Towards measuring the effect of flow in blood T₁ assessed in a flow phantom and in vivo

Measurement of the blood T ₁ time using conventional myocardial T ₁ mapping methods has gained clinical significance in the context of extracellular volume (ECV) mapping and synthetic hematocrit (Hct). However, its accuracy is potentially compromised by in-flow of non-inverted/non-saturated spins and in-flow of spins which are not partially saturated from previous imaging pulses.

Bloch simulations were used to analyze various flow effects separately. T ₁ measurements of gadolinium doped water were performed using a flow phantom with adjustable flow velocities at 3 T. Additionally, in vivo blood T ₁ measurements were performed in 6 healthy subjects (26 ± 5 years, 2 female). To study the T ₁ time as a function of the instantaneous flow velocity, T ₁ times were evaluated in an axial imaging slice of the descending aorta. Velocity encoded cine measurements were performed to quantify the flow velocity throughout the cardiac cycle.

Simulation results show more than 30% loss in accuracy for 10% non-prepared in-flowing spins. However, in- and out-flow to the imaging plane only demonstrated minor impact on the T ₁ time. Phantom T ₁ times were decreased by up to 200 ms in the flow phantom, due to in-flow of non-prepared spins. High flow velocities cause in-flow of spins that lack partial saturation from the imaging pulses but only lead to negligible T ₁ time deviation (less than 30 ms). In vivo measurements confirm a substantial variation of the T ₁ time depending on the flow velocity. The highest aortic T ₁ times are observed at the time point of minimal flow with increased flow velocity leading to reduction of the measured T ₁ time by up to $130\pm 49ms$ at peak velocity.

In this work we attempt to dissect the effects of flow on T ₁ times, by using simulations, well-controlled, simplified phantom setup and the linear flow pattern in the descending aorta in vivo. (paper)