A mathematical model for signal from spins flowing during the application of spin echo pulse sequences.

Models are presented for both laminar and plug flow that predict the signal from spins flowing during the application of slice-selective spin echo pulse sequences. The models permit calculation of the total signal from a cylindrical vessel lying perpendicular to the slice and incorporate the effect of the physical displacement of the spins between successive excitations. This time-of-flight effect gives a signal which is composed of contributions from a finite number of spin populations, with each population signal weighted by the fractional volume of that spin population within the cylindrical vessel segment. The signal and fractional volume from each spin population are derived analytically for ten different spin echo pulse sequences. The models for plug and laminar flow have important application for predicting and interpreting flow effects observed in clinical images. They are shown to be useful for selecting pairs of pulse sequences that can be used to obtain digitally subtracted MR images which provide optimum contrast for flowing blood with essentially complete suppression of stationary anatomy. These models provide a means for quantitatively comparing the expected signal from flowing spins for the many techniques presently being investigated for MR angiography.