Computing diffusion rates in T2-dark hematomas and areas of low T2 signal

BACKGROUND AND PURPOSE: It has been suggested that restricted diffusion is present within hematomas with intact red cell membranes; however, computing apparent diffusion coefficient (ADC) values in areas of low T2 signal can be problematic. Our purpose was to show the pitfalls of measuring diffusion within hematomas with intracellular blood products and to present a framew ork based on the properties of expected values for computing ADC values fro m regions with signal intensities close to that of the background noise (ie , T2-dark hematomas). METHODS: Twelve patients with intracranial hematomas who had undergone diff usion imaging were retrospectively identified during a 2-year period (four intracellular oxyhemoglobin, seven intracellular deoxyhemoglobin, one intra cellular methemoglobin), Regions of interest were drawn on the hematomas, t he contralateral white matter, and over the background. ADC values were com puted using a variety of methods: 1) using expected values incorporating th e variance of the background, 2) computing the mean of the regions of inter est before taking the natural log, 3) masking negative values, and 4) maski ng the background at 0.5% increments from 0.5 to 5.5% and including the mas ked voxels (an intrinsically flawed method). Two-tailed Student's t test wa s performed between the white matter and the hematoma ADC values. RESULTS: There was no statistically significant difference between the hema tomas and the white matter for methods 1 through 3 (P = .14, P = .23, and P = .83, respectively). Only method 4 revealed a statistically significant d ifference, beginning at 0.5% masking (P = .04) and becoming progressively m ore significant with increased masking (P = 4.14 x 10(-7) at 5.5% masking). The effect of masking was limited to the T2-dark hematomas. CONCLUSION: There is no restriction of diffusion for in vivo hematomas with intracellular blood products. The T2 blackout effect for T2-dark hematomas on diffusion-weighted images should not be interpreted as fast diffusion. The method of expected values can be used to obtain measurements for region s with signal intensities near the background noise. Using literature value s for RBC self-diffusion, we computed lower limits of diffusion for hematom as with intracellular blood products to be 0.3 x 10(-3) mm(2)/s.