The Utilization of MOSFET Dosimeters for Clinical Measurements in Radiology
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The Utilization of MOSFET Dosimeters for Clinical Measurements in Radiology David Hintenlang, Ph.D., DABR, FACMP Medical Physics Program Director J. Crayton Pruitt Family Department of Biomedical Engineering J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Conflict of interest statement: The presenter holds no financial interest in, and has no affiliation or research support from any manufacturer or distributor of MOSFET Dosimetry systems. J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program The MOSFET Dosimeter • Metal oxide silicon field effect transistor • Uniquely packaged to serve as a radiation detector – developed as early as 1974 • Applications – Radiation Therapy Dose Verification – Cosmic dose monitoring on satellites – Radiology • ~ 1998 - present J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Attractive features • Purely electronic dosimeter • Provides immediate dose feedback • Integrates over short periods of time • Small size and portability • Simultaneous measurements (up to 20) J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Demonstrated radiology applications – Patient dose monitoring/evaluation • Radiography • Fluoroscopic and interventional procedures • CT • Mammography J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Principles of operation • Ionizing radiation results in the creation of electron-hole pairs • Holes migrate and build up at the gate oxide interface • Changes the voltage required to turn on the transistor • The difference in turn-on voltage provides a measure of radiation dose. • Fundamentally measures dose in voltage (mV) J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Useful Life • “dose” is delivered to SiO2 • Damage accumulates in the device resulting in a limiting dose before transistor no longer functions • Dosimeters have a limited life-time – 7000 mV or ~ 230 cGy (2.3 Gy) – (as used in radiology applications) J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Commercial Development • Originally Developed by Thomson- Nielson Inc. • Currently manufactured and distributed by Best Medical Canada • www.mosfet.ca • Variety of dosimeter packaging and read-out systems • Other manufacturers for therapeutic and applications J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Performance in Clinical Radiology Applications: • Originally developed for RT applications (high energy, high dose), so issues of interest include: • Sensitivity at low dose range • Low energy response • Linearity • Angular response • Reproducibility • Stability J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Configurations for radiology applications • Multiple dosimeter models • Choices of bias supply • Software J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Sensitivity & Energy Response Ref. 8, Bower and Hintenlang 1998 J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Sensitivity & Energy Response for Mammography Range 30 mV/R Ref. 1, Benevides and Hintenlang, 2006 J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Linearity @ 90 kVp Ref. 8, Bower and Hintenlang, 1998 J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Linearity - Mammography Ref. 1, Benevides and Hintenlang, 2006 J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Angular Response Free-in-air TN-1002 RDM Ref. 1, Benevides and Hintenlang, 2006 J. Crayton Pruitt Family Department of Biomedical Engineering20 Medical Physics Program In scatter medium angular response Ref. 8, Bower and Hintenlang, 1998 J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Sensitivity & Reproducibility • Large exposures - 5-10% • typical clinical exposures ~30% • Latest software resolves measurements to 0.01 mV • 1e-7 C/kg~0.4 mR • Reproducibility introduces much greater variation J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Temporal Stability • Short term - best to wait a few minutes, or standard interval between exposure and performing read-out • Long term - recalibration recommended if not used over a period of months J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Calibration Process • Performed for each dosimeter • Calibrate against ion chamber • Backscatter media recommended • Apply 200 mV for calibration (6.7 cGy) • Typical calibration 30 mV/ cGy • Recalibrate after accumulated 7000 mV J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program User Interface for Calibration J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Useful Life • Manufacturer specifies 230 cGy when using high bias supply • In practice appears to be well less than this, ~ 30 cGy, in our applications. J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Measurement Interface J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Measurement of Tissue Dose • Convert mV to tissue dose With M = Measurement in mV S = Calibration to Exposure mV/R J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Clinical Radiography Applications • Radiographic • Use redundant dosimeters • Use higher than clinical exposure techniques • Fluoroscopy Procedures • Use redundant dosimeters • Small size may be out of beam J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Clinical Radiography Applications • CT • Angular response is usually good • Doses large enough to provide consistent results • Mammography • Careful attention to energy response induced by kV and target/filter variations J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program Summary of MOSFETs in Radiology • Convenient implementation • Simultaneous measurement • Very linear response • Good angular response • Energy response good at higher energy- sensitive at lower energy • Poor reproducibility at low doses • Limited lifetime - disposable dosimeter J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program References 1. L A Benevides and DE Hintenlang, Characterization of MOSFET dosimeters for application in clinical mammography, Medical Physics, 33 (2): 514-520, 2006. 2. MD Lina, G Tonchevad, G. Nguyend, S Kime, C Anderson-Evans, GA Johnson, and TT Yoshizumic, Application of MOSFET detectors for dosimetry in small animal radiography using short time exposure times, Radiation Res. 170(2) 260-263, 2008. 3. AK Jones, FD Pazik, DE Hintenlang, and WE Bolch, MOSFET dosimeter depth-dose measurements in heterogeneous tissue-equivalent phantoms at diagnostic x-ray energies, Medical Physics, 32, 10, 3209-3213, 2005. 4. J Roshau and DE Hintenlang, Characterization of the angular response of an “isotropic” dosimeter, Health Physics, 84(3):376-379, 2003. 5. JB Sessions, JN Roshau, MA Tressler, DE Hintenlang, MM Arreola, JL Williams, LG Bouchet, and WE Bolch, Organ doses in pediatric radiology: A comparison of an anthropomorphic physical phantom with MOSFET dosimetry and a computational patient model, Medical Physics, 29, 6, June 2002. 6. BD Pomije, MA Tressler, CH Huh, WE Bolch, and DE Hintenlang, “Comparison of free- in-air and tissue-equivalent phantom response measurements in p-MOSFET dosimeters,” Health Physics 80(5):497-505,2001. 7. DJ Peet and MD Pryor, Evaluation of a MOSFET radiation sensor for the measurement of entrance surface dose in diagnostic radiology. BJR 72, 562-568, 1999. 8. M. Bower and D.E. Hintenlang, “The Characterization of a Commercial MOSFET Dosimeter system of use in Diagnostic X-Ray,” Health Physics, 75, 2, 197-204, 1998. J. Crayton Pruitt Family Department of Biomedical Engineering Medical Physics Program.