J.N. Aarsvold

Simulation of imaging with sodium iodide crystals and position-sensitive photomultiplier tubes

The imaging characteristics of miniature gamma cameras that consist of a single sodium iodide (NaI(Tl)) crystal coupled to a position-sensitive photomultiplier tube (PSPMT) have been studied via Monte Carlo simulations. Images obtained with such cameras with the use of conventional position calculations exhibit considerable distortions, particularly compression. This study demonstrates that the distortions result primarily from nonuniform sensitivities of PSPMTs and secondarily from nonlinear responses of PSPMTs, light-reflection properties resulting from the treatments of crystals, and light-refractive properties of glass interfaces between crystals and photocathodes. Simulation results are compared to images obtained with a prototype miniature gamma camera. >

A Single-tube Miniature Gamma Camera

A Single-Tube Miniature Gamma Camera NJ Yasillo, RA Mintzer, JN Aarsvold, KL Matthews, SJ Heimsath, CE Ordonez, X Pan, C Wu, TA Block, RN Beck, C-T Chen, and M Cooper Franklin McLean Memorial Research Institute, The University of Chicago 5841 S. Maryland Ave., MC-1037, Chicago, IL 60637 In certain medical gamma imaging applications, conventional cameras are too large for practical use. To address these situations, we have constructed a clinically usable miniature gamma camera for photon energies up to 160 keV. The camera has outer dimensions of 92 mm x 92 mm x 190 mm and weighs 5 kg. Included in that mass is a collimator, an internal high-voltage power supply, and all shielding. The camera also has a position-sensitive photomultiplier tube (PSPMT) coupled to an 8-mm thick NaI(Tl) crystal; this crystal/tube combination provides signals to internal processing circuits. External electronics and analog-to-digital conversion circuits are connected to a Macintosh Quadra computer where additional processing, storage, and display of the signals takes place. Because the PSPMT has non-uniform energy response that results in geometric distortion if conventional position-arithmetic calculations are used, we have implemented a maximumlikelihood position-estimation calculation that produces undistorted images. The present version of the camera has a usable field of view of 48 mm x 48 mm and an intrinsic spatial resolution of 2.4 to 3.8 mm full width at half maximum.

Optical microscopy system for 3D dynamic imaging

We describe a prototype 3D optical microscopy system that utilizes parallel computing and high-speed networks to address a major obstacle to successful implementation of 3D dynamic microscopy: the huge computational demand of real-time dynamic 3D acquisition, reconstruction, and display, and the high-bandwidth demand of data transfer for remote processing and display. The system comprises image acquisition hardware and software, high- speed networks between acquisition and processing environments, parallel restoration using wavelet algorithms, and volume rendering and display in a virtual environment.

Maximum-likelihood calibration of small gamma cameras for 511 keV positron annihilation radiation

The feasibility of employing maximum-likelihood (ML) position estimation in small scintillation cameras for use in FDG coincidence imaging was investigated. A small camera consisting of a Hamamatsu R-2487 position-sensitive photomultiplier tube (PSPMT) coupled to a single 8 mm thick NaI(Tl) crystal was calibrated with a tungsten and lead shielded 511 keV /sup 18/F source. The same calibration method has been previously employed at 140 keV using /sup 99m/Tc. The source was positioned at each location of the image space to determine detector-signal distributions used in look-up table (LUT) generation. Corrected 511 keV point-array and flood images were obtained using the resulting 511 keV calibration LUT. Resolution was 2.0 mm-2.5 mm full-width at half maximum (FWHM). This demonstrates that it may be feasible to use ML estimation to obtain accurate event positioning in similar imaging detectors employing more suitable scintillators.

Count-rate dependent sensitivity in position-sensitive photomultiplier tubes

A significant count-rate dependence of mean scintillation-pulse amplitude has been observed in prototype small gamma cameras employing NaI(Tl) crystals coupled to Hamamatsu R2487 and R3941 position-sensitive photomultiplier tubes (PSPMTs). The magnitude of the effect is position dependent. Within each cameras 56 mm/spl times/56 mm field-of-view, as the scintillation rate is increased from 500 to 10,000 counts per second, the response to 140 keV gamma emissions from a collimated source of Tc-99m increases by approximately 2-8% in the R2487 and 1-6% in the R3941. The count-rate dependent sensitivities of both tubes were investigated by varying the rate and position of LED pulses directed at the face of the tubes with a 1 mm diameter optical fiber. Within the specified effective area of the R2487, the increase in response from a 500 Hz rate to a 100 kHz rate varied from 1.5% in the center to 20% at one edge; corresponding increases in the R3941 were only 1% and 8%. In both cases, most of the response increase occurred between 500 Hz and 10 kHz. Similar LED tests on conventional photomultiplier tubes resulted in no more than 2% increases.<<ETX>>

Modular detector systems for nuclear medicine imaging

Modular detectors provide system design flexibility that makes possible the development of many imaging systems not possible through the use of more traditional approaches such as employing fixed ring configurations or large FOV (field-of-view) detectors. We have developed two such modular detector systems. The first is a small FOV modular gamma camera based on a position-sensitive photomultiplier tube (PSPMT) and single contiguous NaI(Tl) scintillation crystal. This detector can be used, for example, in the development of systems for planar single-photon emission imaging and for single-photon emission computed tomography (SPECT). The second is a modular panel based on an array of small FOV photomultiplier tubes (PMTs) and a pixilated array of LSO crystals. This detector can be used in development of systems for positron emission tomography (PET). For each modular detector, special signal-processing and computer-interface electronics have been designed and implemented in conjunction with its own radiation shielding, crystal or crystal array, PSPMT or PMTs, and computing processors. Such modular detectors can be employed as components of general-purpose or application-specific emission imaging systems for human (clinical or research) imaging or for laboratory animal imaging. Applications for which systems with modular detectors may be relevant include diagnostic imaging of humans in specialized clinical procedures and research imaging of animals for drug discovery and development, and for biotechnology development.

Effective compensation for physical effects in 3D single-photon-emission-computed tomography

SPECT can potentially be used for quantitative imaging of in vivo 3D radiopharmaceutical distributions. Attempts for accurate quantitation in 3D SPECT images have been compromise not only by the physical effects of photon attenuation, distance-dependent spatial resolution, and scattering, but also by the lack of effective and efficient methods that will correct for these effects. In this work, we introduce a one-step method that can effectively compensate for the effects of photon attenuation and distance-dependent spatial resolution in 3D SPECT. The correction for these effects requires only a very limited amount of computation in addition to that for 3D reconstruction and hence has the potential for routine clinical application. We use both computer-generated simulations and real data to validate the approach. The results demonstrate that the proposed one-step compensation method results in reconstructed 3D SPECT images with good quantitative information.

Analytical Considerations of Photon Attenuation and System Response Function in SPECT Reconstruction

A unified framework for image reconstruction in single-photon emission computed tomography (SPECT) is proposed for modeling of the effects of photon attenuation and of the imaging-system point-spread function (PSF). This framework provides a basis for analysis of the mathematical properties of the SPECT reconstruction problem. We introduce an average-attenuation factor that is calculated along each projection line. Under certain conditions, a one-step correction method employing this average-attenuation factor compensates completely for the effects of both photon attenuation and PSF. In addition, a new hybrid approach is proposed that uses the one-step correction method at the initial step and shifts to a modified iterative Changs method in the subsequent iterations. We implemented both the iterative Changs method and our hybrid method in computer simulation studies. The results demonstrate that (a) our one-step correction method significantly improves the quality of reconstructed images over those with Changs one-step method; (b) our hybrid approach converges faster than does the iterative Changs method and produces reconstructed images of high quality.