PCMRI

Phase-contrast magnetic resonance imaging (PC-MRI) is an MRI method to quantify blood flow by encoding velocity into the phase of the MR signal. It allows non-invasive measurement of flow velocity and, by integration across a vessel cross-section, volumetric flow rates. PC-MRI is used in cardiac, vascular, and cerebrospinal fluid studies.

Principle: Velocity-encoding gradients create phase shifts proportional to motion along a selected direction. A reference scan subtracts static tissue phase, yielding velocity maps. The maps can be used to compute time-averaged or time-resolved flow. Four-dimensional flow (4D flow) extends PC-MRI to time-resolved three-dimensional velocity fields for comprehensive hemodynamics.

Acquisition: A gradient-echo phase-contrast sequence is run with a velocity encoding value (VENC) chosen near the expected maximum velocity to avoid aliasing. The imaging plane is placed perpendicular to the target vessel. Breath-holding or respiratory gating may be used. Post-processing involves lumen segmentation and calculation of forward, reverse, net flow, and regurgitant fraction.

Applications: Quantification of cardiac output and valvular regurgitation, and shunt measurement in congenital heart disease, and flow in major vessels; CSF flow can also be assessed. 4D flow enables visualization of flow patterns and estimation of wall-shear stress in research and select clinical contexts.

Limitations: Accuracy depends on appropriate VENC selection; aliasing occurs if VENC is too low, and SNR may be limited in small vessels. Susceptibility, motion, and arrhythmias can degrade data. Post-processing requires expertise, and acquisition can be time-consuming; however, PC-MRI provides a non-contrast, quantitative hemodynamic assessment useful in routine and research settings.