A validation of a flow quantification by MR phase mapping software
Introduction
Techniques of flow quantification (FQ) by MR phase mapping have recently been validated 1, 2, 3, 4. The present study is a complementary contribution to these previous validations concerning a commercial Siemens software. Although this work involved a particular software/hardware (derived from a major manufacturer), we expect the results to be useful for the radiologic community. Current use of FQ software necessitates knowing and understanding the uncertainties of the computed values. We have evaluated these uncertainties in the various conditions that we describe in detail.
We first evaluated phantom measurement uncertainties in a very simple case, an experimental permanent laminar flow model. Then we performed measurements of pulsatile flow in healthy volunteers. Various comparisons were made between FQ performed in ascending aorta, FQ performed in right and left pulmonary arteries (sum) and also left ventricular stroke volumes (end diastolic and end systolic volume difference: EDV-ESV) that were measured from left ventricular outlines. The major part of the work deals with FQ measurements in the ascending aorta for assessment of intraobserver, interobserver and interexamination variability. Choice of FQs in the ascending aorta was made because of highest velocities and hence, the maximal variations of velocity are found there. This choice was made also because of the large size of this vessel and its constant quasicylindrical shape over the cardiac cycle.
Section snippets
Flow quantification software
The FQ software used is a Siemens product (version 3b31a) that was developed on a field even echo rephasing (FEER) concept, flow encoding and flow compensation being achieved alternatively. Use of a proper encoding velocity avoids aliasing and the software thus allows quantification of mean flow over a region of interest (ROI) sketched in images that are acquired every 30 ms. The number of successive images available cannot exceed 64. Both a magnitude image and a phase image are shown on the
Results
Results of experiments using a laminar flow model are shown in Table 1. A comparison between expected velocities and velocities measured by means of three encoding velocities is presented. All measured velocities (n=15), except one, were lower than expected velocities. The results also show that uncertainty for a given encoding velocity did not significantly vary with the value of the measured velocity. For 150, 75 and 40 cm/s encoding velocities averaged uncertainties were 3, 1 and 0.8 cm/s,
Laminar flow phantom experiments
Nine different velocity measurements performed with 150 cm/s encoding velocity showed that the FQ software by phase mapping underestimated expected (actual) velocities by an average of 8%. This was close to Burkart's results [4], who found a 6% underestimation with flow phantom experiments (encoding velocity was not precisely reported in this particular experiment). We suggest this underestimation could be correlated with intravoxel phase dispersion due to the velocity gradient that is found at
Acknowledgements
This work was made possible by anonymous volunteers. S. Dupond's flow-phantom work was very useful for the present experiments.
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