Correspondence WALTON, L., BOMFORD, C. K. & RAMSDEN, D., 1987. The Shef-
field stereotactic radiosurgery unit: physical characteristics and principles of operation. British Journal ofRadiology, 60,897-906.
Osteoporosis Research Group, Department of Radiology, University of California San Francisco, San Francisco, CA 94143-0628, USA (Received 29 June 1992 and accepted 16 July 1992) References GLUER, C. C , STEIGER, P., SELVIDGE, R., ELLIESEN-KLIEFOTH, K.,
HAYASHI, C. & GENANT, H. K., 1990a. Comparative assessment of dual-photon absorptiometry and dual-energy radiography. Radiology, 174, 223-228.
Acronyms in bone densitometry
GLUER, C. C , STEIGER, P. & GENANT, H. K., 1990b. Reply.
Radiology, 176, 875-876. NORD, R. H., STEIN, J. A., MAZESS, R. B. & POMMET, R. P., 1991.
The Editor—Sir, In response to our publication (Gluer et al, 1990a), Wilson and colleagues (1990) proposed the adoption of a uniform terminology and corresponding abbreviation for Dual X-ray Absorptiometry (DXA). We supported this suggestion (Gluer et al, 1990b). Since then, the number of papers on DXA has increased considerably. Still, different abbreviations (DEXA, DER, DRA, QDR and DPX) for this technique for bone densitometry are used, several of which are proprietary in origin. DXA is the upgraded version of DPA (Dual Photon Absorptiometry). The nuclear source has been replaced by an X-ray source and this improved technology has gained widespread acceptance and distribution. Recently, similar developments in SPA (Single Photon Absorptiometry) have occurred, which will add yet another abbreviation to densitometry-terminology: SXA for Single X-ray Absorptiometry, not the alternative "SEXA". Users and manufacturers of these techniques have now joined efforts in an International Standards Committee (Nord et al, 1991). In this Committee, standards of calibration, measurement units and terminology are discussed. There is a general agreement that standard abbreviations for bone densitometry techniques are necessary and the term DXA has been accepted. Therefore, again we would like to emphasize the use of the acronym DXA for Dual X-ray Absorptiometry and add the acronym SXA for Single X-ray Absorptiometry. When we confine ourselves to SPA, SXA, DPA and DXA, next to QCT for Quantitative Computed Tomography, both researchers and clinicians will understand what we mean. Still, good acronyms for ultrasound attenuation and velocity measurements and for magnetic resonance measurements, both for quantifying bone mineral density and structure, have to be agreed upon. Acronyms such as QUS for quantitative ultrasound and QMR for quantitative magnetic resonance would be consistent with QCT. Such terminology, however, would clearly need consensus development. Yours etc., H. K. GENANT C. C. GLUER K. G. FAULKNER S. MAJUMDAR S. T. HARRIS K. ENGELKE C . VAN KUIJK
Letter. Bone Mineral, 13, 85. WILSON, C. R., COLLIER, D., CARRERA, G. F. & JACOBSON, D. R.,
1990. Acronym for dual-energy X-ray absorptiometry. Radiology, 176, 875
Latent image fading and mean glandular dose in mobile mammographic screening
The Editor—Sir, The phenomenon of latent image fading, although well known (e.g. Kofler & Gray, 1991) is not often considered as a cause of film optical density variations in conventional radiography because of the short times between exposure and processing normally encountered. In mammography used for breast screening, however, the range of times between film exposure and processing can be much greater than in conventional radiology. When a mammogram is taken at a screening or assessment centre where processing is undertaken on site, the delay will be only a few minutes or less, but when films from mobile breast screening trailers are being returned to the centre for processing on the same working day, the delay could be in the region of 7 h. At some centres, processing of films from mobile units may be carried out on the next working day, giving a delay ranging from approximately 16 h, up to a maximum of 5 days for an extended bank holiday weekend. One reason for using centralized film processing is that the processor can be kept under strict quality control to ensure optimal imaging performance for all films pasing through the centre, but it is possible that this may not be achieved with working practices that can lead to long delays between film exposure and processing. An investigation was performed into the effect of delayed processing in mammography. Kodak MinR-E film was used and processed on a Kodak M6B processor operating on a 180 second
The British Journal of Radiology, December 1992