Quantitative Performance Evaluation of Commonly Used Colormaps for Image Display in Myocardial Perfusion Imaging: Analysis based on Perceptual Metrics

INTRODUCTION: To evaluate performance of mostly used colormaps in image display quantitatively using perceptual metrics as well as to what extent these measures are congruent with true intensity or uptake of pixels in different levels of defect severity in simulated cardiac images.
METHODS: Six colormaps, labeled as “Gray”, “Thermal”, “Cool”, CEqual”, “Siemens” and “S Pet” are included. Colormap are converted CIELAB colorspace. Perceptual metrics for measuring “color difference” are calculated including difference (∆E76) and “speed”. Curves are plotted for each colormap and are compared. Simulated short-axis cardiac slices with incremental defect severity (10% grading) are employed. The circumferential profile curves of true pixel intensity, lightness or luminance and color difference are plotted simultaneously.
RESULTS: In the phantom with 10% defect, discrepancies among curves appear. In “siemens”, the ∆E76 drops sharply. In “siemens” colormap, the ∆E76 drops sharply. In 80% defect, ∆E76 curve, in “gray” colormap drops more slowly than other the curves of other colormaps. In 70% defect, again curves are in good agreement in “thermal”, “siemens” and “cequal”. But consistent lag exists in “gray”. Up to 50% defect, curves maintain their expected pattern, but in defects more severe than 40%, lightness and ∆E76 curves in “cool” and “cequal” rises paradoxically and in “thermal” it starts to slow down in descent.
DISCUSSION AND CONCLUSION: Of 6 colormaps investigated for estimating defect severity, “grayscale” is less favorable than others and “thermal” performs slightly better. “s pet” or rainbow which are used traditionally by many practitioners is strongly discouraged. The “siemens” colormap suffers from decreased discriminating power in the range of mild to moderate/severe. In contrast, the “cool” and “cequal” colormaps outperform other colormaps employed in this study to some extent, although they have some shortcomings.