Dose reduction in lateral cephalometry using rare-earth screens

Review article: radiation protection in dental radiology

Dental radiology represents the most frequent diagnostic radiological investigation in the industrialized world, with over 16 million examinations performed annually in England and Wales alone. Although individual doses and risks are low in dental radiography, the collective dose is not inconsiderable and many examinations are performed in younger age groups. Radiation protection of patients in dental radiology is achieved in three ways: by appropriate selection criteria for patients and equipment, methods of dose limitation and quality assurance procedures. There is a lack of agreed radiographic selection criteria to guide British dentists and this may lead to overuse of certain techniques, principally panoramic radiography. In intraoral radiography the use of fast (E-speed) film and rectangular collimation offer dose reductions of approximately 50% and 60%, respectively. Constant potential X-ray units and rare-earth filtration permit further reductions. In panoramic and cephalometric radiography, improved collimation offers a simple means of dose limitation, while doses can be reduced by up to one-eighth by combining the use of constant potential X-ray units, rare-earth intensifying screens and rare-earth filtration. Lead protection of the abdomen has little relevance to radiation protection; however, thyroid shielding has some value. Concern has been expressed about the poor diagnostic quality of radiographs taken in the general dental services. Consequently a quality assurance programme plays an essential part in dental radiation protection by improving diagnostic yield and limiting repeat examinations.

Quality assessment of lateral cephalograms amongst radiologists and orthodontists

This study investigated technical errors in the production of lateral cephalograms necessitating a retake and the extent to which the retake, as ordered by the radiologist, actually produced a gain in quality as assessed by the orthodontist. All the rejected lateral cephalograms taken at the Department of Oral Radiology over a period of 1 year were saved, and 20 such radiographs and their approved counterparts were then studied. Ten orthodontists were asked to decide whether the radiographs were acceptable or not and to provide reasons for those judged unacceptable. Considerable variation was found between the 10 orthodontists in the rate of acceptance of the radiographs. One orthodontist accepted only 13, while another accepted 37 out of the 40 radiographs. The main cause for rejection by the orthodontists of the radiologically approved films was: patients positioning error, and for the rejected: poor (or no) intercuspation. The results show large discrepancies between the radiologists and the orthodontists in quality criteria for cephalometric radiographs, as well as large interindividual variations among the orthodontists. In half of the cases, the radiologists ordered retakes that, according to the orthodontists, were not necessary.

[A comparative study of the possibility of reducing the dosage in lateral teleradiographs]

Thanks to their capability for post-processing of image data, the digital radiological techniques introduced from 1970 onwards permit dose reduction with no loss of clinical information. The aim of this study was to establish the extent of possible dose reduction in orthodontic lateral cephalography. For this purpose, lateral cephalograms of a head phantom taken with varying radiation doses (5 to 150 mAs, constant 70 kV) were evaluated. The evaluations of ten judges were compared statistically. Conventional lateral cephalograms showed a statistically proven superiority of a number of exposure parameters for the sagittal and vertical localisation of reference points. This correlated with the radiological density of the structures. In contrast to the conventional technique, the reference points in all images obtained by digital luminescence radiography showed no such tendency. Digital luminescence thus meets the clinical need to reduce radiation dose (to about 20% of the usual dose) while providing no less diagnostic information.

Exposure factors and screen-film combinations in temporomandibular joint radiography

Exposure factors and screen-film combinations providing optimal quality are identified for transcranial and transpharyngeal temporomandibular joint views, using conventional intra-oral radiographic equipment without grids. Standardized transcranial and transpharyngeal views, using a fixed whole cadaver head, were performed. Ten readily available screen-film combinations, ranging in nominal speed 20-600, were exposed over 40-100 kV. Films were blindly and independently order ranked by three observers on the basis of sharpness and contrast of cortical outline, trabecular detail, and visualization of adjacent bony structures. Preferred screen-film combinations as a function of kV, preferred kV levels for each screen-film combination, and overall ranking irrespective of kV or screen-film combination, were established. Accepting the use of the lowest radiation dose possible for diagnostically useful radiographs but imposing arbitrarily an upper limit of 20 mGy, it was found that exposures between 50 kV and 70 kV gave the optimal result for both techniques. The amount of scattered radiation in the emergent beam differs greatly between the two techniques. The most favoured combinations for the transpharyngeal technique used screens of fine resolution. Min-r/ortho M screen and film with nominal speed 40 at 60 kV gave 8.0 mGy skin dosage at 0.8 seconds exposure; the same combination at 50 kV was the most favoured, but with skin dosage calculated at 16.7 mGy for 3.0 seconds exposure. For the transcranial technique, medium speed screens providing better differentiation of scattered radiation beams and increased speeds were preferred. Most favoured for image quality was the Lanex Fine/T-Mat G combination at 60 kV giving 17.5 mGy skin radiation dose at 1.75 seconds exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Dose reduction in dental radiography

All exposures to ionizing radiation for medical and dental purposes carry risks to both the patient and staff. It is now accepted practice that all exposures should be kept as low as reasonably practicable to minimize these risks. The various methods currently available for dose limitation in intraoral and panoramic radiography and their effects on image quality are considered. In intraoral radiography the use of fast (E-speed) film and rectangular collimation offer dose reductions of approximately 50 per cent and 60 per cent respectively. Constant potential X-ray units, longer focus-to-skin distances and rare-earth filtration will permit further dose reductions. In panoramic radiography the radiation dose to the patients can be reduced by up to one-eighth by combining the use of constant potential X-ray units, rare-earth intensifying screens and rare-earth filtration.