[A method for quantitative evaluation of the physical quality of radiologic devices]

The evaluation of the quality of a radiological device depends on both clinical judgement about diagnostic imaging and physical judgement about technical performance. The latter is the final result of a set of measurements of several physical quantities. The aim of the present paper is to make a synthesis of such measurements through a parameter whose comparison with a threshold value would allow to establish whether a given radiological machine is acceptable from the point of view of physical quality.

The parameter, which we called quality index, was obtained by considering, for each operating condition, the values of physical quantities which exceed their limits and by giving them a different weight, depending on their influence on image quality, patient dose, or both. Further analysis led to assign a gravity index to such quantities as a function of the extent of the discrepancies with respect to their limits.

The method was illustrated through the example of a simple radiological equipment with two separate hypothetical cases corresponding to different degrees of fault gravity. The method gave very different values of quality index, according to the extent of discrepancies found in the two cases. We gave suggestions about the way to follow in order to determine a proper threshold value for each kind of equipment. The invariance of the method with respect to the choice of the physical quantities and their limits was also shown.

The proposed method appears to be useful because it makes a synthesis, through a single parameter, of a series of measurements of several physical quantities and allows to discriminate, through direct comparison with a threshold value, about the physical quality of a radiological device; in addition, it may be easily implemented into programs for the automated analysis of quality controls. The quality index may contribute, together with some other parameters to be defined in a forthcoming paper, in establishing a quantitative criterion in order to define equipment replacement priorities within the context of the technological improvement requested by the laws currently in force.