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Schlagwörter:
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Zusammenfassung:
Purpose/Introduction: ASL at ultra-high field benefits from higher SNR and longer longitudinal relaxation times [1]. However, stronger B1 + and B0 field inhomogeneities are challenging for the preparation and readout of FAIR-ASL
[2]. We were able to perform the first ASL experiments in the human brain at 9.4T using an optimized adiabatic inversion pulse in combination with a GRE-EPI readout [3].
Subjects and Methods: The protocol consisted of a FAIR preparation containing a time resampled FOCI (TR-FOCI) pulse followed by a 2D GREEPI readout with minimized temporal spacing between the acquisitions of the different slices. The TR-FOCI was optimized using a genetic algorithm
described by Hurley et al. but with modified constraints to benefit from the capabilities of our scanner. A single subject (male, 28yo.) was measured on a Siemens Magnetom 9.4T scanner with a home-built 16ch transmit / 31ch receive array [4]. The required voltage of the TR-FOCI was estimated based on an actual flip angle (AFI) B1
+ map. Additionally, the slice profile of the TR-FOCI was determined with an IR-GRE sequence. For the ASL experiment, forty label-control pairs (16 slices, 2x2x2mm3, no slice gap) were acquired at four different inversion times with
flow crusher gradients. M0 was measured using the same EPI readout but with a longer TR. Image reconstruction and calculations were performed offline in Matlab (MathWorks, USA).
Results: Fig. 1a shows the B1 + map scaled with the voltage of the inversion pulse. The measured slice profile at the same slice position is depicted in Fig. 1b. Fig. 2 displays the calculated perfusion weighted images for all TIs and
every third slice. Note that the three most inferior slices were neglected due to the imperfect inversion slice profile.
Discussion/Conclusion: This work shows that FAIR-ASL can be performed at 9.4T with 2mm isotropic resolution. An optimized adiabatic inversion pulse was used for the FAIR preparation in order to improve B1 + stability. In a future
study, it may be beneficial to replace the non-selective tagging pulse with a spatially confined pulse to reduce the influence of the inferior transmit field variations [5]. For quantitative perfusion mapping with a single subtraction
(QUIPSS), a further prolongation of the repetition time may be necessary since the SAR level was already close to the safety limit.
