Synopsis

Proper design of the arterial spin-labeled (ASL) readout trajectory can minimize signal loss, reduce artifacts, and consequently improve the quality of ASL-derived perfusion maps. High performance gradients can improve MR image quality in spiral acquisition to reduce susceptibility and off-resonance effect. However, the eddy current and the system delay can degrade image quality by causing image rotation and blurring effects. In this work, the image artifact was corrected with a truer trajectory measured by a dynamic field camera. The effects of the trajectory correction on ASL images are investigated on a low-cryogen, compact 3T MRI system.

Introduction

Methods

Five healthy volunteer subjects (3 male, 2 female, 30.2 ± 7.7 y.o.) were scanned on the compact 3T after obtaining written informed consent under an IRB-approved protocol.

Subtractions between controlled and labeled images (ΔM) and proton-density-weighted (PDw) images were obtained with a pseudo-Continuous Arterial Spin Labeling (pCASL) sequence^11,12 using a series of maximum slew rate values ($$$S_{max}$$$) as described in Table 1. The order of pCASL acquisitions was randomized for each subject. The in-plane resolution was set to approximately 3.5 mm by adjusting the number of samples. To correct the trajectory deviation,⁴ the actual trajectories of 1^st-order k-space terms (k_x & k_y) were separately measured using a dynamic field camera with NMR field probes containing ¹⁹F (Skope, Zurich, Switzerland).^13,14 Image reconstruction via density compensated gridding was performed with Orchestra (GE Healthcare,Waukesha,WI,US) in Matlab (The MathWorks, Inc.,Natick,MA,US). The CBF map was calculated by the following equation.¹ $$CBF=\frac{6000\cdot\lambda\cdot{S}_{\Delta{M}}\cdot\exp\left(T_{Post\_labeling\_delay}/T_{1,blood}\right)}{2\cdot\alpha\cdot{T}_{1,blood}\cdot{S}_{PDw}\cdot\left(1-\exp\left(-T_{Labeling\_duration}/T_{1,blood}\right)\right)}[ml/100g/min]$$, where $$$\lambda$$$ and $$$\alpha$$$ were set to 0.9 ml/g and 0.85, respectively.¹ For comparison, two ROIs (red boxes in Figure 1(c)) were placed on occipital region and in the background, respectively.

To study the effect of $$$S_{max}$$$ on CBF maps, the trajectory-corrected PDw images were used to realign all the other trajectory-corrected images. The T₂-weighted fast spin-echo image was co-registered with the PDw images and segmented to make masks for gray and white matter (GM and WM) by SPM12 toolbox (Statistical Parametric Mapping, Wellcome Trust Centre for Neuroimaging,UK) with Matlab. The perfusion SNR¹⁵ and the CBFs on GM and WM were evaluated.

Results

Figure 1 shows sample images of PDw, ΔM and CBF. In the images with the nominal, uncorrected trajectory, the structures in ΔM and the corresponding CBF map were rotated counterclockwise as white boxes in Figure 1(c, d). However, this artefactual rotation was corrected by accounting for the measured trajectory during reconstruction. As shown in Table 2, the trajectory correction enhanced the signal in all of ASL-related images, regardless of $$$S_{max}$$$.

Figure 2 shows examples of an anatomical image co-registered with the trajectory-corrected PDw and two masks of GM and WM. CBFs on GM and WM were evaluated, respectively.

As shown in Figure 3(a), the average values of CBF on GM and WM differed from individual to individual. Thus, CBFs of GM and WM in each subject were normalized by intra-subject averages and then averaged across subjects depending on $$$S_{max}$$$. In Figure 3(c, d), statistically significant difference between $$$S_{max}$$$ of 70 and 300 was observed in CBF of GM while significant difference between in $$$S_{max}$$$ of 200 and 400 in CBF of WM.

Discussion

The effect of high gradient performance, especially the slew rate, was investigated on ASL-related images acquired with a spiral acquisition. Since a high $$$S_{max}$$$ in spiral imaging not only caused image rotation but also the shear distortion on the image, the realignment across images was performed only for the trajectory-corrected images. It appears that trajectory measurement was needed to correct the image distortion as well as to improve SNR in ΔM images.

Based on images with corrections, the effect of $$$S_{max}$$$ on CBF map was investigated. Although the perfusion SNR tended to reduce as $$$S_{max}$$$ increased as shown in Figure 3(b), the CBFs on GM and WM were slightly high when $$$S_{max}$$$ were 300 and 400 T/m/s, respectively. This could be attributed to the shorter TE enabled by high $$$S_{max}$$$, but further study will be required.

Conclusion

Acknowledgements

References

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