Study of PET Detector Performance with Varying SiPM Parameters and Readout Schemes.

The spatial resolution performance characteristics of a monolithic crystal PET detector utilizing a sensor on the entrance surface (SES) design is reported. To facilitate this design, we propose to utilize a 2D silicon photomultiplier (SiPM) array device. Using a multi-step simulation process, we investigated the performance of a monolithic crystal PET detector with different data readout schemes and different SiPM parameters. The detector simulated was a 49.2mm by 49.2mm by 15mm LYSO crystal readout by a 12 by 12 array of 3.8mm by 3.8mm SiPM elements. A statistics based positioning (SBP) method was used for event positioning and depth of interaction (DOI) decoding. Although individual channel readout provided better spatial resolution, row-column summing is proposed to reduce the number of readout channels. The SiPM parameters investigated include photon detection efficiency (PDE) and gain variability between different channels; PDE and gain instability; and dark count noise. Of the variables investigated, the PDE shift of -3.2±0.7% and gain shift of -4±0.9% between detector testing and detector calibration had the most obvious impact on the detector performance, since it not only degraded the spatial resolution but also led to bias in positioning, especially at the edges of the crystal. The dark count noise also had an impact on the intrinsic spatial resolution. No data normalization is required for PDE variability of up to 12% FWHM and gain variability of up to 15% FWHM between SiPM channels. Based upon these results, a row-column summing readout scheme without data normalization will be used. Further, we plan to cool our detectors below room temperature to reduce dark count noise and to actively control the temperature of the SiPMs to reduce drifts in PDE and gain.

Design of an FPGA-Based Algorithm for Real-Time Solutions of Statistics-Based Positioning.

We report on the implementation of an algorithm and hardware platform to allow real-time processing of the statistics-based positioning (SBP) method for continuous miniature crystal element (cMiCE) detectors. The SBP method allows an intrinsic spatial resolution of ~1.6 mm FWHM to be achieved using our cMiCE design. Previous SBP solutions have required a postprocessing procedure due to the computation and memory intensive nature of SBP. This new implementation takes advantage of a combination of algebraic simplifications, conversion to fixed-point math, and a hierarchal search technique to greatly accelerate the algorithm. For the presented seven stage, 127 × 127 bin LUT implementation, these algorithm improvements result in a reduction from >7 × 10(6) floating-point operations per event for an exhaustive search to < 5 × 10(3) integer operations per event. Simulations show nearly identical FWHM positioning resolution for this accelerated SBP solution, and positioning differences of <0.1 mm from the exhaustive search solution. A pipelined field programmable gate array (FPGA) implementation of this optimized algorithm is able to process events in excess of 250 K events per second, which is greater than the maximum expected coincidence rate for an individual detector. In contrast with all detectors being processed at a centralized host, as in the current system, a separate FPGA is available at each detector, thus dividing the computational load. These methods allow SBP results to be calculated in real-time and to be presented to the image generation components in real-time. A hardware implementation has been developed using a commercially available prototype board.

Comparison of Detector Intrinsic Spatial Resolution Characteristics for Sensor on the Entrance Surface and Conventional Readout Designs.

We report on a high resolution, monolithic crystal PET detector design concept that provides depth of interaction (DOI) positioning within the crystal. Our design utilizes a novel sensor on the entrance surface (SES) approach combined with a maximum likelihood positioning algorithm. We compare the intrinsic spatial resolution characteristics (i.e., X, Y and Z) using our SES design versus conventional placement of the photo-sensors on the rear surface of the crystal. The sensors can be any two-dimensional array of solid state readout devices (e.g., silicon photomultipliers (SiPM) or avalanche photodiodes (APD)). SiPMs are a new type of solid-state photodetector with Geiger mode operation that can provide signal gain similar to a photomltipiler tube (PMT). Utilizing a multi-step simulation process, we determined the intrinsic spatial resolution characteristics for a variety of detector configurations. The SES design was evaluated via simulation for three different two-dimensional array sizes: 8×8 with 5.8×5.8 mm(2) pads; 12×12 with 3.8×3.8mm(2) pads; and 16×16 with 2.8×2.8 mm(2) pads. To reduce the number of signal channels row-column summing readout was used for the 12×12 and 16×16 channel array devices. The crystal was modeled as a 15 mm monolithic slab of a lutetium-based scintillator with the large area surface varying from 48.8×48.8 mm(2) up to 49.6×49.6 mm(2) depending upon the dimensions of the two-dimensional photo-sensor array. The intrinsic spatial resolution for the 8×8 array is 0.88 mm FWHM in X and Y, and 1.83 mm FWHM in Z (i.e., DOI). Comparing the results versus using a conventional design with the photo-sensors on the backside of the crystal, an average improvement of ~24% in X and Y and 20% in Z is achieved. The X, Y intrinsic spatial resolution improved to 0.67 mm and 0.64 mm FWHM for the 12×12 and 16×16 arrays using row-column readout. Using the 12×12 and 16×16 arrays also led to a slight improvement in the DOI positioning accuracy.

New Continuous Miniature Crystal Element (cMiCE) Detector Geometries.

Continuous miniature crystal element (cMiCE) detectors are a potentially lower cost alternative to high resolution discrete crystal designs. We report on the intrinsic spatial resolution performance for two cMiCE PET detector designs with depth of interaction (DOI) positioning capability. The first detector utilizes a 50 mm by 50 mm by 8 mm LYSO crystal coupled to a 64 channel, multi-anode PMT. It provides 4 layers of DOI information. The crystal has beveled edges along two of its sides to improve the detector packing when placed in a ring geometry. The second detector utilizes a 50 mm by 50 mm by 15 mm, rectangular LYSO crystal coupled to a 64 channel, multi-anode PMT. It provides up to 15 layers of DOI information. The average intrinsic X, Y spatial resolution for the 8 mm thick, truncated crystal detector was 1.33 +/- 0.31 mm FWHM (45.6 mm by 46.6 mm useful imaging area). The average DOI resolution was 3.5 +/- 0.22 mm. The average intrinsic X, Y spatial resolution for the 15 mm thick crystal detector was 1.74 +/- 0.35 mm FWHM (44.6 mm by 44.6 mm useful imaging area). In addition, the average DOI spatial resolution for 56 test points spanning a 26.4 mm by 12.2 mm region of the crystal was 4.80 +/- 0.36 mm. We believe the 8 mm thick truncated crystal design is suitable for mouse imaging while the 15 mm thick crystal design is more suited for human organ specific imaging systems (e.g., breast and brain).

Impact on the Spatial Resolution Performance of a Monolithic Crystal PET Detector Due to Different Sensor Parameters.

The performance characteristics of a monolithic crystal PET detector utilizing a novel sensor on the entrance surface (SES) design is reported. To facilitate this design, we propose to utilize a 2D silicon photomultiplier (SiPM) array device. SiPMs are a form of Geiger-Muller mode avalanche photodiodes (GMAPD) that can provide signal gain similar to a photomultiplier tube (PMT). Since these devices are still under active development, their performance parameters are changing. Using a multi-step simulation process, we investigated how different SiPM parameters affect the performance of a monolithic crystal PET detector. These parameters include gain variability between different channels; gain instability; and dark count noise. The detector simulated was a 49.6 mm by 49.6 mm by 15 mm LYSO crystal detector readout by a 16 by 16 array of 2.8 mm by 2.8 mm SiPM elements. To reduce the number of signal channels that need to be collected, the detector utilizes row-column summing. A statistics based positioning method is used for event positioning and depth of interaction (DOI) decoding. Of the variables investigated, the dark count noise had the largest impact on the intrinsic spatial resolution. Gain differences of 5-10% between detector calibration and detector testing had a modest impact on the intrinsic spatial resolution performance and led to a slight bias in positioning. There was no measurable difference with a gain variability of up to 25% between the individual SiPM channels. Based upon these results we are planning to cool our detectors below room temperature to reduce dark count noise and to actively control the temperature of the SiPMs to reduce drifts in gain over time.

Design of a High Resolution, Monolithic Crystal, PET/MRI Detector with DOI Positioning Capability.

We report on a high resolution, monolithic crystal PET detector design concept that provides depth of interaction (DOI) positioning within the crystal and is compatible for operation in a MRI scanner to support multimodal anatomic and functional imaging. Our design utilizes a novel sensor on the entrance surface (SES) approach combined with a maximum likelihood positioning algorithm. The sensor will be a two-dimensional array of micro-pixel avalanche photodiodes (MAPD). MAPDs are a new type of solid-state photodetector with Geiger mode operation that can provide signal gain similar to a photomltipiler tube (PMT). In addition, they can be operated in high magnetic fields to support PET/MR imaging. Utilizing a multi-step simulation process, we determined the intrinsic spatial resolution characteristics for a variety of detector configurations. The crystal was always modeled as a 48.8 mm by 48.8 mm by 15 mm monolithic slab of a lutetium-based scintillator. The SES design was evaluated via simulation for three different two-dimensional MAPD array sizes: 8×8 with 5.8×5.8 mm(2) pads; 12×12 with 3.8×3.8 mm(2) pads; and 16×16 with 2.8×2.8 mm(2) pads. To reduce the number of signal channels row-column summing readout was explored for the 12×12 and 16×16 channel array devices. The intrinsic spatial resolution for the 8×8 MAPD array is 0.88 mm FWHM in X and Y, and 1.83 mm FWHM in Z (i.e., DOI). Comparing the results versus using a conventional design with the photosensors on the backside of the crystal, an average improvement of ~24% in X and Y and 20% in Z is achieved. The X, Y intrinsic spatial resolution improved to 0.66 mm and 0.65 mm FWHM for the 12×12 and 16×16 MAPDs using row-column readout. Using the 12×12 and 16×16 arrays also led to a slight improvement in the DOI positioning accuracy.

A high resolution, monolithic crystal, PET/MRI detector with DOI positioning capability.

We report on a high resolution, monolithic crystal PET detector design that provides depth of interaction (DOI) positioning within the crystal and is compatible for operation in a MRI scanner to support multimodal anatomic and functional imaging. Our design utilizes a novel sensor on the entrance surface (SES) design combined with a maximum likelihood positioning algorithm. The sensor will be a two-dimensional array of micro-pixel avalanche photodiodes (MAPD). MAPDs are a new type of solid-state photodetector with Geiger mode operation that can provide signal gain similar to a photomltipiler tube (PMT). In addition, they can be operated in high magnetic fields to support PET/MR imaging. Utilizing a multi-step simulation process, we determined the intrinsic spatial resolution characteristics of a detector using the proposed design. For a 48.8 mm by 48.8 mm by 15 mm LSO crystal detector readout by an 8 by 8 array of 5.8 mm by 5.8 mm MAPD elements the intrinsic spatial resolution is 0.83 mm FWHM in X, 0.92 mm FWHM in Y and 1.83 mm FWHM in Z (i.e., DOI) for normally incident photons. Comparing the results versus using a conventional design with the photosensors on the backside of the crystal, an average improvement of 25% in X, 23% in Y, and 20% in Z is achieved.

Calibration procedure for a continuous miniature crystal element (cMiCE) detector.

We report on methods to speed up the calibration process for a continuous miniature crystal element (cMiCE) detector. Our cMiCE detector is composed of a 50 mm by 50 mm by 8 mm thick LYSO crystal coupled to a 64-channel, flat panel photomultiplier tube (PMT). This detector is a lower cost alternative to designs that use finely pixilated individual crystal detectors. It achieves an average intrinsic spatial resolution of ~1.4 mm full width at half maximum (FWHM) over the useful face of the detector through the use of a statistics based positioning algorithm. A drawback to the design is the length of time it takes to calibrate the detector. We report on three methods to speed up this process. The first method is to use multiple point fluxes on the surface of the detector to calibrate different points of the detector from a single data acquisition. This will work as long as the point fluxes are appropriately spaced on the detector so that there is no overlap of signal. A special multi-source device that can create up to 16 point fluxes has been custom designed for this purpose. The second scheme is to characterize the detector with coarser sampling and use interpolation to create look up tables with the desired detector sampling (e.g., 0.25 mm). The intrinsic spatial resolution performance will be investigated for sampling intervals of 0.76 mm, 1.013 mm, 1.52 mm and 2.027 mm. The third method is to adjust the point flux diameter by varying the geometry of the setup. By bringing the coincidence detector array closer to the point source array both the spot size and the coincidence counting rate will increase. We will report on the calibration setup factor we are able to achieve while maintaining an average intrinsic spatial resolution of ~1.4 mm FWHM for the effective imaging area of our cMiCE detector.