Spectral analysis of coronary bypass doppler blood flow signals

Dissertação de mest., Engenharia Electrónica e Telecomunicações, Faculdade de Ciências e Tecnologia, Univ. do Algarve, 2011The pulsed Doppler ultrasound (DU) is one of the important tools in the study of vessel diseases and the investigation of flow conditions. Due to its non-invasive nature, it has been increasingly used in medicine in the last few decades. Accurate estimation of DU spectral center frequency and bandwidth parameters are extremely important for blood flow diagnostic purposes. Under real-time data acquisition conditions the DU signal is generally corrupted with different types of noise. In these situations the identification of signal components solely belonging to the blood flow signal is a difficult task. This thesis was aimed to study spectral techniques to enhance spectral parameter estimation, in particular the center frequency. Spectral estimates were obtained using the Short Time Fourier Transform (STFT) and Continuous Wavelet Transform (CWT). STFT was applied to short duration data segments, respecting signals’ stationary properties. Two CWT functions have been studied: varying bandwidth filter and fixed bandwidth filter. Since different filter bandwidth values yield different results, bandwidths for fixed bandwidth filter were investigate and the most proper one has been used on the performance comparative studies. To enhance the blood flow signal content of noise-embedded clinical Doppler signals, a STFT-based technique was proposed to reduce the signals’ noise components. Quantitative evaluation of the spectral methods was primarily performed on simulated signals with deterministic center frequency and bandwidth. Different signal to noise ratio signals were simulated. It has been observed that STFT spectral center frequency and bandwidth estimators were less biased than the CWT ones, although the last ones were less sensitive to the center frequency variations. Applying the proposed noise cancellation technique to simulated signals reduces the spectral estimators’ errors. As an example, a typical noisy signal with 10dbSNR, a reduction of 88% and 97% was obtained on the RMS bias of the estimation of the center frequency and bandwidth estimators respectively

Integrated optics for astronomical interferometry - VI. Coupling the light of the VLTI in K band

Our objective is to prove that integrated optics (IO) is not only a good concept for astronomical interferometry but also a working technique with high performance. We used the commissioning data obtained with the dedicated K-band integrated optics two-telescope beam combiner which now replaces the fiber coupler MONA in the VLTI/VINCI instrument. We characterize the behaviour of this IO device and compare its properties to other single mode beam combiner like the previously used MONA fiber coupler. The IO combiner provides a high optical throughput, a contrast of 89% with a night-to-night stability of a few percent. Even if a dispersive phase is present, we show that it does not bias the measured Fourier visibility estimate. An upper limit of 0.005 for the cross-talk between linear polarization states has been measured. We take advantage of the intrinsic contrast stability to test a new astronomical prodecure for calibrating diameters of simple stars by simultaneously fitting the instrumental contrast and the apparent stellar diameters. This method reaches an accuracy with diameter errors of the order of previous ones but without the need of an already known calibrator. These results are an important step of integrated optics and paves the road to incoming imaging interferometer projects

Comprehensive vascular imaging using optical coherence tomography-based angiography and photoacoustic tomography

Studies have proven the relationship between cutaneous vasculature abnormalities and dermatological disorders, but to image vasculature noninvasively in vivo, advanced optical imaging techniques are required. In this study, we imaged a palm of a healthy volunteer and three subjects with cutaneous abnormalities with photoacoustic tomography (PAT) and optical coherence tomography with angiography extension (OCTA). Capillaries in the papillary dermis that are too small to be discerned with PAT are visualized with OCTA. From our results, we speculate that the PA signal from the palm is mostly from hemoglobin in capillaries rather than melanin, knowing that melanin concentration in volar skin is significantly smaller than that in other areas of the skin. We present for the first time OCTA images of capillaries along with the PAT images of the deeper vessels, demonstrating the complementary effective imaging depth range and the visualization capabilities of PAT and OCTA for imaging human skin in vivo. The proposed imaging system in this study could significantly improve treatment monitoring of dermatological diseases associated with cutaneous vasculature abnormalities

Enhancing time-frequency parameters estimation for Doppler ultrasound blood-flow signals

Doppler Ultrasound (DU) blood flow signals, particularly when collected under intra-operative conditions are noisy; accurate extraction of clinical parameters from their spectra becomes a difficult task. The spectral center frequency and bandwidth were estimated using two estimators with alternative time-frequency resolutions: a fixed resolution method, the Short-Time Fourier Transform (STFT) and the multi-resolution Continuous Wavelet Transform (CWT). Their performance was also assessed when the DU signals were pre-processed by a recently proposed Noise Cancellation Technique (NCTech). The NCTech algorithm enables quantification of the magnitude of the canceled noise in the form of percentage, called Cancellation Level (CL). Quantitative comparisons have been performed in terms of bias of the estimators when four signal-to-noise (SNRs) on DU simulated signals are employed: infinity, 20 dB, 10 dB and 5 dB. Results prove that CWT produced spectral parameters estimates with less bias than STFT; however these estimates were less consistent than the STFT ones. When NCTech is primarily applied to the signal, the STFT is the method to benefit most from this pre-processing technique. The CWT combined with NCTech produced estimates of both spectral parameters with better accuracy over the majority of the cardiac cycle, except where the frequency varies within a small range of frequencies during a short period of time. © 2011 IEEE

Noise cancellation technique for Doppler ultrasound spectrogram enhancement

Doppler ultrasound (DU) automatic detection of clinically relevant blood flow spectral parameters may be a difficult task when the spectrogram is corrupted with different types of noise. This paper introduces a technique to eliminate the spectrogram background noise without damaging the blood flow signals' content. Tests were performed on simulated signals with known spectral characteristics. Different signal to noise ratios were tested. Results of 100 signals were averaged. Based on the STFT spectrogram the center frequency and bandwidth of the simulated noisy signals were computed and evaluated against the deterministic curve. Results obtained by defining the background area of DU simulated signals and applying 7% of Cancellation level on signals with 10 dB of signal-to-noise ratio (SNR) produced mean frequency estimations with a maximum bias of 106 Hz. The maximum bias of the bandwidth estimations in this case was 124 Hz. Since the sampling frequency of the DU simulated signals were 12800 Hz, usage of the proposed noise cancelation technique enabled spectral parameters estimations with meaningless errors demonstrating the effectiveness of the proposed technique. © 2011 IEEE

Solving practical issues of a portable Doppler ultrasound system for blood flow assessment during coronary graft surgery

This paper presents some practical issues regarding the development of a dedicated Doppler Ultrasound system (DUS) for the assessment of blood flow signal on coronary grafts during heart surgery. The DUS is composed of several processing units. This paper concentrates on front-end units: the transducer and a particular issue of the software interface for clinical evaluation, the noise cancellation technique (NCTech). The experimental set-up implemented to evaluate the transducer response is presented. The procedure employed to eliminate the noise components embedded in the DUS data is described. Each of these units was primarily tested in laboratory. Results show their effectiveness in achieving their specific goals. Comments on the overall system's performance are presented denoting the usefulness of such dedicated DUS during bypass assessment at heart surgery. © 2011 IEEE

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