Synopsis

Keywords: Pulse Sequence Design, Arterial spin labelling

Velocity-selective pulses include VS saturation pulses (VSS) and VS Inversion (VSI) pulses. Previous study (1) concluded that both dual-sBIR8-VSS and sinc-VSI achieved the highest SNR efficiency among the VS labeling schemes. Overall, the dual-sBIR8-VSS pulse was the most robust against field imperfections, whereas sinc-modulated VSI pulse showed greater tSNR and was the best among the VSI methods. In this study, VSI sequence with rectangular small flip-angle RF pulses (rect-VSI), sinc-VSI with and without VS gradients during the control condition are compared. Bloch simulation and in vivo experiments for their robustness against B₁, B₀ variation and eddy current (EC) are investigated.

PURPOSE

To compare three different vessel-selective inversion (VSI) pulses for noninvasive arterial spin labeling (ASL) measurement of perfusion in human brain.

METHODS

Numerical simulations using the Bloch equations based on matrix rotation were performed to assess the properties of the Fourier transform based velocity-selective inversion (FT-VSI) pulse trains with three different settings using Matlab (MathWorks, Inc., Natick, MA, USA). The M_z profile in the presence of B₁ (0.6 to 1.4 of the nominal value, step size 0.57) and B₀ (-200 to 200 Hz, step size 40 Hz) variations, with arterial T₁ (1932 ms) and T₂ (275 ms) relaxation were simulated. To investigate the sensitivity to the eddy current (EC) effects, Bloch simulations were performed for these different VS pulses with EC amplitude of 0.25% and time constants of 0.1 to 1000 ms. Measurements were performed using a 20-channel head/neck receiver coil on a 3T scanner (Prisma, Siemens Healthineers AG, Erlangen, Germany). Pulse sequence diagrams for VSI-ASL with 3D acquisition are same as demonstrated in previous study (2). It includes five blocks within each repetition: slab-selective saturation, label/control modules using FT-VSI pulse trains, background suppression (BGS), flow-dephasing for suppressing large-vessel signal, and 3D GRASE readout (2). Three FT-VSI pulse trains are compared: 1) traditional VSI sequence with rectangular small flip-angle RF pulses (rect-VSI); 2) sinc-modulated FT-VSI pulse (sinc-VSI) with VS gradients during control condition, for sinc-VSI the amplitude of the nth pulse is given by sinc(n/5), where n = -4:1:4). The flip angles of these 9 rectangular pulses add up to 180°; 3) sinc-modulated FT-VSI pulse (sinc-VSI) without VS gradients during control condition. The design of pulse trains are compared in Figure 1. Sequence parameters were as follows: TE =17.2 ms, FOV=220×220 mm3, FA=120°, slice and in-plane partial Fourier = 6/8, segments=2, matrix size=64×64×24, slice oversample = 16.7%, resolution=3.4×3.4×5 mm³, bandwidth = 2298 Hz, labeling/control duration = 64 ms, cutoff velocity =2.8 cm/s, delay between the pre-saturation and the labeling modules = 2 s, post-labeling delay (PLD) =1200 ms, TR = 4000 ms, timing for BGS = 0.52, 0.91 and 1.08 s; control-labeling pairs = 20, TA = 5.2 min. Each pulse scheme was tested in three healthy subjects.

RESULTS

The simulated M_z-velocity responses are shown in Figure 2. Both rect-VSI and sinc-VSI based pulses were capable of labeling spins in a narrow velocity band with reasonable robustness against field inhomogeneities. The sinc-VSI pulse produced much smoother response in the “unperturbed” velocity bands. The control profile of sinc-VSI without VS gradients during control condition showed the best performance despite B₀/B₁⁺ inhomogeneities. Figure 3 shows the simulated magnetization of the static tissue (V = 0) under the label condition showing EC sensitivity of VSI pulses to EC components with different time constants (ranging from 0.1 ms to 1000 ms) at different distances to the iso-center of the magnet. The sinc-VSI without VS gradients during control condition demonstrated good resistance to EC effects. Normalized perfusion signal equal to the averaged subtraction of label/control pairs (∆S) divided by S₀,tissue, (∆S/S₀,tissue), was calculated as an indicator of perfusion signal level (Figure 4). In the in vivo experiment, the sinc-VSI without VS gradients gave the highest ASL signals at the gray matter but lower ASL signals at the region of basilar arteries.

DISCUSSION

The reason for this phenomenon is likely due to the unbalanced selection of moving spins with different flow direction. In the third scheme all moving spin with all flow directions were inverted in control condition. However, only the moving spins along z direction were selected in label condition. After applying VS gradients along all three directions during labeling, the signal at the region of basilar arteries was restored, as shown in Figure 5.

CONCLUSION

This study compared three FT-VSI ASL schemes. In simulation, the sinc-VSI without VS gradients during the control condition demonstrated the best performance against B₀/B₁⁺ inhomogeneity and EC effects. The in vivo experiment showed that the sinc-VSI without VS gradients gives the highest ASL signals. However, in the in vivo experiment this scheme showed ASL signal voids at the level of basilar arteries. To avoid this, a VS gradient along x and y may need to be added in the labeling section.

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), 507778062.