Turbo-FLASH Based Arterial Spin Labeled Perfusion MRI at 7 T

Abstract

Motivations of arterial spin labeling (ASL) at ultrahigh magnetic fields include prolonged blood T1 and greater signal-to-noise ratio (SNR). However, increased B0 and B1 inhomogeneities and increased specific absorption ratio (SAR) challenge practical ASL implementations. In this study, Turbo-FLASH (Fast Low Angle Shot) based pulsed and pseudo-continuous ASL sequences were performed at 7T, by taking advantage of the relatively low SAR and short TE of Turbo-FLASH that minimizes susceptibility artifacts. Consistent with theoretical predictions, the experimental data showed that Turbo-FLASH based ASL yielded approximately 4 times SNR gain at 7T compared to 3T. High quality perfusion images were obtained with an in-plane spatial resolution of 0.85×1.7 mm2. A further functional MRI study of motor cortex activation precisely located the primary motor cortex to the precentral gyrus, with the same high spatial resolution. Finally, functional connectivity between left and right motor cortices as well as supplemental motor area were demonstrated using resting state perfusion images. Turbo-FLASH based ASL is a promising approach for perfusion imaging at 7T, which could provide novel approaches to high spatiotemporal resolution fMRI and to investigate the functional connectivity of brain networks at ultrahigh field.

Simultaneous multi-slice Turbo-FLASH imaging with CAIPIRINHA for whole brain distortion-free pseudo-continuous arterial spin labeling at 3 and 7 T

“…Moreover, the imaging acquisition parameters were adjusted at 7T to accommodate the SAR constraint. Theoretically, performing ASL at ultrahigh magnetic field strength can be beneficial because of the increased SNR, as well as the lengthened T 1 -realxation time (Zuo et al, 2013). The latter feature not only loosens the restriction on the length of the PLD and image acquisition window, it also creates less spatial blurring as indicated by the simulated PSF shown in Fig.…”

“…Incorporating the train of RF excitations pulses from the TFL readout into the final perfusion weighted signal presentation, the TFL-based pCASL signal Δ S can be modeled as (Zuo et al, 2013): $$\mathrm{normal\Delta }S=\frac{2\alpha {fS}_{M_0}\left[e^{-{wR}_{1a}}-e^{-false(\tau +wfalse)R_{1a}}\right]{false(E_{1}cos\theta false)}^j}{\lambda R_{1a}\left[{false(E_{1}cos\theta false)}^j+false(1-E_{1}false)\frac{1-{false(E_{1}cos\theta false)}^j}{1-E_{1}cos\theta }\right]}$$ where E 1 = e −TR / T 1 , θ is the FA of the RF excitation pulses in TFL, S M 0 is the M 0 signal (control image intensity), and j is the number of acquired PE lines till it reaches the center of k-space in the TFL readout. Since multiple MB slices are acquired sequentially after each pCASL preparation, the nominal PLD was adjusted for each slice according to w =PLD + n *Tacq, where Tacq is the imaging time for each 2D slice, and n is the index for the slice ordering.…”

“…[2]. The labeling efficiency was previously estimated to be 0.85 at 3T (Wu et al, 2007) and 0.64 at 7T (Zuo et al, 2013). …”

“…These hardware advances provide the foundation for continued technical innovations to improve the SNR and image quality of ASL, as well as of other MRI modalities. For instance, pCASL at 7T should theoretically offer a four-fold SNR gain compared to pCASL at 3T (Zuo et al, 2013). …”

“…In the Human Connectome Project, high-resolution SMS-EPI scans are usually acquired twice with opposite phase-encoding (PE) directions to compensate for image distortions during post-processing (Smith et al, 2013; Sotiropoulos et al, 2013). Turbo-FLASH (TFL) offers a promising alternative imaging sequence at high and ultrahigh magnetic fields (Jahng et al, 2007; Wang et al, 2010; Zuo et al, 2013) because of its reasonably fast imaging time, relatively low specific absorption rate (SAR) of RF power, and minimal sensitivity to susceptibility effects due to the short echo time (TE). In addition, TFL is naturally suited for SMS imaging with the CAIPIRINHA technique.…”