Jeffrey A. Kolthammer

Optimized dynamic framing for PET-based myocardial blood flow estimation

An optimal experiment design methodology was developed to select the framing schedule to be used in dynamic positron emission tomography (PET) for estimation of myocardial blood flow using (82)Rb. A compartment model and an arterial input function based on measured data were used to calculate a D-optimality criterion for a wide range of candidate framing schedules. To validate the optimality calculation, noisy time-activity curves were simulated, from which parameter values were estimated using an efficient and robust decomposition of the estimation problem. D-optimized schedules improved estimate precision compared to non-optimized schedules, including previously published schedules. To assess robustness, a range of physiologic conditions were simulated. Schedules that were optimal for one condition were nearly-optimal for others. The effect of infusion duration was investigated. Optimality was better for shorter than for longer tracer infusion durations, with the optimal schedule for the shortest infusion duration being nearly optimal for other durations. Together this suggests that a framing schedule optimized for one set of conditions will also work well for others and it is not necessary to use different schedules for different infusion durations or for rest and stress studies. The method for optimizing schedules is general and could be applied in other dynamic PET imaging studies.

A Monte-Carlo simulation study to evaluate septal spacing using triple-head hybrid PET imaging

The low coincidence fraction (ratio of trues to singles detected) and a limited count rate capability reduce performance of hybrid PET systems as compared with dedicated PET. Axial collimation (i.e., septa oriented perpendicular to the axis of rotation) has been used to reduce acceptance of photons from outside the field of view, reduce scattered events, and allow 2D image reconstruction algorithms. The major drawback of axial collimation is a reduced coincidence detection efficiency. The goal of this study was to evaluate the effect of axial collimator septal spacing, as well as to evaluate a new hybrid parallel-fan beam axial collimator design. The GEANT Monte-Carlo simulation code was modified to model a Marconi IRIX triple-head hybrid PET system in the 0/spl deg/-90/spl deg/-270/spl deg/ geometry and used to investigate varying axial collimator designs. Basic PET system parameters such as scatter fraction, global trues to singles (T/S) ratio, axial profiles of the T/S ratio, and energy spectra were examined. Comparisons to an experimentally acquired line source phantom showed that the GEANT Monte-Carlo accurately modeled the Marconi IRIX camera. The T/S ratio was observed to increase with increasing septal spacing from 0.77 cm to 2.27 cm and was approximately constant for larger septal spacing. Widening the septal spacing modified the T/S axial profiles making them more non-uniform with a strong peak towards the axial center of the field-of-view. Using the hybrid parallel-fan beam axial collimator provided a more desirable uniform axial T/S profile. In addition, for a given singles rate, a larger proportion of singles was measured within the photopeak energy window as septal spacing decreased. This observation can have implications on the recorded randoms rate for some PET systems.

Dynamic Load Balancing on Distributed Listmode Time-of-Flight Image Reconstruction

A major obstacle in performing listmode reconstruction in PET imaging is the increased computation time compared to a conventional frame or histogrammed reconstruction. To overcome this challenge in a clinical setting, it is desirable to distribute the reconstruction task to multiple nodes. A previous work investigated the impact of high performance communication networks and focused mainly on static distribution. In practice, optimal static load balancing is difficult. Therefore we have developed a dynamic load balancing approach, which is flexible and can easily be adapted to a varying number of nodes, and the performance of which is not constrained by variation of the load levels of nodes needed for other tasks or by asymmetric network. In this approach, one of the nodes is designated as the distributor, whose task is to partition the events into small chunks and then distribute those chunks to other nodes for processing. Other nodes, which do the actual data processing, are called workers. Each worker requests a new chunk of data to process upon completion of an old one. In case of the OSEM algorithm, when all chunks have been processed in a subset, the workers are synchronized and the image is updated. This image forms the basis for the next subset. This system has been deployed in the Philips GEMINI-TF PET/CT system. For a whole-body patient scan of 150M events, the event processing time with 8 Xeon 3.6GHz dual-processor computers amounts to approximately 9 minutes for 3 iterations.

CT-based attenuation correction in PET image reconstruction for the Gemini system

The Gemini system is a combined CT/PET imaging system newly developed by Philips Medical Systems. It has a unique open gantry design that allows for variable separation between the CT and PET gantries. The Gemini system incorporates CT-based attenuation correction (CT-AC) into a three-dimensional row-action maximum likelihood algorithm (RAMLA) for PET image reconstruction. It uses several unique techniques to achieve high accuracy while reducing patient X-ray dose. These new techniques include (1) using low-dose CT protocols to obtain CT images with adequate quality and quantitation for CT-AC while keeping patient X-ray dose low; (2) using a CT truncation compensation technique to improve the accuracy of CT-AC; and (3) using a generalized model for the conversion of CT images to attenuation maps at 511 keV. In this paper, we report the workflow and performance of Gemini CT-AC using phantom and patient studies. For comparison, attenuation maps obtained from PET transmission scans are also used for attenuation correction (TX-AC). Both phantom and patient studies show that PET images with CT-AC have image quality equivalent to or better than those with TX- AC.

Downscatter contamination from high-energy photons of /sup 124/iodine in 2D and 3D PET

Iodine is an attractive isotope for medical imaging; /sup 124/I makes available to PET a large set of compounds and protocols traditionally used in X-ray and SPECT imaging and offers additional clinical opportunities in molecular and genomic imaging. However, /sup 124/I has a very complex decay scheme in which higher energy contaminants that compete for abundance with the positron decay. This paper analysis, via Monte Carlo studies, the impact of contaminates from /sup 124/I on 2D and 3D PET imaging systems. Various geometries and detector materials have been simulated. Results show two trends. The presence of a significant amount of lead or tungsten in the field-of-view in the 2D scanner makes it more sensitive to this contamination by causing downscatter into the PET energy window; improved energy resolution decreases the sensitivity to the contamination. Overall, the contamination in the primary energy window is in the range of 20% higher in a 2D imager then in a comparable 3D geometry, an effect that increases with improving energy resolution. A significant proportion of the gamma events are emitted in coincidence with positrons and present an additional and challenge to imaging, but the physical advantage of 3D remains, such that the noise in the coincidence data relative to 2D is reduced from the /sup 18/F or pure-beta emitter imaging case. Varying with energy resolution, the relative improvement in SNR over the 2D geometry ranges from 11%-61%.

Evaluation of a fully 3D, big bore TOF PET scanner with reduced scatter shields

Traditionally, PET scanners have annular lead shielding at the axial ends that extends beyond the crystals to reduce the number of photons from outside of the axial field of view (FOV) hitting the detector. In recent years there has been a trend toward reducing the end shielding in order to increase the patient port diameter of the PET scanner. The reduction of the lead shielding could have performance effects that could affect overall image quality. The University of Pennsylvania has both a Philips Gemini TF Big Bore and a standard Gemini TF, with the major difference between the two systems being the reduction of the end shielding on BigBore. We evaluated the count-rate performance between the two systems to determine differences in performance characteristics. We also performed phantom measurements to determine the impact of performance differences on the scatter correction and reconstructed image quality. While there are differences in the performance of BigBore as compared to TF, the overall image quality of studies obtained on BigBore are comparable to those obtained on TF.

Randoms distributions for triple-head gamma-camera PET systems

Understanding the effects of random coincidences in PET is important to the analysis of emission images. The noncircular detector structure of triple-head systems gives rise to different randoms distributions than ring tomographs. Using a simulation method, it is found that the shape of the randoms distributions for triple-head systems is more sensitive to the configuration of the detector heads than to the source distribution. The distribution for triple-head systems is spread across the field-of-view, peaking off-center; the dual-head case peaks in the center of the FOV. We also discuss the difference in the axial distribution for 2D and 3D acquisition and rebinning; the introduction of septa and an axial acceptance angle reduces randoms, illustrating an important trade-off between imaging with and without axial septa.