Abstract:
A single interconnection unit provides all of the signal connections required to operate a group of PMTs and to transmit signals therefrom to acquisition electronics exterior to an enclosure for the PMTs in a nuclear scanner. In a preferred embodiment, the interconnection unit provides a shielded differential pair connection for RF signals; power supply connections for the PMTs, providing all of the voltages required to operate PMTs and the associated circuits, as well as connections for those signals and appropriate ground planes. Preferably, interconnection unit comprises a flex circuit with end connectors designed to connect to a power supply unit and an acquisition unit remote from the PMTs. Connection points are provided along the length of the flexible circuit for connection to a PMT and provide all of the necessary connections for that PMT, including shared power supplies and ground buses, as well as connections dedicated to a particular PMT, including a dedicated RF differential pair, a dedicated digital line, and an associated ground plane.

Description:
BACKGROUND 
   1. Field of the Invention 
   The present invention relates generally to electrical interconnection structures, and more particularly, concerns an interconnection structure capable of providing connections for diverse signals among a plurality of units. 
   2. Background of the Invention 
   There are several distinct types of imaging systems utilized in contemporary nuclear medicine. One type may employ gamma scintillation cameras (GSCs), so-called “position sensitive” continuous-area detectors, or simply, nuclear detectors. An exemplary GSC is a single photon emission computed tomography (SPECT) scanner. Another type of imaging system involves computed tomography (“CT”) of X-ray imaging. In contrast, magnetic resonance imaging (MRI) visualizes the inside of living organisms by making use of the relaxation properties of excited hydrogen nuclei in water placed in a powerful, uniform magnetic field. 
   Gamma rays are an energetic form of electromagnetic radiation produced by radioactive decay or other nuclear or subatomic processes such as electron-positron annihilation. Gamma rays form the highest-energy end of the electromagnetic spectrum. They are often defined to begin at an energy of 10 keV, a frequency of 2.42 EHz, or a wavelength of 124 pm, although electromagnetic radiation from around 10 keV to several hundred keV is also referred to as hard X-rays. 
   Gamma scintillation cameras, GSCs, are primarily used to measure gamma events produced by very low-level radioactive materials (called radionuclides or radio-pharmaceuticals) that have been ingested by, or injected into, a patient. The signals from the GSCs are used to generate images of the anatomy of organs, bones or tissues of the body and/or to determine whether an organ is functioning properly. The radiopharmaceuticals are specially formulated to collect temporarily in a certain part of the body to be studied, such as the patient&#39;s heart or brain. Once the radio-pharmaceuticals reach the intended organ, they emit gamma rays that are then detected and measured by the GSCs. Nuclear detectors perform spectroscopy and event X/Y positioning by processing signals from a constellation of Photo-multiplier Tubes (PMTs). One current series of detectors contains 59 PMTs. 
   To guard against the deleterious effects of stray X-rays blinding the scintillation crystal and the PMTs and possibly damaging the associated electronics, a GSC has a lead enclosure, i.e., tub, to block the stray X-rays. Furthermore, because the scintillation crystal emits faint amounts of light upon scintillation, the interior of the GSC must be shielded from ambient light that would blind the photosensors in the GSC used to measure the scintillation light emissions. 
   Owing to the large number of interconnections between PMT preamplifiers and an acquisition electronics system, portions of the acquisition electronics system have typically been packaged inside the tub. Because these connections generate heat, a significant amount of heat accumulates in the tub, detrimentally affecting the GSC&#39;s reliability. Likewise, the connections involve numerous printed circuit boards (PCBs) and cables, which must be disassembled to access a PMT needing replacement, making the GSC hard to service. Similarly, because the detectors need to be built in test stands, disassembled and then reassembled in the tubs, this arrangement of the connections compounds the effort needed to manufacture the GSCs. 
   While this arrangement has been functional, reliability, serviceability and manufacturability would all benefit if the electronics could be relocated outside of the tub. For these electronics to be placed outside the tub, an electrical penetration of the tub must support all of the PMT interconnections, including high and low voltage power supply voltages, balanced (differential pair) RF signals, and a serial data stream for analog-to-digital converters. 
   SUMMARY 
   In accordance with embodiments of the present invention, a single interconnection unit provides all of the signal connections required to operate the PMTs and to transmit signals therefrom to exterior circuitry (the acquisition electronics). In a preferred embodiment, the interconnection unit provides a shielded balanced differential pair connection for RF signals to exterior circuitry, as well as power supply connections for the PMTs, thus is providing all of the voltages required to operate PMTs and the associated circuits, as well as connections for those signals and appropriate ground planes. Preferably, the interconnection unit comprises a flex circuit with an end connector design to connect to a power supply unit and a processing unit remote from the PMTs. Connection points are provided along the length of the flexible circuit for connection of a PMT and provide all of the necessary connections for that PMT, including shared power supplies and ground buses, as well as connections dedicated to a particular PMT, including a dedicated RF differential pair, a dedicated digital line, and an associated ground plane. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing brief description and further objects, features, and advantages of the embodiments of the present invention will be understood more completely from the following detailed description of a presently preferred, but nonetheless illustrative, embodiment, with reference being had to the accompanying drawing in which: 
       FIG. 1  is a schematic block diagram illustrating exemplary Gamma ray detection circuitry of a SPECT scanner as the environment for an embodiment in accordance with the present invention; 
       FIG. 2  is a schematic block diagram illustrating how an interconnection subsystem in accordance with an embodiment of the present invention is made up of a plurality of interconnection units, each dedicated to a group of PMTs; 
       FIG. 3  is a schematic representation of a remote connection and a flexible circuit embodying an interconnection unit in accordance with an embodiment of the present invention, this unit being a two-layer circuit and  FIG. 3  representing the first layer of the circuit; 
       FIG. 4  is a schematic diagram similar to  FIG. 5  illustrating the second layer of the flexible circuit; 
       FIG. 5  is a schematic diagram similar to  FIG. 3  illustrating the superposition of the two layers of the flexible circuit; 
       FIG. 6  is a schematic diagram of the flexible circuit of  FIG. 3  an immediate point remote from the end connection, at which a connection is made to a PMT, this figure representing the first layer of the flexible circuit; 
       FIG. 7  is a schematic diagram similar to  FIG. 6  illustrating the second layer of the flexible circuit; and 
       FIG. 8  is a schematic diagram similar to  FIG. 6  illustrating the superposition of the first and second layers. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   In accordance with one or more other embodiments of the present invention, an apparatus may comprise means for electrically connecting a plurality of photo-multiplier tubes inside of a tub enclosure of a nuclear detector and an acquisition electronics system outside of the tub enclosure and also providing power supply connectors to the interior of the tub. The means for electrically connecting may include means for electrically interconnecting the plurality of PMTs. The means for electrically interconnecting the plurality of PMTs may include a high-density configuration of interconnection circuitry. 
   For illustrative purposes, the present invention will be described in terms of an interconnection structure for a single photon emission computed tomography (SPECT) scanner for nuclear medicine applications. 
     FIG. 1  is a block diagram depicting a gamma ray detection circuit of the SPECT scanner  100 . Portions of the gamma ray detection circuitry are similar to those of Anger, U.S. Pat. No. 3,011,057, the entire disclosure of which is incorporated herein by reference. In particular, the gamma ray detection circuit includes a parallel hole collimator  112 , a scintillation detector (crystal)  114 , a light guide  116 , a plurality of photo multiplier tubes (PMTs)  118 , a high voltage power supply  120  for the PMTs  118 , and an acquisition electronics system  122  for processing the data from the PMTs  118 . Interconnection circuitry  124  is used to connect the PMTs  118  to the power supply unit (PSU)  120  and the acquisition electronics system  122 . 
   The parallel hole collimator  112  acts as a guide to channel the gamma rays  110  through the tub  104  to the scintillation crystal  114 . The scintillation crystal  114  functions as a gamma ray detector by converting the high-energy photons of the gamma rays  110  into visible light (i.e., lower energy photons). When a gamma ray  110  strikes and is absorbed in the scintillation crystal  114 , the energy of the gamma ray  110  is converted into flashes of light  110 A (i.e., a large number of scintillation photons) that emanate and spread from the point at which the gamma ray  110  is absorbed. The scintillation crystal  114  may be formed from any suitable materials known in the art, such as sodium iodide doped with a trace of thallium (NaI(TI)) or CsI(TI). The scintillation photons  110 A emitted from the scintillation crystal  114  are typically in the visible light region of the electromagnetic spectrum (and may have a mean value of about 3 eV for NaI(TI)). 
   The light guide  116  assists in focusing the scintillation photons  110 A from the scintillation crystal  114  to the PMTs  118 . The plurality of PMTs  118  is located adjacent to the light guide  116 . In one or more embodiments, the number of PMTs may be on the order of about 50 to 100 tubes arranged in a two dimensional array. The basic function of the PMTs  118  is to detect and amplify the scintillation photons  110 A (events). 
   Each PMT  118  is operable to detect a fraction of the scintillation photons  110 A emanating from the scintillation crystal  114  and produce an analog output signal (e.g., a current or voltage pulse) having an amplitude that is proportional to the number of detected scintillation photons  110 A. Each PMT  118  includes a light sensitive surface, called the photocathode, which emits electrons in proportion to the number of incident scintillation photons  110 A. The emitted electrons, also called photoelectrons, are then electrostatically accelerated into an electron multiplying structure of the PMT  118 , which causes an electrical current (or voltage) to be developed at an output of the PMT  118 . 
   The amplitude of the output signal is proportional to the number of photoelectrons generated in the PMT  118  during the time period that scintillation photons  110 A are incident. More specifically, the amplitude of the output signal from each PMT  118  is proportional to two basic factors: (i) the number of scintillation photons  110 A detected by the PMT  118 , and (ii) the gain of the electron multiplying structure of the PMT  118 . Thus, after a gamma ray  110  absorption event at the scintillation crystal  114 , a given PMT  118  outputs a signal that can be used (with other signals from other PMTs  118 ) to determine the location of the gamma ray  110  absorption event. 
   Assuming that the analog output signals from the PMTs  118  are current signals, such output signals are subject to a current-to-voltage conversion to yield an analog voltage signal. The analog voltage signals are then digitalized using analog to digital (“A/D”) converters prior to (in the tub) or as an initial stage in the acquisition electronics system  122 . The interconnection subsystem  124  communicates the signals from the PMTs  118  to the acquisition electronics system  122 . In the preferred embodiment, the signals from the PMTs are analog, and the conversion to digital occurs in the acquisition electronics system  122 . Also, gain control signals for the PMTs are provided from the acquisition electronics in digital form. At the PMTs, each such signal is converted to analog form, in order to control the PMT. Thus there is a digital-to-analog converter dedicated to each PMT. 
   A basic function of the acquisition electronics system  122  is to calculate the spatial location and energy level of the incident gamma rays  110  based on the digitized analog output signals from the PMTs  118 . From such location information, the acquisition electronics system  122  is then operable to produce a two dimensional image of the anatomy of a patient, which may be displayed on a CRT or other display mechanism. The number of scintillation photons producing output in each PMT  118  is inversely related to the distance of the PMT  118  from the point of gamma ray absorption, or event location, within the scintillation crystal  114 . Thus, the acquisition electronics system  122  uses this relationship to compute the position of the gamma event from the output signals of a number of the PMTs  118  surrounding the event location. Further details regarding the basic operation and structure of the gamma ray detection circuitry of the SPECT scanner  100  may be found in U.S. Patent Application Publication No. US2004/0036026 and/or U.S. Pat. No. 6,124,595, the entire disclosures of which are incorporated herein by reference. 
   As shown schematically in  FIG. 1 , the interconnection subsystem  124  provides all the necessary power supply voltages P to each PMT. In the preferred embodiment, the PSU  120  is actually part of the processing subsystem  122 . In addition, the interconnection subsystem  124  receives an analog signal A from each PMT. The interconnection subsystem  124  may also receive digitized analog output for each PMT and various other signals from tub  104 , or it may provide them to the tub. For example, as explained above, gain control signals for the PMTs are provided from subsystem  122  in digital form, received by the PMTs, and digital-to-analog converters at the PMTs convert them to an analog signal useable by the PMT. The interconnection subsystem  124  then provides communication of the various of the signals between the PMTs and the processing subsystem  122 . 
     FIG. 2  is a block diagram illustrating further details of a preferred embodiment of interconnection subsystem  124 . In this embodiment, interconnection subsystem  124  is made up of a plurality of interconnection units  200 , each of which is dedicated to four PMTs. Those skilled in the art will appreciate that an interconnection unit could be dedicated to fewer or more PMTs. The number of interconnection units  200  making up the interconnection subsystem  124  will depend upon the total number of PMTs to be accommodated.  FIG. 2  details a single interconnection unit, but it also indicates that others are present. In a preferred embodiment, there are 59 PMTs  118  and each interconnection unit  200  may service either four or five PMTs, for a total of 14 interconnection units  200 . 
   Interconnection unit  200  incorporates a technology for building electronic circuits, called flexible electronics, in which electronic devices are deposited on a flexible substrates such as plastic. In the simplest case, flexible electronics can be made by using many of the same components used for rigid printed circuit boards, with exception of the substrate, is flexible rather than rigid. Flex circuits are often used for connections in various applications where flexibility, space savings, or production constraints limit the serviceability of rigid circuit boards or hand wiring. In addition to cameras, a common application of flex circuits is in computer keyboard manufacturing; most keyboards made today use flex circuits for the switch matrix. 
   In the preferred embodiment, each interconnection unit is a flexible circuit with a two-layer structure on opposite sides of the flexible circuit, and it may have the general appearance of a ribbon cable.  FIGS. 3-5  are schematic representations of the terminal end T of an interconnection unit  200  (the end connected to processing subsystem  122 ). In this case,  FIG. 3  represents the first layer,  FIG. 4  represents the second layer and  FIG. 5  represents the superposition of the two layers. 
   In the preferred embodiment, an interconnection unit has 38 pin-out connection points represented by ovals. Pin  1 , indicated by the reference character  202 , is represented by a rectangle. The odd numbered pins appear above  202  in increasing order. For example,  204  is pin  3  and  206  is pin  5 . Pin  2  ( 208 ) is immediately to the left of pin  1  and the even numbered pins appear above it in increasing order. 
   Initially, it should be noted that the first layer has a system of buses which serve all the connected PMTs. The heavy interconnected buses  210 ,  212  and  214  are part of the grounding system and also serve as a ground plane for underlying conductors that lie on the second layer. Also part of the grounding system are guard conductors (discussed further below), which straddle conductors that carry an RF signal. The guard conductors are provided to contain the RF signal. 
   Layer  1  also includes a number of power supply buses. For example, bus  224  is a high voltage bus, bus  226  is a 12-volt bus, and bus  228  is a 2.5 volt bus. Conductors  230 ,  258 ,  232  and  264 , connected to pins  1 ,  8 ,  7 , and  13 , respectively, carry digital signal lines. In the preferred embodiment, these are a series digital signal corresponding to a digitized version of the gain control signal for a PMT. To be used by the PMTs, these digital signals need to be converted to analog (e.g. by a digital-to-analog converter). The conductors connected to pin  3  ( 204 ) and pin  5  ( 206 ) carry an RF signal from a PMT. This is a balanced signal provided differentially between the two conductors. It should be noted that grounded conductor  220 ,  222  bounds the RF conductors on each side. In addition, an underlying grounded bus  236  on layer  2  acts as a ground plane for the two RF conductors. In combination, the ground plane and the guard conductors on each side of the RF conductors, serve to minimize radiation from the RF conductors. Similarly, pin  9  ( 238 ) and pin  11  ( 240 ) connect to a balanced differential pair of RF conductors which are bounded on each side by a grounded guard conductor and from below by a ground plane  242  on layer  2 . 
   Turning to layer  2  ( FIG. 4 ), conductors  244 ,  252  are power supply buses. Conductor  254  is a temperature sensing line and conductors  256  and  258  are digital lines. Conductors  260  and  262  are an RF differential pair and are bounded on each side by a grounded conductor. In addition, bus  214  on layer  1  serves as a ground plane for these conductors. Similarly, conductor  264  is a digital line and conductors  266  and  268  are a differential RF pair which is bounded on each side by a grounded conductor, and bus  212  on layer  1  serves as a ground plane. 
   When superposed, layers  1  and  2  appear as in  FIG. 5 . As may be seen, in the preferred embodiment, the layout is such that the DC buses are in the upper half of the structure, and the four RF buses are in the lower half, each accompanied by the respective digital gain control signal for the respective PMT (on pins  1 ,  7 ,  8  and  11 ). This arrangement of the buses improves the isolation of the DC buses from the RF signals. It should also be noted that adjacent RF buses are on opposite sides of the flexible circuit, to improve the isolation between RF buses 
     FIG. 6  is a representation of layer  1  and  FIG. 7  represents layer  2  of interconnection unit  200  in the vicinity of a connection point for one of the PMTs. At that point, a connector for the PMT is secured to the flex circuit comprising interconnection unit  200  and makes connection to the various pins. Specifically, the connectors connect to power lines  224 ,  226  and  228  on layer  1 , to digital line  234 , to the ground bus  210 , and to the ground conductors  216 ,  218 . On layer  2 , the connector connects to power buses  244 - 252 , to the digital line  264 , to the differential pair  266 ,  268 , and to the grounded conductors guarding them, and to the ground bus  242 . Preferably, the signal on bus  248  is an analog signal common to all PMTs on the respective flex circuit, and the signal is used to inject arbitrary signals (e.g. DC or RF) into a PMT transimpedance amplifier, in order to check for operational integrity. 
     FIG. 8  represents the appearance of interconnection unit  200  of that location, with layers  1  and  2  superposed. It should be noted that the connector for a PMT connects to all of the power line buses on each layer. In addition, it connects to a dedicated differential pair, with its guard conductors, and dedicated ground plane bus (an RF bus unit) and a dedicated digital line. The particular PMT involved is the one closest to terminal end T. Additional PMT connectors are provided at locations progressively further away from terminal end T. It should be noted that the RF bus and digital line for the closest PMT are closest to the center of the flexible circuit, whereas those for progressively further PMTs are located progressively closer to the margin of the flexible circuit. This permits a particularly compact and efficient layout for the flexible circuit. 
   Although not specifically mentioned, it will be appreciated that unit  200  must also have a connector for power supply unit  120 , in order to power the various voltage supply buses. Since power supply unit  120  is located in processing subsystem  122 , this could be provided, for example, at the end of the flexible circuit illustrated in  FIGS. 3-5  as part of the end connector. 
   Although preferred exemplary embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that many additions, modifications and substitutions are possible without departing from the scope and spirit of the invention as defined by the accompanying claims.