A simple and reliable perfusion phantom to measure precise and repeatable arterial spin labeled quantitative perfusion
Joshua S. Greer1,2, Keith Hulsey2, Robert E. Lenkinski2,3, and Ananth J. Madhuranthakam2,3

1Bioengineering, University of Texas at Dallas, Richardson, TX, United States, 2Radiology, UT Southwestern Medical Center, Dallas, TX, United States, 3Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, TX, United States

### Synopsis

Arterial spin labeling (ASL) is a rapidly growing area of interest, primarily because of its ability to provide quantitative perfusion maps non-invasively. But, for the technique to be adopted for clinical use, these quantitative measurements need to be accurate and robust, which will require a quality controlled perfusion phantom to ensure consistency for different magnet strengths and manufacturers. In this study, we demonstrate a simple perfusion flow phantom that can be used to test the precision and repeatability of ASL perfusion measurements.

### Introduction

Arterial spin labeling is a rapidly growing area of interest with clinical potential, primarily because it can provide quantitative perfusion maps non-invasively. ASL measured quantitative perfusion provides a direct measurement of tumor vasculature and has been shown to have clinical utility in measuring therapy response in various indications including glioblastoma multiformae (GBM) [1] and renal cell carcinoma (RCC) [2]. However, in order to have reliable measurements of quantitative perfusion, independent of the field strength or MR scanner manufacturer, it is imperative to establish a quality controlled perfusion phantom. In this study, we demonstrate a simple and reliable perfusion phantom that can be used to test the precision and repeatability of quantitative perfusion measurements using pseudo-continuous ASL (pCASL).

### Methods

Phantom Design: The perfusion phantom was designed similar to a previously described pseudo-diffusion phantom [3,4] using sponges with different densities embedded within poly-vinyl carbon (PVC) pipes submerged in a water bath (fig. 1a). The three tubes containing the sponges were split from a common plastic tubing. The entire set-up was submerged in a water bath to minimize B0 inhomogeneities. A pump outside the MR scanner room circulated water through the tubing. The water was forced through the sponges using the flow pump at different rates to create different perfusion effects in the sponges, which were measured with 2D pCASL using a single shot turbo spin echo (SShTSE) [5]. Subsequently, quantitative perfusion values were calculated using the standard ASL model (fig. 5) [6]. The variables in this equation are: label duration ($\tau$); post-label delay ($\omega$); blood-tissue partition coefficient (λ=0.9); transit delay from the labeling plane to the tissue (δt); inversion efficiency (α = 0.9) for pCASL without background suppression. ΔM is the measured perfusion difference image between label and control and M0 is the proton density image. Axial perfusion-weighted images were acquired through the center of the sponges using a SShTSE after 6000 ms of labeling (see fig. 1). The pump flow rate was varied from 0 to 300 mL/min to vary perfusion in the sponges. A proton-density image was acquired using same acquisition parameters without any preparation pulses for perfusion quantification. Perfusion in each sponge was estimated by measuring the flow into each sponge using phase contrast to estimate flow in mL/min, and dividing this flow by the sponge volume to get perfusion in mL/mL/min. Quantitative perfusion values measured by pCASL were correlated against quantitative perfusion values estimated by phase contrast to evaluate the precision of pCASL measured perfusion. Additionally, the pCASL measurements were repeated with same pump flow rates on two different days (about a week apart) to evaluate the repeatability of pCASL measured quantitative perfusion.

### Results

Figure 1 shows a coronal T2W image of the 3 sponges and the pCASL labeling plane shown in red (a). To show the effects of labeling duration, labeling was applied for 2000ms (fig. 1b) and 6000ms (fig. 1c), showing the flow of labeled water further with longer labeling duration (fig. 1c). Figure 2 shows the correlation between the pump flow rate and the measured quantitative perfusion using 2D pCASL, showing good correlation for each sponge. Sponge 3 has the highest density and serves as a control measurement providing close to zero perfusion. Figure 3 shows the correlation between the measured quantitative perfusion using 2D pCASL and the estimated perfusion using phase contrast and the sponge volume, showing an excellent correlation. Different perfusion values were achieved using different pump flow rates. Figure 4 shows excellent repeatability of 2D pCASL measurements on two different days, acquired one week apart (R2=0.93).

### Discussion

Accurate and reliable measurements of quantitative perfusion using pCASL is of utmost importance in establishing ASL as a quantitative measurement for therapy response in clinical trials. We demonstrated a simple but reliable perfusion phantom that provides precise and reliable measurements of quantitiative perfusion using pCASL. This phantom can be used as a quality control tool to measure ASL perfusion independent of the field strength or the manufacturer of the MR scanner.

### Acknowledgements

No acknowledgement found.

### References

[1] Qiao, X. J., et al. "Arterial spin-labeling perfusion MRI stratifies progression-free survival and correlates with epidermal growth factor receptor status in glioblastoma." American Journal of Neuroradiology 36.4 (2015): 672-677.

[2] de Bazelaire, Cedric, et al. "Magnetic resonance imaging–measured blood flow change after antiangiogenic therapy with PTK787/ZK 222584 correlates with clinical outcome in metastatic renal cell carcinoma." Clinical Cancer Research14.17 (2008): 5548-5554.

[3] Cho, Gene Y., et al. "A versatile flow phantom for intravoxel incoherent motion MRI." Magnetic Resonance in Medicine 67.6 (2012): 1710-1720.

[4] Hulsey, Keith, et al. "Comparison of Results Obtained by Fitting DWI Data to a Model Including IVIM and Kurtosis using Nonlinear Least Squares and Maximum Likelihood Estimation." Proceedings of the International Society for Magnetic Resonance in Medicine, 23rd Scientific Meeting, Toronto, Ontario, Canada. 2015; p.2911

[5] Robson, Philip M., et al. "Strategies for reducing respiratory motion artifacts in renal perfusion imaging with arterial spin labeling." Magnetic Resonance in Medicine 61.6 (2009): 1374-1387.

[6] Buxton, Richard B., et al. "A general kinetic model for quantitative perfusion imaging with arterial spin labeling." Magnetic resonance in medicine 40.3 (1998): 383-396.

### Figures

Figure 1: (A) Coronal T2W image of the flow phantom. The labeling plane is shown in red. (B) perfusion-weighted image with labeling applied for 2 seconds (C) perfusion-weighted image with labeling applied for 6 seconds. Both perfusion images had a post-label delay of 100ms and a flow rate of 300mL/min. The sponge on the right is denser than the other two, and serves as a control with close to zero perfusion.

Figure 2: Pump flow rate vs. measured pCASL perfusion in three sponges, showing increased perfusion with increasing flow rate in each sponge.

Figure 3: Estimated perfusion vs. measured pCASL perfusion. Estimated perfusion was based on the phase contrast flow measured at the inflow to each sponge (mL/min) divided by the sponge volume.

Figure 4: pCASL perfusion measurements in the same sponges at the same flow rates on different days, performed a week apart.

Figure 5: Standard model for perfusion quantification

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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