Very high field fMRIMany researchers are doing fMRI at 7T and higher, so there must be some advantages. What are they?
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A well recognized benefit of 7T imaging is the concomitant increase in BOLD signal and signal-to-noise ratio (SNR). Improved SNR translates into better spatial resolution, allowing voxels ≤ 1 mm³ to be obtained in reasonable imaging times.
High-resolution single-shot spin-echo fMRI at 7T with 0.5 mm in-plane resolution using visual activation paradigm. (From Olman et al 2011 open access)
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Additionally, BOLD signal at 7T more closely reflects the concentration of deoxyhemoglobin in capillaries rather than in larger veins (as it does at 1.5T). This phenomenon results from a field-dependent shortening of blood T2 and T2* relative to brain described in a prior Q&A and in the references below. fMRI at 7T thus offers the potential of improved spatial specificity since cortical neuronal activity is more closely linked to blood flow changes in the microvasculature than in larger vessels. This effect can be further accentuated by using spin-echo (SE) instead of gradient echo (GRE) acquisition.
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Notwithstanding these advantages, some limitations at 7T are well recognized: 1) increased susceptibility artifacts near the frontal sinuses and skull base (producing spatial distortion and signal drop out); 2) increased dielectric artifacts (manifest by signal inhomogeneity); 3) increased specific absorption rate (SAR, restricting number of slices); and 4) more difficult RF-coil design and shimming (causing inhomogeneous excitation and signal variations).
References
Barth M, Poser BA. Advances in high-field BOLD fMRI. Materials 2011; 4:1941-1955.
Harel N. Ultra high resolution at ultra-high field. NeuroImage 2012; 62:1024-1028.
Olman CA, Yacoub E. High-field fMRI for human applications: an overview of spatial resolution and signal specificity. Open Neuroimaging J 2011; 5 (Suppl 1-M3):74-89.
Siero JCW, Ramsey NF, Hooguin H, et al. BOLD specificity and dynamics evaluated in humans at 7T: Comparing gradient-echo and spin-echo hemodynamic responses. PLOS One 2013; 8(1) e354560:1-8.
Uludag, Müller-Bierl B, Ugurbil K. An integrative model for neuronal activity-induced signal changes for gradient and spin echo functional imaging. NeuroImage 2009; 48:150-165.
Barth M, Poser BA. Advances in high-field BOLD fMRI. Materials 2011; 4:1941-1955.
Harel N. Ultra high resolution at ultra-high field. NeuroImage 2012; 62:1024-1028.
Olman CA, Yacoub E. High-field fMRI for human applications: an overview of spatial resolution and signal specificity. Open Neuroimaging J 2011; 5 (Suppl 1-M3):74-89.
Siero JCW, Ramsey NF, Hooguin H, et al. BOLD specificity and dynamics evaluated in humans at 7T: Comparing gradient-echo and spin-echo hemodynamic responses. PLOS One 2013; 8(1) e354560:1-8.
Uludag, Müller-Bierl B, Ugurbil K. An integrative model for neuronal activity-induced signal changes for gradient and spin echo functional imaging. NeuroImage 2009; 48:150-165.
Related Questions
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How is image contrast produced by BOLD fMRI?