Magnetization transfer effects on the efficiency of flow-driven adiabatic fast passage inversion of arterial blood

Abstract

Continuous arterial spin labeling experiments typically use flow-driven adiabatic fast passage inversion of the arterial blood water protons. In this article, we measure the effect of magnetization transfer in blood and how it affects the inversion label. We use modified Bloch equations to model flow-driven adiabatic inversion in the presence of magnetization transfer in blood flowing at velocities from 1 to 30 cm/s in order to explain our findings. Magnetization transfer results in a reduction of the inversion efficiency at the inversion plane of up to 3.65% in the range of velocities examined, as well as faster relaxation of the arterial label in continuous labeling experiments. The two effects combined can result in inversion efficiency reduction of up to 8.91% in the simulated range of velocities. These effects are strongly dependent on the velocity of the flowing blood, with 10 cm/s yielding the largest loss in efficiency due to magnetization transfer effects. Flowing blood phantom experiments confirmed faster relaxation of the inversion label than that predicted by T(1) decay alone.

Figure 1

Simulations of adiabatic inversion of flowing spins and their subsequent relaxation in the presence of off-resonance RF. The parameters used for both simulations are shown in table 1, unless otherwise specified. The spins cross the inversion plane at t=0. (A) Simulation of the effect of including magnetization transfer effects in the simulation. When the magnetization transfer effects are not considered, a greater inversion is achieved and the longitudinal relaxation rate is considerably slower. (B) Simulation of the decay of the spins at different velocities, demonstrating that slower moving spins experience greater amount of MT than faster ones. Note that the inversion efficiency at 1, 5, 10, 15, 20 and 30 cm/s. was 0.50, 0.62, 0.73, 0.80, and 0.84, and 0.90 respectively, when magnetization transfer was considered. The loss of inversion efficiency due to magnetization transfer was 0.21, 3.65, 3.50, 2.85 and 2.24 and 1.43 %.

Figure 2

Magnetization transfer spectrum of water and blood. The curve shows the typical frequency dependence observed in tissues and blood.

Figure 3

Magnitude (left) and phase (right) images showing laminar flow in phantom. Inversion pulses lasted 200 ms, while image acquisition parameters were TR= 300, TE =21 ms. The rectangle identifies the plane used for measurement of inversion efficiency as a function of velocity. Arrows indicate plane of application of the adiabatic inversion pulse. Multiple banding patterns in both the magnitude and phase images arise from the alternating control and labeling pulses applied prior to imaging.

Figure 4

(A) Maximum inversion achieved as a function of velocity. At low flow velocities, the maximum inversion observed is lower than at higher flow velocities because of saturation due to magnetization transfer. (B) Plot of power required for optimum inversion efficiency. Inversion Efficiency (alpha) was measured using equation 2. The optimum inversion power depends not only upon the velocity, but also upon the contribution of magnetization transfer, as demonstrated by the difference between the water and blood phantoms.

Figure 5

Relaxation of inverted spins in blood (A) and water (B) as they move away from the inversion plane. The data are compared to T₁ and T_1sat relaxation, as well as the model presented here. The relaxation rate appears to match the T_1sat relaxation curve more closely than the T₁ relaxation curve.

Figure 6

Illustrated is the dependence on flow velocity of FPA inversion efficiency in both blood (triangles) and water (circles). The simulations (blue curves) incorporated the effects of T1 and T2 relaxation on the inversion process, while the in vitro data (red curves) included a correction for T1 relaxation between the inversion plane and detection plane. The difference for the simulated and in vitro curves obtained for blood are likely due to MT relaxation.