Aorta wall motion monitoring by 1-d MRI of perpendicular diameters

Manganese-enhanced MRI of mouse heart during changes in inotropy

Recently the dual properties of manganese ion (Mn(2+)) as an MRI contrast agent and a calcium analogue to enter excitable cells has been used to mark specific cells in brain and as a potential intracellular cardiac contrast agent. Here the hypothesis that in vivo manganese-enhanced MRI (MEMRI) can detect changes in inotropy in the mouse heart has been tested. T(1)-weighted images were acquired every minute during an experimental time course of 75 min. Varying doses of Mn(2+) (3.3-14.0 nmoles/min/g BW) were infused during control and altered inotropy with dobutamine (positive inotropy due to increased calcium influx) and the calcium channel blocker diltiazem (negative inotropy). Infusion of MnCl(2) led to a significant increase in signal enhancement in mouse heart that saturated above 3.3 +/- 0.1 nmoles/min/g BW Mn(2+) infusion. At the highest Mn(2+) dose infused there was a 41-47% increase in signal intensity with no alteration in cardiac function as measured by MRI-determined ejection fractions. Dobutamine increased both the steady-state level of enhancement and the rate of MRI signal enhancement. Diltiazem decreased both the steady-state level of enhancement and the rate of MRI signal enhancement. These results are consistent with the model that Mn(2+)-induced enhancement of cardiac signal is indicative of the rate of calcium influx into the heart. Thus, the simultaneous measurement of global function and calcium influx using MEMRI may provide a useful method of evaluating in vivo responses to inotropic therapy.

In vitro validation of MR measurements of arterial pulse-wave velocity in the presence of reflected waves

A magnetic resonance imaging projective velocity encoding sequence was used to determine the pulse-wave velocity in an artery model. To this end, a well-defined flow phantom simulating flow propagation in large arteries was used. In order to validate the measurement method in the presence of large reflected waves, these were deliberately created in the phantom. The projective sequence was applied to two measurement sites and the wave velocity was determined from the spatial and temporal separations of the foot of the velocity waveform. A theoretical model describing reflection and attenuation phenomena was compared with experimental velocity waveforms. The model showed that reflections and attenuation can explain the important changes in velocity waveforms. The model also confirmed that in the presence of reflecting waves, the foot of the waveform can be used as a characteristic point for measurements through changes in the waveform.