Abstract
PurposeTo improve the labeling efficiency of pseudo‐continuous arterial spin labeling (PCASL) at 7T using parallel transmission (pTx).MethodsFive healthy subjects were scanned on an 8‐channel‐transmit 7T human MRI scanner. Time‐of‐flight (TOF) angiography was acquired to identify regions of interest (ROIs) around the 4 major feeding arteries to the brain, and B1+ and B0 maps were acquired in the labeling plane for tagging pulse design. Complex weights of the labeling pulses for each of the 8 transmit channels were calculated to produce a homogenous radiofrequency (RF) ‐shimmed labeling across the ROIs. Variable‐Rate Selective Excitation (VERSE) pulses were also implemented as a part of the labeling pulse train. Whole‐brain perfusion‐weighted images were acquired under conditions of RF shimming, VERSE with RF shimming, and standard circularly polarized (CP) mode. The same subjects were scanned on a 3T scanner for comparison.ResultsIn simulation, VERSE with RF shimming improved the flip‐angles across the ROIs in the labeling plane by 90% compared with CP mode. VERSE with RF shimming improved the temporal signal‐to‐noise ratio by 375% compared with CP mode, but did not outperform a matched 3T sequence with a matched flip‐angle.ConclusionWe have demonstrated improved PCASL tagging at 7T using VERSE with RF shimming on a commercial head coil under conservative SAR limits at 7T. However, improvements of 7T over 3T may require strategies with less conservative SAR restrictions.
Highlights
Arterial spin labeling (ASL) is a noninvasive perfusion imaging technique which offers various clinical applications in areas such as stroke, dementia, and chronic vascular diseases.[1]
For brain imaging using pseudo-continuous arterial spin labeling (PCASL), blood is typically labeled in the 4 main feeding arteries: the right internal carotid artery (RICA), right vertebral artery (RVA), left internal carotid artery (LICA), and left vertebral artery (LVA)
Simulation results predicted that Gaussian shimming would improve the mean FA in the regions of interest (ROIs) across all subjects by 37%, and that Variable-Rate Selective Excitation (VERSE) shimming would improve the mean FA by 90% (Figure 2, right; Figure 3A-C)
Summary
Arterial spin labeling (ASL) is a noninvasive perfusion imaging technique which offers various clinical applications in areas such as stroke, dementia, and chronic vascular diseases.[1] During an ASL scan, spins in blood vessels upstream of the imaging plane are inverted or saturated (labeled). After a postlabeling delay to allow the labeled blood water to reach the tissue, an image is acquired, for example with an echo planar imaging (EPI) readout.[2] ASL labeling. Consortium for ASL in Dementia due to its relatively high signal-to-noise ratio (SNR) and clinical applicability.[1] For brain imaging using PCASL, blood is typically labeled in the 4 main feeding arteries: the right internal carotid artery (RICA), right vertebral artery (RVA), left internal carotid artery (LICA), and left vertebral artery (LVA)
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