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

Zhentao Zuo,Danny J J Wang,Keith S St Lawrence,Rong Xue,Rui Wang,Yan Zhuo,Xiaoliang Zhang

doi:10.1371/journal.pone.0066612

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.

Highlights

During the past two decades, arterial spin labeled (ASL) perfusion MRI has been developed into a class of noninvasive methods for direct measurement of cerebral blood flow (CBF)
The relative signal-to-noise ratio (SNR) acquired by Turbo-FLASH for 7T pseudo-continuous ASL (pCASL), 7T pulsed ASL (PASL), 3T pCASL and 3T PASL is 3.7:3.1:1.3:1, i.e., an approximate factor of 3 when performing ASL at 7T versus the same technique at 3T using Turbo-FLASH as the readout
Resting-state pCASL and PASL Figure 3 shows a direct comparison of Turbo-FLASH based pCASL perfusion images acquired at 7T and 3T with 3 spatial resolutions (1.763.4 mm2, 1.761.7 mm2 and 0.8561.7 mm2 at 7T, 3.463.4 mm2, 1.763.4 mm2 and 1.761.7 mm2 at 3T)

Summary

Introduction

During the past two decades, arterial spin labeled (ASL) perfusion MRI has been developed into a class of noninvasive methods for direct measurement of cerebral blood flow (CBF). It has found a broad range of applications in both basic and clinical neuroscience [1,2]. The drawbacks of ASL, include the relatively small fractional signal of labeled arterial blood (,1%) as well as the transit artifact resulting from intravascular labeled blood yet to reach brain tissue by the time of image acquisition [1,3]. While a general benefit of SNR gain has been demonstrated, the implementation of ASL at ultrahigh fields presents several challenges

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Results

Conclusion