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13:00   Biomedical Signal Processing II Chair: Marcel Rutten 13:00 15 mins PULSE-BY-PULSE PHOTOPLETHYSMOGRAPHY QUALITY INDEX FOR SIGNAL RELIABILITY ASSESSMENT BASED ON PULSE MORPHOLOGY Gabriele B. Papini, Pedro Fonseca, Rik Vullings, Sebastiaan Overeem, Jan Bergmans Abstract: In the last decades, photoplethysmography (PPG) has been employed in a wide spectrum of applications, ranging from consumer devices to medical equipment. The pulsatile PPG signal recorded contains valuable information on the cardio-vascular and -respiratory system. Several features can be derived from the PPG and these can be used to infer physiological states of the human system, for instance to estimate the sleep structure or the fitness level of a subject. As every sensing technology, the PPG can be corrupted by artifacts so it needs to be enhanced and, sometimes, parts of it have to be rejected. The rejection becomes fundamental especially when features are based on the shape of the signal. In literature, several methods are available to determine a quality index (QI) of parts of the PPG and use this QI as criterion for segment rejection. However, these methods exclude entire segments of the signal rather than single pulses, or they calculate a QI susceptible to the physiological PPG variation. In this study, a new algorithm for single PPG pulse QI calculation is proposed. This QI ranges between 0 and 1 and it is assessed by comparing each pulse with a template. The template is derived from PPG by using dynamic time warping (DTW) barycenter averaging. Each pulse is warped, using DTW, to maximize the match with the template and the QI is calculated as a normalized root mean square error of the remaining mismatch. The QI is resilient to physiological pulse deformations, but still able to quantify the pulse morphology corruption and to recognize artifacts. The algorithm is validated on the Complex System Laboratory database, according to the ANSI/AAMI standards. For each pulse the beat location is calculated and it is rejected if QI is lower than 0.5. The positive predictive value and sensitivity (PPV, SEN) are calculated with respect to human beat annotations, with a true positive detection criterion of 30 ms distance from the annotated beat. The developed algorithm has a PPV of 99.43% and a SEN of 95.43% while the one in [3] has, respectively, 97.98% and 98.99%. The QI thresholding allows to obtain a significantly higher PPV, a consequence of an improved corrupted beat rejection, at the expense of a lower sensitivity. The proposed algorithm provides a single pulse QI, resilient to physiological deformation, based on the morphology comparison with a data-derived template. The algorithm gives the possibility to choose the QI depending on the features that will be derived. For instance, in case of morphology related features, a high QI threshold can increase the likelihood that only uncorrupted beats are kept. 13:15 15 mins A DISCRIMINATIVE EVENT BASED MODEL FOR DISEASE PROGRESSION MODELING Vikram Venkatraghavan, Esther Bron, Wiro Niessen, Stefan Klein Abstract: Understanding the progression of neurodegenerative diseases like Alzheimer’s disease is extremely important for early diagnosis and disease staging in clinical practice. Disease progression can be represented by an ordering of events, where an event is defined by a clinical biomarker becoming abnormal. To estimate this ordering of events, one could set up a longitudinal study including subjects at high risk of developing the disease and repeatedly measuring all clinical biomarkers over a long period of time. However, this would be a costly and time-consuming approach, since disease progression might be slow and not all subjects might develop the disease after all. Event based modeling (EBM) [1] is an emerging data-driven approach to estimate an ordering of events that best describes the disease progression, based on cross-sectional study data. The assumption made in [1] is the existence of a single ordering common in all the subjects within a cohort. This assumption was relaxed in [2] which estimates a distribution of event orderings using a generalized Mallows model. All the variants of EBM developed so far are generative in nature, and consequently do not scale well to the large number (>1000) of biomarkers resulting from e.g. advanced brain imaging experiments. In this work we propose a novel discriminative approach to EBM that is more accurate and computationally more efficient than the existing state-of-the-art EBM approaches. The method first estimates subject-wise orderings by sorting the biomarkers of each subject based on the probability that the event had occurred. Subsequently the central ordering is estimated by fitting a generalized Mallows model to these subject-wise orderings. To evaluate the accuracy, extensive simulation experiments were performed using the framework proposed in [3]. Subsequently, the method was applied to public Alzheimer's Disease Neuroimaging Initiative (ADNI) data. In the simulation experiments, the accuracy of our discriminative EBM was on average 11% higher than those of [1] and [2]. The average computation time was 3.5 times faster than that of [1] and 680 times faster than that of [2]. Results on ADNI data were in line with existing knowledge of disease progression. In conclusion, we proposed a novel method for data-driven disease progression modeling based on cross-sectional data, which outperformed state-of-the-art EBM methods. REFERENCES [1] H.M. Fonteijn, M.Modat, M.J. Clarkson, J. Barnes, M. Lehmann, N.Z. Hobbs, R.I. Scahill, S.J. Tabrizi, S.Ourselin, N.C. Fox, and D.C. Alexander, “An event-based model for disease progression and its application in familial Alzheimer’s disease and Huntington’s disease,” NeuroImage, vol. 60, no. 3, pp. 1880 – 1889, 2012. [2] J. Huang and D. Alexander, “Probabilistic event cascades for Alzheimer’s disease,” in Advances in Neural Information Processing Systems 25, F. Pereira, C. J. C. Burges, L. Bottou, and K. Q. Weinberger, Eds., pp. 3095–3103. Curran Associates, Inc., 2012. [3] A.L. Young, N.P. Oxtoby, S. Ourselin, J.M. Schott, and D. Alexander, “A simulation system for biomarker evolution in neurodegenerative disease,” Medical Image Analysis, vol. 26, no. 1, pp. 47 – 56, 2015. 13:30 15 mins A NOVEL TECHNIQUE FOR ASSESSMENT OF MECHANICAL PROPERTIES OF VASCULAR TISSUE Stefan Sanders, Frans van de Vosse, Marcel Rutten Abstract: Rupture of atherosclerotic plaques in the carotid artery is a major cause for stroke. Currently, the level of lumen occlusion caused by the stenosis is used to estimate the risk of plaque rupture. However, plaque rupture occurs when the mechanical stresses in the cap of the plaque exceed the local tissue strength, not necessarily when the luminal occlusion exceeds a certain level. Therefore, a biomechanical model of the plaque and its mechanical properties may help to better assess rupture risk. To determine the risk of rupture, mechanical properties of plaque components and cap strength are measured in 2D inflation experiments and assessed using inverse numerical modelling. In this study, we develop a method to assess material properties of (diseased) vascular tissue in a quasi-2D setting. Thin slices of material were cut and slightly compressed between two glass plates. Fluid was injected into the lumen, through a hole in the bottom plate, to inflate the sample. A pressure sensor monitored the intraluminal pressure, while a high speed camera recorded the displacement of the sample. The lumen diameter was calculated from these images. Material properties were assessed using a 1D model, describing radial displacements. For validation purposes, thin rings of silicone rubber, with known material properties, were inflated in the experiments. The obtained material properties (using a Neo-Hookean material model) were compared to the properties calculated from tensile tests on rectangular samples of the same material. Next, for validation on fresh carotid tissue, ultrasound measurements have been performed on 4 healthy porcine carotid arteries. Thin slices were cut from these arteries, measured in the inflation experiment and the material properties (using a Holzapfel-Gasser-Ogden material model) were compared to the ultrasound measurements. The results reveal a good agreement between properties found by classic tensile tests and our new inflation method, for rubber rings. For the fresh carotid tissue, a good fit of the model on the data was obtained. However, a different set of parameters was obtained for the ultrasound and ring-inflation measurements. Application of the Holzapfel-Gasser-Ogden model gives parameter estimations that depend on the experimental boundary conditions. This needs further research. In future applications, heterogeneous properties, like in atherosclerotic plaque material, may be assessed as well, using vital staining techniques to distinguish different tissue components without affecting their mechanical behaviour. 13:45 15 mins PHASE-RECTIFIED SIGNAL AVERAGING FOR AUTOMATIC DETECTION OF QRS FRAGMENTATION Griet Goovaerts, Bert Vandenberk, Carolina Varon, Rik Willems, Sabine Van Huffel Abstract: QRS fragmentation is visible in the ECG signal as the presence of one or more deflections, notches or slurs in the QRS complex. The presence of QRS fragmentation is strongly related with the myocardial fibrosis or scarrings and has been associated with adverse outcome in patients. Since detection of fragmented QRS complexes is mainly done on a visual basis, its practical use is limited. We propose an automatic method to detect the QRS fragmentation based on Phase-Rectified Signal Averaging (PRSA). The method calculates the PRSA curve, approximates it with a linear fit and derives four parameters related to the slope and linear fit. Analysis of a dataset from the University Hospitals of Leuven with 268 patients shows that all four parameters are significantly different in fragmented channels compared to normal channels (p<0.001). Furthermore, Cohen’s d suggests that three out of four parameters have at least a large effect size (d > 0.8). 14:00 15 mins AN INNOVATIVE IMPULSE CROSSING TOOL FOR CHRONIC TOTAL OCCLUSIONS Aimée Sakes, Dimitra Dodou, Paul Breedveld Abstract: Chronic Total Occlusions (CTOs) are the most challenging physical barriers interventionists face during Percutaneous Coronary Interventions (PCIs). During PCI, a guidewire and deflated balloon catheter are fed from the femoral artery (groin) or radial artery (wrist) towards and through the CTO in the coronaries. Subsequently, the balloon catheter is inflated to reopen the artery, and a stent is placed to prevent collapse. The most common failure mode during PCIs of CTOs is the inability to cross the lesion with a guidewire, accounting for approximately 63% of the failure cases, mainly due to guidewire buckling [1]. The goal of this study was to explore the use of a dynamic impulse (J = ∫F ∙dt, with F being the impact force [N] and dt being the time interval for which the force acts [s]) to improve the chance of a successful crossing procedure. Applying an impulse on a CTO has several advantages over the static loading that is currently applied, including an increase in the buckling resistance of the guidewire, a decrease in displacement of the CTO and surrounding tissues during crossing, and a decrease in the penetration load of the CTO. A proof-of-principle prototype (Ø2 mm) of a CTO crossing tool was developed that generates translational momentum (p=m∙v, with m being the mass of the indenter [kg] and v being the velocity of the indenter [m/s]) and converts it to an impulse J during impact (see Fig. 1). To generate this dynamic impulse, the prototype included a distal spring-loaded indenter (with interchangeable tip shapes) and a novel compliant (re)load mechanism. The maximum indenter momentum and impact peak force were experimentally determined at 1.3 mNs (m = 0.39 grams, v = 3.4 m/s) and 20 N, respectively, which is well over the measured puncture force of real CTOs of 1.52 N [2]. Furthermore, the puncture performance was tested on three CTO models made out of different weight percentages of gelatin (23, 25, and 50 weight%) and calcium (50, 75, and 77 weight%) using six different tip shapes: stamp, spherical, wedge, pointed (Ø0.4 mm tip fillet), spherical with guidewire lumen (Ø0.4 mm), and ringed (1 mm lumen and Ø0.4 mm filleted edge). It was found that the number of punctures (efficacy) increased and the number of strikes for puncture (efficiency) decreased with increasing model calcification. For the brittle, most calcified, model the most efficient and effective tip shapes were the spherical (2.7 strikes on average), spherical with guidewire passage (2.7 strikes on average), and the ringed (2 strikes on average) tip shape. As a continuation of this proof-of-principle work, we are currently manufacturing a flexible and faster (Ø2 mm) version of this device, allowing for endovascular crossing of CTOs. Fig. 1. Proof-of-principle prototype. REFERENCES [1] G.W. Stone, N.J. Reifart, I. Moussa, A. Hoye, D.A. Cox, et al. “Percutaneous recanalization of chronically occluded coronary arteries: A consensus document: Part II”, Circulation, Vol. 112, pp. 2539-2537, 2005. [2] A. Thind, B. Strauss, A. Teitelbaum, R. Karshafian, et al. “A novel method for the measurement of proximal fibrous cap puncture force in chronic total occlusions: the effect of increasing age”, EuroIntervention, Vol. 6, pp. 997-1002, 2011. 14:15 15 mins EVALUATION OF NON-INTUITIVE EMG CONTROL INTERFACE AS COMMAND INPUT FOR A 2-D FITTS’S LAW STYLE TASK Stergios Verros, Arjen Bergsma, Edsko Hekman, Bart Verkerke, Bart Koopman Abstract: The most common form of muscular dystrophy in humans is Duchenne muscular dystrophy (DMD), affecting 1 in every 3,500 boys[1]. DMD causes progressive degeneration of muscles which leads to progressive loss of muscle strength[1]. The mean age of death was 20 years but due to improved health care practices and ventilation the life expectancy is increased to 25-30 years[2][3]. By increasing the life expectancy, the function of upper extremity becomes more important for giving DMD patients more independence to perform daily tasks[4]. The loss of muscle function not only affects the arms, but it also causes instability of trunk and head. Furthermore, assisting the arm function with an assistive device, like the A-gear[5], can cause extra instability of the trunk and loss of visual feedback of the arm. So, a trunk assistive device is essential to stabilize and support the trunk during arm movements whereas a head assistive device is essential to stabilize, support and provide visual control during arm movement. In this project we focus on the development of assistive devices that will stabilize and support the trunk and the head and on the development of control interfaces that can realize user’s intention of movement in a trunk assistive device. A useful control interface is the surface electromyography (sEMG) but intuitive mapping of sEMG from trunk muscles results in a poor performance due to body fat concentrated in the trunk area, ECG noise etc. This problem can be avoided by using non-intuitive mapping from other muscle areas in healthy subjects. In this study we compared the intuitive sEMG mapping from trunk muscles with the non-intuitive mapping of leg muscles under isometric conditions for both trunk and leg. We evaluated the two control interfaces using Fitts’s law in terms of throughput and path efficiency. Both control interfaces showed poor performance at the beginning (path efficiency: 54.9% for trunk, 61.3% for leg) but during the experiments, both control interfaces had a learning effect leading to better performance after 5 trials (76.6% for trunk, 77.5 for leg). In the throughput performance measure, non-intuitive mapping performed better whereas in the path efficiency both non-intuitive and intuitive control performed similar in the whole experiment. It can be concluded that with more training the non-intuitive mapping can perform better than intuitive control(p-value 0.026 between fourth and fifth task). Future work will include more control interfaces for evaluation such as joystick and Force control.