Calibration and error analysis of metal‐oxide‐semiconductor field‐effect transistor dosimeters for computed tomography radiation dosimetry

Abstract

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...

High correlation between radiation dose estimates for 256‐slice CT obtained by highly parallelized hybrid Monte Carlo computation and solid‐state metal‐oxide semiconductor field‐effect transistor measurements in physical anthropomorphic phantoms

“…The main contribution to the estimated error comes from the systematic error in the MOSFET measurement due to the calibration. Its contribution is 5% (Materials and Methods: α = 0.05) . The contribution from the random error in the MOSFET measurement is typically about 1‐2% at higher values of CTDI vol to 3‐4% for the lower values of CTDI vol .…”

“…The reason was that the calibration is used for both the MOSFETs and the MC simulation so the error cancels out in the ratio. When we look at doses instead of dose ratios we do need to take into account this additional systematic error of 5% . For the MOSFETs we then have an estimated error in the dose of 7% (8 or 9% in the cases where the estimated error in the ratio was 6 or 7%).…”

“…The error in the average ratio is due to the random error in the MC result, the random error in the MOSFET voltage measurement, the systematic error due to the MOSFET to MOSFET variation, and the systematic error in the MC to MOSFET conversion factor . We thus estimate the error in the mean ratio for a single MOSFET as:…”

“…Here denotes variance, the first term in the last sum on the right denotes the random error of the MOSFET measurement, with c = 0.11 mV, and the last term denotes the systematic error of a MOSFET measurement, with α = 0.05 . The second term is the random error in the MC result (Appendix ).…”

Cardiac-Specific Conversion Factors to Estimate Radiation Effective Dose From Dose-Length Product in Computed Tomography

“…Organ dosimetry was performed using a mobileMOSFET dose verification system (TN-RD-70W; Best Medical, Ottawa, Canada), associated with high-sensitivity MOSFETs (TN-1002RD-H; Best Medical, Ottawa, Canada). Voltage (in mV) readings were translated to dose (in mGy) by calibration of the MOSFETs using an ion chamber (10×6–3CT; Radcal, Monrovia, California) with a control unit (Accu-Dose 2186; Radcal), and a standard 32cm diameter cylinder polymethylmethacrylate phantom (West Physics Consulting, Atlanta, GA), according to the calibration scheme of Trattner et al (16) Separate calibration factors were determined for each x-ray tube potential used for cardiac scan modes, due to MOSFET sensitivity to energy spectrum. MOSFETs were positioned within the phantom in all 27 internal organs contributing to ED determination (10).…”