Organ dose and inherent uncertainty in helical CT dosimetry due to quasiperiodic dose distributions

Purpose : The purpose of this study was to characterize relative magnitudes of peripheral dose modulations present in helical MDCT resulting from variation in x-ray tube starting position and phantom position relative to isocenter using a novel methodology that employs direct dosimetric measurements and knowledge of scan geometry. The magnitudes of potential dose savings to specific radiosensitive tissues related to the phase of the quasiperiodic dose distribution are also quantified and compared to similar Monte Carlo based studies. Methods : For this study, a Siemens SOMATOM Sensation 16 helical MDCT scanner and a tomographic adult anthropomorphic phantom from the University of Florida phantom series were used for all scans. In addition, a plastic scintillator-based fiber-optic-coupled dosimetry system was used to record real-time axial dosimetric measurements. These direct measurements were used to derive cumulative point doses and tissue doses for helical MDCT using different pitch values and surface to isocenter distances. Results : Cumulative point doses and doses for the lens of the eye and thyroid showed strong variation with both the phase of the dose distribution and phantom positioning relative to isocenter. Depending on the phantom positioning relative to isocenter, individual in-phantom cumulative point dose values were shown to vary anywhere from 0 to 25% lower than the maximum value for scans of pitch 1, and greater than 60% for scans of pitch 1.5. Reduction in total tissue dose to the lens of the eye (thyroid) varied from 0 to 20% (4%) lower than the maximum value for pitch 1, and from 59 (14%) to 71% (19%) lower than the maximum value for scans using pitch 1.5. These values are similar to those found in previous Monte Carlo based studies. The reduction in average total tissue dose between the extremes (±3 cm from nominal) of phantom positioning relative to isocenter for the eye (thyroid) was 16% (13%) for pitch 1, and 14% (12%) for pitch 1.5. Conclusions : 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.