Date Published: April 25, 2019
Publisher: Public Library of Science
Author(s): María Guadalupe Mora Álvarez, Robert Wayne Stobbe, Christian Beaulieu, Quan Jiang.
For standard clinical applications, ASL images are typically acquired with 4–8 mm thick slices and 3–4 mm in-plane resolution. However, in this paper we demonstrate that high-resolution continuous arterial spin labeling (CASL) perfusion images can be acquired in a clinically relevant scan time using current MRI technology. CASL was implemented with a separate neck coil for labeling the arterial blood on a 4.7T MRI using standard axial 2D GE-EPI. Typical-resolution to high-resolution (voxels of 95, 60, 45, 27, or 7 mm3) images were compared for qualitative and quantitative cerebral blood flow analysis (CBF) in nine healthy volunteers (ages: 24–32 years). The highest resolution (1.5×1.5×3 = 7 mm3) CASL implementation yielded perfusion images with improved cortex depiction and increased cortical CBF measurements (53 ± 8 ml/100g/min), consistent with reduced partial volume averaging. The 7 mm3 voxel images were acquired with 6 cm brain coverage in a clinically relevant scan of 6 minutes. Improved spatial resolution facilitates CBF measurement with reduced partial volume averaging and may be valuable for the detection of perfusion deficits in small lesions and perfusion measurement in small brain regions.
Arterial spin labeling (ASL) measures cerebral perfusion using radiofrequency tagged blood as an endogenous contrast agent, and offers comparable perfusion deficit detection to intra-venous Gd bolus tracking in acute stroke [1–3]. In an effort to standardize ASL measurements for clinical applications, an ASL “white paper”  recommends 4–8 mm thick slices and 3–4 mm in-plane resolution yielding voxel volumes from 36 mm3 to 128 mm3 in 4 minutes maximum scan time (see Table 2 in Reference ). While ASL protocols of this sort may be effective for large lesions, they are likely inadequate for the detection of smaller regions of perfusion deficit as may be expected in transient ischemic attack or minor stroke . They are also likely inadequate for the assessment of perfusion in small brain structures . The need for higher spatial resolution has been recognized in two recent ASL stroke studies at 3T that used smaller voxels of 27 mm3 (i.e. 3 mm isotropic) and 23 mm3 [7,8].
The review board of our institution approved this study, and written informed consent was obtained from all volunteers who participated. Perfusion imaging was performed on a 4.7T scanner (Varian, Walnut Creek, CA) using a butterfly neck coil for continuous arterial spin labeling with housing dimensions: length 110 mm, width 150 mm, and height 75 mm (Fig 1) (Rapid Biomedical, Rimpar, Germany). This coil weighed 350 g and was designed to handle a maximum mean power of 5W. Active decoupling enabled isolation from the transmit coil and receive-array during imaging. A dedicated amplifier (Communication Power Corporation, Hauppauge, New York, USA) capable of operation in continuous-wave mode provided RF power. The RF coil used for head excitation was a 16-leg detunable quadrature birdcage design with a 27 cm diameter and 25 cm leg lengths (XL-Resonance, London, Ontario, Canada) . It also possessed an integral RF shield with 35.5 cm diameter. The RF coil used for reception had 4 detunable elements closely fitting the average adult head, and each element was connected to a low impedance, low noise-figure preamplifier (Pulsteq Ltd, Guildford, Surrey, UK). The irradiation range of the labeling coil was tested to be outside the brain and the transmit and receive coils were detuned during the labeling period. Thus, magnetization transfer effects in brain were avoided, a known advantage of separate labeling coils [16,17,19].
The labeling efficiency of arterial blood tagging at 4.7T with the butterfly neck coil and continuously applied RF was measured to be 0.83, 0.81, and 0.85 in the carotid arteries of 3 volunteers.
Continuous arterial spin labeling with a separate neck coil facilitates the acquisition of high-resolution CBF images due to high perfusion contrast , good labeling efficiency and low SAR which is particularly important for high fields. In this study, we demonstrate that (1.5×1.5×3 mm3) images of cerebral blood flow are feasible in a 6 minute scan time at 4.7T. These higher-resolution images reduce partial volume effect and facilitate improved cortex depiction.
High-resolution CASL images (7 mm3) with a separate labeling neck coil were shown to be feasible at 4.7T in 6 minutes with a 6 cm multi-slice coverage. Higher spatial resolution depicted perfusion in the cortical gray matter much better than in the typical ASL protocols and also yielded greater CBF values in line with less partial volume effects. High-resolution ASL images could facilitate improved detection and quantification of small perfusion deficits.