R. I. Wiener

The imaging performance of a LaBr3-based PET scanner

A prototype time-of-flight (TOF) PET scanner based on cerium-doped lanthanum bromide [LaBr(3) (5% Ce)] has been developed. LaBr(3) has a high light output, excellent energy resolution and fast timing properties that have been predicted to lead to good image quality. Intrinsic performance measurements of spatial resolution, sensitivity and scatter fraction demonstrate good conventional PET performance; the results agree with previous simulation studies. Phantom measurements show the excellent image quality achievable with the prototype system. Phantom measurements and corresponding simulations show a faster and more uniform convergence rate, as well as more uniform quantification, for TOF reconstruction of the data, which have 375 ps intrinsic timing resolution, compared to non-TOF images. Measurements and simulations of a hot and cold sphere phantom show that the 7% energy resolution helps to mitigate residual errors in the scatter estimate because a high energy threshold (>480 keV) can be used to restrict the amount of scatter accepted without a loss of true events. Preliminary results with incorporation of a model of detector blurring in the iterative reconstruction algorithm not only show improved contrast recovery but also point out the importance of an accurate resolution model of the tails of LaBr(3)s point spread function. The LaBr(3) TOF-PET scanner demonstrated the impact of superior timing and energy resolutions on image quality.

Comparison of diffuse optical tomography of human breast with whole‐body and breast‐only positron emission tomography

We acquire and compare three-dimensional tomographic breast images of three females with suspicious masses using diffuse optical tomography (DOT) and positron emission tomography (PET). Co-registration of DOT and PET images was facilitated by a mutual information maximization algorithm. We also compared DOT and whole-body PET images of 14 patients with breast abnormalities. Positive correlations were found between total hemoglobin concentration and tissue scattering measured by DOT, and fluorodeoxyglucose (18F-FDG) uptake. In light of these observations, we suggest potential benefits of combining both PET and DOT for characterization of breast lesions.

DOI Determination by Rise Time Discrimination in Single-Ended Readout for TOF PET Imaging

Clinical TOF PET systems achieve detection efficiency using thick crystals, typically of thickness 2-3 cm. The resulting dispersion in interaction depths degrades spatial resolution for increasing radial positions due to parallax error. Furthermore, interaction depth dispersion results in time pickoff dispersion and thus in degraded timing resolution, and is therefore of added concern in TOF scanners. Using fast signal digitization, we characterize the timing performance, pulse shape and light output of LaBr3:Ce, CeBr3 and LYSO. Coincidence timing resolution is shown to degrade by ~50 ps/cm for scintillator pixels of constant cross section and increasing length. By controlling irradiation depth in a scintillator pixel, we show that DOI-dependence of time pickoff is a significant factor in the loss of timing performance in thick detectors. Using the correlated DOI-dependence of time pickoff and charge collection, we apply a charge-based correction to the time pickoff, obtaining improved coincidence timing resolution of <; 200 ps for a uniform 4 × 4 × 30 mm3 LaBr3 pixel. In order to obtain both DOI identification and improved timing resolution, we design a two layer LaBr3[5%Ce]/LaBr3[30%Ce] detector of total size 4 × 4 × 30 mm3, exploiting the dependence of scintillator rise time on [Ce] in LaBr3:Ce . Using signal rise time to determine interaction layer, excellent interaction layer discrimination is achieved, while maintaining coincidence timing resolution of <; 250 ps and energy resolution <; 7% using a R4998 PMT. Excellent layer separation and timing performance is measured with several other commercially-available TOF photodetectors, demonstrating the practicality of this design. These results indicate the feasibility of rise time discrimination as a technique for measuring event DOI while maintaining sensitivity, timing and energy performance, in a well-known detector architecture.

An investigation of waveform sampling for improved signal processing in TOF PET

The development of fast photodetectors and of fast, high light output scintillation crystals has placed an increased emphasis on the need for fast readout electronics used for Time-Of-Flight (TOF) PET detectors. These improvements have paralleled developments in analog sampling technology that makes fast waveform digitizing of photodetector signals an attractive alternative to custom PCB and IC designs. Waveform digitization offers a flexible means for evaluating and implementing more complex signal processing algorithms. We used a commercial 1GHz 2Gs/s waveform digitizing system to acquire coincident pulses from LYSO and LaBr3 (5%Ce) scintillator crystals coupled to a fast PMT. By measuring the time pickoff based on a linear fit to the rising edge of the pulse, we show an improvement in coincident timing resolution from ∼200ps to ∼80ps FWHM for LaBr3(5%Ce) coupled to Photonis XP20D0 PMTs, and from ∼160ps to ∼80ps Full Width at Half Maximum (FWHM) for LaBr3(5%Ce) coupled to Hamamatsu H4998 PMTs. Our results show a consistent improvement in timing resolution as well as reduced sensitivity to signal risetime as compared to leading edge (LE) pickoff techniques. Measurements with partial detector modules indicate that system timing resolution of ≪200ps is attainable with LaBr3 (5%Ce) pixelated detectors designed for TOF PET.

Signal analysis for improved timing resolution with scintillation detectors for TOF PET imaging

Clinical TOF PET systems offer significantly degraded timing performance in comparison to that measured with small crystal geometries of the same scintillators. The usage of long, narrow crystal geometries is a major contributor to the degradation in system timing performance. We explore the effect of increased pixel length on timing performance of LaBr3[Ce], CeBr3 and LYSO detectors. Using fast signal digitization we characterize the response of the detector to 511keV photons interacting at different depths. Systematic shift in time pickoff with interaction depth is shown to degrade the timing performance of long crystals. Collected light, signal shape and signal arrival time are shown to correlate with interaction depth. The depth dependence of detector response shows strong dependence on surface treatment, with finer surface treatment corresponding to reduced depth sensitivity. The correlated dependencies of time pickoff, energy and signal shape offer a method for recovering timing information lost due to interaction depth dispersion. An energy-based correction to the time pickoff is shown to improve detector timing resolution. The improvement in timing performance of thick detectors with single sided readout demonstrates the benefit of inclusion of additional signal information attainable by fast signal digitization.

Evaluation of local PMT triggering electronics for a TOF-PET scanner

The development of high light output scintillation crystals with fast decay times has made clinical whole body TOF-PET scanners possible. Current clinical systems based on LYSO have timing resolutions near 600ps (FWHM), but considerably better resolution is possible with faster scintillators. We have previously reported results for a prototype TOF-PET scanner using LaBr3 (5% Ce) as the scintillation crystal. These preliminary results were obtained using a “local” triggering electronics scheme, in which the seven PMTs used for energy and positioning measurements are also used to produce a trigger. The local triggering design is intended to reduce the effects of pulse pileup and noise, reducing deterioration of the timing resolution at high count rates, and allowing us to take full advantage of the timing properties of LaBr3. In this work we present the details of the implementation of our design, and demonstrate the impact of the size of the trigger zone on the system timing resolution with increasing count rate. Optimization of the system has allowed us to achieve a 375ps (FWHM) system timing resolution of our prototype scanner.

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.

Timing and energy characteristics of LaBr 3 [Ce] and CeBr 3 scintillators read by FBK SiPMs

Lanthanum Halide scintillators exhibit high light output and fast decay times. The resulting high radiant flux has enabled the measurement of annihilation photon coincidence timing resolution of &#60;100ps FWHM as well as with energy resolution&#60;4% FWHM using conventional PMTs. Taking full advantage of these characteristics in a TOF PET detector is greatly affected by the performance of the photodetector. The ability to fabricate Silicon Photomultipliers (SiPM) pixels of comparable size to the cross section of scintillator pixels used in clinical scanners (4×4mm2) allows the direct coupling of a crystal array to an array of SiPM pixels. SiPMs fabricated at FBK have been designed to offer high Photon Detection Efficiency (PDE), high spatial and temporal transit time uniformity, fast risetime and low noise. The high gain and low noise of the FBK SiPM result in signal amplitude of ∼200mV, enabling excellent performance using passive readout circuits. We characterize the response of LaBr3[Ce] and LYSO scintillators read out by FBK SiPM to annihilation photons. 511KeV photopeak energy resolution of 5.6% and 12.5% is measured with LaBr3 and LYSO, respectively, despite of the saturation behavior evident at that energy. Coincidence timing resolution of 163ps and 249ps FWHM are measured for 5mm long and 30mm long pixels, respectively. While measured performance with LaBr3 does not quite yet match performance measured with PMTs, these results are very encouraging considering that QE is not yet well matched to shorter wavelength emission of LaBr3.

Waveform-sampling electronics for time-of-flight PET scanner

Waveform sampling (WFS) is an appealing technique for instruments requiring precision time and pulse-height measurements. Recent advances in switched-capacitor-array ASICs such as the Domino Ring Sampler (DRS4) have made WFS affordable for large systems. LAPET is a whole-body time-of-flight PET scanner using 38880 LaBr3(5% Ce) scintillator crystals of dimension 4 × 4 × 30 mm3, imaged by 432 Photonis XP20D0 PMTs, grouped into 24 identical detector modules. High light yield (61000 photons/MeV) and fast decay time (20 ns) make LaBr3 an excellent scintillator for TOF PET. Our group previously reported coincidence timing resolution 315–330 ps (fwhm) in benchtop measurements and 375 ps in full-system measurements using semi-custom electronics. This contribution reports on a complete redesign of the LAPET electronics, trigger, and data acquisition system. Our design uses 240 DRS4 chips to obtain oscilloscope-quality sampling of each PMT waveform at 2 GSPS. The 7 PMTs with which each crystals scintillation light is collected map cleanly into the 8 analog inputs of a DRS4 chip, facilitating a redundant and nearly deadtime-free (at clinical rates) trigger design, in spite of the ∼ 3 µs required for DRS4 readout. An FPGA-based trigger using analog pulse shaping and 100 MSPS sampling provides coarse energy and timing measurements used to detect coincident pairs and to select DRS4 chips for readout. Simulation studies show that oscilloscope-quality readout of each PMT signal will permit more flexible handling of detector calibrations, PMT waveform baseline offsets, and pulse pile-up effects. We thus expect the upgraded electronics to permit system-level performance that more closely approximates single-module benchtop results and to preserve that performance at clinical count rates. Our goals are both to explore the feasibility of WFS for a large scanner and to improve the overall performance of the LAPET research scanner. We present initial tests using prototype units of our redesigned electronics.

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.