K.R. Pollard

Improving the efficiency of emission tomography simulations using variance reduction techniques

The development for public distribution of a general-purpose code, SIMSET, for the modeling of positron-emission and single-photon-emission tomographs (PET and SPECT) is discussed. An important part of the SIMSET project is the development of methods for efficient photon simulation which can handle heterogeneous distributions of activity and attenuation. For both positron and single-photon tomographs, the solid angle of acceptance of the detector array is small, due to collimation and to the size of the detector array itself. This results in significant computational inefficiencies with conventional Monte Carlo simulation, because only a few percent of the photons generated and tracked will actually be detected. A similar kind of problem arises in radiation shielding calculations, where the flux through the shield, although significant, may correspond to only a tiny fraction of the initial emitted photons. To improve the efficiency problem of simulations, some techniques from the radiation shielding literature have been adapted to increase the yield of detected photons. >

Monte Carlo simulation of BaF/sub 2/ detectors used in time-of-flight positron emission tomography

To better understand the performance of the SP3000 time-of-flight PET (positron emission tomography), machine, the authors performed Monte Carlo calculations of photon interactions in a BaF/sub 2/ detector module and calculations of crystal optics for BaF/sub 2/ UV scintillation light. Evaluated were photon detection sensitivities, the amount of photon spillover, crystal crosstalk, interring miscodes, and true-to-random coincidence ratios as a function of the lower-level energy discriminator setting and crystal/photomultiplier tube energy resolution. UV light collection efficiencies for the timing tube and decode tube were calculated assuming different crystal geometries, reflective materials, and reflectances. For a typical SP3000 detector module the detection efficiency for single 511-keV photons varied between 20 and 60% depending on the angle of incidence. >

Post therapy imaging in high dose I‐131 radioimmunotherapy patients

The biodistribution of a trace-labeled I-131 antibody is used to predict the biodistribution of a high dose I-131 antibody for therapy. Internal radiation dose estimates derived from the trace-labeled antibody have been used to determine the I-131 doses in a phase I escalating dose therapy trial for hematologic malignancy. To confirm the hypothesis that the distribution of a trace- and high-dose labeled antibodies are similar, both trace (7-11 mCi, 259-407 MBq) and high-dose (100-800 mCi, 3700-29600 MBq) I-131 radiolabeled antibody infusion were imaged in 12 patients who were treated for leukemia or lymphoma. With specialized imaging techniques using lead attenuation sheets, clearance data from organs were obtained from serial gamma camera images. Biological clearance half times of I-131 from both trace and therapy level doses were in agreement. An exception was a patient who developed human antimouse antibody before therapy, and subsequently had rapid clearance of the therapy dose. The method was feasible, yielded reproducible results, and provided critical data for relating therapy toxicity to radiation absorbed dose estimates.

A new clinical scintillation camera with pulse tail extrapolation electronics

The performance of a novel scintillation camera, designed for high-event-rate capability was evaluated. The system consisted of a 400-mm field-of-view NaI(Tl) camera with 61 photomultiplier tubes and modified General Electric Starcam electronics. A significant feature of the system was circuitry for performing pulse tail extrapolation and separation of individual pulses involved in pulse pile-up events. System deadtime, flood field uniformity, energy resolution, linearity, spatial resolution bar phantom image quality, and misplaced events were evaluated for count rates up to 200 kcps in a 20% photopeak window. Results indicate that this camera design does not compromise image quality at normal clinical count rates and that at higher event rates, it can provide better image quality and increased sensitivity over many Anger cameras employed in nuclear medicine. >

Quantitative evaluation of an energy-based scatter correction using planar Rollo phantom images

An energy-based scatter correction for SPECT and planar images is quantitatively evaluated using measured Rollo phantom data. The correction relies on model fits to the energy spectra at each pixel in a given projection image. For analysis purposes, a numerical model of the Rollo phantom is employed to construct a projection image corresponding to that expected from an ideal imaging system. This is then convolved with the measured spatial point response (in air) of the actual imaging system to produce a realistic estimate of the scatter-free image. Summary statistics derived from comparisons between this image and the raw and scatter-corrected images are extremely reliable quantitative measures of the detrimental effects of scatter. Position and energy (xyE) list mode data were acquired for several isotopes, each with 0, 5, 10, and 15 cm of Lucite intervening between the phantom and the collimator face to induce varying amounts of scatter. Analysis of the /sup 99m/Tc and /sup 201/Tl data demonstrates a significant improvement in quantitation for high count images as well as for images of clinical count densities. >

Scatter Correction In Single-photon Imaging Using Multiple Energy Bins And Minimum-risk Estimators

We are investigating methods of scatter correction that use multiple (typically, 615) energy bins for scatter correction in single photon imaging. We have previously reported preliminary results of this work with simple phantoms. In this article, we present new experimental results using the Rollo phantom, a complex phantom designed for evaluation of quantitative imaging techniques. We have refined our correction techniques by allowing the use of Poisson weighting and by developing a technique for progressive basis function refinement. We have studied a new class of methods, the “minimum risk” estimators, which are specifically designed to minimize the mean-square error (MSE) of primary and scatter estimates (much like the goal of the Wiener filter, but in the energy domain). The performance of minimum-risk estimators and our earlier constrained minimization techniques both improve on that of simple deconvolution, particularly with respect to noise sensitivity. We have applied these techniques to the removal of scatter from images of the Rollo phantom filled with Tc-99m, 1-131, and T1-201; studies with variable-depth scatterers and other isotopes are currently underway.

A Method For Quantitative Evaluation Of Scatter Corrections To Rollo Phantom Images

A method to quantify the degradation of planar gammacamera images of a phantom (applied in this case to a Rollo phantom) due to Compton-scatter is presented. This provides an essential tool for testing and optimizing an energy-based scatter-correction currently under development. We use a numerical model of the Rollo phantom to construct a projection image corresponding to that expected from an ideal imaging system. This is then convolved with the measured spatial point response (in air) of the actual imaging system to produce a realistic estimate of the scatter-free image. Summary statistics derived from comparisons between this image and the raw and scatter-corrected images are extremely reliable quantitative measures of the detrimental effects of scatter. Position and energy (xyE) list mode data were acquired for several isotopes, each with 0, 5, 10, and 15 cm of Lucite intervening between the phantom and the collimator face to induce varying amounts of scatter. Quantitative results based on these data and the numerical Rollo model are presented.

Applications Of Importance Sampling To Simulations Of Emission Tomography

Some refinements in the use of importance sampling for increasing the computational speed of emission tomography simulations are described. They have been applied to investigating the feasibility of a new scatter correction method for SPECT, which is illustrated here. I. INTRODUCTION The Imaging Research Laboratory at the University of Washington is developing a Simulation System for Emission Tomographs (SimSET) for public distribution. An important part of the SimSET project is the development of photon tracking methods for simulation of heterogeneous distributions of activity and attenuation in reasonable computational times. The solid angle of acceptance for positron and single-photon tomographs is small because of collimation and the size of the detector array. This results in significant computational inefficiencies with conventional Monte Carlo simulation, because only a small proportion of the photons generated and tracked will actually be detected. Last year we reported on our adaptation of some techniques from the radiation shielding literature to simulations of emission tomography. We found that we could improve the efficiency of our simulations by 1 to 2 orders of magnitude. Since then we have made several improvements in our application of these techniques and have seen further gains in efficiency. Below we review the importance sampling techniques we previously applied and note the improvements we have made over the past year. We also report some preliminary work on the application of our simulation to an objectspecific scatter correction technique.