Patent Publication Number: US-6903344-B2

Title: Baseline correction in PET utilizing continuous sampling ADCs to compensate for DC and count rate errors

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention pertains to the field of gamma ray detection. More specifically, the invention relates to a method for determining and correcting the baseline of a continuously sampled signal for use in positron emission tomography. 
     2. Description of the Related Art 
     In the field of positron emission tomography (PET), it is well known that to measure the energy absorbed from a gamma ray interacting in a scintillating crystal, the total light from a crystal must be determined by integrating the photomultiplier tube (PMT) current. This current signal represents the rate of light collected by the sensing PMTs or photodiodes. The integration to determine the total light is traditionally performed using analog circuitry via a gated integrator. This method is graphically illustrated in FIG.  1 . Shown are the PMT current signal i(t) from a scintillation event and the integration of the current signal i(t), or:
 
 e ( t )=∫ 0   t   i ( t ) dt 
 
It will be understood by those skilled in the art that either voltage signals or current signals may be measured.
 
     Alternately, as graphically illustrated in  FIG. 2 , the integration may be performed by using a uniformly weighted summation of digital samples of the signal. In this method, the level of the signal at time t( 0 ) is precisely at zero volts. However, when the PMT signal is AC-coupled to an analog-to-digital converter (ADC), the level at time t( 0 ) is not zero volts, but varies with the count rate and electronic offset errors. 
     A gamma ray from an annihilation event interacts with a scintillator crystal, which produces a light output sensed by a PMT. For pulse applications, it is advantageous to use positive bias at the PMT, which results in high voltage bias applied to the anode contact of the PMT. To isolate the processing electronics, AC coupling is required between the PMT and the preamplifier stage. AC coupling between multistages reduces the DC offset errors that accumulate throughout the data processing chain. In the schematic illustration of  FIG. 3 , an isolation capacitor  102  is disposed between the PMT  100  and a pre-amplifier  104  because of the high voltage bias of the PMT  100 . Isolation capacitors  102  are also disposed between each output of the pre-amplifier  104  and the inputs of an amplifier  106  in order to reduce DC offset errors. 
     Although AC coupling is effective in isolating the high voltage PMT signals from the low voltage processing electronics, the average signal level at the input of the ADCs  108  is dependent on the count rate through the isolation capacitors  102  due to charge buildup. For example,  FIG. 4  illustrates a baseline shift due to charge buildup in isolating capacitors. Since the differential pulse height for a mono-energy input is repeatable, the change in average value, the common mode level, with count rate results in an error in pulse height measurement. Because position normalization is also dependent on pulse height measurement, final crystal position used to localize the annihilation is ultimately affected. 
     It is necessary to determine the baseline prior to an event and correct the baseline to a fixed level. Traditionally, this has been performed by using analog negative feedback baseline correction schemes which correct the baseline when a pulse has not been detected by evidence that a constant fraction discriminator (CFD) has not fired. However, with such a scheme it is possible for the CFD to not register an event if the energy of the pulse is low enough not to trigger the CFD. This results in the negative feedback of the analog baseline circuits attempting to incorrectly adjust the baseline since the error signal used to correct the baseline is derived from an event and not the desired average value. 
     Other methods have been developed to overcome these and similar problems associated with energy measurement associated with a crystal scintillation event. Typical of the art are those devices disclosed in the following U.S. Patents: 
                                                 U.S. Patent No.   Inventor(s)   Issue Date                          5,585,637   Bertelsen et al.   Dec. 17, 1996           5,608,221   Bertelsen et al.   Mar. 4, 1997           5,841,140   McCroskey et al.   Nov. 24, 1998           6,072,177   McCroskey et al.   Jun. 6, 2000           6,160,259   Petrillo et al.   Dec. 12, 2000           6,252,232   McDaniel et al.   Jun. 26, 2001           6,255,655   McCroskey et al.   Jul. 3, 2001           6,291,825   Scharf et al.   Sep. 18, 2001                        
Also of interest is Takahashi, et al., in “A New Pulse Height Analysis System Based on Fast ADC Digitizing Technique,” Conference Record of the Nuclear Science Symposium &amp; Medical Imaging Conference, 1992, Vol. 1, pp. 350-352.
 
     Of these patents, the &#39;140, &#39;177 and 655 patents issued to McCroskey et al., disclose a gamma camera modified to perform PET and Single Photon Emission Computed Tomography (SPECT) studies. These devices utilize SPECT electronics to generate triggering pulse signals for photons indicative of a positron annihilation event which are corrected, on a bundled basis, for position, linearity and uniformity by the same digital processors used by the camera for SPECT studies. While these patents specifically set forth methods to correct for timing delays, McCroskey et al., do not address baseline correction of DC and count rate offsets. 
     Petrillo et al., in the &#39;259 patent, and Scharf et al., in the &#39;825 patent, disclose a method and apparatus for selectively integrating PMT channel signals in a gamma camera system. In the &#39;259 method, a trigger word is decoded to determine which of multiple PMT channels are affected by a given scintillation event. When two scintillation events overlap both spatially and temporally, only those channels which are affected by both events stop integrating in response to the second event. Pre-pulse pile-up is corrected by removing the tail of a preceding pulse from a current pulse using an approximation of the tail of the preceding pulse based upon the instantaneous energy of the current pulse and the current count rate. 
     In the &#39;232 patent issued to McDaniel et al., a detector is disclosed as including opposed detector heads having anode signal processors. The anode signal processors perform a sliding box car integration of each PMT anode signal, as well as correct for baseline shifts and pileup from the tails of previous events, vary the length of the box car based on the time between events, and use a peak detection circuit to reduce the dependence of the integrated value on timing differences between the asynchronous events and the synchronous ADC conversion. 
     Takahashi, et al., discuss a digitizing system using a pulse height analysis system in nuclear spectroscopy, concluding that a technique disclosed therein has a possibility to analyze individual signals with required accuracies and to be used as an advanced signal processing method. It was noted by the authors that one problem is that noted in the present disclosure—that it was often observed that the baseline of the preamplifier output changed greatly due to the tail of the previous pulse. In order to estimate the baseline value under the signal pulse, an averaging method was employed wherein M points of sampled data are summed and averaged. In order to accomplish this method, the summed data is averaged with equally weighted coefficients. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for determining and correcting the baseline of a continuously sampled signal for use in positron emission tomography (PET) is disclosed. The method of the present invention serves to reduce the cost of and improve the performance and reliability of PET. The method of the present invention employs continuous signal sampling to determine the signal level at time t( 0 ) so that an accurate determination of the integrated signal may be calculated, resulting in an accurate energy estimate for an ac-coupled, continuously-sampled signal at various count rates. 
     The device of the present invention includes a front-end electronics processing channel which consists primarily of an analog CMOS Application Specific Integrated Circuit (ASIC), a bank of Analog-to-Digital Converters (ADCs), a Field Programmable Gate Array (FPGA) based digital sequencer, and two Random Access Memories (RAMs). 
     The analog CMOS ASIC includes an integrated constant fraction discriminator (CFD) and a time-to-digital converter (TDC). Also included are semi-Gaussian shaped photomultiplier (PMT) channels which are continuously sampled to determine energy on a per channel basis. The processing electronics perform continuous digital integration of the PMT current signals to obtain normalized position and energy for each event. 
     The position and energy channels utilize a baseline restoration (BLR) algorithm wherein the baseline of the signal pulses are placed at mid-scale by continuously sampling the ADC, thus always making available the past history in discrete steps. By performing an average calculation of the current baseline prior to the associated CFD firing (signifying an event of interest), a correction signal is generated for use in negative feedback control of the baseline. A feedback signal is used to control an analog constant current source across a fixed capacitor to alter the analog baseline to the desired operating point. The baseline voltage offset adjust circuit is controlled by a Gate signal along with an Up/Down control signal to control the baseline. 
     The CMOS ASIC includes a baseline control circuit wherein signals from the PMTs are amplified and ac-coupled to front-end amplifiers to normalize variations in light yield. The analog CMOS ASIC is used to generate timing information for each event and shape the high bandwidth PMT signals for use in qualifying the energy of each detected event. The CFD generates a time mark signal which indicates the detection of a gamma ray and triggers event processing by the FPGA. 
     Each analog CMOS ASIC shapes four PMT signals using two-pole low pass filters. Each shaped signal is differentially driven to a continuous sampling ADC in a mode that allows digital baseline restoration with a minimal loss in dynamic range. Each PMT is digitized by a single ADC. PMT gain information is shared between necessary channels so that ADC corrections are performed prior to energy and position calculations. 
     After the continuous panel setup information has been stored in the analog ASIC and lookup RAMs, the FPGA is programmed with the sequencer logic equations. The FPGA is re-programmable and performs crystal detection, pileup detection, energy qualification, time correction, and baseline restore functions. The FPGA digitally integrates the PMT signals and controls the processing routines. The FPGA is also used to perform setup functions such as loading the analog ASIC registers via an I 2 C bus interface, loading the position, energy and time correction lookup tables, as well as determining the event rates for each individual detector area. 
     Detection by the FPGA of a synchronous time mark from the analog ASIC starts the FPGA and ensures that framing is correct for all detected events. The FPGA controls the ADC mode logic, the digital integration and division for the energy output and the shape discrimination and digital division for the normalized X and Y signals. The FPGA also controls the lookup RAMs for crystal type identification and event location, as well as event energy qualification and time correction. Finally, the FPGA frames the data and sends the event data serially to the next set of event processors. 
     The RAMs perform energy qualification, determine the event position, and perform event time correction. The events from a continuous detector panel are energy-qualified and time corrected per crystal using a lookup table. 
     In an alternate method of adjusting the baseline, no negative feedback to the ADC common mode output is used. In the alternate method, the measured baseline is subtracted from the pulse height measurement. In a simplified example, placing the baseline near the A/D mid-scale allows the baseline to shift in either direction without loss of measurement. The measured baseline is then subtracted from the measured pulse height of each sample and the energy of the pulse is determined from the integral of the samples. Alternately, the baseline may be adjusted to below mid-scale to allow a greater dynamic range to be utilized for unipolar shaping. 
     The method and apparatus of the present invention is useful in ac-coupled systems utilizing a wide variety of scintillation materials. Such scintillation materials include, but are not limited to, LYSO, LGSO, GSO, BGO and LSO. Further, ADC sampling rates and energy shaping filter time constants may be adapted as required for various environments in which the present invention is employed. To wit, specific sampling rates and energy shaping filter time constants are disclosed as exemplary of such that have been found acceptable. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The above-mentioned features of the invention will become more clearly understood from the following detailed description of the invention read together with the drawings in which: 
         FIG. 1  is a graphical illustration of one prior art method of integrating a light signal from a crystal scintillation event in order to measure the energy absorbed from a gamma ray interacting with a detector; 
         FIG. 2  is a graphical illustration of an alternate prior art method of integrating a light signal from a crystal scintillation event; 
         FIG. 3  is a schematic diagram of prior art circuitry incorporating isolation capacitors due to a high voltage bias of a PMT and to reduce DC offset errors; 
         FIG. 4  is a graphical representation of a baseline shift due to charge buildup in the isolation capacitors of  FIG. 3  due to the event rate; 
         FIG. 5  is a block diagram of the front-end electronics processing channel incorporating several features of the device of the present invention; 
         FIG. 6  is a block diagram of a CMOS ASIC incorporated in the front-end electronics of  FIG. 5 ; 
         FIG. 7  is a schematic illustration of the baseline control circuit in accordance with the present invention; 
         FIG. 8  is a graphical illustration of raw ADC output of a typical signal from the ASIC shaping filter sampled at 100 MHz using the method and apparatus of the present invention; 
         FIG. 9  is a graphical illustration of a measured energy histogram for a single LSO crystal/PMT with a Na-22 source using 16 contiguous samples by means of the method and apparatus of the present invention; 
         FIG. 10  is a graphical illustration of a sampled signal wherein the baseline is set at an approximate mid-scale of the analog-to-digital converters; and 
         FIG. 11  is a graphical illustration of a sampled signal wherein the baseline is adjusted to below the mid-scale of the analog-to-digital converters to allow utilization of a greater dynamic range. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A method for determining and correcting the baseline of a continuously sampled signal for use in positron emission tomography (PET) is disclosed. The method of the present invention serves to reduce the cost of and improve the performance and reliability of PET. The method of the present invention employs continuous signal sampling to determine the average signal level for each event at time t( 0 ) so that an accurate determination of the integrated signal may be calculated. The result is an accurate energy estimate for ac-coupled, continuously-sampled photomultiplier tube (PMT) signals at various count rates. A gamma ray from a positron annihilation interacts with a scintillator crystal such as LSO, which produces light output proportional to the absorbed gamma energy. The light output is sensed by a PMT which produces a current signal which is shaped and digitally sampled to accurately determine the total energy relative to each event. An apparatus for performing the method of the present invention is also disclosed. 
     The device of the present invention uses a combination of digital and analog electronics techniques to determine the baseline prior to an event and correct the baseline to a fixed level. Included is a front-end electronics processing channel  10 , such as illustrated in  FIG. 5 , consisting primarily of four functional blocks. These functional blocks include an analog CMOS Application Specific Integrated Circuit (ASIC)  12 , a bank of Analog-to-Digital Converters (ADCs)  14 , a Field Programmable Gate Array (FPGA) based digital sequencer  16 , and two Random Access Memories (RAMs)  18 . 
       FIG. 6  is a block diagram of a CMOS ASIC  12  used in the present invention. The illustrated analog ASIC  12  includes an integrated constant fraction discriminator (CFD)  20  and a time-to-digital converter (TDC)  22  with intrinsic minimum timing resolution of 312 ps. Also included are semi-Gaussian shaped photomultiplier (PMT) channels  24  which are continuously sampled to determine energy on an area. The processing electronics perform continuous digital integration of the PMT signals to obtain normalized position and energy. It will be understood that the present invention is not limited to either this configuration or timing resolution. 
     The CMOS ASIC  12  architecture of the present invention eliminates gated baseline restoration, as the CFD  20  utilizes a continuous baseline restoration (BLR). The baseline of the shaped PMT signal pulses are ideally placed at mid-scale by continuously sampling the ADC  14 . As a result, the past history is always available. By performing an average calculation of the current baseline prior to the associated CFD  20  firing, a correction signal is generated for use in negative feedback control of the baseline. As illustrated in  FIG. 7 , a feedback signal  26  is used to control an analog constant current source  28  across a fixed capacitor  30  to alter the baseline to the desired operating point. The baseline voltage offset adjust circuit  32  is controlled by a Gate signal  34  along with an Up/Down control signal  36  to control the baseline. Using the present architecture, the continuous CFD  20  BLR function allows channel processing times of approximately 100 nanoseconds. Because the energy channels utilize continuous sampling ADCs  14 , energy channel dc-correction is estimated by examining the baseline history immediately prior to an event. The CMOS ASIC  12  allows for additional baseline correction via digital pulse correction, which alters the energy channel offset voltage by utilizing constant charging currents through a fixed off-chip capacitor. 
     Again referring to  FIG. 7 , which is a block diagram of a portion of the CMOS ASIC  12  including a baseline control circuit incorporated therein, signals  24  from the PMTs are amplified and ac-coupled to the analog ASIC front-end amplifiers  38  to normalize variations in light yield. The analog ASIC  12  is used to generate timing information for each event and shape the high bandwidth PMT signals  24  for use in qualifying the energy of each detected event. The CFD  20  generates a time mark signal which indicates the detection of a gamma ray and triggers event processing by the FPGA  16 . 
     Referring again to  FIG. 5 , each analog ASIC  12  shapes four PMT signals  24  using two-pole low pass filters  40 . Each shaped signal is differentially driven to a continuous sampling ADC  14  in a mode that allows digital baseline restoration with a minimal loss in dynamic range. The overall scanner architecture is designed such that each PMT is digitized by a single ADC  14 . PMT gain information is shared between necessary continuous panel channels so that ADC corrections are performed prior to energy and position calculations.  FIG. 8  illustrates raw ADC output of a typical signal from the ASIC shaping filter sampled at 100 MHz. The signal baseline is set near the mid-range of the differential ADC  14  at bin  530 , but is not yet optimized for full dynamic range. In one application of the present invention, differential ADCs  14  having a differential range of 2 volts (+1 v full scale positive, −1 v full scale negative) were used. A 15 ns two-pole low pass filter  40  is used to shape the PMT signals  24 . While differential ADCs  14  are used in this example, the present invention is not limited to differential ADCs  14 . 
     After the continuous panel setup information has been stored in the analog CMOS ASIC  12  and lookup RAMs  18 , the FPGA  16  is programmed with the sequencer logic equations. The FPGA  16  is re-programmable and performs crystal detection, pileup detection, energy qualification, time correction, and baseline restore functions. The FPGA  16  digitally integrates the PMT signals  24  and controls the processing routines. The FPGA  16  is also used to perform setup functions such as loading the analog ASIC registers via an I 2 C bus interface, loading the position, energy and time correction lookup tables, as well as determining the event rates for each individual continuous detector panel. 
     Detection by the FPGA  16  of a synchronous time mark from the analog ASIC  12  starts the FPGA  16  and ensures that framing is correct for all detected events. The FPGA  16  controls the ADC mode logic, the digital integration and division for the energy output and the shape discrimination and digital division for the normalized X and Y signals. The FPGA  16  also controls the lookup RAMs  18  for crystal type identification and event location, as well as event energy qualification and time correction. Finally, the FPGA  16  frames the data and sends the event data serially to the next set of event processors. 
     The RAMs  18  perform energy qualification, determine the event position, and perform event time correction. The events from a continuous detector panel are energy-qualified and time corrected per crystal using a lookup table. 
       FIG. 9  illustrates a measured energy histogram for a single LSO crystal/PMT with a Na-22 source using 16 contiguous samples. The overall LSO crystal energy resolution at a sample rate of 50 MHz has been found to be 17.6%. At sample rates of 100 MHz and 250 MHz, the energy resolution was found to be 16.0%. The overall energy resolution of 16% compared well to the crystal energy resolution measured using analog based NIM electronics and an independent MCA. The energy resolution shown illustrates that minimal degradation in energy resolution added by the digital electronics processing. 
     In an alternate method of adjusting the baseline, no negative feedback is used. In the alternate method, the measured baseline is subtracted from the pulse height measurement. Setting the baseline near the A/D mid-scale allows the baseline to shift in either direction without loss of measurement. As illustrated in  FIG. 10 , the measured baseline is set at approximately mid-scale. The measured baseline is then subtracted from the measured pulse height and the energy of the pulse is determined from the integral of the samples. 
     While the alternate method has described the baseline, being set at mid-scale, it is anticipated that the other advantages may be attained by setting the baseline away from mid-scale so that the full dynamic range of the ADCs  14  may be used. For example, as illustrated in  FIG. 11 , the baseline is adjusted to below mid-scale, thereby allowing a greater dynamic range to be utilized. 
     The method and apparatus of the present invention, as described, is useful in ac-coupled systems utilizing a wide variety of scintillation materials. Such scintillation materials include, but are not limited to, LYSO, LGSO, GSO and LSO. Further, ADC sampling rates and energy shaping filter time constants may be adapted as required for various environments in which the present invention is employed. To wit, specific sampling rates and energy shaping filter time constants are disclosed as exemplary of such that have been found acceptable. 
     From the foregoing description, it will be recognized by those skilled in the art that a method for determining and correcting the baseline of a continuously sampled signal for use in PET offering advantages over the prior art has been provided. Namely, the method of the present invention serves to reduce the cost of and improve the performance and reliability of PET. The method and apparatus of the present invention generates an accurate energy estimate for an ac-coupled, continuously-sampled signal at various count rates. 
     While the present invention has been illustrated by description of several embodiments and while the illustrative embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the general inventive concept.