This work established Clinical Diagnostic Reference levels (DRLci) for adult Computed Tomography (CT) examination and its application in the practice of medical imaging and Radiology. We also assessed post-optimization image quality as a panacea for dose optimization. The study was a prospective study conducted in five centres in Enugu, Nigeria with ethical certificate no NHREC/05/01/2008B-FWA00002485-1RB00002323. Dose values and technical parameters were obtained using International Commission on Radiological Protection (ICRP) Publication 135.Protocols were used as specified in Siemens machine for helical pre and post contrast CT examination. Scan parameters such as Field of view, pitch, rotation time, tube output, Kv, mAs and slice thickness were documented. Quality control checks were carried out on the machines. Adult patients from 18 years and above that presented to for CT investigation of the head, chest or abdomen from November 2018 to April 2021 were included in the study. A total of 2490 patients with distribution 1200 brain scans (48.2%), 420 chest scans (16.9%) and 870 abdominal scans (34.9%) were studied from the 5 participating centres (one public and four private CT centres). The distribution were as follows: Head - CVA (300), Trauma (300), Metastasis/abscess (300), Infection (180), Seizure (120). Chest - Lung disease (240), Lung cancer/mass (180). Abdomen/Pelvis - Abdominal cancer/metastasis (120), Abdominal mass (300), Liver disease (240), Urography (210). The adult patient's weight was 50 ± 10 kg. Objective image quality assessment was carried out via signal to noise ratio (SNR) analysis prior to data collection. Pearson correlation was used to compare the mean dose quantities, age and sex. Dose values for CT centres and between previous literatures were determined using student t-test at 0.05 level of significance. 476(39.7%) were females and 724(60.3%) were males. Out of the 420 patients that underwent the chest scan, 204(48.6%) were females and 216(51.4%) were males. For the Abdomen/Pelvis scan with 870 patients, 411(47.2%) were females and 459(52.8%) were males. For head CT, the CTDIvol for trauma, CVA, metastasis, infection and seizure are 43, 43, 43, 34 and 28 mGy respectively. Their corresponding DLP includes; 907, 879, 1689, 969, 995 mGy cm. In the chest, the CTDI for lung disease and mass are 13 and 13 mGy. Their corresponding DLP includes; 763 and 1531 mGy cm. In the abdomen, the CTDIvol for liver disease, malignancy, mass and urography are 16, 12, 16, and 15 mGy. Their corresponding DLP includes; 2723, 2011, 2011, 2047 mGy cm. The CTDIvol values in this study for head, chest and abdomen/pelvis region are below the European commission value (1999) with a p value of 0.024, 0.018, and 0.000 respectively All the images were accepted by the radiologist and has comparable signal to noise ratio (SNR) dose reduction from the optimization process. Almost all the centres have their dose value below the estimated DRL for Enugu and up to 50% dose reduction. Clinical indications DRL for 10 common clinical indications in CT examination were established. The study showed wide variations in patient dose between hospitals and even between identical scanners for different clinical indications. Clinical Indication DRLs developed for pre and post contrast studies are useful for dose optimization.
