A survey of CT doses in Northern Ireland in the period between October 1995 and March 1997 was carried out. The survey included all but one of the 10 scanners in use at the time, and, additionally, two others that were replacement machines. The method used was to study standard protocols and calculate doses to the NRPB mathematical phantom, so that a direct comparison could be made with other surveys carried out in a similar fashion elsewhere. The survey addressed the patient radiation dose but not image quality or clinical outcomes. It is estimated that in Northern Ireland the contribution to collective dose to the population from CT is about 40% of that from all medical X-rays. The proposed European Commission reference quantities, weighted CT dose index and dose-length product were computed and their potential use evaluated. A full study of mean values of effective dose per examination revealed the average dose per examination was not significantly different from that found in the 1989 UK survey, although several procedures gave rise to doses that were high enough to be investigated with a view to justification or reduction. One of the scanners was found to give consistently high doses. It is likely that a revision of the mAs values used on this scanner will produce a significant reduction in patient doses without compromising image quality. When compared with the draft EC reference levels, fewer procedures were found to have excessively high dose values. The proposed EC reference levels would therefore be useful for continual monitoring of CT dose status, but do not appear to provide as comprehensive an assessment of patient exposure as that given by consideration of effective doses.

New data based on calculated X-ray spectra for X-ray tubes of different target angles and various magnitudes of kilovoltage ripple are presented. Currently available published data relating the total filtration of diagnostic X-ray tubes to the measured half-value thickness (HVT) are of limited use to those carrying out such measurements. Electronic penetrameters are now widely available, which allow improved accuracy in the measurement of tube kilovoltage, and the monitoring of waveforms. This has highlighted the necessity to refer to data which apply to the correct waveform ripple value, and to the correct tube target angle. A wide range of data referring to 10 degrees and 16 degrees target angles has been calculated. An experimental example is presented where measured values of HVT on nine X-ray tubes are converted to values of total filtration, using the calculated data.

The purpose of this study was to investigate the proportion of patients currently being investigated by CT that could be investigated by MRI with a potential reduction in exposure to ionizing radiation. The health detriment arising from the radiation dose associated with CT has been quantified in terms of the number of likely cases of serious health effects. The results show that a significant saving in the collective radiation dose is possible, with an associated detriment of between 0.23 and 0.33 cases of cancer or severe hereditary effects averted in one imaging department every year. In selecting the balance of provision of MRI and CT facilities, the health detriment associated with the radiation dose from CT should be considered.

Cochlear implantation with a multi-channel electrode array which provides stimulation via the auditory nerve has become a standard treatment for profound deafness. Postoperative radiography demonstrates electrode position and enables confirmation of satisfactory intra-cochlear electrode placement. The number of active electrodes which have been inserted can be determined and possible complications such as electrode kinking or slippage can be assessed. We evaluated digital radiography with confirmation of electrode position by intermittent fluoroscopy and assessed the relative radiation dose of the digital technique, conventional radiography and CT scanning. Radiation dose for this method usually ranges between 40 microGy and 440 microGy compared with a single exposure on the skull stand which produces a dose to the region of the cochlea of 470 microGy and a CT exposure of 950 microGy. The digital technique is comfortable for the patient, easily reproducible and provides images of high diagnostic quality enabling each electrode to be identified, which is especially valuable in association with postoperative electrode mapping. It also involves a lower radiation dose than conventional radiography. We now use digital radiography for all postoperative cochlear implant assessment.

There are many sources of error in the traditional method involving measurement of the half-value layer (HVL) and use of appropriate conversion data to determine the total filtration of X-ray tube assemblies. Optimal technique and an analysis of errors are essential for interpreting whether the total filtration of a diagnostic X-ray tube is acceptable. This study approaches these problems from a theoretical viewpoint utilising recent theoretical HVL-total-filtration data relating to 10' and 16' tungsten target angles and 0-30% kilovoltage ripples. The theory indicates the typical accuracy to which each of the appropriate parameters must be determined to maintain acceptable errors in total filtration. A quantitative approach is taken to evaluate systematic errors in a technique for interpolation of HVL from raw attenuation curve data.