Magnetic resonance imaging (MRI) provides a versatile tool to determine a variety of fluid mechanical properties, but similarly the quantification is biased by the acquisition itself and the technique has limitations among which are limited spatial resolution and low signal-to-noise-ratio.
To overcome such issues, phase contrast MRI methods have been investigated at 7 T. At this field strength variations of the transmit field pose a substantial problem. Additionally, so-called displacement artifacts become important, since they scale with spatial resolution. Transmit field variations are addressed in this work by multi-spoke RF pulses, which are not straightforwardly applicable to velocity quantification. They require a detailed investigation of displacement artifacts that arise due to differences in the encoding time points of velocity and space. This work investigates three different encoding schemes, for conventional excitation as well as for multi-spoke excitation, which yield different displacement artifacts. Their impact on derived haemodynamic parameters, such as wall shear stress, which had been unknown so far, are investigated. Moreover, spoke pulses are further fine-tuned by using asymmetric pulse shapes. Besides the correct determination of the velocity at 7 T, the precise quantification of acceleration is another important factor, which is solved in this work by developing an acceleration-encoded sequence free of artifacts. Furthermore, another confounding factor in MRI-based velocitmetry, the intravoxel velocity distributions, affect the velocity encoding process. This effect has been investigated and, based on measured velocity spectra, noise-optimized velocity encoding sensitivity (VENC) values have been proposed. Finally, the potential of precise MR-based velocity measurements is demonstrated for a well-known fluid dynamic test case (flow over periodic hills) with a Reynolds number of 60,000. For this case, the Reynolds stress tensor has been quantified.
In conclusion, the presented techniques improve the precision at which fluid mechanical properties can be quantified by means of MRI
