Abstract

A subtractive time of flight technique for MR angiography and quantitative blood flow measurement. Proton spins of water in the arterial supply to a tissue or organ are inverted in a steady-state fashion by applying constant amplitude off-resonance radio frequency pulses in the presence of a constant linear magnetic field gradient to effect adiabatic fast passage transport-induced inversion of spins which move in the direction of the gradient. An angiogram is formed by subtracting an image acquired with the arterial inversion pulse from a control image acquired with no arterial inversion. By inverting the spins in a steady-state manner, no cardiac gating is necessary for imaging organs. However, cardiac gating is desirable when imaging the heart so that spins of blood passing through the coronary arteries can be inverted during systole, when most of the blood is in the left ventricle, and imaged at end diastole, when most of the blood is in the coronary arteries. A coronary angiogram is then formed by subtracting images acquired with and without the inversion pulse. Also, by applying several inverting and imaging pulses during a cardiac cycle in accordance with the technique of the invention, a characteristic banding pattern may be formed in the fluid whereby each band corresponds to a population of spins that experienced inversion due to a single RF pulse. Since the width of the inversion band is proportional to the duration of the RF pulse and the velocity of the spin, measurement of the thickness of the inverted and uninverted bands allows for calculation of flow velocity. By gating such a pulse sequence to the cardiac cycle, time resolved in vivo velocity measurements may be made.

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