This proposal deals with the development and characterization of an NMR imaging tool for the measurement of slow fluid flow.
The aim i s for the technique to be able to measure capillary flow. Two methods will be investigated. The first is a subtraction of two images each with a different sensitivity to flow. The subtraction cancels the signal from stationary spins and leaves a signal from a range of velocities. Precise cancellation of the static signal requires a high signal to noise ratio; hence the technique will be applied on a high field imager (1.5 T). Steady State Free Precession (SSFP) imaging will be used. SSFP produces a periodic magnetization response in the presence of a gradient. The flow sensitivity is changed by altering the wavelength Delta r of the periodicity. An extensive calibration of the flow sensitivity will be done with flow phantoms that have velocity distributions whose mean and width are adjustable.
The aim of the subtraction is to obtain a signal from spins with a velocity distribution of zero mean and a standard deviation of -2mm/s. A theoretical model of the flow sensitivity of SSFP will be pursued to provide insight and feedback into the development process. Numerical simulations using Bloch's equations will be used to model a group of spins with a particular velocity distribution to allow their magnetization response to evolve in time. The second flow visualization method will be to develop a moving reference frame (mrf). The signal will be from spins moving with the reference frame so only one image is acquired. This technique will be used to measure velocity distributions with a non-zero mean. In the SSFP version of the mrf, both the mean and width of the velocity distribution will be controlled by varying the speed of the reference frame and Delta r respectively. To establish the mrf, a Helmholtz coil will be used to track the frequency and phase of moving spins. A linearly mrf will be characterized using a flow phantom and then a reference frame moving in a nonlinear fashion will be developed.
