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Purpose: The purpose of this study was to explore the effects of a paraffin screen located at various positions in the maze on the neutron dose equivalent at the maze door. Methods: The neutron dose equivalent was measured at the maze door of a room containing a 15 MV linear accelerator for x-ray therapy. Measurements were performed for several positions of the paraffin screen covering only 27.5% of the cross-sectional area of the maze. The neutron dose equivalent was also measured at all screen positions. Two simple models of the neutron source were considered in which the first assumed that the source was the cross-sectional area at the inner entrance of the maze, radiating neutrons in an isotropic manner. In the second model the reduction in the neutron dose equivalent at the maze door due to the paraffin screen was considered to be a function of the mean values of the neutron fluence and energy at the screen. Results: The results of this study indicate that the equivalent dose at the maze door was reduced by a factor of 3 through the use of a paraffin screen that was placed inside the maze. It was also determined that the contributions to the dosage from areas that were not covered by the paraffin screen as viewed from the dosimeter, were 2.5 times higher than the contributions from the covered areas. This study also concluded that the contributions of the maze walls, ceiling, and floor to the total neutron dose equivalent were an order of magnitude lower than those from the surface at the far end of the maze. Conclusions: This study demonstrated that a paraffin screen could be used to reduce the neutron dose equivalent at the maze door by a factor of 3. This paper also found that the reduction of the neutron dose equivalent was a linear function of the area covered by the maze screen and that the decrease in the dose at the maze door could be modeled as an exponential function of the product [formula] at the screen. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this paper was to take the first steps toward applying noncontact ultrasound (NCU) to the tasks of monitoring osteoporosis and quantitative ultrasound imaging (QUS) of cortical bone. The authors also focused on the advantages of NCU, such as its lack of reliance on a technologist to apply transducers and a layer of acoustical coupling gel, the ability of the transducers to operate autonomously as specified by preprogrammed software, and the likely reduction in statistical and systematic errors associated with the variability in the pressure applied by the clinician to the transmitting transducer that NCU might provide. The authors also undertook this study in order to find additional applications of NCU beyond its past limited usage in assessing the severity of third degree burns. Methods: A noncontact ultrasound imaging system using a pair of specially designed broadband, 1.5 MHz noncontact piezoelectric transducers and cortical bone phantoms, were used to determine bone mineral density (BMD), speed of sound (SOS), integrated response (IR), and ultrasonic transmittance. Air gaps of greater than 3 cm, two transmission and two reflection paths, and a digital signal processor were also used in the collection of data from phantoms of nominal mass densities that varied from 1.17 to 2.25 g/cm3 and in bone mineral density from 0 to 1.7 g/cm3. Results: Good correlations between known BMD and measured SOS, IR, and transmittance were obtained for all 17 phantoms, and methods for quantifying and minimizing sources of systematic errors were outlined. The BMD of the phantom sets extended through most of the in vivo range found in cortical bone. A total of 16-20 repeated measurements of the SOS, thickness, and IR for the phantom set that were conducted over a period of several months showed a small variation in the range of measurements of ±1%-2%. These NCU data were shown to be in agreement with similar results using contact ultrasound to be within 1%-2%. Transmittance images of cortical bone phantoms showed differences in the nominal overall BMD values of the phantoms that were large enough to be distinguished by a visual examination. A list of possible sources of errors in quantitative NCU was also included in this study. Conclusions: The results of this paper suggest that NCU might find additional applications in medical imaging, beyond its original and only previous usage in assessing third degree burns. The fact that the authors' phantom measurements using conventional, gel coupled ultrasound are in agreement with those obtained with NCU demonstrates that in spite of large additional levels of attenuation of up to 150 dB and new error sources, NCU could have comparable levels of accuracy to those of conventional quantitative ultrasound, while providing the medical and patient comfort-related advantages of not involving direct contact. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study was to develop a new diagnostic technique for measuring bone mineral density (BMD) for the assessment of osteoporosis, which improves upon the coherent to Compton scattering ratio (CCSR) method, which was first developed in the 1980s. To help the authors achieve these goals, they have identified and studied two new indices for CCSR, the forward scattered to backward scattered (FS-BS) and the forward scattered to transmitted (FS-T) ratios. They believe that, at small angles, these two parameters can offer a practical in vivo determination of BMD that can be used to overcome the limitations of past CCSR systems, including high radiation dosages, costs, and examination durations. Methods: In previous CCSR studies, a high-activity radioactive source with a long half-live (usually 241Am) and an expensive and bulky cryogenic HPGe detector were applied to both in vivo and in vitro measurements. To make this technique more suitable for clinical applications, the possibility of using a standard diagnostic x-ray tube generating a continuous spectrum was investigated in this paper. Scattered radiation from trabecular bone-simulating phantoms containing various mineral densities that span the normal range of in vivo BMD was collected in this study using relatively inexpensive noncryogenic CdTe or NaI detectors. Results: The initial results demonstrate that a modified version of CCSR can be successfully applied to trabecular bone assessment using a diagnostic x-ray tube with a continuous spectrum in two variations, the FS-BS and the FS-T ratio. When FS-BS is measured, intensity spectra in the forward and backward directions must be collected while FS-T requires only the integral intensity of the scattered and transmitted (T) spectra in the energy region above 40 keV. For both of these methods, forward scattering angles less than or equal to 15° and backward scattering angles greater than or equal to (165°= 180° - 15°) are needed. Conclusions: The authors determined that FS-T is more sensitive to changes in BMD than transmission or absorption alone and that the FS-BS method can yield an absolute measurement of the mean atomic number of the scattering medium, after a correction for path-dependent attenuation. Since this study determined that the FS-T ratio is independent of the incident energy over a broad energy region, it will be possible to apply FS-T to bone densitometry using inexpensive integral photon detectors. The authors believe that, by replacing the radionuclide source with an x-ray tube and the cryogenically cooled HPGe detector with a single solid state CdTe, NaI, or silicon detector or an annular array of detectors, as suggested in this study, the past difficulties of CCSR concerning high radiation exposure, costs, and durations as well as lack of convenience can be overcome and that CCSR could eventually become popular in clinical settings. [ABSTRACT FROM AUTHOR]