The value of b required to avoid T2 shine-through from old lacunar infarcts in diffusion-weighted imaging

Multiple small infarcts of different ages are common in small-vessel disease. Diffusion-weighted imaging (DWI) is a powerful method for discriminating new from chronic lesions. This can be done on the diffusion-weighted images provided that b is sufficiently high. Our purpose was to determine that critical value of b. We reviewed DWI from a previous study of acute, mainly lacunar strokes, and selected 18 old lacunar infarcts, well defined on uncoded images with b 0 s/m(2) (i.e., T2-weighted images) but invisible on DWI with b 1200 x 10(6) s/m(2). We used a 1.5 tesla imager and sin,ale-shot echo-planar technique. We had seven separate acquisitions with echo time 123 ms and b in steps between 0 and 1200 x 10(6) s/m(2). Two neuroradiologists blinded to the selection of lesions carried out two different lesion-detection procedures, thereby testing each lesion four times, giving a total of 72 tests of b values. The results were consistent, indicating a level for detection of 800 x 10(6) s/m(2) in two tests, 400-600 x 10(6) s/m(2) in 65 tests and at lower values in the remainder. For imagers up to 1.5 tesla, at long repetition times and an echo time up to 120 ms T2-shine through of old lacunar infarcts can be avoided using b of 1000 x 10(6) s/m(2).