Optimisation of density and contrast yielded by multiformat photographic imagers used for scintigraphy

The maximum optical density (MOD) and contrast (characterised by an effective gamma, gamma eff) yielded by multiformat photographic imagers used for scintigraphy, was determined by densitometry of a computer image, which consisted of a pattern of uniform squares containing known counts. A series of clinical scintigrams were photographed using various MOD and gamma eff values, and were graded by three experienced clinicians. It was found that a MOD of 1.6 and a gamma eff of 2.9 yielded optimal images for general use if bone scintigrams are also to be photographed. Lesion detectability in phantom images using extreme MOD and gamma eff values that were still clinically acceptable was determined. At very low false-positive rates (less than or equal to 1%) the higher gamma eff yielded a significantly greater lesion detectability (p = 0.02) at an information density of 2200 counts cm-2 in accordance with White-head's theory. It is concluded that quantification and optimisation of density and contrast factors are important and that the technique described enables quality control of multiformat imagers with respect to these factors.