The effect of gamma on subjective quality and contrast of X-ray images

We have studied the influence of the parameter gamma on the perception of digital X-ray images. Gamma is the exponent of the function describing how grey values are transformed into luminance. We have applied several values of gamma to angiographic images, and we have presented these images on a monitor (soft copy) and also as films on a light box (hard copy). The perceived contrast and quality have been assessed with the aid of category scaling techniques. Radiologists, technical experts (people involved in the development of X-ray systems) and nonexperts participated in the experiments. We found that perceived contrast increases with gamma. Gamma also affects the perceived quality, but the effect depends on the image contents. Furthermore, we found slightly different judgments for different groups of subjects. Technical experts appreciated higher gamma values than nonexperts did. In the case of radiologists, the perceived quality depended on the image (and on its diagnostic purpose) to a much greater extent than in the case of nonexperts. No differences were found between the results for hard and soft copies.