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Calibration of a scanning whole body counter for measurement of the activity of gamma emitting radionuclides in the human body

Al-Musawi, Thaer 2018. Calibration of a scanning whole body counter for measurement of the activity of gamma emitting radionuclides in the human body. PhD Thesis, Cardiff University.
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Sources of ionising radiation in the environment are mainly naturally occurring radionuclides, such as potassium-40 (40K) and carbon-14 (14C) and man-made radionuclides, such as caesium-137 (137Cs) and iodine-131 (131I). Both sources increase radiation dose to human beings and other living organisms and have the same biological effects. Exposure to these radionuclides creates a demand for feasible methodologies to measure their body burdens. Whole body counters (WBC) are well-established instruments to detect, identify and quantify radioactivity in the human body. In order to perform reliable measurements of the body burden of radionuclides, a whole body counter should be calibrated with applicable phantoms whose sizes are similar to those of human subjects. The Cardiff WBC in the Medical Physics and Clinical Engineering Department at the University Hospital of Wales, is composed of six large-volume thallium-doped sodium iodide (NaI(Tl)) scintillation detectors in a shielded room. In this work, the counter was calibrated empirically for three radionuclides, namely 40K, 137Cs and 131I, using an anthropomorphic phantom technique. Prior to the calibration experiments, preliminary work was done to characterise the performance of the Cardiff WBC. Optimum photopeak counting windows were determined by various methods, and a window of twice the FWHM was chosen as the best one. Energy resolution was determined for the three radionuclides, and it was found that for 137Cs, the value was consistent with the British Standard for scintillation detector-based instruments. The theoretical minimum detectable activity (MDA) of the counter for the three radionuclides was determined and shown to increase linearly with phantom mass. The effect of varying the scan speed was measured and it was found that count rate increased significantly (p<0.05) with increase in speed. Background count rate inside the counter room was studied in terms of its stability and the effect of phantom mass. Stability was determined over a two-month period at five different positions along the length of the room, with and without the patient bed. It was found that background count rate was less at the closed end of the room that at the centre or near the doors. Furthermore, it was greater with the iii bed in place than in the absence of the bed. Phantom mass had a significant influence on the background count rate for the 40K and 137Cs counting windows. Finally, the counting efficiency of the WBC decreased logarithmically with photon energy. As regards calibration of the WBC, the Department has three sets of Bush phantoms, the compartments of which were arranged to produce phantoms with a wide range of height and weight. Potassium-40 was calibrated using uniformly distributed and point source techniques, whereas only the point source technique was used for 137Cs and 131I. The measurement of 131I count rate was corrected for overlap of the Compton region with the photopeak in the pulse height spectrum. There was a significant negative correlation (P<0.01) of the calibration factor F (sensitivity) with phantom mass for the three radionuclides, whereas phantom height had very little impact on F. The ratio of count rate in the Compton energy band to that in the photopeak was considered as an index of size. This ratio had a significant positive correlation (P<0.01) with phantom mass and the variation of F with the Compton to photopeak count rate ratio was used as an alternative method for determining the total body radioactivity for the three radionuclides. In general, this alternative method gave satisfactory results compared with the conventional approach. It has the advantage of being a self-contained technique that makes a correction for subject size based on just information from the gamma pulse height spectrum, with no need for additional measurements such as height and weight.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Calibration scanning Whole Body Counter; Measurement of the activity of gamma emitting radionuclides in the human body; K40, Cs-137 and I-131.
Date of First Compliant Deposit: 12 September 2018
Last Modified: 04 Aug 2022 02:06

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