(*) Summary: Computed tomography (CT) is an invaluable diagnostic tool in current medical practice. Unfortunately, the radiation dose imparted during a CT scan can be significant. Over the last few years due to recent advances such as spiral and multidetector-row CT technology, CT examinations have rapidly increased in number resulting in increased collective radiation dose as reported by many international organizations. It has also been stated that image quality in CT exceeds the level required for confident diagnosis.  
This research seeks to establish, verify, and validate the radiation dose to patients and estimate the diagnostic reference levels during selected routine CT examinations in UCH Ibadan, Nigeria’s largest tertiary hospital; using appropriate computational methods for computing this dose accurately and efficiently.
The dose characteristic of the CT scanner used was determined by using the scanning parameters of 100 previously examined patients. For each scan technique, patient doses were estimated using two methods; (i) the software developed by the ImPACT scan group in conjunction with the NRPB –S250 conversion coefficients data, (ii) the displayed scanner DLP against the conversion factors determined the European commission (2003).
The results show that the largest mean CTDIvol, DLP and effective dose was obtained for lumbosacral CT, Head CT, and Abdomen/pelvis, respectively. The average patient dose varied from one study to another. The largest range was found for CT of the abdomen. Reference dose values were calculated for each exam. Among studies, except head scan there was considerable variation in the DLP and patient radiation dose for a specific exam. Reference doses and patient doses were higher than the European commission recommendation. The higher dose levels, which are possibly associated with significant risks, would justify an extensive similar study at the national level in order to unify different approaches towards optimisation of CT examinations.
The need to train radiology personnel, establish standard dose reduction protocols and continuously monitor the performance of CT equipment to control patient CT doses cannot be overemphasized.

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(**) Biodata: Biodata:  Rachel Ibhade Obed, PhD, is a Junior Associate of the ICTP in the Applied Physics Scientific Section, Medical Physics. She is a Lecturer in the Department of Physics, University of Ibadan, Nigeria. She is also an Adjunct Lecturer at the National Institute for Radiation Protection and Research (NIRPR) in Ibadan. She was also an associate staff of the defunct Federal Radiation Protection Service (FRPS) in Nigeria for 7 years and was involved in radiation monitoring of many medical and industrial establishments. She was the Medical Physicist in the Department of Radiotherapy, University College Hospital Ibadan from August 1996 to September 1997. She has attended many IAEA training workshops and also had a 3-month IAEA fellowship award for training in Radiation Protection in Medical Physics tenable in the United Kingdom in 2001. She is the newly elected Honourable Treasurer of the Science Association of Nigeria (SAN) and belongs to many learned societies both locally and internationally. Currently she works to ensure radiation protection of patients in diagnostic radiology and radiotherapy in Nigeria. Amongst her duties also is to ensure radiation protection of the general public through her vast research works in environmental radioactivity such as soil radioactivity concentration levels in relation to cancer incidence in Nigeria, measurements of radon in drinking water and indoor radon in dwellings and workplaces in Nigeria using CR-39 track etch detectors. She has published 10 international papers in the area of radiation protection of patients and the general public and has supervised many undergraduate and postgraduate (M.Sc., PGDRP, MRP, MPhil/PhD and PhD) students. Her main interest is in imaging techniques and patient dosimetry in diagnostic radiology and radiotherapy.