Patent Publication Number: US-10760960-B2

Title: Coincidence resolving time readout circuit

Description:
FIELD OF THE INVENTION 
     The invention relates to a coincidence resolving time (CRT) readout circuit. In particularly but not exclusively the present disclosure relates to a CRT readout circuit for implementing a multi-threshold readout approach for CRT applications. 
     BACKGROUND 
     Silicon Photomultipliers (SPMs) are compact, high performance solid state detectors which are of growing importance for nuclear medicine and radiation detection systems. The disclosure presented herein uses SPM detectors and is of particular relevance to medical imaging scanners which use scintiliation radiation detection methods, such as Positron Emission Tomography (PET) including Time-of-Flight (TOF), gamma cameras and Positron Emission Mammography (PEM). 
     The quality of clinical images in these systems is dependent on many parameters including coincidence resolving time (CRT). Digital SiPMs exhibit a good photon resolution due to the digitization at the cell level of the received signal. However, this requires CMOS integration and affects the cell fill factor due to the inclusion of in-cell logics. Analogue SiPMs do not require internal logic since the output is the analogue sum of the current of each cell. The digitization is external, after the amplification stage. The traditional way to digitize the SiPM output consists in the use of a configurable single-threshold comparator which converts the SiPM output into digital pulses. However, this approach has the primary limitation of using a single threshold. When this is set at a low value, higher peaks of the signal, at high photon rates, are registered with the loss of information of the intensity, i.e. the photon number. For example, in a system with single-photon threshold, any two or three photon-events will be counted as single photon-events. 
     There is therefore a need to provide for a CRT readout circuit which addresses at least some of the drawbacks of the prior art. 
     SUMMARY 
     In one aspect a coincidence resolving time readout circuit is provided comprising:
         an analog SiPM sensor for detecting photons and generating an SIPM output signal;   an ADC configured to provide multiple threshold values for converting the analogue SiPM output signal to digital values; and   a time to digital converter configured to receive the digital values from the ADC and timestamp them.       

     In another aspect, an amplifier is provided for amplifying the SiPM output signal in advance of the SiPM signal being received by the ADC. 
     In one aspect, the ADC is a flash ADC. 
     In a further aspect, the ADC is configured to have a voltage range. 
     In another aspect, the voltage range is determined based on a plurality of parameters. 
     In a further aspect, at least one of the parameters is associated with a parameter of a CRT application. 
     In one aspect, at least one of the parameters is associated with a characteristic of a scintillator. 
     In a further aspect at least one of the parameters is associated with a characteristic of a laser pulse shape. 
     In one aspect, at least one of the parameters is associated with a bandwidth of an amplifier. 
     In another aspect, at least one of the parameters is associated with a dynamic range of the SiPM. 
     In one aspect, at least one of the parameters is associated with system jitter. 
     In a further aspect, the voltage range of the ADC is used to set the ADC resolution in addition to predefined CRT algorithm requirements. 
     In one aspect, the number of timestamps from the TDC is sufficient to reach an ideal Cramer limit. 
     In another aspect, all the timestamps from the TDC feed dedicated algorithms proven to reach the minimum CRT values. 
     In one aspect, the SiPM sensor is a single-photon sensor. 
     In a further aspect, the SiPM sensor is formed of a summed array of Single Photon Avalanche Photodiode (SPAD) sensors. 
     In another aspect, the SiPM sensor comprises a matrix of micro-cells. 
     These and other features will be better understood with reference to the followings Figures which are provided to assist in an understanding of the present teaching. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present teaching will now be described with reference to the accompanying drawings in which: 
         FIG. 1  illustrates an exemplary structure of a silicon photomultiplier. 
         FIG. 2  is a schematic circuit diagram of an exemplary silicon photomultiplier. 
         FIG. 3  illustrates a schematic of a prior art CRT readout circuit. 
         FIG. 4  is graph of a CRT output from CRT readout circuits. 
         FIG. 5  is a flow chart illustrating exemplary steps of the prior art CRT readout circuit in operation. 
         FIG. 6  is a schematic circuit diagram of an CRT readout circuit in accordance with the present teaching. 
         FIG. 7  is a flow chart illustrating exemplary steps of the CRT readout circuit of  FIG. 6  in operation. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure will now be described with reference to an exemplary CRT readout circuit. It will be understood that the exemplary CRT readout circuit is provided to assist in an understanding of the teaching and is not to be construed as limiting in any fashion. Furthermore, circuit elements or components that are described with reference to any one Figure may be interchanged with those of other Figures or other equivalent circuit elements without departing from the spirit of the present teaching. It will be appreciated that for simplicity and clarity of illustration, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. 
     Referring initially to  FIG. 1 , an analog silicon photomultiplier  100  comprising an array of Geiger mode photodiodes is shown. As illustrated, a quench resistor is provided adjacent to each photodiode which may be used to limit the avalanche current. The photodiodes are electrically connected to common biasing and ground electrodes by aluminium or similar conductive tracking. A schematic circuit is shown in  FIG. 2  for a conventional silicon photomultiplier  200  in which the anodes of an array of photodiodes are connected to a common ground electrode and the cathodes of the array are connected via current limiting resistors to a common bias electrode for applying a bias voltage across the diodes. 
     The analog silicon photomultiplier  100  integrates a dense array of small, electrically and optically isolated Geigermode photodiodes  215 . Each photodiode  215  is coupled in series to a quench resistor  220 . Each photodiode  215  is referred to as a microcell. The number of microcells typically number between 100 and 3000 per mm 2 . The signals of all microcells are then summed to form the analog output of the SiPM  200 . A simplified electrical circuit is provided to illustrate the concept in  FIG. 2 . Each microcell detects photons identically and independently. The sum of the discharge currents from each of these individual binary detectors combines to form a quasi-analog output, and is thus capable of giving information on the magnitude of an incident photon flux. 
     Each microcell generates a highly uniform and quantized amount of charge every time the microcell undergoes a Geiger breakdown. The gain of a microcell (and hence the detector) is defined as the ratio of the output charge to the charge on an electron. The output charge can be calculated from the over-voltage and the microcell capacitance. 
             G   =         C   ·   Δ     ⁢           ⁢   V     q           
Where:
 
     G is the gain of the microcell; 
     C is the capacitance of the microcell; 
     ΔV is the over-voltage; and 
     q is the charge of an electron. 
     An SiPM sensor converts the detected laser photons and some detected photons due to noise to electrical signals that are then timestamped by timing electronics. The average number of detected photons k in a typical output pulse width τ is calculated from the incident rate Φ and the photon detection efficiency (PDE) as:
 
 k=Φ×PDE×τ   Equation 1
 
     Typically, the threshold for the digital readout of an SiPM is set to k to maximize the probability of detecting events. The probability of detecting X photon events when the average number is k is given by: 
     
       
         
           
             
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     Which has a maximum for X=k, as illustrated in  FIG. 3 . When a single threshold, using the comparator readout circuit of  FIG. 4 , is set to a certain value h, the single event per pulse is registered with a probability given by: 
     
       
         
           
             
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     All the events occurring with probability P(X≥h′) when h′&gt;h will not be distinguished and therefore not counted (or timed) as separate events. 
     Referring to  FIG. 3  which illustrates a prior art CRT readout circuit  300 . The CRT readout circuit  300  comprises an SiPM detector  305 , an amplifier  310 , a comparator  315  and a Time-digital-converter (TDC)  320 . The SiPM sensor  305  may be employed in Coincidence Resolved Time measurements for PET applications. In a typical PET system, two 511 keV photons are emitted from the body which are approximately 180 degrees out of phase with one another. The single high energy photon is converted to a number of lower energy photons in the visible spectrum which are detected using a standard photomultiplier tube (PMT) in the current state of the art. For LSO approximately 20,000 to 30,000 lower energy visible wavelength photons are emitted for each of the 511 keV photons incident on the crystal. The decay time of the LSO output is approximately 40 ns. The challenge for the detection system in a PET system is two fold. First the detectors must be of sufficient high detection efficiency to be able to convert the photons into a measurable response to allow sufficient photons to be obtained. Second the detector must be sufficiently fast to allow the detection of the incident optical pulse with sufficient accuracy to allow the pulse to be measured and analysed. Thus the detector must be fast. Coincidence resolving time is the time resolution that is obtained from detecting the coincidence between the two photons emitted during a positron annihilation in PET. Because of the speed of light which is a constant given by c=3×10 8  m/s the detector, including both the scintallator crystal and the optical detector must be suitably fast to allow detection of the pulses under with a resolution under 500 ps. This fast resolution is required to allow position of the photon emission to be resolved to a 7.5 cm resolution. To increase the resolution, faster detectors are required. It is possible to increase the resolution through repetitive measurements and signal processing, but this slows the data acquisition process and decreases overall system performance. High timing resolution detectors are therefore a requirement for PET. To perform this coincidence timing requires both a fast scintillator crystal, for high energy to lower energy conversion, and a subsequently fast and accurate readout circuits with the ability to reconstruct the output pulse from the crystal. 
     In CRT readout circuits known heretofore the SiPM  305  may be used to provide an optical detector as illustrated in  FIG. 3 . The SiPM output is typically connected to the comparator  315  which allows the recording of the time when the SiPM signals exceeds a fixed threshold. The threshold of the comparator  315  is required to be calculated and fixed in advance of operating the readout circuit  300  which delays the speed at which data may be acquired. The readout circuit  300  may be utlised in single threshold approach, and the choice of the threshold value is set to an optimal value which minimise the CRT full width half maximum (FWHM), as shown in the dotted line in graph of  FIG. 4 . The flowchart of  FIG. 5  illustrates exemplary steps for setting the optimum threshold value for readout circuit  300 . An initial voltage level threshold is set to V 0 , block  510 . CRT standard deviation is then measured for V 0 , block  515 . The threshold is then incremented by the step of interest, block  520 . The CRT standard deviation is remeasured and this loop continues until the upper limit of the threshold if reached,  525 . Once all the values of the CRT standard deviation are collected, block  530 , the threshold value giving the minimum CRT is selected, block  535 . 
     Referring to  FIG. 6  which illustrates a CRT readout circuit  600  in accordance with the present teaching. The CRT readout circuit  600  comprises an analog SiPM  605  which operates as an optical detector. An amp lifer  610  amplifies the analog output signal from the SiPM  605 . A flash ADC  615  is operable for converting the amplified SiPM analog output signal to digital values. The TDC  620  provides timing circuitry which time stamps the digital values from the ADC  615 . The CRT readout circuit  600  implements a multi threshold approach by appropriately configuring the ADC  615  to have multiple threshold levels which are used for converting the analog SiPM output signal to digital values which are then relayed to the TDC  620 . The inventors realised that setting the optimum threshold level as described in the flowchart of  FIG. 5  was slow which limited the ability of the readout circuit  300  for fast readout applications such as CRT applications. The readout circuit  600  addressed this problem by preconfiguring the ADC  615  to have multiple threshold levels. As a consequence, the readout circuit  600  does not require a sweep to be performed as indicated in the flowchart of  FIG. 5  in order to set the threshold value since automatically all the threshold values from the ADC are available in parallel. Therefore the threshold value giving minimum CRT is always available. 
       FIG. 7  illustrates exemplary steps for configuring the flash ADC  615 . The ADC voltage range is determined based on number of parameters, block  720 . In the exemplary arrangement, the input pulse shape of the laser used in a CRT application is provided as a parameter, block  710 . The bandwidth of the amplifier  610  is provided as another parameter, block  730 . The dynamic range of the SiPM  605  is provided as a further parameter, block  715 . The dymanic range is the ratio between maximum input detectable and minimum input detectable. 
     System jitter is also provided as a parameter, block  725 . Block  720  determines the ADC voltage range using the parameters provided by blocks  710 ,  715 ,  725  and  730 . It will be appreciated that other parameters may be used for determining the ADC voltage range other than those described with reference to blocks  710 ,  715 ,  725 , and  730 . In block  735  the ADC voltage range from block  720  is used to set the ADC resolution in addition to predefined CRT algorithm requirements provided by block  740 . 
     It will be appreciated by those skilled in the art that by the characterization of the system, i.e. light source, sensor and amplifier, the operating voltage range may be determined. The maximum and minimum values of the voltage range are then used to configure the ADC  615 . The number of timestamps obtained, i.e. the resolution of the ADC  615  may be chosen according to the algorithm for CRT extraction. The number of timestamps from the TDC  620  needs to be “sufficient” to reach the ideal Cramer limit, therefore it can be set to a high value (100+). This multi-threshold approach eliminates the need of tuning the single threshold setting through a voltage sweep as illustrated in the traditional approach of  FIG. 5  since the useful range of voltages from the ADC  615  operates in parallel. All these timestamps from the TDC  620  may be arranged to feed dedicated CRT algorithms proven to reach the minimum CRT values. 
     The present disclosure provides means to obtain multiple timestamps from the analogue SiPM signal allowing the analogue SiPM signals to be better exploited in CRT measurements. The output of the analogue SiPM  605  is a signal whose amplitude depends on the incident light coming, in CRT measurement, from a scintillator. Setting different analogue thresholds using the ADC  615  enables multiple timestamping without the need of digitization circuity being provided on the SiPM  605 . The flash ADC  615  divides a certain range of voltages into a series of intervals according to their resolution and the output of each level can be timestamped by the TDC  620 . In this way, as many timestamps as ADC bits may be obtained. Compared to a single-mode of operation as described with reference to the readout circuit  300 , where only one threshold is active, the multiple threshold mode of operation, where up-to-N levels are enabled in parallel, the CRT measurement is proved to show a lower standard deviation increasing therefore the CRT measurement quality as illustrated in solid line in the graph in  FIG. 4 . 
     It will be appreciated by the person of skill in the art that various modifications may be made to the above described embodiments without departing from the scope of the present invention. In this way it will be understood that the teaching is to be limited only insofar as is deemed necessary in the light of the appended claims. The term semiconductor photomultiplier is intended to cover any solid state photomultiplier device such as Silicon Photomultiplier [SiPM], MicroPixel Photon Counters [MPPC], MicroPixel Avalanche Photodiodes [MAPD] but not limited to. 
     Similarly the words comprises/comprising when used in the specification are used to specify the presence of stated features, integers, steps or components but do not preclude the presence or addition of one or more additional features, integers, steps, components or groups thereof.