Conventional cine phase-contrast MRI (Conv-PC) is the gold standard for blood flow measurements but can only provide flow measurements for an averaged heartbeat cycle (using cardiac gating). In contrast to Conv-PC, phase-contrast echo-planar MRI (EPI-PC) can be used to quantify the blood flow rate in real-time – giving the technique great potential for clinical and research applications. The objectives of the present in vitro study were to compare the accuracy of EPI-PC and Conv-PC and to assess the influence of pixel size and velocity encoding on flow measurements obtained with the two sequences.
Flow quantification was assessed using a pulsatile flow phantom. The flow rate curves were extracted with in-house post-processing software, and the flow curves from EPI-PC were reconstructed to give an average pulse cycle with 32 sampling points with the same waveform as that obtained with Conv-PC. Firstly, the accuracy of the EPI-PC was checked by comparing it with the flow rate in the calibrated phantom and the pulsation index from Conv-PC. Secondly, flow data from the two sequences were compared quantitatively as a function of the pixel size and the velocity encoding.
The mean percentage difference between the EPI-PC flow rate and calibrated phantom flow rate was -2.9 %, which was within the confidence interval. The pulsatility indices for EPI-PC and Conv-PC were respectively 0.64 and 0.59. In order to keep the flow rate measurement error within 10%, the ROI in Conv-PC had to contain at least 13 pixels, while the ROI in EPI-PC had to contain at least 9 pixels. Furthermore, Conv-PC had a higher velocity-to-noise ratio and could use a higher velocity encoding than EPI-PC (20cm/s and 15cm/s, respectively).
The result of an in vitro study confirmed the accuracy of EPI-PC. Furthermore, EPI-PC was found to be less sensitive than Conv-PC at a low spatial resolution but was more sensitive than Conv-PC to velocity encoding.