Abdominal perfusion imaging of the kidneys using CT or MRI contrast media is costly and potentially nephrotoxic. These limitations have sparked the desire to develop abdominal perfusion techniques that do not require contrast media injection. Arterial spin labeling (ASL) MRI is a candidate technique that employs the endogenous contrast of labeled protons in the blood to image abdominal perfusion. Previously, the feasibility of abdominal ASL has been investigated using spatially selective, pulsed (PASL) and pseudo-continuous (pCASL) labeling strategies on the kidneys^1,2 and liver³. Recently, feasibility velocity selective ASL (VS-ASL)⁴ was investigated for the kidneys⁵ to eliminate the delicate aspects of planning the spatially selective label slab. VS-ASL also potentially improves SNR by labeling closer to the tissue of interest, which reduces the time of the label to reach the target tissue and thus preserving its signal. However, this recent study⁵ called for motion compensation strategies to suppress labeling artifacts due to typical abdominal motion. Therefore, we investigated the reproducibility of pCASL and VS-ASL for renal and hepatic perfusion mapping in healthy volunteers under different triggering conditions.

Imaging

Five healthy volunteers (mean age 32yo, range [22-47], 2 female) were scanned on a 1.5T MR system (Ingenia, Philips Healthcare, Best, the Netherlands), equipped with a 32-channel torso coil. Written informed consent was obtained. Volunteer preparation included intake of 500ml juice to suppress susceptibility artifacts near the stomach and duodenum. In each exam, three coronal ASL scans were acquired twice; 1. pCASL scan with labeling slab positioned on the aorta and a fixed TR of 6500ms, 2. VS-ASL with a BIR-4 based tagging pulse⁶ and a fixed TR of 6500ms (fVS-ASL), and 3. tVS-ASL, triggered on expiration (tVS-ASL).

Volunteers were instructed to shortly prolong their end-expiratory state to match the fixed TR of 6500ms, allowing for in- and expiration after the acoustically easily recognizable readout, such that labeling and readout were performed in exhaled position.

The velocity selective labeling employed a velocity threshold of 2.5 cm/s in the craniocaudal direction and a post labeling delay of 500ms. Pseudo-continuous labeling duration was 1450ms. Coronal imaging was performed with a multi-slice single shot gradient-echo EPI Look-Locker readout, with four repetitions for pCASL (post-label delay 50ms) and five for VS-ASL (post-label delay 500ms).

Imaging parameters were TE 4.7ms, acq matrix 96x86, 3.9x4.0mm, 14 slices, slice thickness 8mm, gap 1.0mm, parallel imaging factor 2.5, EPI factor 57, flip angle 30°, for all ASL scans, with 20 repetitions of label and control pairs.

To confirm constant perfusion conditions, phase-contrast flow measurements on the left renal artery were interleaved with the ASL scans throughout the scan session. Anatomical T1 and T2 weighted scans were acquired for segmentation of the liver and kidneys.

Analysis

The kidneys and liver were manually segmented. To assess the consistency of the labeling for each of the methods, temporal SNR (temporally averaged signal intensity/temporal standard deviation) was measured for the 20 control-label repetitions. Pairwise (control-label)-subtractions were averaged and perfusion maps were calculated by fitting the data to the general kinetic model for Look-locker readout⁷ and expressed as ml/100ml/min. Reproducibility of the VS-ASL techniques was assessed for the liver and kidneys. All processing was implemented using MeVisLab (v2.6.2, MeVis Medical Solutions AG, Bremen, Germany).

Temporal SNR (tSNR) measurements (Fig. 1) did not reveal significant differences between the labeling methods on any of the regions of interests. Overall tSNR (mean ± SD) were 33.5 ± 2.0, 31.6 ± 2.1 and 32.5 ± 2.7 for pCASL, fVS-ASL and tVS-ASL respectively. Renal artery flow measurements confirmed constant perfusion conditions over the scan sessions. Although the feasibility of abdominal VS-ASL was confirmed by good visual correspondence (Fig. 2 and 3), VS-ASL reproducibility measurements were hindered by motion artifacts caused by differences in exhalation level at readout. This study on the use of abdominal VS-ASL in healthy volunteers confirms the feasibility of the technique. Although motion during labeling was avoided by pacing the volunteers breathing cycle or triggering the labeling, differences in exhalation level at readout were still present and hindered reproducibility measurements. However, these breathing motion artifacts might be suppressed by retrospective motion correction. This is under investigation. Future research will also focus on rejection strategies and comparison of the resulting perfusion maps with DCE-MRI and vascular input flow measurements. Abdominal perfusion imaging using VS-ASL is feasible and promising. In order to improve the reproducibility of the technique, future research should be directed towards retrospective motion correction to suppress motion artifacts introduced during readout.