Characterization of MOSFET dosimeters for low-dose measurements in maxillofacial anthropomorphic phantoms.

Juha H Koivisto,Timo Kiljunen,Mika Kortesniemi,Jan E Wolff,Dirk Schulze

doi:10.1120/jacmp.v16i4.5433

Abstract

The aims of this study were to characterize reinforced metal‐oxide‐semiconductor field‐effect transistor (MOSFET) dosimeters to assess the measurement uncertainty, single exposure low‐dose limit with acceptable accuracy, and the number of exposures required to attain the corresponding limit of the thermoluminescent dosimeters (TLD). The second aim was to characterize MOSFET dosimeter sensitivities for two dental photon energy ranges, dose dependency, dose rate dependency, and accumulated dose dependency. A further aim was to compare the performance of MOSFETs with those of TLDs in an anthropomorphic phantom head using a dentomaxillofacial CBCT device. The uncertainty was assessed by exposing 20 MOSFETs and a Barracuda MPD reference dosimeter. The MOSFET dosimeter sensitivities were evaluated for two photon energy ranges (50–90 kVp) using a constant dose and polymethylmethacrylate backscatter material. MOSFET and TLD comparative point‐dose measurements were performed on an anthropomorphic phantom that was exposed with a clinical CBCT protocol. The MOSFET single exposure low dose limit (25% uncertainty, k=2) was 1.69 mGy. An averaging of eight MOSFET exposures was required to attain the corresponding TLD (0.3 mGy) low‐dose limit. The sensitivity was 3.09±0.13 mV/mGy independently of the photon energy used. The MOSFET dosimeters did not present dose or dose rate sensitivity but, however, presented a 1% decrease of sensitivity per 1000 mV for accumulated threshold voltages between 8300 mV and 17500 mV. The point doses in an anthropomorphic phantom ranged for MOSFETs between 0.24 mGy and 2.29 mGy and for TLDs between 0.25 and 2.09 mGy, respectively. The mean difference was −8%. The MOSFET dosimeters presented statistically insignificant energy dependency. By averaging multiple exposures, the MOSFET dosimeters can achieve a TLD‐comparable low‐dose limit and constitute a feasible method for diagnostic dosimetry using anthropomorphic phantoms. However, for single in vivo measurements (<1.7 mGy) the sensitivity is too low.PACS number: 87.50.wj

Highlights

Monitoring of the radiation exposure induced by dental examinations has become more important due to the rapidly increasing number of X-ray devices in use
One possible alternative to thermoluminescent dosimeters (TLD) is Metal Oxide Semiconductor Field Effect Transistor (MOSFET) dosimeters that can be used for near real-time, point-dose measurements in anthropomorphic phantoms
MOSFETs were mainly used in radiotherapy,(10-13) but are being more commonly used in different fields of diagnostic radiology.[14,15,16,17,18] A drawback with MOSFET dosimeters is that they have limited sensitivity when compared with TLD dosimeters

Summary

Introduction

Monitoring of the radiation exposure induced by dental examinations has become more important due to the rapidly increasing number of X-ray devices in use. The main drawback with using TLDs is that they need to be replaced and read after every exposure. This can become tedious and time-consuming when a series of different measurements are performed. One possible alternative to TLDs is Metal Oxide Semiconductor Field Effect Transistor (MOSFET) dosimeters that can be used for near real-time, point-dose measurements in anthropomorphic phantoms. MOSFETs were mainly used in radiotherapy,(10-13) but are being more commonly used in different fields of diagnostic radiology.[14,15,16,17,18] A drawback with MOSFET dosimeters is that they have limited sensitivity when compared with TLD dosimeters

Objectives

Methods

Results

Conclusion