Comparison of the return-to-the-origin probability and the apparent diffusion coefficient of water as indicators of necrosis in RIF-1 tumors.

Two model-independent measures of diffusion, the apparent diffusion coefficient (ADC) and return-to-the-origin probability enhancement (R) were compared for their ability to detect tissue necrosis in RIF-1 murine tumors. Both reflect the degree of restriction experienced by the endogenous water molecules; however, the ADC is calculated from the initial linear slope of the diffusion attenuation curve, while R is calculated from data that includes the non-monoexponential part of the curve. In spectroscopic studies (n = 9), neither the ADC nor R showed a strong correlation with tumor volume. In imaging studies (n = 14), ADC, R, and T(2) were calculated on a pixel-by-pixel basis. There, the mean ADC and mean R for the entire imaging slice showed reasonable correlation with necrotic tumor fraction (r(2) = 0.679 and -0.665, respectively). The mean T(2) value yielded a poor correlation (r(2) = 0.436). Regions-of-interest were chosen from areas identified as either necrotic or viable and the resulting sets of ADC and R-values were subjected to discriminant analysis to determine the identification error rate. The error was greater for R than for the ADC (P < 0.001). Therefore, in this application, the use of the non-monoexponential part of the diffusion attenuation curve does not provide additional identification power.