Measurement of focal spot size in mammography X-ray tubes

Three different measurement techniques for estimating the size of focal spots in X-ray tubes are described. These are the pin-hole, the slit and the star pattern or resolution pattern, and all are well known. Results are reported for a number of modern mammography X-ray tubes employed in a screening programme, using all three techniques. The results are compared with each other and with makers' specifications, IEC tolerances and UK Department of Health guidelines. Agreement between slit and star pattern results is generally within a few per cent, while pin-hole results are usually appreciably smaller. Although the slit technique is the most reliable for estimating focal spot size, both the others have a useful role, especially in revealing focal spot condition. Compliance with makers' specifications is usually demonstrated, but compliance with Department of Health guidelines is only found on the width of broad foci.