Verification and evaluation of internal flow and motion. True magnetic resonance imaging by the phase gradient modulation method.

Abstract

Qualitative and quantitative evaluation and verification studies of the bipolar phase gradient modulation method for true MR [magnetic resonance] imaging of internal flow and motion velocities are presented. Velocity encoding modulations provide speed-of-motion and direction-sensitive images using special phase-sensitive reconstructions. True motion MR imaging does not depend upon subject parameters, T1 or T2, nor upon selective active-volume time-of-flight calculations, nor is it limited strictly to fluid-flow velocities. Conventional MR sequences often induce strong accidental phase gradient modulations that can cause severe artifacts in conventional MR scans and can limit the useful sensitivities of true motion MR. Multiple steps of velocity encoding allow resolution of separate elements of the velocity spectrum, and they enable suppression of all such phase-artifact difficulties. Some view-to-view phase inconsistencies are intrinsic to the subject being scanned, e.g., strong motion variations during the heart cycle; limitations due to such effects require external modifications in the scanning, such as cardiac gating. Since conventional density information remains in the data independent of velocity encoding modulations, a multiple encoding sequence and salvage of the MR raw data are suggested. These evaluations and verifications demonstrate exciting potential in clinical application for the phase gradient modulation method of true flow and motion MR imaging.