The Characterization of a Commercial Mosfet Dosimeter System for Use in Diagnostic X-Ray

Abstract

A commercial patient dose verification system utilizing non-invasive metal oxide semiconductor field effect transistor (MOSFET) dosimeters originally designed for radiotherapy applications has been evaluated for use at diagnostic energy levels. The system features multiple dosimeters that may be used to monitor entrance or exit skin dose and intracavity doses in phantoms in real time. We have characterized both the standard MOSFET dosimeter designed for radiotherapy dose verification and a newly developed "high sensitivity" MOSFET dosimeter designed for lower dose measurements. The sensitivity, linearity, angular response, post-exposure response, and physical characteristics were evaluated. The average sensitivity (free in air, including backscatter) of the radiotherapy MOSFET dosimeters ranged from 3.55 x 10(4) mV per C kg(-1) (9.2 mV R(-1)) to 4.87 x 10(4) mV per C kg(-1) (12.6 mV R(-1)) depending on the energy of the x-ray field. The sensitivity of the "high sensitivity" MOSFET dosimeters ranged from 1.15 x 10(5) mV per C kg(-1) (29.7 mV R(-1)) to 1.38 x 10(5) mV per C kg(-1) (35.7 mV R(-1)) depending on the energy of the x-ray field. The high sensitivity dosimeters demonstrated excellent linearity at high energies (90 and 120 kVp) and acceptable linearity at lower energies (60 kVp). The angular response was significant for free-in-air exposures, as illustrated by the sensitivity differences between the two sides of the dosimeter, but was excellent for measurements within a tissue equivalent cylinder. The post-exposure drift response is a complicated but reproducible function of time. Real-time monitoring requires little if any corrections for the post-exposure drift response. The MOSFET dosimeter system brings some unique capabilities to diagnostic radiology dosimetry including small size, real-time capabilities, nondestructive measurement, good linearity, and a predictable angular response.

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

“…The MOSFET dosimeters can be used without dose dependency correction factors for low‐dose ($<17.5\text{mGy}$) measurements. The results were comparable with the findings of Dong et al (9) and Bower and Hinterlang (19) …”

“…The lower sensitivity results may have been caused by the different orientation of the MOSFET epoxy bulbs that faced towards the water backscatter material. In our study, the epoxy bulbs were directed towards the X‐ray source to attain the highest sensitivity, based on a previous study by Koivisto et al (22) Furthermore, Bower and Hinterlang (19) characterized MOSFET dosimeter energy dependency using a (roughly) constant dose and a tube voltage ranging between 40–140 kVp. Their choice of the constant dose, PMMA backscatter material, and RADCAL 1015 reference dosimeter resulted in comparable values to those attained in our study when the results of both studies were normalized to 90 kVp tube voltage.…”

“…To date, most studies focusing on MOSFET characterization of diagnostic radiology use TLDs 9 , 13 , 19 . MOSFET dosimeters should ideally be characterized similarly to TLDs, and calibrated at the radiation qualities close to those for which measurements are needed (20) .…”

COMPARISON OF ANGULAR FREE-IN-AIR AND TISSUE-EQUIVALENT PHANTOM RESPONSE MEASUREMENTS IN p-MOSFET DOSIMETERS

“…Due to the small size and non-invasiveness of MOSFET dosimeters, they can be placed in radiation therapy or fluoroscopy beams without interfering with these procedures (Bower and Hintenlang 1998;Thomson and Nielsen 1996a). Typical sizes of the active region of MOSFET dosimeters are 400 ϫ 500 ϫ 100 m (Ramani et al 1997).…”

Optimization of dose and image quality of paediatric cardiac catheterization procedure

“…As described by Balter et al (2002) [14] and based on the investigations of Bower and Hintenlang (1998) [15] and Peet and Pryor (1999) [16], solid-state (MOSFET) detectors are reliable for real-time measurements of incident dose (without backscatter). Ionization chambers should be used however if backscatter has to be included [17].…”