DISPLAY OF MERGED MULTIMODALITY BRAIN IMAGES USING INTERLEAVED PIXELSWITH INDEPENDENT COLOR SCALES

This article reviews common methods for two-dimensional display of reg istered multimodality brain images and describes a software package fo r presentation of merged MRI and PET images that runs on a workstation with an eight-bit color display. The software package displays merged brain images from multiple modalities in a way that is readily manipu lated, visually pleasing and easy to interpret. The display method use d, i.e., interleaving of alternate pixels with independent color scale s, is effective in producing merged images with high contrast-detail f or each volume. Interleaving images from different volumes creates unu sual perceptual effects, one of which is the apparent camouflage of lo w-contrast signals by high values in the paired volume. Methods: The c amouflage effect was thought to arise from perceptual merging of adjac ent pixels. An observer experiment was conducted to investigate this t endency of high-activity PET data to obscure low-contrast detail in in terleaved MRI data in spite of the digital independence of neighboring pixels. Four observers were presented with 20 combinations of signal plus background targets with uniform mask images, using a two-alternat ive forced-choice experimental design with 50 trials per combination. Results: The psychophysical evaluation of the ability of human observe rs to detect the simple test objects in an interleaved image presentat ion indicated a statistically significant camouflage effect of one vol ume on the other for some combinations of target and mask. The concept of perceptual merging of adjacent pixels was able to predict which co mbinations caused the greatest degradations in performance. Conclusion s: The image interleaving approach to the display of two-dimensional s lices from registered image volumes makes efficient use of an eight-bi t color display. Contrast resolution of both individual volumes is hig h compared with that in other techniques and the volumes are presented in familiar color scales. However, the method yields an unexpected ca mouflage effect that tends to obscure low-contrast signals. The practi cal effect of such camouflage on the interpretation of clinical images remains to be investigated.