Infographics / MRI Physics

MR angiography & flow phenomena (TOF/PC/CE)

A side-by-side comparison of two MR angiography methods. Time-of-flight (left) is inflow based: a gradient-echo slab saturates static tissue to low signal while fresh unsaturated blood flowing into the slab stays bright; a saturation band suppresses inflow from one direction, and slow flow can saturate in-slab. Phase-contrast (right) is velocity encoded: a bipolar gradient gives stationary spins zero net phase while moving spins accrue a phase shift proportional to velocity, so it measures both flow speed and direction, tunable by the velocity-encoding value VENC. A footer notes spin-echo flow void versus gradient-echo bright blood.

MR angiography: time-of-flight versus phase-contrast flow techniques A side-by-side comparison of two MR angiography methods. Time-of-flight (left) is inflow based: a gradient-echo slab saturates static tissue to low signal while fresh unsaturated blood flowing into the slab stays bright; a saturation band suppresses inflow from one direction, and slow flow can saturate in-slab. Phase-contrast (right) is velocity encoded: a bipolar gradient gives stationary spins zero net phase while moving spins accrue a phase shift proportional to velocity, so it measures both flow speed and direction, tunable by the velocity-encoding value VENC. A footer notes spin-echo flow void versus gradient-echo bright blood. MR angiography — two flow-based techniques bright-blood signal depends on how each method separates moving from static spins Time-of-flight (TOF) inflow enhancement · magnitude image flow saturation band static tissue: saturated → dark unsaturated inflow → bright – spoiled GRE, very short TR (~15–40 ms) – background saturated; inflow stays bright – slow flow saturates in-slab → signal loss – short-T1 fat can stay bright (needs fat-sat) Phase-contrast (PC) velocity-encoded · speed and direction + − bipolar gradient Δφ = γ·v·M1 static spins → net 0 moving spins → ∝ v – encodes velocity as signal phase – stationary spins: zero net phase – gives flow speed and direction – low VENC → slow flow (may alias) – high VENC → fast flow, less aliasing Spin-echo: spins wash out before the 180° refocusing pulse → flow void (dark blood). Gradient-echo inflow gives bright blood; both TOF and PC use GRE readouts.
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