2D AND 3D HIGH-RESOLUTION GRADIENT-ECHO FUNCTIONAL IMAGING OF THE BRAIN - VENOUS CONTRIBUTIONS TO SIGNAL IN MOTOR CORTEX STUDIES

Recent emphasis on high resolution gradient echo studies in functional imaging has led to the conclusion that there are likely three domains of response to the blood circulation in the brain when considering fi eld inhomogeneity effects of the venous blood pre- and during activati on. The first is a coherent effect due to large or macroscopic vessels on the order of the size of the voxel (ca 200-500 mu m in most studie s). These can lead to very large signal changes (up to 100%). The seco nd is at the venule level (ca 50-200 mu m) and is associated with smal ler parenchymal changes (usually ca 10% or less). The third is at the capillary level and is associated with much smaller signal changes at 1.5 T and even up to 4 T. The actual signal changes depend on field st rength and sequence design. In this paper, we present our experience i n detecting the first two domains with 2D and 3D gradient echo imaging at 1.5 T. We find that high resolution enables visualization of the l arger small veins in motor cortex studies and that, on occasion, it is possible to see the venule effects as well. We suggest a simple model to explain the large signal changes based on susceptibility changes a nd partial volume effects. Comparisons of the functional imaging data to this model and to MR angiographic studies are also shown as evidenc e of the venous sources of the susceptibility changes.