Automatic Exposure Control Devices Research Articles

An Initial Report on the Investigation of High Patient Doses for the Lateral Lumbosacral Projection in the Lumbar Spine Examination

Lumbar spine investigations in our diagnostic imaging department were included in a recent European study of patient dose and image quality. On average, for views other than the lateral projection of the lumbosacral junction, the study found a relatively good image quality at a reasonable average patient dose. For the lumbosacral view the average dose was high and the spread of doses involved was broader than expected. As this is a relatively high dose examination (approx. 40 mGy to skin) a formal investigation of the methodology involved in this projection was undertaken. A reject analysis for this projection revealed that it was discarded with a high frequency, given the extent to which it is performed and the fact that automatic exposure control (AEC) devices are commonly used for it. This paper presents results for (a) a survey of the pattern of use of AEC and pre-programmed exposure control facilities by radiographers for the lumbosacral view and (b) the aforementioned reject analysis of lumbosacral junction and related projections. From the survey of radiographers and related studies, the following are identified as possibly being of importance: (i) use of AEC and related devices for this view, (ii) the occasions on which such devices are or are not used, and (iii) the reasons why such systems are often not trusted. In the case of the reject analysis, the relatively high reject rate for the projection and the impact and significance of such a rate are highlighted.

Measurements of the frequency distribution of optical density in screening mammography.

A method for obtaining histograms of the frequency of occurrence of optical densities in mammograms has been implemented. Averaged optical density distributions were produced for approximately 50 screening mammograms at each of two centres using different film-screen and processing chemistry combinations. Additional measurements were made on films of two commercially available anthropomorphic breast phantoms. The results show that screening mammograms, on average, exhibit a strongly peaked optical density distribution which corresponds closely to the target optical density measured using the current recommended test protocol for automatic exposure control devices.

Evaluation of the use of a niobium filter for patient dose reduction in chest radiography

The performance characteristics of a 50 microns niobium filter and a copper filter of equivalent thickness (0.15 mm) were compared for postero-anterior chest X-ray examination. Radiographs of 20 patients obtained using an automatic exposure control device showed no significant differences in image quality and exposure setting between the filters. Radiation dose distributions measured in a Rando phantom revealed a higher reduction in the patient dose with the niobium filter. The reduction in the effective dose equivalent, produced by the addition of the niobium and copper filters to the standard filter of 2.5 mm aluminium, of 17% and 10%, was relatively small compared with the reductions in the entrance surface dose of 31% and 26%. Our study confirms that drastic alterations in the patient dose by the addition of a dose reduction filter to the usual filtration cannot be expected.

Radiotherapy Verification Film for Estimating Cumulative Entrance Skin Exposure for Fluoroscopic Examinations

Measurement of skin entrance exposures during fluoroscopic procedures is complicated by the use of automatic exposure control devices and the presence of contrast media. Due to variability in positioning spot films from patient to patient, standard dosimeters, such as thermoluminescent, cannot be properly placed on the skin prior to examination. Prepackaged film of the type used for portal verification in radiation therapy held next to the patient's skin in a specially modified patient examination gown was found to be useful for determining the entrance skin exposure from both fluoroscopy and spot films during air contrast barium enema exams. The usable sensitivity range of this film has been found satisfactory for exposure measurements at exposures and kVps typically used for gastrointestinal fluoroscopic procedures. Errors in exposure estimates due to changes in film speed and contrast with kVp are less than 5% for the range of kVps used. Errors from variations in beam quality due to the adjacency of scattering material are approximately 5%. Entrance exposures determined with film agreed with those determined from TLD measurements to within 21%, with an average difference of 9%.

Variation in repeat rates between 18 radiology departments.

A comprehensive analysis of repeat rates has been obtained from an observational study of radiological practice in diagnostic X-ray departments throughout Wales. Interdepartmental and intradepartmental repeat rates are investigated for chest, abdomen, pelvis, sinus, lumbar spine, cervical spine and intravenous pyelographic examinations. The confounding effects upon observed repeat rates of trainee practitioners and automatic exposure control devices are also considered. The overall proportions of total film cost and patient exposure attributable to repeat radiographs are estimated to be 10% and 14% respectively.

Evaluation of an automatic exposure control device for mobile radiography.

The mobil-AID automatic exposure control device was evaluated quantitatively and clinically for use with mobile radiographic units. It was found that such a device can improve the consistency and quality of mobile radiographs and also reduce the retake rate. Problems in calibration and clinical use are discussed.