Quantitative and Functional Pulsed Arterial Spin Labeling in the Human Brain at 9.4 T
Arterial spin labeling (ASL) can benefit twofold from ultrahigh-field MRI. First, the higher field strength leads to an increase in the intrinsic signal-to-noise ratio (SNR), and second, a higher and longer-lasting perfusion-related signal change can be measured due to the longer longitudinal relaxation times. The great potential of ultrahigh-field ASL in humans was demonstrated by a number of studies performed at 7 T with diverse ASL preparations and signal sampling strategies. Some of these studies used the pulsed arterial spin labeling (PASL) technique known as flow-sensitive alternating inversion recovery (FAIR) which has comparatively low power deposition and is virtually unsusceptible to magnetization transfer effects. The feasibility of quantitative FAIR-ASL at ultrahigh fields was confirmed by a comparative study performed at 3 T and 7 T, where an increase in temporal SNR of 62% and a 78% higher image SNR was found for the data obtained at 7 T. However, the implementation of PASL at field strengths above 3 T is not straightforward. The FAIR technique uses a nonselective inversion pulse to create the label and a selective inversion around the image plane for the control condition. The commonly used signal models for quantification of perfusion assume that a spatially well-defined bolus is created during the labeling phase of the sequence. On ultrahigh-field scanners, however, the labeling is typically performed with a head coil that produces a spatially limited transmit field that weakens toward the inferior end of the coil. A label created with such a coil will result in an underestimation of perfusion if not considered in the model. This source of error can be avoided by spatially confining the labeling slab to a region with sufficient B1, which can also help to reduce spatial variations of the inversion efficiency caused by static magnetic field inhomogeneities.
Because all these restrictions scale with field strength, PASL at 9.4 T can be expected to be even more challenging than at 7 T. In this study, the first results of pulsed ASL with FAIR at 9.4 T are presented. For the implementation, an adiabatic inversion pulse and a multipulse in-plane presaturation scheme were optimized. Both were evaluated by measuring their spatial selectivity and efficiency. In a quantitative experiment, multislice perfusion-weighted images (PWI) were acquired at several inversion times (TIs) and at two different field strengths (3 T and 9.4 T) to analyze the characteristics of the ASL signal.