Quantitative analysis of adiabatic fast passage for steady laminar and turbulent flows

Abstract

Adiabatic fast passage (AFP) is used in noninvasive quantitative perfusion experiments to invert (or label) arterial spins. Continuous arterial spin labeling (CASL) experiments conducted in vivo often assume the inversion efficiency based on the labeling field and steady flow conditions, without direct verification. In practice, the labeling field used in CASL is often amplitude-and duty cycle-limited due to hardware or specific absorption rate constraints. In this study, the effects of the labeling field amplitude and duty cycle, and flow dynamics on the inversion efficiency of AFP were examined under steady flow conditions in a saline flow phantom. The experimental results were in general agreement with models based on Zhernovoi's theory except at high labeling field amplitudes, when the spin inversion times are at least half of the duration of the labeling pulse. The nonlinear relation observed between the inversion efficiency and the labeling duty cycle implies that the practice of linear derating the inversion efficiency with the labeling duty cycle may be prone to significant error. A secondary finding was that the Key words: arterial spin labeling; adiabatic fast passage; inversion efficiency; steady flow; labeling duty cycle During the last decade there has been a significant amount of development in noninvasive quantitative perfusion MRI using arterial spin labeling (ASL), with applications including the human brain (1-5) and kidney (6 -8). Continuous ASL (CASL) employs adiabatic fast passage (AFP) (9) to invert arterial spins (i.e., in blood) prior to their arrival at the tissue region of interest (ROI). The perfusion signal is directly proportional to the inversion (or labeling) efficiency of AFP within a finite inversion region and is attenuated by T 1 relaxation that occurs in transit from the inversion region to the imaging region (5,6,10). It is desirable to maximize the inversion efficiency (and, where possible, minimize T 1 relaxation in transit) since the perfusion signal is small in humans: approximately 1-2% of the image signal for gray matter (11) and 4 -6% in the renal cortex (6,7).Measurement of inversion efficiency in vivo is difficult since it may vary during the cardiac cycle (12) and the ability to synchronize the measurement with the spin labeling (or tagging) is challenging (6). Inversion efficiencies have been measured directly in the rat carotids (13) and rat descending aorta (14), human descending aorta (6) using gated acquisitions, and indirectly in the human brain (4,5,11,15,16) by varying the labeling field.However, the labeling fields and inversion efficiencies are not typically measured in every subject or study (3,17), since such measurements significantly extend the exam time and are prone to large errors. Instead, the inversion efficiency is assumed based on earlier experimental measurements or simulations. Sources of error include the assumptions of the spin hemodynamics (e.g., pulsatile vs. steady flow), relaxation times, labeling field amplitude, and the cycling of the ...

Amplitude-modulated Continuous Arterial Spin-labeling 3.0-T Perfusion MR Imaging with a Single Coil: Feasibility Study

“…In a recent study (36), investigators measured the continuous labeling efficiency in a saline flow phantom by using the single-section approach. The results showed an efficiency of 0.85 for a mean flow velocity of 26.4 cm/sec with 2.5- With an approximate 7% increase in efficiency by using a gradient of 1.6 instead of 2.5 mT/m, the predicted efficiency with the imaging parameters used in our experiments will be 0.91, which is a very close match with our assumption.…”

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

“…First, the velocity dependence of the adiabatic condition introduces variability in the degree of inversion obtained by a given RF pulse with velocity fluctuations. As indicated by others (16,19,17,18), appropriate choice of gradient magnitude and H 1 strength will minimize this effect, although at the expense of greater specific absorption rate.…”

“…Formulations for optimization of the continuous arterial spin labeling experiment have been presented previously for both the single (15) and double (16) coil experiments, providing general guidelines for choice of the adiabatic slice selection gradient, RF transmitter power and RF offset frequency for optimizing the inversion process, and minimizing the RF power required, and thus specific absorption rate (17,18). Using numerical solutions to the modified Bloch equations, Maccotta et al (19) indicated that measured adiabatic efficiencies in humans approach simulated levels, provided that one accounts for T 1 relaxation occurring during vascular transit periods.…”

Efficiency of flow‐driven adiabatic spin inversion under realistic experimental conditions: A computer simulation

“….n. [7] We assume that the magnetization is at equilibrium at the onset of inversion, M(0) ϭ M 0 ϭ (0, 0, M 0 ). The number of time steps of the simulations was increased until the results were stable, and a decrease in the step size yielded changes in the efficiency prediction of Ͻ0.05%.…”

“…In the present work, we investigated the influence of a nonlinearity of the frequency offset using a local magnetic field gradient coil (12) on ␣. We also addressed the differences between laminar and plug flow (7), and the realistic behavior of the bloodflow velocity during the cardiac cycle (8).…”