ANALYSIS OF NOISE IN PHASE-CONTRAST MR-IMAGING

In this work we analyze the effects of inherent random noise on the de tectability of low-contrast vessel structures that possess slow flow. When flow is encoded in more than one direction, the number of indepen dent noise contributions increases in addition to the scan time. In a fast-flow scenario, only the noise contribution from sampling along th e direction of flow is of any significance. At slow flow rates, howeve r, it becomes necessary to account for the noise in each encoded Carte sian direction. The degree to which noise affects low-contrast detecta bility also depends on the method of phase contrast image processing e mployed. A theoretical analysis of the statistical properties of signa l and noise in processed phase contrast magnitude images is presented and verified from experimental MR image data. Results show a progressi vely increased bias in the processed phase contrast image magnitude at slow flow rates due to contributions from inherent random noise. The amount of this bias increases with the number of physical directions i n which flow is encoded and is larger for complex difference processed images than for phase difference processing. Correspondingly, the out put signal-to-noise ratio associated with flow is compromised. (C) 199 6 American Association of Physicists in Medicine.