Interpulse phase corrections for unbalanced pseudo‐continuous arterial spin labeling at high magnetic field

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The pseudo‐continuous arterial spin labeling (pCASL) technique imitates continuous arterial spin labeling (CASL) by applying a train of short radiofrequency (RF) pulses in rapid succession 1. To match the phase evolution of the flowing spins during labeling, a theoretical interpulse phase increment (Δϕth,L) is applied: JOURNAL/mrim/04.02/01445475-201803000-00010/math_10MM1/v/2018-01-24T161827Z/r/image-png where γ is the gyromagnetic ratio, Gmean is the mean gradient, Δz is the labeling slice offset, and Δt is the time interval between two pCASL RF pulses. Two different gradient schemes have been proposed for the control experiment 2: balanced pCASL, with similar mean gradient for label and control conditions, and unbalanced pCASL, with null mean gradient during control. For standard pCASL measurements, the unbalanced method is recommended 3, as it is known to be less sensitive to off‐resonance effects 2. The theoretical interpulse phase increment during control (Δϕth,C) is shifted by 180° relative to the one used in the label condition to avoid labeling. For unbalanced pCASL, because the mean gradient is zero, Δϕth,C = 180°.
At higher magnetic fields, B0 inhomogeneities in the labeling plane, away from the isocenter, increase and affect the spins' phase. Therefore, the theoretical interpulse phase increments Δϕth,L and Δϕth,C may not be optimal anymore. In this condition, arterial blood magnetization may not be fully inverted during labeling and some inversion may occur during control. Altogether, this lowers the inversion efficiency (IE) and the relative ASL signal, yields unstable ASL signals across subjects, and leads to interhemispheric asymmetry, as previously reported 5.
Placing the labeling plane at the isocenter is a way to obtain high IE 4, but at the cost of image quality, as the readout is not located at the isocenter anymore. Several correction strategies were developed for balanced pCASL. In multiphase pCASL 10, images with different pCASL phase offsets are acquired and are fitted to a model to retrieve the CBF. However, this method is based on a blood velocity dependent model and yields a lower signal‐to‐noise ratio per unit of time. Jahanian et al 12 presented a field–map–based approach at 3 T: The mean gradient and RF phase are corrected based on the measurement of the off‐resonance field and gradient. However, at higher magnetic field, it can become challenging to obtain accurate B0 maps in areas away from the isocenter and where the magnetic field is heterogeneous. Luh et al 13 demonstrated for balanced pCASL on humans at 7 T, that varying the phase during a prescan to measure the optimal phase increment was a robust way to improve the overall perfusion signal while keeping the imaging plane close to the isocenter. Moreover, other studies showed that correcting the control condition separately improves the ASL signal 14. However, the latter control correction strategies resulted in applying different frequencies for the label and the control conditions, leading to residual magnetization‐transfer effects. Even if these residual effects may be negligible for magnetic fields up to 4 T, they are much more pronounced at high magnetic fields, and may therefore bias the ASL signal.
Here, we investigated a phase‐optimization prescan approach for unbalanced pCASL on rats at 9.4 T. We analyzed the effect of separately optimizing the label and the control interpulse phases on the relative ASL signal and on its interhemispherical symmetry. We challenged the robustness of this phase‐correction approach by manually changing the shims. In a second step, the dependence of the obtained optimized phase values on the shim settings and on the Larmor frequency at the location of the carotids in the labeling plane was evaluated.

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