Joseph Panetta

Waveform-Sampling Electronics for a Whole-Body Time-of-Flight PET Scanner

Waveform sampling is an appealing technique for instruments requiring precision time and pulse-height measurements. Sampling each photomultiplier tube (PMT) waveform at oscilloscope-like rates of several gigasamples per second enables one to process PMT signals digitally, which in turn makes it straightforward to optimize timing resolution and amplitude (energy and position) resolution in response to calibration effects, pile-up effects, and other systematic sources of waveform variation. We describe a system design and preliminary implementation that neatly maps waveform-sampling technology onto the LaPET prototype whole-body time-of-flight PET scanner that serves as the platform for testing this new technology.

Development of a high-resolution and depth-of-interaction capable detector for time-of-flight PET

PET with its quantitative powers is becoming increasingly popular in the clinic. While the detector spatial resolution and sensitivity directly affect its ability, it has been shown that including the time-of-flight information further enhances its powers. Currently numerous approaches are being pursued to improve spatial resolution and timing, but most involve trade-offs. We describe here the development of a high-resolution PET detector with time-of-flight capabilities. The detector design is based on our previously developed pixelated Anger-logic detector where an array of individual crystals is readout by an array of larger photomultiplier tubes (PMTs) coupled to it via a light-guide. Depth-of-interaction (DOI) measurement in this design is accomplished by making use of a dual crystal-layer offset relative to each other. With a target spatial resolution of 1–2 mm, we have carefully evaluated the performance of several 1.5 × 1.5 and 2.0 × 2.0 mm2 and 10–20 mm long LYSO crystals readout by several appropriately sized PMTs. Experiments and simulations were used to investigate the design, and optimize performance of the detector. An experimental prototype using a single 8 × 7 array of 1.5 × 1.5 × 12 mm3 LYSO crystals readout by a 7-PMT array of the Hamamatsu R4124 PMTs was developed. A high-speed waveform sampling data acquisition system based on the DRS4 switched-capacitor that digitizes data at 5 GS/s was also built. Experimental evaluations demonstrate that the detector provides very good timing and also successfully discriminates 1.5 × 1.5 mm2 cross-section scintillation crystals. Bench-top timing measurements with a dual-layer detector demonstrate that relatively good timing can be maintained in a stacked crystal arrangement, suggesting the feasibility for extending the approach to incorporate DOI with this design.

Validation of phantom‐based harmonization for patient harmonization

Purpose To improve the precision of multicenter clinical trials, several efforts are underway to determine scanner‐specific parameters for harmonization using standardized phantom measurements. The goal of this study was to test the correspondence between quantification in phantom and patient images and validate the use of phantoms for harmonization of patient images. Methods The National Electrical Manufacturers’ Association image quality phantom with hot spheres was scanned on two time‐of‐flight PET scanners. Whole‐body [18F]‐fluorodeoxyglucose (FDG)‐PET scans were acquired of subjects on the same systems. List‐mode events from spheres (diam.: 10–28 mm) measured in air on each scanner were embedded into the phantom and subject list‐mode data from each scanner to create lesions with known uptake with respect to the local background in the phantom and each subjects liver and lung regions, as a proxy to characterize true lesion quantification. Images were analyzed using the contrast recovery coefficient (CRC) typically used in phantom studies and serving as a surrogate for the standardized uptake value used clinically. Postreconstruction filtering (resolution recovery and Gaussian smoothing) was applied to determine if the effect on the phantom images translates equivalently to subject images. Three postfiltering strategies were selected to harmonize the CRCmean or CRCmax values between the two scanners based on the phantom measurements and then applied to the subject images. Results Both the average CRCmean and CRCmax values for lesions embedded in the lung and liver in four subjects (BMI range 25–38) agreed to within 5% with the CRC values for lesions embedded in the phantom for all lesion sizes. In addition, the relative changes in CRCmean and CRCmax resulting from the application of the postfilters on the subject and phantom images were consistent within measurement uncertainty. Further, the root mean squared percent difference (RMSpd) between CRC values on the two scanners calculated over the three sphere sizes was significantly reduced in the subjects using postfiltering strategies chosen to harmonize CRCmean or CRCmax based on phantom measurements: RMSpd of the CRCmean values in subjects was reduced from 36% to < 8% after harmonizing CRCmean, while RMSpd for CRCmax was reduced from ˜33% to < 6% after harmonizing CRCmax with a different strategy. However, with this strategy designed to harmonize CRCmax, the RMSpd for CRCmean only improved to ˜14% in subjects. Conclusions The consistency of the CRC measurements between the phantom and subject data demonstrates that harmonization strategies defined with phantom studies track well to patient images. However, quantitative agreement between different scanners as represented by the RMSpd depends on the metric chosen for harmonization.

Combined analog/digital approach to performance optimization for the LAPET whole-body TOF PET scanner

LAPET is a LaBr3-based whole-body time-of-flight PET scanner. We previously reported coincidence timing resolution 315-330 ps (fwhm) in benchtop measurements and 375 ps in full-system measurements. We are currently testing prototype units for a complete redesign of LAPETs electronics, aimed at further improving full-system timing performance and at preserving that performance at high count rates. We report on four facets of the new design. First, PMT-by-PMT high-voltage control at two points per dynode chain permits both gains and timing offsets to be equalized across the scanner. Second, analog pulse shaping reduces the duration of each PMT pulse from 75 ns to 35 ns, reducing pile-up effects. Third, custom circuit boards use the DRS4 waveform-sampling ASIC to provide oscilloscope-quality readout of each PMT signal, enabling digital processing of PMT waveforms. Finally, an FPGA-based trigger provides the coarse energy and timing measurements used to detect coincident pairs. Tests are underway of prototype High Voltage Control boards, Shaper/Analog Mezzanine cards, and the DRS4-based Module Readout Board; the Master Coincidence Unit design is in progress.

Evaluation of the imaging performance of continuous detectors etched with laser induced optical barriers

Several groups are actively investigating the performance of thick continuous crystals to determine the potential of this detector design. One method to alter the behavior of this detector is to etch laser induced optical barriers (LIOBs) into the crystal. This work explores the performance of thick continuous detectors etched partly into the crystal in a grid pattern. As a motivation for this study, system Monte Carlo simulations were used to investigate the added benefit in imaging performance of improved spatial resolution. Detector simulations were used to qualitatively study the potential of etching crystals in the entrance region in a grid pattern, by varying the opacity of the etchings. The behavior of the LIOBs in response to optical light was also investigated by etching several small cubes of LYSO with a single LIOB, varying the etching parameters among the cubes, and measuring the transmission of optical light through the cubes as a function of interaction position and incident angle. Last, thick crystals were etched in this pattern and the performance characterized. The results showed that the opacity of the etchings may be altered by varying the laser power and etching pattern, and that greater opacity results in more restricted light spread within crystals etched in a grid pattern. Thick crystals etched with LIOBs in this pattern showed slightly improved performance in the etched region of the crystals, and slightly degraded performance in the unetched region. This study illustrates, therefore, that etching crystals in a grid pattern is possible, and may result in improved overall spatial resolution, though the etching parameters must still be optimized.

Image-Based Modeling of PSF Deformation With Application to Limited Angle PET Data

The point-spread-functions (PSFs) of reconstructed images can be deformed due to detector effects such as resolution blurring and parallax error, data acquisition geometry such as insufficient sampling or limited angular coverage in dual-panel PET systems, or reconstruction imperfections/simplifications. PSF deformation decreases quantitative accuracy and its spatial variation lowers consistency of lesion uptake measurement across the imaging field-of-view (FOV). This can be a significant problem with dual panel PET systems even when using TOF data and image reconstruction models of the detector and data acquisition process. To correct for the spatially variant reconstructed PSF distortions we propose to use an image-based resolution model (IRM) that includes such image PSF deformation effects. Originally the IRM was mostly used for approximating data resolution effects of standard PET systems with full angular coverage in a computationally efficient way, but recently it was also used to mitigate effects of simplified geometric projectors. Our work goes beyond this by including into the IRM reconstruction imperfections caused by combination of the limited angle, parallax errors, and any other (residual) deformation effects and testing it for challenging dual panel data with strongly asymmetric and variable PSF deformations. We applied and tested these concepts using simulated data based on our design for a dedicated breast imaging geometry (B-PET) consisting of dual-panel, time-of-flight (TOF) detectors. We compared two image-based resolution models; i) a simple spatially invariant approximation to PSF deformation, which captures only the general PSF shape through an elongated 3D Gaussian function, and ii) a spatially variant model using a Gaussian mixture model (GMM) to more accurately capture the asymmetric PSF shape in images reconstructed from data acquired with the B-PET scanner geometry. Results demonstrate that while both IRMs decrease the overall uptake bias in the reconstructed image, the second one with the spatially variant and accurate PSF shape model is also able to ameliorate the spatially variant deformation effects to provide consistent uptake results independent of the lesion location within the FOV.

Do phantom harmonization efforts translate into harmonized patient images

Scanners with different performance characteristics and reconstruction protocols can produce images that show large variations in uptake in small lesions. There have been a number of studies aimed at reducing this variability for clinical trials using phantom data to optimize the acquisition and/or reconstruction protocols. The underlying assumption is that the protocols that harmonize the phantom images will also result in reduced variability in patient images among scanners/sites. The goal of this study was to use patient data with embedded lesions of known uptake to test this assumption. The NEMA image quality (IQ) phantom with hot spheres (diam.: 10-37 mm) was scanned on two time-of-flight (TOF) PET scanners and reconstructed using a list-mode TOF ordered subsets expectation maximization (OSEM) algorithm. Patient FDG data were also acquired on these scanners and the data stored in list-mode format. List-mode events from spheres measured in air on these systems were embedded into the phantom and patient list-mode data to insert 5-6 spheres in the phantom background region and the patient liver and lung regions with a known (9.7:1) uptake with respect to the local uptake. The impact of applying post-reconstruction filtering (both resolution recovery and Gaussian smoothing) on the phantom and patient images was also studied to determine if changes measured with the phantom images using these post-filters would be the same as those measured with patient images. Contrast recovery coefficient (CRC) values measured from lesion embedding studies on the phantoms and patients agree with one another, although CRCmax showed greater variability. Additionally, relative changes in CRC brought about by application of the reconstruction post-filters to the patient images were consistent with those observed for the phantom images. This study illustrates, therefore, that these methods may be used to harmonize patient studies by first optimizing (and harmonizing) the reconstruction approach with the NEMA IQ phantom.