Coded excitation improves vascular wall shear rate estimation

There has been great interest in developing accurate ultrasonic velocity estimators to measure wall shear rate (WSR), in vivo. In this study, we implemented coded pulse excitation methods onto a commercial scanner and used flow-angle-corrected correlation methods to obtain WSR estimates with high spatial resolution near vessel walls. We tested the method on flow phantoms that generated a similar time-averaged shear range to human arteries (100-600 1/s), and on carotid arteries of a volunteer. Phase-modulated (PM = 13-bit Optimal) code and frequency-modulated (FM = 2.1μs pseudo-chirp) pulse with similar excitation energies were programmed into a Siemens Antares system. RF data recorded using ultrasound research interface (URI) was decoded offline. The accuracy and precision of WSR measurements were compared among broadband (BB) /narrowband (NB) and coded/uncoded data. eSNR was enhanced 15 dB using codes compared with BB pulses, and 5 dB compared with NB pulses. Experimental results show that coded pulses overall yield the minimal errors in WSR estimation (-10 % bias (B) and 4% standard deviation (SD)), compared with those of BB (30% B, 15% SD) and NB pulses (20% B, 10% SD). Errors increase with WSR for all pulses. ECG-gated acquisitions from the carotid artery of a volunteer yielded the comparable WSR values at peak systole of 853 1/s (PM), 820 (FM), 729 1/s (NB), and 590 1/s (BB), while the coded pulses provided the lowest variances. Our study shows that coded excitation techniques can increase both the precision and accuracy in WSR estimation over a broad range of shear rate. The study also demonstrates the feasibility of in vivo application of coded excitation in WSR flow imaging.