Biomedical subjects

M M Urie

Publications and source records attributed to M M Urie.

22 records · Page 2Linked to original sources

PubMedDate not supplied

Rotating laminar emission camera with GE-detector: experimental results.

Experimental results of a prototype rotating laminar emission camera (ROLEC) for nuclear medicine imaging are reported. A 11.5 mm thick, 45 x 45 mm high-purity germanium detector is segmented into 30 1.47 mm wide parallel channels and collimated with 39 mm high parallel plates. Projection data acquired at multiple angular orientations as the detector-collimator assembly is rotated about its center are mathematically reconstructed to image the activity distribution. The spatial resolution of the ROLEC is at least twice as good, at all distances, as that of gamma cameras with high resolution collimators. The better energy resolution of the germanium enhances the detection and resolution of the ROLEC in comparison to gamma cameras with NaI(T1) crystals, the relative superiority increasing with greater volumes and with greater depths. Adequate sensitivity is maintained while achieving these improvements in spatial resolution and in practice; ROLEC images are acquired in less time than pinhole collimator images with gamma cameras.

Animals↗

PubMedDate not supplied

Rotating laminar emission camera with GE-detector: an analysis.

Design criteria for a germanium detector emission camera with laminar collimator-detector geometry are reported. In this design, parallel plates above a grooved germanium slab form detector channels which accumulate data in multiple projections through 180 degrees. Activity distribution is determined through mathematical reconstruction from the projections. The laminar design has greater efficiency than a hole-collimated device due to a favorable open area ratio and due to collimation in only one dimension. The relative sensitivity at the center of the detector strip increases with detector length and distance from collimator face. Spatial resolution depends upon the design geometry (septal penetration and scatter within the crystal are of such small magnitude that degradation of the point spread function is minimal). A 30 channel protype device (ROLEC) shows good correlation with theoretically determined PSF and signal-to-noise ratio (SNR). Measuring times projected for a clinical sized device (300 mm diam) are about 4.6 times that of the Anger camera (with same SNR) and high resolution collimator.

Germanium↗

PubMedDate not supplied

Proton beam penumbra: effects of separation between patient and beam modifying devices.

The sharp lateral penumbra of a proton beam is often used to spare sensitive normal structures in treating clinical sites in which the target volume abuts, or even wraps around, these structures. Using Monte Carlo calculations and measurements, the factors which influence the penumbra of the proton beam at the Harvard Cyclotron Laboratory were investigated, with particular emphasis on the effects of separation between the patient and any beam modifying devices. Penumbra broadening, characterized by the distance over which the dose rises from 20% to 80% of the central dose, increases with greater amounts of scatterer introduced into the beam line. The broadening due to separation of the beam modifying devices and the patient is essentially linear with increasing air gap; the rate of increase depends on the details of these devices and on the depth of interest in the patient. For a particular portal, most of the parameters which affect the penumbra width are fixed by the patient's anatomy and the target volume. Only the thickness of the compensating bolus around the aperture edge and any air gap between the patient and the beam modifying devices can vary. Families of curves relating combinations of bolus thickness and air gap that maintain a constant penumbra width have been developed for guidelines during patient setup.

Biophysical Phenomena↗

PubMedDate not supplied

Variable versus fixed modulation of proton beams for treatments in the cranium.

Dose distributions in the cranium with fixed and with variably modulated proton beams were compared. The variable modulation was designed to tailor the proximal high-dose region of each field to the target volume surface whereas the fixed modulation beams had a constant modulation determined by the greatest extent of the target. Dose-volume histograms of normal tissues were compared, as were the estimated complication probabilities. Twelve patients with chordomas or chondrosarcomas of the base of skull who had been treated to approximately 70 cobalt Gray equivalent (CGE) were evaluated. Dose distributions of the actual treatments were compared to those which would have been delivered had the proton beams been variably modulated; two patients for whom x-ray plans were available were also evaluated. The greatest difference in dose between the variable and fixed modulation proton beams, averaged over all the patients, was 13.8 CGE (8.0-18.0 CGE range). Much of this reduction occurred in the brain, particularly the temporal lobes. In those temporal lobes receiving significant doses, variable modulation reduced the volume receiving more than 54 CGE by 3.0 cc; all temporal lobes had at least a 5 CGE difference to some portion, half had more than 10 CGE and three more than 15 CGE difference to some portion. The optic structures, brainstem and spinal cord received from 1 to 3 CGE less dose with the variability modulated beams. Eight of the parotid glands received more than 20 CGE to more than half their volume with the fixed modulation beams; in these, variable modulation reduced the mean dose by 5.3 CGE. The reduction in integral dose with variable as compared to fixed modulation was in the range 3 to 12%; this gain was considerably less than the gain for uniformly modulated proton beams over x-rays in the two patients for whom x-ray plans were available.

Brain Stem↗