Measurement of flow in small vessels by magnetic resonance phase mapping techniques: an in vitro and in vivo study.

The measurement of blood flow in small arteries is a potential extension of magnetic resonance (MR) angiography. We have compared flow measurements based on MR phase mapping with those obtained by Doppler ultrasound and electromagnetic flowmetry in both phantom and animal models. Correlation between modalities was high for in vitro studies (R2 = 0.93-0.98). In vivo, electrocardiogram-gated MR and Doppler ultrasound flow measurements compared to electromagnetic flowmetry showed fair correlation coefficients (R2 = 0.73 and 0.66, respectively). However, limits of agreement indicated that in small vessels flow measured by the three modalities could differ by up to +/-90 mL/min. For both models, arteries in the range of 3-6.5 mm in diameter produced complementary errors in area and velocity measurements in MR studies. Ungated MR studies showed a reduced agreement (R2 = 0.88 in vitro, 0.54 in vivo), which may in part be due to poor sampling of the velocity pattern. The results show that the high correlation obtained in vitro cannot be extrapolated to the in vivo situation, where additional physiological and anatomical variables are encountered.