摘要:

In a homogeneous medium of soft tissue the radiation dose distribution due to a nonuniformly distributed beta source can be calculated by convolution of the beta dose point kernel of the nuclide with the source distribution. A possible extension of the technique to the calculation of the dose distribution in heterogeneous media involving relatively simple geometric interfaces requires the knowledge of the resulting perturbation to the beta point kernels in individual media. We simulated a soft‐tissue–bone planar interface by a polystyrene (PST)–aluminum junction and measured the change in beta dose from the dose value in homogeneous PST due to a point source of32P using7LiF thermoluminescent dosimeters. With the point source at the interface, the dose rates at 0–31, 125–156, and 283–314 mg/cm2separations from the interface were increased by (12±3)%, (8±2)%, and (3±2)%, respectively, compared with homogeneous PST. With the point source at a PST–air planar interface to simulate a soft‐tissue–air junction, the dose rates at 0–31, 139–170, and 283–314 mg/cm2from the interface were decreased by (25±4)%, (11±7)%, and (5±2)%, respectively. The changes in dose rates for these two interfaces have also been measured with degraded spectra of32P. Comparison of the experimental data with Monte Carlo calculation for a point source and the two‐group method of calculation for a plane source is also presented.

ISSN:0269-3127    

DOI:10.1118/1.596112

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Uniformity artifacts in rotating gamma camera tomography will result if there are errors in the correction factors which are routinely calculated from a static uniformity flood image. The accuracy of the correction factors is a function of the statistics in the collected flood image. Since the factors are applied to each projection view, an error in a correction factor will propagate as a projection error at the same pixel location for each view. For circular detector motion, the error in each projection is reconstructed as a ring whose maximum amplitude varies approximately inversely proportional to the square root of the distance of the projection error from the center of rotation. For noncircular detector motion the artifacts are not rings but are more complicated geometric curves. Simulations show that statistical fluctuations in the reconstructed image will mask the uniformity artifacts provided the correction flood satisfies minimum count requirements. An analytical expression is derived for the percent root‐mean‐square (% rms) error in the reconstruction and is compared with the percent relative amplitude error (% RAE) of the reconstructed artifacts in order to obtain expressions for uniformity flood counting statistics. For an elliptical source distribution with total counts equal toCT, the uniformity statistics required to reconstruct elliptical disks is inversely proportional to the square root of the area:Ū≥KCT/area1/2. The constantKdepends on the filter function and type of detector motion.

ISSN:0269-3127    

DOI:10.1118/1.596130

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The detection efficiency of a high‐pressure, gas scintillation proportional chamber (GSPC), designed for medical imaging in the 30–150 keV energy range, has been investigated through measurement and Monte Carlo simulation. Measurements were conducted on a GSPC containing 4 atm of pure xenon separated from a hexagonal array of seven ultraviolet‐sensitive photomultiplier tubes by 1.27‐cm‐thick fused‐silica windows. Experimental measurements of the photopeak efficiency, fluorescence escape efficiency, and the energy collection efficiency were obtained. Results were also obtained for different photon energies and different values of temporal resolution. The measurements were compared with the results obtained from a Monte Carlo simulation designed specifically for investigating the imaging of low‐energy photons (below 150 keV) with a gas‐filled detector. The simulation was used to estimate photopeak efficiency, fluorescence escape efficiency, photopeak‐to‐fluorescence escape peak ratio, quantum interaction efficiency, energy collection efficiency, and local energy collection efficiency. The photopeak efficiency of the GSPC relative to that of a 3‐in. (7.62‐cm)‐thick sodium iodide crystal was measured to be 0.284±0.001 at 60 keV and 0.057±0.001 at 140 keV. Of the 60‐keV photons incident upon the detector, 70%±4% interacted in the detector, with 28%±1% being in the photopeak, as estimated both by experimentation and through the simulation. The maximum energy collection efficiency was found to be 65% at 60 keV, with 46% being deposited within 0.2 cm of the initial photon interaction. The information gained from this study is being used to design an optimized detector for use in specialized nuclear medicine studies.

ISSN:0269-3127    

DOI:10.1118/1.596098

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The optical densities of vessel absorption features in small bandwidth fundus photographs have been used by many people for finding the oxygenation of the blood in retinal vessels. The purpose of this article is to show that the information found in this way can be made more accurate, and more information can be found by considering the details of the absorption profile (intensity as a function of distance across the vessel). The “wings” of the profile are primarily determined by the line‐spread function of the video camera or film. A four‐parameter curve fitting procedure using the line‐spread function and an “ideal” vessel profile can be used to eliminate the effects both of the line‐spread function and of any central reflection (vessel reflex) that may be present, yielding estimates of the vessel diameter and central absorption. The conclusions concerning the reflection are supported by studies of reflections from capillary tubes and wires in a model eye.

ISSN:0269-3127    

DOI:10.1118/1.596131

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Signal intensity (SI) variability was evaluated on a 0.15‐T resistive instrument using a phantom and was found to increase with time, repetition time (TR), and echo time (TE), ranging from 0.24% standard deviation (SD) over 34 min for 500/30 (TR/TE) images, to 2.1% SD over 5 days for 2000/30 images. Signal‐to‐noise (S/N) variability increased with time andTEbut notTRand ranged from 4.2% SD over 34 min to 7.1% SD over 5 days in 500/30 images. Variability inT1andT2measurements on phantoms ranged from 1.8% to 4.8% SD forT1and 3.6% to 6.5% SD forT2in the biological range over 5 h. High reproducibility of SI,T1, andT2measurements was demonstrated over a 6‐week period in normal muscle measurements.

ISSN:0269-3127    

DOI:10.1118/1.596099

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Attenuation properties of Lipowitz's metal have been studied for narrow and broad beams of cobalt‐60 gamma rays and 4–10 MV x rays. The measured transmitted fraction for geometries used in radiotherapy depends on the field size and depth of measurement. Therefore a calculation of dose for partially attenuated beams based on narrow beam attenuation coefficients can cause large errors in dosimetry. Our simple calculation of transmitted fractions based on primary attenuation and scattered radiation agrees quite well with the measured data for therapeutic geometries. Also given is a table for linear, mass attenuation, and mass energy absorption coefficients of Lipowitz's metal in the photon energy range from 10 keV to 10 MeV.

ISSN:0269-3127    

DOI:10.1118/1.596100

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The inclusion of air‐filled spaces in treatment fields creates a potential dosimetric problem due to the loss of charged particle equilibrium near the air–tissue interface. We have used a simulated larynx phantom and a small buildup/extrapolation chamber to compare the magnitude and spatial extent of underdosing and overdosing at the distal surface for two linear accelerators (10‐ and 6‐MV x rays) and a cobalt‐60 machine. Surface doses were compared to doses measured in a similar but homogeneous phantom to give observed/expected ratios (O/E), which were greater than 1.0 for large field sizes and less than 1.0 for small field sizes on all machines. The minimum field sizes which produce no surface underdosing for a simulated 2‐cm‐diam larynx are roughly 7×7 cm for 10‐MV x rays, 6×6 cm for 6‐MV x rays, and 5×5 cm for cobalt‐60. In addition, the depth over which underdosing occurs is seen to increase with increasing energy.

ISSN:0269-3127    

DOI:10.1118/1.596101

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.596103

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.596105

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.596132

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY