Cardiac k-factors for all scanners and protocols are considerably higher than the k-factor currently used to estimate ED of cardiac CT studies, suggesting that radiation doses from cardiac CT have been significantly and systematically underestimated. Using cardiac-specific factors can more accurately inform the benefit-risk calculus of cardiac-imaging strategies.

Purpose Metal-oxide-semiconductor field-effect transistors (MOSFETs) serve as a helpful tool for organ radiation dosimetry, and their use has grown in computed tomography (CT). While different approaches have been used for MOSFET calibration, those using the commonly-available 100 mm pencil ionization chamber have not incorporated measurements performed throughout its length, and moreover no previous work has rigorously evaluated the multiple sources of error involved in MOSFET calibration. In this paper we propose a new MOSFET calibration approach to translate MOSFET voltage measurements into absorbed dose from CT, based on serial measurements performed throughout the length of a 100 mm ionization chamber, and perform an analysis of the errors of MOSFET voltage measurements and four sources of error in calibration. Methods MOSFET calibration was performed at two sites, to determine single calibration factors for tube potentials of 80, 100, and 120 kVp, using a 100 mm long pencil ion chamber and a cylindrical computed tomography dose index (CTDI) phantom of 32 cm diameter. The dose profile along the 100 mm ion chamber axis was sampled in 5 mm intervals by nine MOSFETs in the nine holes of the CTDI phantom. Variance of the absorbed dose was modeled as a sum of the MOSFET voltage measurement variance and the calibration factor variance, the latter being comprised of three main subcomponents: ionization chamber reading variance, MOSFET-to-MOSFET variation and a contribution related to the fact that the average calibration factor of a few MOSFETs was used as an estimate for the average value of all MOSFETs. MOSFET voltage measurement error was estimated based on sets of repeated measurements. The calibration factor overall voltage measurement error was calculated from the above analysis. Results Calibration factors determined were close to those reported in the literature and by the manufacturer (∼ 3mV/mGy), ranging from 2.87 to 3.13 mV/mGy. The error σV of a MOSFET voltage measurement was shown to be proportional to the square root of the voltage V: σV=cV where c = 0.11 mV. A main contributor to the error in the calibration factor was the ionization chamber reading error with 5% error. The usage of a single calibration factor for all MOSFETs introduced an additional error of about 5-7%, depending on the number of MOSFETs that were used to determine the single calibration factor. The expected overall error in a high dose region (∼30 mGy) was estimated to be about 8%, compared to 6% when an individual MOSFET calibration was performed. For a low-dose region (∼3 mGy) these values were 13% and 12%. Conclusions A MOSFET calibration method was developed using a 100 mm pencil ion chamber and a CTDI phantom, accompanied by an absorbed dose error analysis reflecting multiple sources of measurement error. When using a single calibration factor, per tube potential, for different MOSFETs, only a small error was introduced into absorbed dose determinations, thus supporting the use of such a calibration factor for experim...

Since its introduction in the 1970s, diagnostic computed tomography (CT) imaging has grown rapidly and developed into a standard diagnostic test for a wide variety of cardiovascular conditions. Although this has undoubtedly led to improved medical care, it has also been associated with a significant increase in population-based radiation exposure and the potential downstream increase in cancer is a justified concern. For cardiovascular CT, new CT scanner technologies were initially directed toward maximizing image quality rather than minimizing radiation exposure. Only more recently have technologic advances yielded dose-saving protocols for cardiovascular applications, with impressive reduction of radiation exposure. The achievable limits of population-based exposure are dependent on responsible, evidence-based use of CT for cardiovascular imaging as well as exploitation of available and emerging dose-saving strategies.