the scanning components used to form an image representative of the scanned area of interest in the patient is known to those skilled in the art, and for brevity is not explained in further detail herein. The scanner  10  also includes a cover structure including an axial exterior cover  14  and a patient scanning field cover  16 , both of which are generally annular-shaped and supported by the gantry  20 . 
         [0025]    Referring to  FIGS. 3 and 5 , gantry  20  has an annular shaped transaxial spatial volume  22  that is bounded by axial end shields  24 ,  26 , the patient scanning field cover  16  and the circumferential (radial and axial) array of a plurality of detectors  30 . Each detector  30  has a detector crystal  31  that may comprise lutetinium oxy-orthosilicate (LSO), or other suitable known material. Detectors  30  have spatial gaps  32  between each other along the array circumference that are in communication with the interior volume of the gantry  20 . Each detector  30  is of known construction, with a scintillation detector crystal  31  coupled to a detector enclosure  34  that includes light pipes and photomultiplier tubes and/or charge coupled devices for converting detector light scintillations into electrical signals, for further processing by a respective detector electronics assembly (DEA)  36  that is associated with each detector  30 . DEAs  36  and other components within the gantry  20  generate heat during scanner  10  operation. Heat generated by gantry components  20  within the gantry enclosure is transferred to and absorbed by heat exchanger ring  40 . Water or other coolant circulating within the heat exchanger ring  40  transfers heat away from the gantry  20 . Generally the gantry  20  enclosure is sealed from the ambient environment in the patient scanning room, in order to avoid light pollution and to isolate patients from the scanner  10  apparatus. Therefore the coolant ring  40  is used as the heat transfer mechanism, rather than by direct venting to the ambient atmosphere. 
         [0026]    As shown schematically in  FIGS. 4 and 5 , the transaxial space  22  forms a cooling duct through which is directed forced cooling gas, such as air, by a cooling system  50 , by way of cooling duct intake air duct  52 . Cooling air is forced to circulate within the cooling duct transaxial space  22 , passing over detector crystals  31  in the detector  30  array, and returning to the cooling system by cooling duct outlet air duct  54 . Air flow is also directed radially outwardly through the gaps  32  formed between detectors  30  in the array. The radial airflow additionally benefits detector crystal temperature stabilization by forming a circumferential thermal isolation barrier between the crystals  31  and heat generated by the detector electronics assemblies  36 . Hot air in the outer radial periphery of the gantry  20  enclosure advantageously may be routed to the cooling system  50  by way of gantry outlet duct  56 , as was air from cooling duct outlet  54 . Forced airflow cooling air is circulated by motorized fan  58 , where it transfers heat to heat exchanger  60  that contains a circulating fluid from a coolant source  62  (e.g., a fluid to fluid coolant heat exchanger) by way of coolant inlet  64  and coolant outlet  66 . Coolant flow rate optionally may be regulated by a valve  68 . The valve  68  may be a manually operated valve or a remote actuated valve under control of a known controller  70 . In  FIG. 4  both the fan  58  and the valve  68  receive actuation signals from controller  70  by way of respective communication pathways  72 ,  74 . The controller has a temperature sensing input pathway  76  coupled to temperature sensor  78  that is in communication with the cooling duct transaxial space  22 . The controller  70  regulates cooling air flow rate and/or temperature by selectively operating the fan  58  and/or valve  68  in response to temperature information received from the temperature sensor  78 . 
         [0027]      FIGS. 6 and 7  show schematically another embodiment of the detector cooling system of the present invention wherein the cooling pipes  40  within the gantry function as the cooling air heat exchanger that is coupled to the coolant external source  162  by coolant inlet  164  and outlet  166 . An annular cooling pipe enclosure  150  envelops cooling pipes  40  and is in direct fluid communication with the transaxial space cooling duct  22  by way of inlet air duct  152 . Cooling air flows circumferentially within cooling duct  22  and returns to the cooling pipe enclosure  150  by way of outlet air duct  154 . As in the prior embodiment of  FIGS. 4 and 5 , cooling air is also directed radially outwardly in the gaps  32  between detectors  30 , forming an isolation layer between the detector crystals  31  and the heat emitting gantry components, such as the DEAs. Warm air in the gantry  20  returns to the cooling pipe enclosure  150  through outlets  156  formed in its periphery. The warm air entering the enclosure  150  is re-cooled and recycled through the cooling duct  22 . 
         [0028]    Air cooling and circulation in the embodiment of  FIGS. 6 and 7  are regulated as in the embodiment of  FIGS. 4 and 5 . Motorized fan  158  is in fluid communication with the enclosure  150  and cooling duct  22 , causing cooling air circulation therein. Coolant flow rate optionally may be regulated by a valve  168 . The valve  168  may be a manually operated valve or a remote actuated valve under control of a known controller  170 . In  FIG. 6  both the fan  158  and the valve  168  receive actuation signals from controller  170  by way of respective communication pathways  172 ,  174 . The controller has two temperature sensing input pathways  176 A and  176 B coupled to respective temperature sensors  178 A in communication with the cooling duct transaxial space  22 , and  178 B that is in communication with the cooling pipe  40  enclosure  150 . The controller  170  regulates cooling air flow rate and/or temperature by selectively operating the fan  158  and/or valve  168  in response to temperature information received from the temperature sensors  178 A,  178 B. The controller  170  may be coupled to fewer or more temperature sensors than shown in the  FIG. 6  embodiment. While the exemplary embodiments show use of a controller  70 ,  170  to regulate heat exchanger coolant temperature and cooling air fluid flow rates, the present invention can be practiced without use of a controller. For example coolant flow rate can be manually set through use of a manually actuated valve  68  or  168 , or a permanently regulated flow restrictor, such as an orifice. Similarly, the forced cooling air flow motorized fan circulation rate can be manually set, or the motor powering the fan can be operated at a fixed speed. 
         [0029]    Embodiments of the present invention circulate cooling air directly in contact with detector crystals  31  within the cooling duct transaxial space  22 , and thereby enabling localized temperature regulation in the spatial volume proximal the crystals. Thus detector crystals  31  can be located in a stable temperature environment that is optimized for higher intensity scintillation and greater detector  30  sensitivity. By directing cooling air radially away from the cooling duct transaxial space  22  through the gaps  32  between detectors  30  the detector crystals  31  are thermally isolated from other heat generating components within the gantry  20 . Thus energy necessary to power the coolant source  62 ,  162  and the air circulation fan  58 ,  158  is reduced compared to known gantry cooling systems that attempt to cool the entire gantry interior by convective and radiant heat transfer. While the forced cooling gas flow (e.g., air) in the embodiments of  FIGS. 4-7  is performed by powered cooling fans  58 ,  158 , a powered pressurizing pump (with or without a storage bladder or other reservoir) can be substituted for the powered fan. The cooling gas flow between the detector crystals  31  and the patient does not interfere with detector  30  sensitivity and in fact enhances detector sensitivity and stability. 
         [0030]    Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings. For example, detector crystal material other than LSO may be utilized in the PET scanner, and the cooling system is configured to maintain a stable operating temperature suitable for those crystals. Similarly, as noted above, forced cooling gas flow over the crystals can be accomplished with a pressurizing pump rather than with a powered fan.