جستجوی مقالات مرتبط با کلیدواژه "scatter" در نشریات گروه "پزشکی"

The percent depth dose (PDD) is used as a dose calculation method in radiation therapy. PDD varies with source surface distance (SSD) according to inverse square law. In most clinical situations it is necessary to convert standard SSD to that must be used in practice. Therefore; this research proposes a new factor to calculate PDD corresponding to SSD.
Electa compact accelerator for 6MV photon beam, Scanditronix blue phantom the size of 50 ×50 ×50cm3 and two ionization chamber with sensitive volume of 0.13 cc were used. Dosimetry was done for field sizes: 8×8, 10×10¡ 6.4×6.4 cm2 at SSD=80 and 100cm. Ks0 and Ks taking into account the collimator scatter, for fields in the standard SSD and the practical SSD, respectively. The ratio of Ks0 and Ks was applied to correct the Maynord F factor.
This corrected Maynord F factor revealed better results for 8×8 and 10×10 cm2 fields further more the corrected Maynord F factor, reached the more precision in buildup region for the 6.4×6.4 cm2 field for PDD calculations.
For small fields with less collimator scatter, Maynord F factor method is more accurate to calculate PDD changes with varying SSD. Although, for large depths or SSDs, using the corrected Maynord F factor will show better results for calculating PDD variations with SSD changes.

One of the most important parameters in x-ray CT imaging is the noise induced by detected scattered radiation. The detected scattered radiation is completely dependent on the scanner geometry as well as size, shape and material of the scanned object. The magnitude and spatial distribution of the scattered radiation in x-ray CT should be quantified for development of robust scatter correction techniques. Empirical methods based on blocking the primary photons in a small region are not able to extract scatter in all elements of the detector array while the scatter profile is required for a scatter correction procedure. In this study, we measured scatter profiles in 64 slice CT scanners using a new experimental measurement. To measure the scatter profile, a lead block array was inserted under the collimator and the phantom was exposed at the isocenter. The raw data file, which contained detector array readouts, was transferred to a PC and was read using a dedicated GUI running under MatLab 7.5. The scatter profile was extracted by interpolating the shadowed area. The scatter and SPR profiles were measured. Increasing the tube voltage from 80 to 140 kVp resulted in an 80% fall off in SPR for a water phantom (d=210 mm) and 86% for a polypropylene phantom (d = 350 mm). Increasing the air gap to 20.9 cm caused a 30% decrease in SPR. In this study, we presented a novel approach for measurement of scattered radiation distribution and SPR in a CT scanner with 64-slice capability using a lead block array. The method can also be used on other multi-slice CT scanners. The proposed technique can accurately estimate scatter profiles. It is relatively straightforward, easy to use, and can be used for any related measurement.

Scattered photons are one of the main causes of degrading the contrast of lesions and resolution in SPECT imaging of the heart that result in error in quantification. The usual technique for the rejection of scattered photons is through energy windowing. However, because of limited energy resolution of current scintillation cameras it is impossible to avoid scatter photons from detection. Modeling of Compton scattering through finding suitable functions was proposed in this study. These functions were used for scatter correction through deconvolution in next step. Monte Carlo simulation was used for creating projections of three different activity sources: a line source passed through left ventricle, a point source placed in left ventricle and finally real activity distribution of Tc-99m in torso organs. All of these sources were placed in a digital attenuation phantom which modeled a real patient body. Images of primary and scattered photons were acquired separately. Convolution and 2D deconvolution in Fourier domain was applied for estimating the primary projections through total ones. In the first step, scatter and total images were modeled as convolution of a modified exponential function with primary image. The best exponent value was determined for each of 64 views (0.115 to 0.150 according to heart to detector distance). In the next step, these functions were used for scatter correction through deconvolution. Sum of the square differences between the primary and scatter corrected images were decreased considerably, myocardium to cavity contrast increased for all of 64 views (34% ± 10%). Good agreement between the real primary and scatter corrected images were also found.Discussion and These results indicate that deconvolution technique for the scatter compensation can significantly reduce the degrading roles of scattering in quantitative SPECT imaging.