Background phase induced steady‐state effects in velocity quantification using phase‐contrast MRI

Purpose: Flow quantification using phase‐contrast (PC) MRI is based on steady‐state gradient echo (GRE) sequences and is hampered by spatially varying background phase offsets. The purpose of this work was to investigate the effect of steady‐state disruptions during PC‐MRI GRE sequences on these background phases. Based on these findings, a specific sequence and timing is suggested, and caution is expressed when using typical correction algorithms. Methods: Steady‐state responses in stationary tissue were investigated in different prospectively triggered through‐plane phase‐contrast MRI sequence. Different spoiling methods (gradient spoiling/FISP versus gradient+RF spoiling/FLASH) and interleaving of flow encoding gradients (every TR vs. every ECG cycle) were investigated using simulations, in phantoms and in vivo. Additionally, the effect of relaxation times on the phase offsets was simulated and measured. The impact on image‐ and phantom‐based background phase correction was studied. Results: Good agreement between simulation and phantom measurements were observed. Different sequences lead to different spatiotemporal and tissue dependent background phases. Average flow rates in the popliteal artery were over‐ and underestimated for ECG‐interleaved and TR‐interleaved FISP acquisitions compared to FLASH, respectively. Conclusion: Background phase measurements are influenced by steady‐state effects leading to potentially false background phase quantification. Current background phase correction methods cannot correct for the disturbance.