Accurate patient dosimetry in modern computed tomography systems
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The most common parameter used to estimate and minimise patient dose in computed tomography (CT) is the CT dose index (CTDI). The CTDI is the average absorbed dose from a series of contiguous irradiations along the long axis. However, it has limitations. For instance, the 14-cm length of the body CTDI phantom does not provide a sufficiently long scatter path relative to the typical length of a human torso; hence, the patient dose may be underestimated depending on imaging parameters. The American Association of Physicists in Medicine (AAPM) task group 111 proposed the new method Equilibrium Dose (DEq) to estimate the dose from the CT scanner by using a small volume ionization chamber positioned in a phantom long enough to establish dose equilibrium at the location of the chamber. In this thesis, a novel phantom was designed and validated using three separate anatomical clinical sequences (head, chest and abdomen), and the equilibrium dose was determined and compared to CTDI dose estimations using a standard pencil chamber. This methodology allowed measurement of the accumulated dose for any clinical scan length and measurement of the equilibrium dose. In addition, DEq and conventional CTDI methods were used to estimated organ dose values, and the output was compared with human phantom embedded the thermoluminescent dosimeter (TLD). Furthermore, the annual reported CTDI volumes of 20 patients were corrected to DEq, and the effective organ dose was estimated. In conclusion, the thesis demonstrates the DEq method provides a closer approximation of dose in modern clinical CT systems, and the CTDI method underestimates dose .