Velocity Encoding of 4-D Flow MR Imaging Measurements Using Straight-Tube and Aortic Aneurysm Phantoms.

Purpose: This phantom-based study aimed to identify acceptable velocity encoding (VENC) settings for flow measurements in straight-tube and aortic aneurysm phantoms using 4-D flow magnetic resonance (MR) imaging.

Methods: A pulse flow pump was connected to 2 types of plastic phantoms (straight-tube and aortic aneurysm) to create steady and pulsatile flow, respectively. The validity of flow measurements with 4-D flow MR imaging using 2 VENC settings (16 and 32 cm/s) was examined, using flow meter measurements as the gold standard. The optimal VENC setting was 16 cm/s. Equivalence tests were conducted to identify acceptable VENC settings using an equivalence margin of 25%. A P value of less than .05 was considered significant.

Results: Using either steady or pulsatile flow in the straight phantom, it was found that the 4-D flow MR imaging measurement results with a VENC of 32 cm/s (P < .008) and 16 cm/s (P < .049) were acceptable (ie, within the range of equivalence). 4-D flow MR imaging measurements with VENC of 32 cm/s showed equivalence with flow meters under all conditions except for the aneurysm form and pulsatile flow conditions (P < .008). None of the VENC settings were acceptable using pulsatile flow in the aneurysm phantom because of overestimation (VENC of 16 cm/s) or underestimation (VENC of 32 cm/s).

Discussion: In the case of straight-tube morphology, acceptable measurement results can be obtained even with VENC settings twice the actual flow velocity. However, in the context of aortic aneurysm morphology, the authors speculated that achieving precise measurements with a VENC setting tailored to a straight tube would be challenging.

Conclusion: Twice the optimal VENC is acceptable in straight vessels. However, an appropriate VENC setting would be difficult for measuring pulsatile flow in an aneurysm.