Comparison of various film densitometers

Abstract

Quality control techniques are increasingly being applied in general radiography and particularly in mammography. A densitometer is an important basic tool for quality control measurements, and may be used for checking film processor performance (in conjunction with a sensitometer) and for measuring optical densities at test points on a radiograph of an image quality phantom (BIR, 1988; IPSM, 1989). Incorrect calibration or drift of such an instrument could give a misleading impression of the performance of the imaging system when performing comparisons with colleagues. The purpose of the present work was to establish whether differences between instruments are likely to be a significant source of error. In a quality control programme, all measuring equipment should be calibrated at a frequency that may be advised by the manufacturer; in the absence of such advice, this would typically be on an annual basis. The densitometers discussed here were not calibrated especially for the study, except where this was a part of the routine method of use. A test strip was produced, having a range of optical densities, for circulation to the authors. The strip was produced by exposing a Kodak MinRE mammography film to a nine-step optical sensitometer (“QA Sensi-Densitometer” Model SDM, MIC Medical Ltd). This resulted in nine areas of the film, each roughly 1 cm square, with various densities in the range approximately 0.2–3.3.