Melani Plett

Transient Detection With Cross Wavelet Transforms and Wavelet Coherence

Effective detection of unknown, transient oscillations in strong, colored, time varying noise and clutter has remained a signal processing challenge. Recently we presented a self-normalized wavelet detector for this purpose. Here, we revisit this detection problem for the case of a tapered, complex, transient oscillation observed coherently in two time series. We develop and compare three detectors based on the following: 1) the self-normalized Morlet cross wavelet spectrum (NCM); 2) the normalized cross discrete wavelet transforms (DWT), extended from Wang and Willets DWT power law detector (NCDWT3); and 3) wavelet coherence (WCOH). Each detector normalizes the cross wavelet spectrum to perform a binary hypothesis test for noise only. Simulated receiver operating curves for complex tapered transient oscillations in colored, slowly time varying, Gaussian noise and 60dB low frequency clutter are included for the three detector types and for one based on the fast Fourier transform (FFT) (NCFFT3) for comparison. Without clutter, detection rates at 0.1% false alarm rates for a (signal on) signal to noise ratio of 0.5 are: 99.4%, 99.1%, 97.0% and 96.3% for the NCFFT3, NCM, WCOH, and NCDWT3 detectors, respectively. When 60 dB clutter is present, the NCM and WCOH detection rates were unchanged, but the NCFFT3 and NCDWT3 detectors became ineffective

In vivo ultrasonic measurement of tissue vibration at a stenosis: a case study

It is known that bruits often can be heard downstream from stenoses. They are thought to be produced by disturbed blood flow and vessel wall vibrations. Our understanding of bruits has been limited, though, to analysis of sounds heard at the level of the skin. For direct measurements from the stenosis site, we developed an ultrasonic pulse-echo multigate system using quadrature phase demodulation. The system simultaneously measures tissue displacements and blood velocities at multiple depths. This paper presents a case study of a severe stenosis in a human infrainguinal vein bypass graft. During systole, nearly sinusoidal vessel wall vibrations were detected. Solid tissue vibration amplitudes measured up to 2 microm, with temporal durations of 100 ms and frequencies of roughly 145 Hz and its harmonics. Cross-axial oscillations were also found in the lumen that correlate with the wall vibrations, suggesting coupling between wall vibration and blood velocity oscillation.

Automated ultrasonic arterial vibrometry: detection and measurement

Since the invention of the stethoscope, the detection of vibrations and sounds from the body has been a touchstone of diagnosis. However, the method is limited to vibrations whose associated sounds transmit to the skin, with no means to determine the anatomic and physiological source of the vibrations save the cunning of the examiner. Using ultrasound quadrature phase demodulation methods similar to those of ultrasonic color flow imaging, we have developed a system to detect and measure tissue vibrations with amplitude excursions as small as 30 nanometers. The system uses wavelet analysis for sensitive and specific detection, as well as measurement, of short duration vibrations amidst clutter and time-varying, colored noise. Vibration detection rates in ROC curves from simulated data predict > 99.5% detections with < 1% false alarms for signal to noise ratios >= 0.5. Vibrations from in vivo arterial stenoses and punctures have been studied. The results show that vibration durations vary from 10 - 150 ms, frequencies from 100 - 1000 Hz, and amplitudes from 30 nanometers to several microns. By marking the location of vibration sources on ultrasound images, and using color to indicate amplitude, frequency or acoustic intensity, new diagnostic information is provided to aid disorder diagnosis and management.

Improved transient oscillation detection with multiwavelets

Multiwavelets have recently been used for analysis of transient oscillations. Here we use them for transient oscillation detection by incorporating them into two existing Morlet-based self-normalizing detectors, one that operates on a single time series and another on two correlated time series. The Morlet-based detectors are effective in detecting weak, transient oscillations, especially in strong clutter where FFT based methods fail. Simulated receiver operating curves reveal that the new multiwavelet detectors show marked improvement when applied to complex, transient oscillations in white Gaussian noise and 60 dB clutter. At 0.1% false alarm rate for a (signal on) signal to noise ratio of 0.25, detection rates are: 95.5% and 99.6% for the single and dual time series detectors, respectively. These compare to 75% and 95.5% for the Morlet wavelet detectors.

Using Doppler ultrasound to detect wall vibrations and flow velocity fluctuations in arteries

Two arterial disorders, stenoses and perforations, result in fluctuations in both wall displacements and blood velocities that are not displayed by conventional ultrasound instruments. Wall vibrations with amplitudes on the order of a micron are too small to be displayed by M‐mode. Narrow‐band velocity oscillations of less than 50‐ms duration can easily be missed in viewing traditional Doppler spectral waveforms. Low‐frequency velocity oscillations are attenuated by wall filters. Furthermore, the temporal relationship between flow velocities and wall displacements is not avail‐ able in conventional systems, yet this relationship provides key information in studying stenoses and locating perforations. To simultaneously examine flow velocities and wall displacements in arteries, a customized ultrasonic, pulse‐echo, multigate, quadrature phase demodulation system has been developed. The use of multigate, quadrature phase demodulation in the absence of a wall filter enables exploration of arterial behaviors t...