Variable flip angle 3D‐GRASE for high resolution fMRI at 7 tesla

Abstract

Purpose To evaluate the use of variable flip angle refocusing pulse trains in single-shot 3D gradient and spin-echo (3D-GRASE) in order to reduce blurring and increase the spatial coverage for high spatial resolution T2-weighted functional MRI at 7 T. Methods Variable flip angle refocusing schemes in 3D-GRASE were calculated based on extended phase graph theory. The blurring along the slice (partition) direction was evaluated in simulations, as well as phantom and in-vivo experiments. Further, temporal stability and functional sensitivity at 0.8 mm isotropic resolution were assessed. Results Variable flip angle refocusing schemes yielded significantly reduced blurring compared to conventional refocusing schemes, with the full width at half maximum being about 30-40 % narrower. Simultaneously, spatial coverage could be increased by 80 %. The temporal signal-to-noise ratio was slightly reduced, but functional sensitivity was largely maintained due to increased functional contrast in the variable flip angle acquisitions. Signal-to-noise ratio and functional sensitivity were reduced more strongly in areas with insufficient RF transmission indicating higher sensitivity to experimental imperfections. Conclusion Variable flip angle refocusing schemes increase usability of 3D-GRASE for high-resolution fMRI by reducing blurring and allowing increased spatial coverage.

Cortical depth profiles of luminance contrast responses in human V1 and V2 using 7 T fMRI

“…T2‐weighted sequences, such as GRASE (De Martino et al, ; Kemper, De Martino, Yacoub, & Goebel, ; Kemper et al, ), and sequences that are not based on the BOLD contrast, such as VASO (Huber et al, ; Huber, Uludağ, & Möller, ) and ASL (Huber et al, ; Pfeuffer et al, ; Uludağ et al, in preparation), are less affected by vascular biases and are therefore expected to yield a better estimation of local neuronal activity compared with GE sequences. However, this advantage comes at the price of a lower sensitivity and/or decreased coverage.…”

“…T2-weighted sequences, such as GRASE (De Martino et al, 2013;Kemper, De Martino, Yacoub, & Goebel, 2015;Kemper et al, 2016), and sequences that are not based on the BOLD contrast, such as VASO (Huber et al, 2014(Huber et al, , 2015Huber, Uluda g, & M€ oller, 2017b) and…”

How to choose the right MR sequence for your research question at 7 T and above?

“…Care should however still be taken to ensure that the blurring in the partition encoding direction is less than one pixel. Both high power deposition and the rate of signal decay during the echo train can be reduced by using a tailored variable refocusing pulses (Kemper et al, 2015). It is also possible to acquire 3D GRASE data in a single shot on a standard 7T scanner: this has to date been used for the acquisition of high resolution fMRI data using a zoomed (inner volume selective) approach (Bennett et al, 2008;De Martino et al, 2013b).…”

“…The spin echo approach that is most commonly employed is to use zoomed GRASE, which has been shown capable of producing layer specific activation patterns (De Martino et al, 2013b). Using tailored flip angles the spatial PSF of this sequence can also be improved (Kemper et al, 2015).…”

Controlling T2 blurring in 3D RARE arterial spin labeling acquisition through optimal combination of variable flip angles and k‐space filtering

“…The design of variable flip angle trains, however, is generally performed using heuristic rather than systematic methods. For example, variable flip angles schemes were proposed in an intuitive way by designing the maximum signal at the last echo or attempting to maintain constant signal until the k‐space center was covered and then increasing flip angles linearly . Such empirical designs are often targeted for very specific problems and may only provide suboptimal performance.…”