PATIENT DOSE ASSESSMENT OF ADULT AND PAEDIATRIC CHEST COMPUTED TOMOGRAPHY EXAMINATION: COMPARISON BETWEEN AUTOMATIC EXPOSURE CONTROL AND FIXED TUBE TECHNIQUES.

ABSTRACT

This study was to examine procedures used to estimate radiation dose delivered to adult and paediatric patients undergoing computed tomography (CT) examination and propose alternative and more efficient ways to establish body CT protocols for optimum protection of patient for clinical use. Automatic exposure control (AEC) and fixed tube current (FTC) techniques were used for Polymethylmethacrylate (PMMA) dosimetry phantoms in a Siemens emotion 16-slice multidetector computed tomography (MDCT) scanner. Chest CT scan was simulated for adult and paediatric patients using CT-Expo software and dose measurements using RTI barracuda system with electrometer and CT dose Profiler probe. Estimates of effective doses to the chest were done based on ICRP publication 103 recommendations and doses to organs at risk in the chest were estimated using CT-Expo software. It was observed that measured absorbed doses for adult chest examinations deviated from the console values by 1.71 and 10.50%, and that for paediatrics deviated by -68.83 and -52.83% for CTDIvol and DLP respectively. Statistical analysis demonstrated a highly significant correlation between measured and console CTDI values of P < 0.001. Simulated doses for adults deviated from the console values by 23.40 and -34.44%, whiles that for paediatrics deviated by -59.47 and -91.55% for CTDIvol and DLP respectively, showing that radiation dose delivered to patients may not always be the same as that displayed by the CT console and would not be reliable in dose estimation and safe scan protocols. Measurement of radiation dose output of a CT scanner would require the right phantom size in order to obtain safe values consistent with good image quality. Also, dose reduction for the adult and paediatric chest examinations were -4.96 and 38.80 % respectively, between AEC and FTC techniques for both CTDI values showing that even

though the paediatric doses were relatively high compared to other studies, there was appreciable dose reduction with the AEC technique. AEC has therefore proven its dose reduction efficacy and has the advantage of providing a more generalized but focused approach that might be relevant to use among paediatrics and small adults. Meanwhile, activation of the AEC in the adult chest CT examination resulted in an increase in radiation dose of 4.96% between the AEC and FTC techniques. The use FTC has therefore shown some level of dose reduction capability that could be used to optimize dose for adults and large patients. In conclusion, both AEC and FTC techniques can be employed in the optimization of chest CT examinations and should be incorporated in the establishment of chest CT protocols with respect to patient body size and according to the clinical task. This emphasises the need for medical physicists and radiographers to seriously consider the use of AEC and FTC as dose reduction techniques for patient radiation protection. There is the need to continue to re-examine and refine CT scan procedures for the optimization of patient protection.