Whole-brain perfusion mapping at 7T by SAR-efficient non-segmented 3D EPI-pCASL

Abstract

Pseudo-continuous arterial spin labeling (pCASL) is one of the most recommended MRI techniques for non-invasive perfusion mapping. Conventionally, pCASL have been conducted by 2D multi-slice or segmented 3D readouts for whole-brain perfusion mapping. However, these readouts have intrinsic problems such as heterogeneous post labeling delays across slices or low temporal resolution. Additionally, pCASL labeling pulses cause high specific abruption rate (SAR), limiting the usage of the conventional readout schemes in the high field of 7T human MRI. Therefore, in this paper, SAR-efficient non-segmented 3D echo planar imaging (EPI)-pCASL is proposed. At the given TR, SAR level of the 3D EPI readout could be reduced down to the order of 0% by successive water-excitation rectangular RF pulses with low flip angle, allowing for more efficient pCASL labeling at the high field. Furthermore, 3D EPI with slice encoding per RF excitation enables non-segmented whole-brain 3D imaging for each labeling/control scan. Therefore, with background suppression and partial Fourier, high quality perfusion maps were obtained by the proposed readout sequence at both 3T and 7T MRI. At 3T, cerebral blood flow (CBF) values and spatial signal to noise ratio (SNR) from the proposed 3D EPI-pCASL were consistent with those from the conventional 2D EPI-pCASL, while having more homogeneous post labeling delays and thus more homogeneous perfusion maps across slices. Furthermore, despite usage of a single transmitter/receiver head RF coil at 7T MRI, decent perfusion maps could be acquired without any additional hardware. Moreover, because of high temporal resolution, functional MRI with the proposed 3D EPI-pCASL could be conducted by tasks simultaneously stimulating visual and motor cortices. The functional z score map showed the activations at both areas properly. Therefore, the proposed SAR-efficient non-segmented 3D EPI-pCASL is a good candidate for perfusion imaging at high field MRI.