Distinguishing pial and laminar gradient-echo BOLD signals at 7 Tesla

Introduction With high field scanners and multi-channel coil arrays becoming more readily available, high resolution fMRI methods may be capable of detecting neural responses at the laminar level. First results in animals indicate that gradient-echo (GE) is not able to depict laminar activation [1-4] and this has been attributed to the large BOLD contribution of the pial veins corrupting the laminar activation profiles. Spin-echo (SE) studies on the other hand have been able to depict layer specific activation in animals [1-3]. At high field strengths (7T and above), the largest contribution to the SE-BOLD effect originates from the capillaries and hence SE is thought to be the method of choice for high resolution fMRI. In this study we show that GE is capable of depicting laminar activation when using a sequence with a high resolution and a high degree of anatomical trueness (i.e. no distortion). When these conditions are met, veins can be separated from the laminar response based upon their position alone and one can benefit from the superior sensitivity and reduced SAR of the GE method.