Whole-brain background-suppressed pCASL MRI with 1D-accelerated 3D RARE Stack-Of-Spirals readout

Abstract

Arterial Spin Labeled (ASL) perfusion MRI enables non-invasive, quantitative measurements of tissue perfusion, and has a broad range of applications including brain functional imaging. However, ASL suffers from low signal-to-noise ratio (SNR), limiting image resolution. Acquisitions using 3D readouts are optimal for background-suppression of static signals, but can be SAR intensive and typically suffer from through-plane blurring. In this study, we investigated the use of accelerated 3D readouts to obtain whole-brain, high-SNR ASL perfusion maps and reduce SAR deposition. Parallel imaging was implemented along the partition-encoding direction in a pseudo-continuous ASL sequence with background-suppression and 3D RARE Stack-Of-Spirals readout, and its performance was evaluated in three small cohorts. First, both non-accelerated and two-fold accelerated single-shot versions of the sequence were evaluated in healthy volunteers during a motor-photic task, and the performance was compared in terms of temporal SNR, GM-WM contrast, and statistical significance of the detected activation. Secondly, single-shot 1D-accelerated imaging was compared to a two-shot accelerated version to assess benefits of SNR and spatial resolution for applications in which temporal resolution is not paramount. Third, the efficacy of this approach in clinical populations was assessed by applying the single-shot 1D-accelerated version to a larger cohort of elderly volunteers. Accelerated data demonstrated the ability to detect functional activation at the subject level, including cerebellar activity, without loss in the perfusion signal temporal stability and the statistical power of the activations. The use of acceleration also resulted in increased GM-WM contrast, likely due to reduced through-plane partial volume effects, that were further attenuated with the use of two-shot readouts. In a clinical cohort, image quality remained excellent, and expected effects of age and sex on cerebral blood flow could be detected. The sequence is freely available upon request for academic use and could benefit a broad range of cognitive and clinical neuroscience research.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1. Illustration of the (A) non-accelerated and (B) 1D-accelerated 3D RARE Stack-Of-Spirals readout employed.

Two spiral interleaves were used to sample each partition (shown in red and blue). In the 1D-accelerated readout, every other kz partition is skipped due to undersampling (highlighted in gray).

Fig 2. (A) Modulation transfer function (MTF) and (B) normalized point spread function (PSF) of the non-accelerated (blue) and 1D-accelerated (1-shot = red, 2-shot = green) readouts.

The width of the PSF decreased from 2.86 to 1.54 (1-shot) and 1.30 (2-shot) with the use of acceleration, increasing the effective voxel resolution.

Fig 3. Mean CBF maps obtained in Study 1 without and with acceleration from a representative subject, in ml/min/100g, alongside the corresponding anatomical dataset.

Note the reduced through-plane blurring and increase in GM—WM tissue contrast with acceleration in the coronal and sagittal views.

Fig 4. Results of the functional activation data analysis in Study 1.

Panels (A) and (B) show the task>rest activation clusters for two representative subjects, obtained with the non-accelerated and the 1D-accelerated data in both visual (left) and motor (right) areas. Panel (C) shows the group activation maps obtained with each readout type. Panel (D) shows the differences in activity across readouts. Non-accelerated data only presented higher activity than accelerated data in the white matter region below the left motor cortex cluster and above the primary visual cortex cluster, likely reflecting the effects of image blurring, while 1D-accelerated data presented higher differential activation within both clusters.

Fig 5. (A) Mean CBF and corresponding (B) temporal SNR maps obtained with 1-shot and 2-shot 1D-accelerated readouts in Study 2 from a representative subject, alongside the anatomical dataset.

Note how the two-shot readout resulted in both an increase in SNR and a further reduction in through-plane blurring, evident in the coronal and sagittal views. White arrows point at orbitofrontal areas, which typically suffer from low SNR due to shortened T2* and T2. CBF units are in ml/min/100g.

Fig 6. Mean CBF and temporal SNR maps of the Study 3 elderly cohort obtained from (A) a representative subject and (B) the group average after registration to MNI space and smoothing.

The lowest SNR values were found in orbitofrontal regions, due to the slight sensitivity of spiral readouts to T2* loses.