Technical note: assessment of X-ray field alignment in mammography.

Abstract

In mammography it is important that the chest wall edge of the X-ray field is accurately aligned with the edge of the film in the film cassette. Too small an X-ray field (which may result in the loss of clinical information) is easily seen on the resulting image, but too large a field is less easy to detect and is potentially a source of unnecessary patient dose. This is of particular concern when mammography is being used for the screening of asymptomatic women. If the X-ray field is oversized, the scattered radiation dose to the thymus, lungs and to the fraction of the red bone marrow in the chest region will be higher. It is also possible for these organs to be partially irradiated by the primary beam if the field size is excessively large. These organs are recognized as being particularly radiosensitive and are included in the list of tissues considered when calculating effective dose (ICRP, 1990). Tests of beam alignment have been suggested (IPSM, 1989) which involve the use of films or fluorescent screens overlapping the front edge of the support table together with markers for the field edge and a steel rule. In practice, this approach has been found to be difficult to reproduce on different designs of mammographic unit and, more importantly, found not to give a measurement easily referenced to the actual film position.