Biomedical subjects

H Michael Gach

Publications and source records attributed to H Michael Gach.

2 recordsLinked to original sources

PubMed2004

Simple model of double adiabatic inversion (DAI) efficiency.

Multislice continuous arterial spin labeling (CASL) using alternating single adiabatic inversion (SAI) and double adiabatic inversion (DAI) (ASD) suffers from control inefficiencies that reduce the perfusion signal-to-noise ratio (SNR). A simple analytic model for DAI efficiency (chi) is presented, and compared with simulation and experimental data. The DAI efficiency (defined as the ratio of uninverted to equilibrium spins) depends primarily on the effective adiabaticity (beta') and the inversion time (both determined from the labeling RF field and gradient amplitudes, and flow velocities). In this study, chi behaved like a damped oscillator, starting from 1 at beta'=0 (no spin inversion), dropping to 0.5 at beta'=0.5 (spin saturation), rising again to a peak at beta' approximately 3, and subsequently oscillating at higher beta'. The simple model matched the flow phantom data for beta' < 2, with an accuracy of better than 10%, but it diverged from the flow phantom data as the data oscillated at higher beta'. The relative inversion efficiencies of ASD and simultaneous proximal and distal irradiation (SPDI) were measured in 28 elderly volunteers. The ratio of CASL difference (control-label) signals measured in gray matter (= 1.16 +/- 0.05), and the simple efficiency models (= 1.13 +/- 0.02) had a correlation coefficient of 0.974.

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PubMed2002

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

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 T1 of the flowing fluid could be calculated based on the flow dynamics after varying the flow rate.

Blood Flow Velocity↗