Jia Guo1
1Bioengineering, University of California Riverside, Riverside, CA, United States
Synopsis
We proposed a novel strategy to improve the temporal resolution
and/or SNR efficiency of perfusion imaging using velocity-selective (VS)
labeling, by purposely labeling spins within a narrow velocity band. This strategy
allows faster recovery/refreshment of the magnetization of arterial spins for improved
SNR efficiency and temporal resolution. A few implementation methods of such labeling
strategy were explored, using modified Fourier-transform based VS inversion
pulses. The SNR efficiency and achievable temporal resolution were examined by ASL
signal modeling, demonstrating a good promise for ultra-fast perfusion imaging
with high SNR efficiency.
Introduction
Velocity-selective arterial spin labeling (VSASL) is insensitive
to inhomogeneous transit times 1, 2. This feature permits a short
post-label delay (PLD), suitable for fast perfusion imaging to study brain
functions 3, 4. Conventional VSASL methods label globally and target
a wide band of velocities for a large bolus duration (BD) for a strong perfusion
signal. When imaging with a short TR for high temporal resolution, multiple
labeling pulses may be applied on the same bolus, resulting in reduced perfusion
signal due to multiple saturation/inversion, as shown in Figure 1A.
A recent design incorporated slice selectivity into VSASL to limit
the BD for fast perfusion imaging 5. However, the slice coverage was
limited with potential dependence on the vasculature orientation, and the
labeling efficiency was suboptimal due to saturation-based labeling. In this
study, we propose a new design to limit the BD by inverting spins with narrow-band
velocity-selectivity (nb-VS), aiming for ultra-fast perfusion imaging with improved
SNR, while keeping the labeling geometry-independent for a good coverage. Theoretical
SNR efficiency of three major categories of ASL methods were also compared in
the context of fast perfusion imaging.Methods
As demonstrated in Figure
1B, labeling spins within a relatively narrow band of velocities allows detection
of the proximal label while keeping the spins at higher velocities unperturbed as
fully relaxed probe for the next measurement.
To implement nb-VS labeling, a sinc-modulated Fourier-transform based velocity-selective
inversion (FT-VSI) pulse (sinc-VSI) 6 was modified with: 1) linearly
increasing phase shift on the small tip-angle pulses to shift the VS profile 7,
so that spins moving at low velocities were inverted while the static and fast
moving spins were unaffected; 2) the control condition was implemented without VS
gradients. The nb-VS labeling using the original FT-VSI pulse (rect-VSI) 8
was also constructed for comparison. Other modification included: composite
refocusing pulses with MLEV-8 phase cycling patterns for improved B1
insensitivity 9; gaps before and after gradient pulses to reduce
eddy current sensitivity 10. The details of the pulses are shown in Figure 2.
Bloch
simulation was performed in MATLAB2020b (The Mathworks, Nantick, MA) to study
the nb-VS profiles in the presence of B1 (0.7 to 1.3 of the nominal value,
step size 0.1) and B0 (-150 to 150 Hz, step size 50 Hz) variations,
with arterial T1 (1650 ms) and T2 (150 ms) relaxation included.
To
examine the SNR benefit of nb-VS quantitatively, the SNR efficiency (SigASL/√acquisition time) was calculated by a
kinetic ASL signal model 11 in the context of fast imaging (TR from
0.3 s to 3 s). Different labeling methods were compared, including: conventional
saturation- and inversion-based VSASL (VSS and VSI, respectively, BD = 2 s) where
the magnetization was assumed to start from saturation; nb-VS labeling (BD = 1
s) with fully relaxed magnetization; pulsed ASL (PASL, PLD = 1 s, BD = 1.2 s, labeling
efficiency (α) = 0.98) with and pseudo-continuous ASL (PCASL, PLD = 1.5 s, BD =
∞, α = 0.85) with fully relaxed magnetization. T1 and T2
relaxation and an imaging time of 0.3 s were assumed.Results
The nb-VS profiles are shown in Figure 3. 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 and slightly wider inversion bands compared to rect-VSI. FT-VSS-based
nb-VS labeling was also feasible, albeit with a higher B1 sensitivity
(Figure 4).
The
SNR efficiencies of different labeling methods are shown in Figure 5. With nb-VS labeling, full
relaxation significantly boosted the SNR efficiencies at very short TRs. The inversion-based
labeling had the highest SNR efficiency, twice of that with the saturation-based
labeling. Conventional VS labeling had much lower SNR efficiency due to the need
for the magnetization to recover. For PASL and PCASL, the temporal resolution
was limited by the PLD and the SNR efficiencies were lower than nb-VS labeling.
For PASL, the optimal TR of around 2.5 s matched with the values typically used
in PASL-based fMRI.Discussion
The nb-VS labeling was designed within ±40 cm/s, likely sufficient
for brain imaging. Because FT-VSI has a periodic inversion pattern, to avoid
potential perturbation of spins at higher velocities, slice selectivity may be applied
5.
The inversion bandwidth affects the BD, and subsequently the
temporal resolution and SNR efficiency. Adjusting the bandwidth would also
change the period of the inversion bands. Optimization of these parameters, and
possibly some tradeoff, are needed with support from in vivo data.
The
temporal resolution and SNR efficiency of PASL can be significantly improved by
Turbo-ASL 12, though the quantification is not straightforward. For
nb-VS labeling, the image acquisition requires a matched VS profile for
quantification. Conventional slice-selective excitation with vascular crushing may
interfere with the spins moving at high velocities. Therefore, an excitation tailored
for VS labeling 13 may be more appropriate for nb-VSASL.Conclusion
We proposed nb-VS labeling for ultra-fast perfusion fMRI with high
SNR efficiency. Its feasibility and SNR efficiency advantage are yet to be verified
by further study, especially in vivo experiments.Acknowledgements
No acknowledgement found.References
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