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Design of an In-Vivo Radiation Measurement Scheme Using a Reliable Wireless Detector - Conclusions/Future Work/References

Module by: Narayana Vedula

Summary: The purpose of this project is to develop a method for monitoring, evaluating the status of a tumor undergoing radiation treatment and transmit the radiation dose received by the tumor in real-time to an external data acquisition unit and evaluate the treatment strategy. The target system to be designed would consist of a miniature wireless sensor module which would be implanted in the region of interest and this unit transmits the data to an external receiver wirelessly. The Modern fabrication process allows us to fabricate CMOS circuits in nanometer range which is highly desirable when used in radiation sensing scheme where size is the main constraint. The heart of the sensing system is a MOS transistor device. The electrical properties of the MOS device (drain to source resistance, threshold voltage) change when exposed to gamma radiation. The objective of this project is to evaluate the MOS transistor parameters change with radiation dose, study the effects with varying transistor widths, lengths and propose the ideal characteristics of the CMOS device. The scheme of the radiation measurement is presented. Some of the questions that will be addressed in this report are – The effect of radiation on channel resistance during & after radiation, temperature dependency of the radiation effects. The complete characterization of the sensor is performed and the results are produced and cross checked with device physics. The characterized CMOS transistor is exposed to radiation and the radiation dosage received by the transistor is sent to a RFID receiver using radio frequency communication. The signal at the RFID receiver is filtered and the actual radiation dosage was determined from the filtered output’s duty cycle. A novel approach is used in determining the radiation measurement in this study.

Design of an In-Vivo Radiation Measurement Scheme Using a Reliable Wireless Detector - Conclusions/Future Work/References
Shivaranjan Vadlapudi, Narayana Rao Vedula
University of Minnesota- Twin cities
10. Conclusions:
The effect of radiation is studied on different MOS devices. The threshold voltage of a MOS transistor decreases with a raise in radiation dosage because of the trapped holes in oxide region. For an NMOS transistor, absolute value of Vt goes down making it to conduct easily, while for a PMOS transistor it works in the other way. We can make use of this cumulative effect by studying the behavior of an inverter. The On time of an inverter decreases with radiation dosage and while the Off time increases with it. With a lower frequency input signals, the range of difference in timing values can be increased. The sensor output signal is transmitted wirelessly using a transmitter/receiver pair. The receiver data signal is obtained and plotted using a DAQ (Data Acquisition) card and a LabVIEW program. The sampled data is filtered to remove the DC and undesired high frequency components. The On and off times for different radiation doses and signal frequencies are used calibrate and measure an unknown radiation dosage value. It is also observed that the irradiated device characteristics do not change with time.
Finally, this project implements the idea of measuring radiation dose in terms of changes in MOS transistor characteristics. Since all of the processing is done at the receiver side, no complex circuitry will be placed next to the sensor (except an RF transmitter). With this the power and size requirements will be lot lesser than other commercialized devices. As the sensors are not custom-made devices, the results did not render high resolution.
11. Future Work:
In this project, we can observe that the timing measurements show a smaller range for the total range of radiation dosages. So, custom-made MOS transistors can be used to improve this range and also the resolution to a greater extent.
The whole design should be fabricated into mm wide ICs which can be implanted inside human body. This is achievable with today’s nano scale fabrication technologies. Also the RF transmitter should reside on the same chip along with the sensor. For this, an elaborated design of the transmitter is required.
Apart from the sensor, no other device should be affected by radiation. Specialized SOI (Silicon On Insulator) or SOS (Silicon on Sapphire) fabrication techniques provide embedded protection against the radiation damage. With this, normal commercial-grade chips can withstand between 5 and 10K CGy which is far above the working range of our sensor.
As the On and Off time values of sensor output depend on the input signal frequency, a radiation sensor array network can be designed in such a way that each sensor in the array operates at a specific frequency. All these signals can be transmitted wirelessly and processed at the receiver side (by using a DSP circuitry) to obtain each sensor’s output signal characteristics separately.
References:
2) Khalil I. Arshak, Member, IEEE, and Olga Korostynska “Gamma Radiation Nose System Based on In2O3=SiO Thick-Film Sensors” IEEE SENSORS JOURNAL, VOL. 6, NO. 2, APRIL 2006
3) M. N Darwish M. C Dolly C. “A Goodwin Modeling of Radiation Induced Burnout in DMOS Transistors “
4) A.S. Beddar, M. Salehpour, T.M. Brire, H. Hamidian, M.T. Gillin,“Preliminary evaluation of implantable MOSFET radiation dosimeters,” Phys.Med. Biol. 50:141-149 (2005).
5) C.W. Scarantino, C.J. Rini, M. Aquino, T.B. Carrea, R.D. Ornitz, M.S.Anscher, R.D. Black, “The initial clinical results of an in vivo dosimeterduring external beam radiation therapy,” Int. J. Radiat. Oncol. Biol. Phys.62(2):606-613 (2005).
6) C.W. Scarantino, D.M. Ruslander, C.J. Rini, G.G.Mann, H.T. Nagle, and R.D. Black, “An implantable radiation dosimeter for use in external beam radiation therapy,” Med. Phys. 31:2658-2671 (2004).
7) R.D. Black, C.W. Scarantino, G.G. Mann, M.S. Anscher, and R.D. Ornitz, “Ananalysis of an implantable dosimeter system for external beam therapy,”Int. J. of Radiat. Oncol. Biol. Phys. 63(1):290-300 (2005).
8) T.M. Briere, A.S. Beddar, and M.T. Gillin, “Evaluation of precalibratedimplantable MOSFET radiation dosimeters for megavoltage photon beams,” Med.Phys. 32(11):3346-9 (2005).
9) Project Website: www.tc.umn.edu/~vadla005
10) George C. Messenger, and Milton S. Ash, "The Effects of Radiation on Electronic Systems," Van Nostrand Reinhold Company, New York, NY, 2005
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