Assessment of 3D velocity vector fields and turbulent kinetic energy in a realistic aortic phantom using multi-point variable-density velocity encoding
Journal of Cardiovascular Magnetic Resonance volume 14, Article number: W50 (2012)
A multi-point velocity encoding approach for the assessment of velocity vector fields and TKE is shown in this work. The method is applied in an aortic arch phantom under different flow conditions.
Three-dimensional Phase Contrast (PC) MRI has emerged as a promising non-invasive acquisition technique for assessing velocity vector fields of blood flow . To address the limited sensitivity when velocities are much lower than the encoding velocity venc, three-point acquisition methods with a high venc and a low venc acquisition to unwrap the low venc scan may be employed . However, by using the high venc data only to control phase unaliasing the approaches are not signal-to-noise ratio (SNR) efficient. This fact becomes relevant in particular when incorporating data undersampling techniques to shorten the long scan times associated with 3D PC-MRI. Accordingly, SNR optimality of encoding and decoding is desired. To this end Bayes’ approaches have been proposed and adapted to PC-MRI [3, 4].
In the present work the feasibility of velocity vector field and turbulent kinetic energy (TKE) mapping based on multi-point variable-density velocity encoding with spatiotemporal undersampling is demonstrated on a realistic aortic phantom .
An elastic cast of an aortic arch equipped with a mechanical aortic valve (St. Jude Medical Inc., St. Paul, MN, USA) was set up in a pulsatile flow conduit and measured using a velocity encoded, cardiac triggered 3D gradient echo sequence on a 3T Philips Achieva System (Philips Healthcare, Best, The Netherlands). Within a scan time of 33 min, 5 velocity encodings according to venc = [200, 100, 50, 28, 20] cm/s in each spatial direction plus a non-encoded reference segment were acquired (Fig 1. red dots) with 5x k-t undersampling and 11x6 training profiles with a temporal resolution of 46 ms. Velocities and TKE values  were computed using Bayesian parameter estimation . In a second experiment, one leaflet of the valve was fixed in order to simulate a stenotic valve.
Mean TKE values in the ascending aorta were found to be about 4 times higher for the stenosed valve experiment compared to a normal heart valve. The jet of high velocities up to 100 cm/s is surrounded by increased TKE areas with TKE values > 50 J/m3 as it is shown in Fig. 2 b) & d).
The presented work shows the assessment of velocity vector fields and TKE in a realistic aortic phantom. Using the identical setup comparison of TKE values to data from Particle Tracking Velocimetry (PTV) is possible hence permitting assessment relative to a method of reference for measuring fluctuating velocities at very high temporal resolution.
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Knobloch, V., Binter, C., Gulan, U. et al. Assessment of 3D velocity vector fields and turbulent kinetic energy in a realistic aortic phantom using multi-point variable-density velocity encoding. J Cardiovasc Magn Reson 14 (Suppl 1), W50 (2012). https://doi.org/10.1186/1532-429X-14-S1-W50