0335

Dual-module velocity-selective arterial spin labeling (dm-VSASL) using velocity-selective saturation and inversion pulses

Jia Guo^1,2
¹Bioengineering, University of California Riverside, Riverside, CA, United States, ²Center for Advanced Neuroimaging, University of California Riverside, Riverside, CA, United States

Synopsis

Recently velocity-selective saturation (VSS) based dual-module (dm-) preparation and velocity-selective inversion (VSI) based preparation have demonstrated improved SNR efficiency in velocity-selective arterial spin labeling (VSASL) than conventional single-module VSS based preparation. In this study, a novel strategy was developed to allow dm-VSI labeling to further improve the SNR efficiency. Implementation of dm-VSASL were evaluated for VSS and/or VSI combinations. The theoretical performance was compared via ASL signal modeling, and then validated by in vivo human experiments with an increase of 6.6%. Dm-VSASL is capable of providing further improved SNR performance, with potentially better suppression of diffusion attenuation/eddy current artifacts.

Introduction

Velocity-selective ASL (VSASL) ¹ is insensitive to inhomogeneous arterial transit times (ATT) among ASL methods ^{1, 2}, offering an SNR advantage when ATT is long. VS saturation (VSS) based labeling has relatively low signal-to-noise ratio (SNR). Recent development on improving the SNR includes: 1) multi-module VSS preparation ², and 2) VS inversion (VSI) preparation ³. An SNR improvement in the range of 20 ~ 30% compared to single-module (sm-) VSS based preparation has been reported using both methods ^2-4. A novel labeling strategy is invented to enable dual-module (dm-) VSI labeling to further improve the SNR efficiency, which also benefits VSS labeling.

Theory and Methods

Similar to the dm-VSS preparation ², two groups of spins are considered for dm-VSI: 1) one labeled by only the first VS module; 2) one labeled by both VS modules. The evolution of the magnetization for dual-VSS and dual-VSI is shown in Figure 1, and Sig_ASL=ΔM_z1·TI₁.+ΔM_z2·TI₂.

Dm-VSI labeling: VSI pulses invert the stationary spins, therefore flipping the label/control condition in the second VSI is required to avoid signal canceling. Additional benefits include: 1) the sensitivity to motion, diffusion attenuation and eddy current (EC) effects are better balanced in the label/control images, potentially reducing artefacts and improving quantification accuracy; 2) the inversion effects allow more efficient background suppression (BS).

Dm-VSS labeling: 1) VSS+VSS as described previously ²; 2) VSS_inv+VSS or VSS_inv+VSS_inv to allow a more balanced label/control design as described above. VSS_inv can be implemented in several ways ^{2, 4}, such as adding a phase of π to the last segment of the symmetric BIR-8 (sBIR8) pulse.

Dm-VSASL using VSS and VSI: VSS+VSI, VSI+VSS and their inverted variants. Label/control condition should be flipped in the second VS module when necessary.

Modeling and simulation
A kinetic ASL model ⁵ was used to model the ASL signals, including Pulsed ASL (PASL) and Pseudo-Continuous ASL (PCASL) ⁶. The SNR efficiency, Sig_ASL/sqrt(2TR), was evaluated with and without T₂ relaxation for VSASL, and realistic labeling efficiencies for PASL and PCASL ⁷. The maximal bolus durations were: 2s for VSASL ², 1s for PASL and unlimited for PCASL.

In vivo experiments
Four healthy human subjects (1F, age 23-38) were studied on a 3T scanner (Siemens Prisma, Erlangen, Germany) under an IRB approval. Symmetric BIR-8 ⁸ and sinc-VSI ⁴ pulses were used for VSS and VSI labeling, respectively. ASL scans were acquired in a randomized order with BS: 1) PASL: FAIR ^{9, 10} Q2Tips ¹¹, TI1=0.8s, TI=2.4s, 2 BS pulses; 2) VSS_inv: TI=1.4s, 1 BS pulses; 3) VSS+VSS_inv: TI_1/2=1.16/0.83s, 1 BS pulses; 4) VSS_inv+VSS: TI_1/2=1.45/0.54s, 1 BS pulses; 5) VSI: TI=1.4s, 1 BS pulses; 6) VSI+VSI: TI_1/2=1.45/0.54s, 2 BS pulses. Other parameters were: 2-interleaved 3D GRASE EPI readout, FOV=220x220mm (64x64), 24 slices, 4mm thickness, TR=4s (PASL) and 5s (VSASL), 15 and 12 label/control pairs for PASL and VSASL, cutoff velocity=2cm/s along S/I in VSASL. Reference images were acquired for quantification.

Results

SNR efficiencies of different ASL methods from simulation are shown in Figure 2. Compared to sm-VSI, the maximal SNR efficiency of dm-VSI was increased by 11.8% without T₂ relaxation; and was the same with T₂ relaxation.

Figure 3a demonstrates the necessity of the label/control flipping in the second VS module in VSS_inv+VSS and VSI+VSI. Figure 3b shows ATT artefacts with PASL in an early testing in sub. 3, while VSASL demonstrated excellent ATT insensitivity.

Normalized ASL signal and cerebral blood flow (CBF) maps are shown in Figure 4. Averaged values in gray matter (GM) and white matter (WM) are summarized in Table 1. After correcting for the signal attenuation from BS (5% reduction each), dm-VSI produced the highest signal in GM, followed by sm-VSI, and VSS+ VSS_inv. Dm-VSI showed a trend of increasing the signal by 6.6% (p=0.062) compared to sm-VSI labeling; VSS+VSS_inv increased ASL signal by 24.4% (p=0.025) compared to sm-VSS, consistent with previous findings ^{2, 4}; VSS_inv+VSS showed a marginal increase of 2.4% (p=0.91). Dm-VSI and sm-VSI increased ASL signal by 50% (p≤0.01) compared to sm-VSS. There was no significant difference in CBF between labeling methods (p=0.19 in GM, and p=0.070 in WM). In addition, the temporal SNR was doubled with dm-VSASL in GM (details reported in another abstract). A regional ASL signal reduction was observed in sub. 4 using VSI labeling (Figure 4).

Discussion

For dm-VSI vs. sm-VSI, in vivo experiments measured a signal increase of 6.6%, higher than that predicted by simulation, likely due to more stable labeling with dual-module labeling. Sinc-VSI is preferred to rect-VSI because of a shorter effective TE (eTE, 29.4ms vs. 37.6ms using the same timings in the study). Further reduction of eTE should increase the SNR advantage of dm-VSASL.

It is important to arrange VS modules properly to accumulate ASL signals constructively. It is required to flip the label/control condition in the second VS module if the first one inverts stationary spins. This can help reduced diffusion attenuation/EC artifacts. Consistent with the findings in a recent study ⁴, further reduction of the sensitivity to field inhomogeneities is needed for VSI pulses.

Conclusion

Dual-module VSS and VSI can be implemented with improved SNR efficiency and/or temporal SNR with potentially reduced artefacts, may be a preferred VSASL labeling strategy for clinical applications.

Acknowledgements

The authors thank Dr. Jason Langley for data acquisition, and Dr. Divya Bolar for sharing code for pulse sequence development.

References

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Figures

Figure 1. Evolution of the magnetization of the moving spins under the label (red) and the control (blue) conditions for dual-module VSS (a) and dual-module VSI (b) preparations. Under the label condition, the spins labeled by only the first VS module and by both VS modules follow the red solid and red dashed lines, respectively. Note that for simplicity and visual clarity, the inversion effect on ΔM_z1 from the second VSI module is not shown in (b).

Figure 2. Maximal ASL SNR efficiencies at TRs from 2 to 8s and an imaging time of 0.5s: a) the labeling efficiency (α) was 1 for all; b) with T2 relaxation for VS labeling (effective TE of 20ms and 30ms for VSS and VSI, respectively), and α = 0.85 and 0.98 for PCASL and PASL, respectively. The labeling timings were optimized for optimal SNR efficiency at each TR. The PLDs were 1.8s and 1.6s for PCASL and PASL, respectively. T₁ of 1660ms and T₂ of 150ms were assumed for arterial blood.

Figure 3. a) ASL images without and with control/label condition flipping in the second VS module; b) transit artefacts with PASL acquisition in sub. 3 (TI₁ = 0.8s and TI = 2s, delayed perfusion and intravascular ASL signals are labeled with black and white arrows, respectively), but not with any of the VS labeling methods (only one example is shown). A later PASL scan with a longer TI of 2.4s (Figure 4) and angiography confirmed that the flow was delayed due to tortuous vasculature.

Figure 4. a) Examples of normalized ASL signal (as a percentage of the reference signal to allow inter-subject comparison) and corresponding CBF maps from human subjects. The ASL signal in the anterior region in sub. 4 was slightly lower using VSI labeling (black arrows), possibly due to its sensitivity to field inhomogeneities.

Table 1. Measured ASL signals (normalized to percentages of the reference scan) and CBF (ml/100g/min) in gray and white matters in vivo. The raw ASL signals corrected for the background suppression (BS) attenuation were also shown. For visual clarity, only mean values were shown for individual subjects.

Proc. Intl. Soc. Mag. Reson. Med. 30 (2022)

0335

DOI: https://doi.org/10.58530/2022/0335