Analysis of the Optimum Conditions for Data Display and Image Perception in Photoscanning

A discussion of display in the visualization of radioisotope distribution requires prior analysis of the data collection system. It is important to know what spatial resolution can be expected in a given situation. Based on work by Rose and Coltman, a simple relation between count density, spatial resolution and modulation transfer function of a collimator can be established for two special but rather typical situations: ( 1) a maximum decrease of gamma-ray flux to zero from a 'cold area' embedded in tissue having a fairly uniform distribution of isotope (i.e. the ideal situation in liver or thyroid scanning), and (2) an increase in flux to twice that from the surroundings (i.e. a typical situation in brain scanning). The optimum spatial resolution in a given situation can thus be determined with this method, and the data presentation system - specifically the size of the lightspot in photographic display - can be adjusted accordingly. This proposal is tested with phantoms using collimators which differ in sensitivity and using lightspots of different sizes. Good correlation with the theoretically-obtained data is found, and it can be easily shown that with an improper choice of lightspot - either too small or too large for a given count density - information is lost. The same result can be achieved by varying the size of the displayed picture, using a fixed size lightspot. This approach is based on studies by several authors which indicate that contrast threshold for visual resolution varies with spatial frequency. When this is tested with minification of the photographic display, the expected improvement in visibility is found. It is therefore suggested that variation of the minification of the displayed picture according to the optimum spatial resolution, and thus according to the count density, in a given situation would give maximum perceptibility of a lesion. The described approach to the analysis of the optimum display system can be extended to other ratios of gamma-ray flux and to stationary detectors. (author)