Characterization of Three Photodetector Types for Computed Tomography Dosimetry

In this study three commercial semiconductor devices were characterized in the laboratory for computed tomography dosimetry: one photodiode and two phototransistors. It was evaluated four responses to the irradiation: dose linearity, energy dependence, angular dependence and loss of sensitivity after X ray exposure. The results showed that the three devices have proportional response with the air kerma; the energy dependence displayed for each device suggests that some calibration factors would be applied for each one; the angular dependence showed a similar pattern among the three electronic components. In respect to the fourth parameter analyzed, one phototransistor has the highest sensitivity however it also showed the greatest loss of sensitivity with the accumulated dose. The photodiode was the device with the smaller sensitivity to radiation, on the other hand, the loss of sensitivity after irradiation is negligible. Since high accuracy is a desired feature for a dosimeter, the photodiode can be the most suitable of the three devices for dosimetry in tomography. The phototransistors can also be used for CT dosimetry, however it would be necessary a correction factor due to loss of sensitivity with accumulated dose.

Authors:

• C. M. M. Paschoal
• D. do N. Souza
• L. A. P. Santos

Keywords:

• Dosimetry
• computed tomography
• phototransistor
• photodiode

References:

[1] R. L. Dixon, "A new look at CT dose measurements: Beyond CTDI,"
Medical Physics, vol. 30, pp. 570-578, June 2003.
[2] K. D. Nakonechny, B. G. Fallone, S. Rathee, "Novel methods of
measuring single scan dose profiles and cumulative dose in CT,"
Medical Physics, vol. 32, pp. 98-109, Jan. 2005.
[3] L. Herrnsdorf, M. Björk, B. Cederquist, C. G. Mattsson, G. Thungström,
C. Fröjdh. "Point dose profile measurements using solid-staate detectors
in characterizaion of computed tomography systems," Nuclear
Instruments and Methods in Physics Research A, vol. 607, pp. 223-225,
March 2009.
[4] L. Aoyoma, S. Koyama, C. Kawaura, "An in-phantom dosimetry system
using pin silicon photodiode radiation sensors for measuring organ doses
in X-ray CT and other diagnostic radiology," Medical Physics, vol. 29,
pp. 1504-1510, July 2002.
[5] S. Kim, T. T. Yashizumi, G. Toncheva, D. P. Frush, F-F Yin,
"Estimation of absorved doses from pediatric cone-beam CT scans:
MOSFET measurements and Monte Carlo simulations," Radiation
Protection Dosimetry, vol. 138, pp. 257-263, Nov. 2009.
[6] L. A. P. Santos, C. M. S. Magalhães, J. O. Silva, J. Antonio Filho, E. F.
Silva Jr., W. M. A. Santos, "A feasibility study of a phototransistor for
the dosimetry of computerized tomography and stereotactic radiosurgery
beams," Radiation Measurements, vol. 43, pp. 904-907, Feb. 2008.
[7] L. A. P. Santos, E. F. Silva Jr., E. Vilela, "Filtered x-ray beam dosimetry
from 10-3 to 102 Gy dose range by using phototransistors," Radiation
Protection Dosimetry, vol. 101, pp. 145-148, 2002.
[8] G. Batignani, S. Bettarini, M. Bondioli, M. Boscardin, L. Bosisio, G-F
D. Betta et al., "Functional characterization of a high-gain BJT radiation
detector," IEEE Transactions on Nuclear Science, vol. 52, no. 5, pp.
1882-1886, Oct. 2005.
[9] IEC 61267, "Medical diagnostic X-ray equipment - Radiation
conditions for use in determination of characteristics," International
Electrotechnical Commissiom (IEC). Genève, 2005.
[10] OP520, OP521, "Silicon Phototransistor in Miniature SMT Package:
OP520, OP521," Optek Technology, Issue 1.1, 4 p, 2005.
[11] BPW34FS, "Silicon PIN Photodiode with daylight filter," Siemens, 4 p,
1997.
[12] TRS 457, "Dosimetry in Diagnostic Radiology: An International Code
of Practice," International Atomic Energy Agency (IAEA). Viena, 2007.