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Influence of labeling parameters of velocity selective arterial spin labeling for renal perfusion imaging
Isabell K. Bones1, Suzanne L. Franklin1,2, Anita A. Harteveld1, Matthias J.P. van Osch2, Jeroen Hendrikse3, Chrit Moonen1, Marijn van Stralen1, and Clemens Bos1

1Center for Image Sciences, University Medical Center Utrecht, Utrecht, Netherlands, 2C.J.Gorter Center for High Field MRI, Leiden University Medical Center, Leiden, Netherlands, 3Department of Radiology, University Medical Center Utrecht, Utrecht, Netherlands

Synopsis

Velocity selective arterial spin labeling (VSASL) is a spatially non-selective method that labels spins based on their flow velocity, thereby labeling closer to the target tissue, reducing the influence of arterial transit time (ATT) and requiring no planning. In the abdomen, motion and complex vascular anatomy might, however, require dedicated VS-labeling parameters. We assessed the feasibility of VSASL for renal perfusion measurement by investigating its dependency on essential labeling parameters, and by comparing it with pseudo-continuous ASL (pCASL) as a spatially-selective reference ASL-technique. Our results show, that with carefully chosen sequence parameters, VSASL is feasible for renal perfusion measurement.

Introduction

Velocity selective arterial spin labeling (VSASL)1 labels spins based on their flow velocity, thereby also labeling in the imaging plane, thus reducing arterial transit time (ATT) artifacts. Moreover, VSASL does not require planning of a labeling slab, making it highly attractive for abdominal perfusion measurements, where planning might be challenging due to complex (vascular) anatomy. However, limited work has been done studying the application of VSASL in the abdomen. In this study, the dependency of renal VSASL on essential labeling parameters is assessed and its sensitivity to renal perfusion evaluated by comparison with renal pseudo-continuous ASL (pCASL).

Methods

Imaging: Kidneys of 15 volunteers (age 23-38, 6 men) were scanned on a 1.5T MRI (Ingenia, Philips, The Netherlands) using a 28-element phased-array receiver-coil. Seven angulated coronal slices were acquired with a gradient echo EPI readout (Table 1) using VSASL2 and pCASL3 (1500ms label duration and PLD), including 10 label-control pairs. This study focused on the following VSASL parameters (Table 2): VS-labeling mode (single (s)VSASL and dual (d)VSASL), labeling cut-off velocity (Vc), labeling gradient orientation (GrOri), post-labeling delay (PLD).
Experiments: Based on pilot experiments, a baseline scan was defined (Vc=5cm/s, GrOri=FH, PLD=1200ms). For each of the parameters Vc, GrOri and PLD, 5 subjects were scanned in which the parameter was systematically varied, for both sVSASL and dVSASL labeling (Table 2). Additionally, baseline-VSASL, pCASL and an M0 equilibrium image were acquired in all volunteers. ASL scans included background suppression.
Analysis: Images were aligned using Elastix4, for each kidney separately. The influence of labeling parameters on VSASL was assessed by calculating the perfusion-weighted signal (PWS=ΔM/M0×100%) and voxel-wise temporal SNR (tSNR) and were reported as a function of Vc, GrOri and PLD. Occurrence of subtraction artifacts (spurious labeling) was visually assessed per dynamic for scans with varying Vc, recognizable by homogeneously high ΔM over the entire kidney. In addition, sensitivity to perfusion signal of VSASL (baseline-settings) was evaluated by PWS comparison with pCASL at the subject level, established by the Pearson correlation-coefficient.

Results

With decreasing Vc, PWS and tSNR increased for both single- and dual-VSASL (Figure 1a). However, at low Vc(≤5cm/s), subtraction artefacts became evident (Figure 2) for 19%/28% (left/right kidney) of the label-control pairs with Vc=2cm/s and 5%/3% with Vc=5cm/s.
GrOri had no effect on PWS for either sVSASL or dVSASL (Figure 1b). tSNR was slightly affected by the orientation, with labeling in FH direction yielding highest tSNR.
Increasing PLD resulted in a PWS decrease for sVSASL, while for dVSASL this effect was considerably smaller (Figure 1c). Nevertheless, for short PLDs(≤800ms), especially for sVSASL, high PWS appeared in the center of the kidneys. Highest tSNR for dVSASL was found using a PLD of 1200ms, while for sVSASL it occurred at shorter PLDs(≤800ms).
Overall, dVSASL yielded consistently lower PWS and tSNR than sVSASL (Figure 1). When correlating the PWS per volunteer, measured with baseline-VSASL to the reference technique pCASL, dVSASL showed a stronger correlation than sVSASL with r=0.60 and r=0.52, respectively, with an offset present for sVSASL (Figure 3).

Discussion & Conclusions

Results showed that renal VSASL is dependent on the Vc and PLD, but is relatively stable for different gradient orientations. Subtraction artefacts were observed with Vc≤5cm/s, likely caused by respiration-induced motion during VS-labeling.1,5
In this study, both dual and single VSASL were studied. dVSASL does not only allow quantification by creating a fixed temporal label width, but also eliminates venous signal. However, sVSASL is the fundamental building-block determining the amount of label created. Signal enhancement of sVSASL in the center of kidneys at short PLDs(<800ms) and the offset when compared with pCASL, were probably caused by venous signal since both were eliminated when using dVSASL6.
A good correlation between dVSASL and pCASL was found, especially when taking into account that the ASL-data was not quantified yet. Compared to dVSASL, pCASL will aggregate more label in the target tissue due to the 1500ms label duration. Moreover, the second module in dVSASL will also crush part of the arterial signal. These aspects make quantification necessary for fair comparison, as it takes T1 differences of tissue and blood as well as label input functions into account.
Our results show, that with carefully chosen sequence parameters, challenges such as subtraction artefacts and high signal from the venous side can be accounted for. In conclusion, (d)VSASL is feasible for renal perfusion measurement, offering a planning-free, non-invasive technique which is less dependent on altered ATT, such as found in elderly and tumor patients.

Acknowledgements

This work is part of the research program Applied and Engineering Sciences with project number 14951 which is (partly) financed by the Netherlands Organization for Scientific Research (NWO). We thank MeVis Medical Solutions AG (Bremen, Germany) for providing MeVisLab medical image processing and visualization environment, which was used for image analysis.

References

1. Wong EC, Cronin M, Wu WC, Inglis B, Frank LR, Liu TT. Velocity-selective arterial spin labeling. Magn Reson Med. 2006;55(6):1334-1341.
2. Schmid S, Ghariq E, Teeuwisse WM, Webb A, Van Osch MJP. Acceleration-selective arterial spin labeling. Magn Reson Med. 2014;71(1):191-199.
3. Dai W, Garcia D, De Bazelaire C, Alsop DC. Continuous flow-driven inversion for arterial spin labeling using pulsed radio frequency and gradient fields. Magn Reson Med. 2008;60(6):1488-1497.
4. Klein S, Staring M, Murphy K, Viergever MA, Pluim JPW. elastix : A Toolbox for Intensity-Based Medical Image Registration. 2010;29(1):196-205.
5. Jao TR, Nayak KS. Demonstration of velocity selective myocardial arterial spin labeling perfusion imaging in humans. Magn Reson Med. 2018;80(1):272-278.
6.Duhamel G, De Bazelaire C, Alsop DC. Evaluation of systematic quantification errors in velocity-selective arterial spin labeling of the brain. Magn Reson Med. 2003;50(1):145-153.
7.Schmid S, Heijtel DFR, Mutsaerts HJMM, et al. Comparison of velocity- and acceleration-selective arterial spin labeling with [15O]H2O positron emission tomography. J Cereb Blood Flow Metab. 2015;35(8):1296-1303.

Figures

Table 1: Imaging parameters.

Table 2: Variations of VSASL parameter settings. Three parameters were investigated, the cut-off velocity (Vc) and gradient orientation (GrOri) of the velocity selective module, as well as the post labeling delay (PLD). Grey highlighted values indicate the reference values.

Figure 1: Individual perfusion weighted signal (PWS) and temporal SNR (tSNR) with mean and standard error bars as a function of varying sequence parameters. Error-bars reflect inter-subject variability which is independent from measurement precision and accuracy. Results for pCASL, single VSASL (sVSASL) and dual VSASL (dVSASL) are presented. (a) With higher cut-off velocities, tSNR and PWS decrease for both sVSASL and dVSASL. (b) While the PWS is unaffected by the gradient orientation, highest tSNR is found with labeling in FH direction (c) With longer PLD’s, sVSASL tSNR and PWS both decrease, while with dVSASL tSNR increases with a constant PWS.

Figure 2: (a) Average perfusion weighted images (PWIs) of one volunteer obtained using pCASL, single VSASL (sVSASL) or dual VSASL (dVSASL) with two Vcs. With Vc=2cm/s, subtraction artifacts were observed in both kidneys, for both VSASL modes. Blue arrows pointing at extreme perfusion values in the center of the kidney. With Vc=10cm/s perfusion patterns match between pCASL and VS-labeling. (b) PWI’s per dynamic per kidney before averaging. With a low Vc=2cm/s subtraction artifacts occur for both VSASL modes (red boxes). Discrepancy in subtraction artifacts between kidneys is observed. Low measurement variability can be appreciated for pCASL and VSASL with Vc=10cm/s.

Figure 3: Perfusion weighted signal (PWS) correlation between spatially selective pCASL (grey circles) and the two VS-labeling methods sVSASL (black circles) dVSASL (black squares) on individual basis, plus identity (grey-dotted) and linear regression lines (continuous-black). Pearson’s coefficient of correlation ‘r’ is larger for dVSASL than for sVSASL and larger linear regression off-set is present for sVSASL.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
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