Matthew E. Werner

Investigation of time-of-flight benefit for fully 3-DPET

The purpose of this paper is to determine the benefit that can be achieved in image quality for a time-of-flight (TOF) fully three-dimensional (3-D) whole-body positron emission tomography (PET) scanner. We simulate a 3-D whole-body time-of-flight PET scanner with a complete modeling of spatial and energy resolutions. The scanner is based on LaBr/sub 3/ Anger-logic detectors with which 300ps timing resolution has been achieved. Multiple simulations were performed for 70-cm long uniform cylinders with 27-cm and 35-cm diameters, containing hot spheres (22, 17, 13, and 10-mm diameter) in a central slice and 10-mm diameter hot spheres in a slice at 1/4 axial FOV. Image reconstruction was performed with a list-mode iterative TOF algorithm and data were analyzed after attenuation and scatter corrections for timing resolutions of 300, 600, 1000 ps and non-TOF for varying count levels. The results show that contrast recovery improves slightly with TOF (NEMA NU2-2001 analysis), and improved timing resolution leads to a faster convergence to the maximum contrast value. Detectability for 10-mm diameter hot spheres estimated using a nonprewhitening matched filter (NPW SNR) also improves nonlinearly with TOF. The gain in image quality using contrast and noise measures is proportional to the object diameter and inversely proportional to the timing resolution of the scanner. The gains in NPW SNR are smaller, but they also increase with increasing object diameter and improved timing resolution. The results show that scan times can be reduced in a TOF scanner to achieve images similar to those from a non-TOF scanner, or improved image quality achieved for same scan times.

Optimization of a fully 3D single scatter simulation algorithm for 3D PET

We describe a new implementation of a single scatter simulation (SSS) algorithm for the prediction and correction of scatter in 3D PET. In this implementation, out of field of view (FoV) scatter and activity, side shields and oblique tilts are explicitly modelled. Comparison of SSS predictions with Monte Carlo simulations and experimental data from uniform, line and cold-bar phantoms showed that the code is accurate for uniform as well as asymmetric objects and can model different energy resolution crystals and low level discriminator (LLD) settings. Absolute quantitation studies show that for most applications, the code provides a better scatter estimate than the tail-fitting scatter correction method currently in use at our institution. Several parameters such as the density of scatter points, the number of scatter distribution sampling points and the axial extent of the FoV were optimized to minimize execution time, with particular emphasis on patient studies. Development and optimization were carried out in the case of GSO-based scanners, which enjoy relatively good energy resolution. SSS estimates for scanners with lower energy resolution may result in different agreement, especially because of a higher fraction of multiple scatter events. The algorithm was applied to a brain phantom as well as to clinical whole-body studies. It proved robust in the case of large patients, where the scatter fraction increases. The execution time, inclusive of interpolation, is typically under 5 min for a whole-body study (axial FoV: 81 cm) of a 100 kg patient.

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.

Implementation and Evaluation of a 3D PET Single Scatter Simulation with TOF Modeling

Accurate quantification in TOF listmode PET reconstruction requires proper modeling of the TOF dependence of scatter. We have extended our non-TOF single scatter simulation to provide this information. We have observed excellent agreement with experimental data and an improvement in the uniformity of scatter correction over scaled non-TOF scatter correction. The method is practical in a clinical setting with existing computational resources. It can also provide simplifications to the reconstruction processing pipeline.

Characterization of a time-of-flight PET scanner based on lanthanum bromide

A proto-type time-of-flight (TOF) 3D PET scanner based on lanthanum bromide detectors has been developed. The LaBr/sub 3/(5%Ce) Anger-logic detectors in this new scanner use 4/spl times/4/spl times/30 mm pixels and continuous light-guide coupled to a hexagonal array of 50-mm PMTs. The scanner consists of 24 modules with a 93-cm detector diameter and 25-cm axial field-of-view. Initial characterization of scanner performance has been performed, including energy and timing performance. We currently measure an overall system energy resolution of 7.5% and a system timing resolution is 460 ps, although we expect these results to improve eventually when the electronics are fully optimized. Since there are not yet standard tests to quantify the benefit of TOF, we designed two phantoms with hot and cold spheres in 27-cm and 35-cm diameter vessels to evaluate the TOF performance as a function of body size. The data from this scanner are reconstructed with a fully 3D list-mode iterative TOF algorithm with all data corrections incorporated into the system model. We find that TOF reconstruction reduces the noise and background variability, especially for the larger phantom representing a large patient. In addition, TOF improves detail and contrast of the spheres (lesions), especially the smallest 10-mm sphere. The TOF reconstruction reaches convergence faster than the non-TOF reconstruction, and the rate of convergence is seen to be more insensitive to object size. These results indicate that TOF will help improve image quality and potentially reduce scan time with clinical patients.

Influence of Time-of-Flight Kernel Accuracy in TOF-PET Reconstruction

In the reconstruction of data from time-of-flight (TOF) PET systems, the timing resolution is assumed to be known accurately; in iterative reconstruction, it is included in the system model, typically as a Gaussian function. The width of the reconstruction TOF kernel is taken to be equal to the measured coincidence timing resolution (tau). If tau changes (e.g., as a function of count rate), the TOF kernel used in reconstruction will not accurately model the measured data. The goal of this work was to assess the effect of using an inaccurate value of tau in the reconstruction TOF kernel. The Alderson phantom with 10-mm hot spheres was imaged on the Philips Gemini TF PET/CT system (585-ps intrinsic timing resolution). Distributions of hot spheres in 27- and 35-cm diameter warm cylinders were also simulated (trues only) for 300- and 600-ps timing resolutions. The data were reconstructed using up to 20 iterations of TOF-OSEM with 20 chronological subsets. For simulated data, reconstruction with a TOF kernel 10-25% narrower than tau led to a 3-12% decrease in contrast for all sphere sizes at the same background noise level because sphere events were misplaced due to the finite timing accuracy. Using a reconstruction TOF kernel 10-25% wider than tau resulted in a 3-7% increase in contrast. Even wider reconstruction kernels led to still higher contrasts for kernel widths up to 2-3 times tau, although at the cost of increased reconstruction time and slower convergence rates. For kernel widths greater than 3tau, the performance decreased, as the reconstruction approached the non-TOF algorithm. These trends were also observed with the measured data, although much less pronounced. The results indicate that contrast/noise performance is fairly insensitive to inaccuracies in the reconstruction TOF kernel, provided that it is not narrower than the actual timing resolution of the data.

Time of flight coincidence timing calibration techniques using radioactive sources

The coincidence timing offset must be measured accurately and robustly in order to elicit the best performance for a time of flight (TOF) PET scanner. The proposed calibration methods involve measuring the timing differences between multiple coincidence line pairs. The absolute time biases for each pixel on the detector are assumed to be independent which greatly reduces the number of counts that are needed for a good estimate. It is not a requirement that each pixel on the detector be measured in coincidence with every other pixel on the detector, but that the coincidences involve a reasonably large number of different pixels on the opposite side. The intrinsic timing offset for each detector crystal pixel is unfolded by forming the average of the offset values of each crystal pixel with opposing pixels to average out crystal variations and photomultiplier (PMT) timing differences. In this work, a comparison is made of different timing calibration techniques using radioactive sources, specifically a rotating line source and a point source in a scattering block. The calibrations using the different methods are performed on a prototype TOF scanner and the results are presented

Design Optimization of a Time-Of-Flight, Breast PET Scanner

A dedicated breast positron emission tomography (PET) scanner with limited angle geometry can provide flexibility in detector placement around the patient as well as the ability to combine it with other imaging modalities. A primary challenge of a stationary limited angle scanner is the reduced image quality due to artifacts present in the reconstructed image leading to a loss in quantitative information. Previously, it has been shown that using time-of-flight (TOF) information in image reconstruction can help reduce these image artifacts arising due to missing angular projections. Our goal in this work is to optimize the TOF, breast scanner design by performing studies for estimating image uniformity and lesion activity uptake as a function of system timing resolution, scanner angular coverage and shape. Our results show that (i) 1.5 × 1.5 × 15 mm3 lutetium oxy-orthosilicate (LSO) crystals provide a high spatial resolution and system sensitivity relative to clinical scanners, (ii) 2/3 angular coverage scanner design with TOF timing resolution less than 600 ps is appropriate for providing a tomographic image with fewer artifacts and good lesion uptake estimation relative to other partial ring designs studied in this work, (iii) a flat scanner design with 2/3 angular coverage is affected more by larger parallax error than a curved scanner geometry with the same angular coverage, but provides more uniform lesion contrast estimate over the imaging field-of-view (FOV), (iv) 2/3 angular coverage, flat, 300 ps TOF scanner design (for short, practical scan times of ≤ 5 min per breast) provides similar precision of contrast recovery coefficient (CRC) values to a full curved, non-TOF scanner, and (v) employing depth-of-interaction (DOI) measuring detector and/or implementing resolution modeling (RM) in image reconstruction lead to improved and more uniform spatial resolution and lesion contrast over the whole FOV.

Implementation of a single scatter simulation algorithm for 3D PET: application to emission and transmission scanning

We describe a new implementation of a Single Scatter Simulation (SSS) algorithm for the prediction and correction of scatter in 3D PET. Comparison of SSS predictions to Monte Carlo simulations and experimental data from uniform, line and cold-bar phantoms showed that the code is accurate for uniform as well as asymmetric objects and can model different energy-resolution crystals and low level discriminator (LLD) settings. The method was also extended to the case of transmission scanning and applied to patient studies. Ongoing work aims at incorporating the algorithm in routine clinical image reconstruction, at evaluating its accuracy in absolute quantitation studies, and its robustness in the case of large patients, where the scatter fraction increases.

An LOR-based fully-3D PET image reconstruction using a blob-basis function

Conventional reconstruction in Positron Emission Tomography (PET) imaging involves a line-of-response (LOR) preprocessing step where the raw LOR data are interpolated to evenly spaced sinogram data. The LOR-based reconstruction eliminates this interpolation step and thus gives rise to better spatial resolution and image quality. In the Philips PET/CT product, Gemini GXL, this approach is combined with a blob basis function that leads not only to substantial suppression of the image noise but also to preservation of the resolution. When projecting along the raw LORs, however, the computational advantage associated with projecting an evenly spaced sinogram is lost. In addition, using blobs to represent an object results in more image elements to trace in the projection because an LOR intersects more blobs than voxels for equivalent image quality. Therefore, the combined use of LOR-based reconstruction and a blob basis function requires significantly more computation time and represents a reconstruction performance challenge. In the Gemini GXL software we have used a system-matrix lookup table. Both multiplicative and additive corrections are modeled in the system matrix but not included in the lookup table. By making use of the scanner symmetry, and, more importantly, by aligning the blob matrix with the axial crystal rings, the lookup table is reduced in size by a factor of more than 100. The reconstruction performance is optimized by continuous memory access and block looping techniques in a hybrid-projection method. Compared to a calculate-on-the-fly approach, it is ~3 times faster on a Xeon 3.06 GHz dual-processor computer, which allows GXL to achieve excellent clinical performance.