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By carefully considering all the challenges of high-field VASO and filling behavior of the relevant vasculature, the proposed method can detect and quantify CBV changes with high CNR in human brain at 7T.

We have discovered a simple and highly robust method for removal of chemical shift artifact in spin-echo MR images, which simultaneously decreases the radiofrequency power deposition (specific absorption rate). The method is demonstrated in spin-echo echo-planar imaging brain images acquired at 7 T, with complete suppression of scalp fat signal. When excitation and refocusing pulses are sufficiently different in duration, and thus also different in the amplitude of their slice-select gradients, a spatial mismatch is produced between the fat slices excited and refocused, with no overlap. Because no additional radiofrequency pulse is used to suppress fat, the specific absorption rate is significantly reduced compared with conventional approaches. This enables greater volume coverage per unit time, well suited for functional and diffusion studies using spin-echo echo-planar imaging. Moreover, the method can be generally applied to any sequence involving slice-selective excitation and at least one sliceselective refocusing pulse at high magnetic field strengths. The method is more efficient than gradient reversal methods and more robust against inhomogeneities of the static (polarizing) field (B 0 ). Magn Reson Med 64:319-326, 2010. V C 2010 Wiley-Liss, Inc. Key words: chemical shift; spin-echo; fat suppression; highfield MRI; specific absorption rate; spin-echo echo-planar imaging; fMRI; diffusion imaging MRI is intended to provide spatially resolved images correctly depicting anatomy. Because MR-visible protons in water and fat have Larmor frequencies that are 3.35 ppm apart, techniques using relatively low receiver bandwidths per voxel suffer from chemical shift artifact, in which the image of fatty tissue can be displaced by several voxels from the water image, with a highly problematic overlap. The magnitude of the displacement depends on magnetic field strength, RF pulse bandwidth, gradient strength, acquisition bandwidth, voxel size, and k-space trajectory. This problem can be particularly severe in echo-planar images, where at 7 T the fat artifact can be displaced by 70% of the field of view from the water image. To avoid the chemical shift artifact, other MRI sequences are run with unnecessarily high bandwidths, with a detrimental effect on signal-to-noise ratio.Since the earliest chemical shift imaging publications (1-3) numerous methods have been used to image water and fat tissue distributions separately (4-6). For functional and diffusion-weighted neuroimaging, the fat distribution is usually of little interest. For both applications, single-shot echo-planar imaging (EPI) is often used, because it acquires entire slices in a fraction of a second and thus avoids distributed artifacts associated with head motion. However, EPI acquisitions are sensitive to chemical shift artifacts, which appear in the phase-encoding direction of the acquisition, due to the relatively low bandwidth per voxel compared to the frequency-encoding direction. This is usually mitigated by including a fat-suppression module precedi...

Cerebral blood volume (CBV) changes significantly with brain activation, whether measured using positron emission tomography, functional magnetic resonance imaging (fMRI), or optical microscopy. If cerebral vessels are considered to be impermeable, the contents of the skull incompressible, and the skull itself inextensible, task- and hypercapnia-related changes of CBV could produce intolerable changes of intracranial pressure. Because it is becoming clear that CBV may be useful as a well-localized marker of neural activity changes, a resolution of this apparent paradox is needed. We have explored the idea that much of the change in CBV is facilitated by exchange of water between capillaries and surrounding tissue. To this end, we developed a novel hemodynamic boundary-value model and found approximate solutions using a numerical algorithm. We also constructed a macroscopic experimental model of a single capillary to provide biophysical insight. Both experiment and theory model capillary membranes as elastic and permeable. For a realistic change of input pressure, a relative pipe volume change of 21±5% was observed when using the experimental setup, compared with the value of approximately 17±1% when this quantity was calculated from the mathematical model. Volume, axial flow, and pressure changes are in the expected range.