In-vivo validation of fast spectral velocity estimation techniques - preliminary results

Spectral Doppler is a common way to estimate blood velocities in medical ultrasound (US). The standard way of estimating spectrograms is by using Welch´s method (WM). WM is dependent on a long observation window (OW) (about 100 transmissions) to produce spectrograms with sufficient spectral resolution and contrast. Two adaptive filterbank methods have been suggested to circumvent this problem: the Blood spectral Power Capon method (BPC) and the Blood Amplitude and Phase Estimation method (BAPES). Previously, simulations and flow rig experiments have indicated that the two adaptive methods can display sufficient spectral resolution for much shorter OWs than WM. The purpose of this paper is to investigate the methods on a larger population and letting a clinical expert evaluate the spectrograms. Ten volunteers were scanned over the right common carotid artery and four different approaches were used to estimate the spectrograms: WM with a Hanning window (WMhw), WM with a boxcar window (WMbw), BPC and BAPES. For each approach the window length was varied: 128, 64, 32, 16, 8, 4 and 2 emissions/estimate. Thus, from the same data set of each volunteer 28 spectrograms were produced. The artery was scanned using the experimental ultrasound scanner RASMUS and a B-K Medical 5 MHz linear array transducer with an angle of insonation not exceeding 60deg. All 280 spectrograms were then randomised and presented to a radiologist blinded for method and OW for visual evaluation: useful or not useful. WMbw and WMhw estimated less useful spectrograms compared to the adaptive methods at OW below 64. The BAPES method performed better than BPC at OW of 16 and 8. Furthermore, BAPES was the only method that estimated spectrograms equally well for OW of 16 compared to OW of 128. All four approaches failed at OW of 4 and 2. The preliminary results indicate that the OW can be reduced to 32 when using the BPC method and to 16 when using the BAPES method for spectral blood estimation. This will - - liberate processing time in spectral US examination and could be used to increase the frame rate of the interleaved B-mode image.