Landsat Thematic Mapper image-derived MTF

Abstract

As recent Monte Carlo simulations have shown, there exists an inherent uncertainty when performing dose measurements within a phantom during helical MDCT scans. The periodic dose distributions in helical MDCT means that low resolution sampling of local phantom doses could result in dose measurement aliasing. For reliable results, these considerations should be accounted for in helical MDCT phantom dosimetry studies. The variability in surface dose has a strong dependence on phantom positioning relative to isocenter. Dose to tissues such as the lens of the eye and thyroid can be minimized by positioning patients, so these tissues are closer to isocenter because the decrease in x-ray intensity due to beam divergence dominates the increases resulting from increased primary beam exposure overlap. Of course, this dose decrease would have to be balanced against any diminished image quality resulting from misalignment of the patient with the bowtie filter. Additionally, significantly reduced dose to small radiosensitive tissues such as the lens of the eye could occur if it were possible to shift the phase of the periodic dose distribution present in helical MDCT. These dose reductions would come at no cost to image quality.