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 (~3 mV/mGy), ranging from 2.87 to 3.13 mV/mGy. The error σ_V 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 a single calibration factor for experiments involving many MOSFETs, such as those required to accurately estimate radiation effective dose.

Keywords: MOSFET; calibration; dosimetry; ion chamber; measurement error.

Figure 1

Illustration of the axial scan radiation dose profile D(z).

Figure 2

CTDI phantom with nine holes, one at the center, four at the periphery, noted as clock positions 3:00, 6:00, 9:00, and 12:00, and four at the midway part, at clock positions 1:30, 4:30, 7:30, and 10:30, between center and periphery. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

Right image: A custom‐made holder for a MOSFET, with lines at 5 mm intervals, that is used to move the MOSFET in discrete steps. Left image: A schematic depiction of the holder with the marked lines at 5 mm intervals containing a MOSFET centered on the z‐axis. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

Illustration showing the movement of the holder with MOSFET inside a single hole in z‐direction in the CTDI phantom. In this illustration, holder and MOSFET had begun in the position denoted by the dashed lines and were moved to the left to the position denoted by the solid lines.

Figure 5

MOSFET voltage V as a function of MOSFET location index $k=1,\dots ,21$, for the nine different holes in the CTDI phantom located at the specified clock positions, for a calibration measurement at 120 kVp. The locations are distributed over 100 mm with 5 mm separation. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 6

Plot of the cumulative distribution function (CDF) of the (start‐angle corrected) data points (gray circles) and maximum likelihood estimation (MLE) fit (black solid line). Data is from all tube voltage measurements (80, 100, and 120 kVp).

Figure 7

Cumulative distribution function (CDF) of $\tilde{V}$ as a function of $X\equiv (\tilde{V}-\widehat{\mathit{\mu }})/s$ (circles) and CDF of the standard normal distribution (solid line).

Figure 8

Calibration factors, $C{F}_{h,V_t}$, for 80 kVp (circles), 100 kVp (crosses), and 120 kVp (triangles) for Site 1. The horizontal axis is the index of the CTDI hole (h): 1 = 3:00, 2 = 6:00, 3 = 9:00, 4 = 12:00, 5 = 1:30, 6 = 4:30, 7 = 7:30, 8 = 10:30, and 9 = center.

Figure C1

Values of c with 95% confidence intervals obtained by grouping MOSFET voltage measurements at 100 kVp by value. The horizontal axis reports the average value of ${\tilde{V}}_j$ for each group. The horizontal lines denote the result obtained by using all measurements: $c=0.113\pm 0.006\text{mV}$.

Figure C2

Values of c with 95% confidence intervals obtained by grouping MOSFET voltage measurements at 120 kVp by value. The horizontal axis reports the average value of ${\tilde{V}}_j$ for each group. The horizontal lines denote the result obtained using all measurements: $c=0.115\pm 0.007\text{mV}$.