Enhancing Multichannel Laser-Doppler Vibrometry Signals with Application to (Carotid-Femoral) Pulse Transit Time Estimation.

Pulse-wave velocity (PWV) can be used to quantify arterial stiffness, allowing for a diagnosis of this condition. Multi-beam laser-doppler vibrometry offers a cheap, non-invasive and user-friendly alternative to measuring PWV, and its feasibility has been previously demonstrated in the H2020 project CARDIS. The two handpieces of the prototype CARDIS device measure skin displacement above main arteries at two different sites, yielding an estimate of the pulse-transit time (PTT) and, consequently, PWV. The presence of multiple beams (channels) on each handpiece can be used to enhance the underlying signal, improving the quality of the signal for PTT estimation and further analysis. We propose two methods for multi-channel LDV data processing: beamforming and beamforming-driven ICA. Beamforming is done by an SNR-weighted linear combination of the time-aligned channels, where the SNR is blindly estimated from the signal statistics. ICA uses the beamformer to resolve its inherent permutation and scale ambiguities. Both methods yield a single enhanced signal at each handpiece, where spurious peaks in the individual channels as well as stochastic noise are well suppressed in the output. Using the enhanced signals yields individual PTT estimates with a low spread compared to the baseline approach. While the enhancement is introduced in the context of PTT estimation, the approaches can be used to enhance signals in other biomedical applications of multi-channel LDV as well.

Template Matching and Matrix Profile for Signal Quality Assessment of Carotid and Femoral Laser Doppler Vibrometer Signals.

Background: Laser-Doppler Vibrometry (LDV) is a laser-based technique that allows measuring the motion of moving targets with high spatial and temporal resolution. To demonstrate its use for the measurement of carotid-femoral pulse wave velocity, a prototype system was employed in a clinical feasibility study. Data were acquired for analysis without prior quality control. Real-time application, however, will require a real-time assessment of signal quality. In this study, we (1) use template matching and matrix profile for assessing the quality of these previously acquired signals; (2) analyze the nature and achievable quality of acquired signals at the carotid and femoral measuring site; (3) explore models for automated classification of signal quality. Methods: Laser-Doppler Vibrometry data were acquired in 100 subjects (50M/50F) and consisted of 4-5 sequences of 20-s recordings of skin displacement, differentiated two times to yield acceleration. Each recording consisted of data from 12 laser beams, yielding 410 carotid-femoral and 407 carotid-carotid recordings. Data quality was visually assessed on a 1-5 scale, and a subset of best quality data was used to construct an acceleration template for both measuring sites. The time-varying cross-correlation of the acceleration signals with the template was computed. A quality metric constructed on several features of this template matching was derived. Next, the matrix-profile technique was applied to identify recurring features in the measured time series and derived a similar quality metric. The statistical distribution of the metrics, and their correlates with basic clinical data were assessed. Finally, logistic-regression-based classifiers were developed and their ability to automatically classify LDV-signal quality was assessed. Results: Automated quality metrics correlated well with visual scores. Signal quality was negatively correlated with BMI for femoral recordings but not for carotid recordings. Logistic regression models based on both methods yielded an accuracy of minimally 80% for our carotid and femoral recording data, reaching 87% for the femoral data. Conclusion: Both template matching and matrix profile were found suitable methods for automated grading of LDV signal quality and were able to generate a quality metric that was on par with the signal quality assessment of the expert. The classifiers, developed with both quality metrics, showed their potential for future real-time implementation.

Silicon photonics-based laser Doppler vibrometer array for carotid-femoral pulse wave velocity (PWV) measurement.

Pulse wave velocity (PWV) is a reference measure for aortic stiffness, itself an important biomarker of cardiovascular risk. To enable low-cost and easy-to-use PWV measurement devices that can be used in routine clinical practice, we have designed several handheld PWV sensors using miniaturized laser Doppler vibrometer (LDV) arrays in a silicon photonics platform. The LDV-based PWV sensor design and the signal processing protocol to obtain pulse transit time (PTT) and carotid-femoral PWV in a feasibility study in humans, are described in this paper. Compared with a commercial reference PWV measurement system, measuring arterial pressure waveforms by applanation tonometry, LDV-based displacement signals resulted in more complex signals. However, we have shown that it is possible to identify reliable fiducial points for PTT calculation using the maximum of the 2nd derivative algorithm in LDV-based signals, comparable to those obtained by the reference technique, applanation tonometry.

Six-beam homodyne laser Doppler vibrometry based on silicon photonics technology.

This paper describes an integrated six-beam homodyne laser Doppler vibrometry (LDV) system based on a silicon-on-insulator (SOI) full platform technology, with on-chip photo-diodes and phase modulators. Electronics and optics are also implemented around the integrated photonic circuit (PIC) to enable a simultaneous six-beam measurement. Measurement of a propagating guided elastic wave in an aluminum plate (speed ≈ 909 m/s @ 61.5 kHz) is demonstrated.

Polymer optical fiber compound parabolic concentrator tip for enhanced coupling efficiency for fluorescence based glucose sensors.

We demonstrate that the light excitation and capturing efficiency of fluorescence based fiber-optical sensors can be significantly increased by using a CPC (Compound Parabolic Concentrator) tip instead of the standard plane-cut tip. We use Zemax modelling to find the optimum CPC tip profile and fiber length of a polymer optical fiber diabetes sensor for continuous monitoring of glucose levels. We experimentally verify the improved performance of the CPC tipped sensor and the predicted production tolerances. Due to physical size requirements when the sensor has to be inserted into the body a non-optimal fiber length of 35 mm is chosen. For this length an average improvement in efficiency of a factor of 1.7 is experimentally demonstrated and critically compared to the predicted ideal factor of 3 in terms of parameters that should be improved through production optimization.

Clinical evaluation of a transcutaneous interrogated fluorescence lifetime-based microsensor for continuous glucose reading.

BACKGROUND: Continuous glucose monitoring is presently used worldwide. Accuracy, precision, durability, invasiveness, and lack of drift of sensors and lag time are key parameters essential to these systems. This article describes a new online minimally invasive biodegradable microsensor for optical, transcutaneous interrogation, which has at least 14 days of functionality. METHOD: Studies were performed in vitro and in vivo on pigs, as well as on type 1 diabetic humans. Functionality has been ensured in laboratory settings, and precision and durability have been tested in vivo. During in vivo studies, venous blood samples were used as reference. Results were based on one single point calibration per experiment. RESULTS: Excellent stability was found in 14-day in vitro trials as well as in vivo in up to 70-hour trials. The overall median relative absolute difference of type 1 diabetic patients was 11.4%. Error grid analysis showed 97.7% of all values in the A+B zone. Comparable results were found in animal studies. No sensor drift was observed in any trial. CONCLUSION: Results point toward the possibility of developing a stable and precise minimally invasive glucose reader for at least 2 weeks of continuous use.