| 1. |
- Sörnmo, Leif, et al.
(författare)
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Non-invasiv analys av förmaksflimmer
- 2006
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Ingår i: Elektrokardiologi : klinik och teknik. ; , s. 287-309
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 2. |
- Sörnmo, Leif, et al.
(författare)
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Signabehandling av EKG
- 2006
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Ingår i: Elektrokardiologi : klinik och teknik. ; , s. 17-40
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 3. |
- Ahlström, Christer, 1977-
(författare)
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Processing of the Phonocardiographic Signal : methods for the intelligent stethoscope
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Phonocardiographic signals contain bioacoustic information reflecting the operation of the heart. Normally there are two heart sounds, and additional sounds indicate disease. If a third heart sound is present it could be a sign of heart failure whereas a murmur indicates defective valves or an orifice in the septal wall. The primary aim of this thesis is to use signal processing tools to improve the diagnostic value of this information. More specifically, three different methods have been developed:• A nonlinear change detection method has been applied to automatically detect heart sounds. The first and the second heart sounds can be found using recurrence times of the first kind while the third heart sound can be found using recurrence times of the second kind. Most third heart sound occurrences were detected (98 %), but the amount of false extra detections was rather high (7 % of the heart cycles).• Heart sounds obscure the interpretation of lung sounds. A new method based on nonlinear prediction has been developed to remove this undesired disturbance. High similarity was obtained when comparing actual lung sounds with lung sounds after removal of heart sounds.• Analysis methods such as Shannon energy, wavelets and recurrence quantification analysis were used to extract information from the phonocardiographic signal. The most prominent features, determined by a feature selection method, were used to create a new feature set for heart murmur classification. The classification result was 86 % when separating patients with aortic stenosis, mitral insufficiency and physiological murmurs.The derived methods give reasonable results, and they all provide a step forward in the quest for an intelligent stethoscope, a universal phonocardiography tool able to enhance auscultation by improving sound quality, emphasizing abnormal events in the heart cycle and distinguishing different heart murmurs.
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| 4. |
- Alcaraz Martinez, Raul, et al.
(författare)
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Application of frequency and sample entropy to discriminate long-term recordings of paroxysmal and persistent atrial fibrillation
- 2010
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Ingår i: Engineering in Medicine and Biology Society (EMBC), 2010 Annual International Conference of the IEEE. - 1557-170X. - 9781424441235 ; , s. 4558-4561
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Konferensbidrag (refereegranskat)abstract
- Atrial fibrillation (AF) is the most common arrhythmia in clinical practice. At an early stage of the disease, AF may terminate spontaneously and is then referred to as paroxysmal AF. On the other hand, when external intervention is required for the arrhythmia to terminate, it is referred to as persistent AF. In this work, a method to discriminate between paroxysmal and persistent AF in the long-term ECGs is presented. The dominant frequency as well as the organization of the atrial activity are employed to characterize AF. The dominant atrial frequency (DAF) is estimated using hidden Markov model based frequency tracking, and organization is estimated by the sample entropy of the main atrial wave (MAW) and the first two harmonics, respectively. Long-term variations in DAF and organization from 50 ECG recordings were evaluated, showing that episodes of paroxysmal AF were consistently associated with lower DAF and organization of the MAW and the harmonics, than was persistent AF. Discrimination of paroxysmal and persistent AF resulted in classification rates of 84.1±26.1%, thus suggesting that it possible to discriminate between paroxysmal and persistent AF in patients without previously known AF history.
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| 5. |
- Alcaraz Martinez, Raul, et al.
(författare)
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Classification of paroxysmal and persistent atrial fibrillation in ambulatory ECG recordings
- 2011
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Ingår i: IEEE Transactions on Biomedical Engineering. - 1558-2531. ; 58, s. 1441-1449
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Tidskriftsartikel (refereegranskat)abstract
- The problem of classifying short atrial fibrillatory segments in ambulatory ECG recordings as being either paroxysmal or persistent is addressed by investigating a robust approach to signal characterization. The method comprises preprocessing, estimation of the dominant atrial frequency for the purpose of controlling the subbands of a filter bank, and computation of the relative subband (harmonics) energy and the subband sample entropy. Using minimum-error-rate classification of different feature vectors, a dataset consisting of 24-h ambulatory recordings from 50 subjects with either paroxysmal (26) or persistent (24) atrial fibrillation (AF) was analyzed on a 10-s segment basis; a total of 212196 segments were classified. The best performance in terms of area under the receiver operating characteristic curve was obtained for a feature vector defined by the subband sample entropy of the dominant atrial frequency and the relative harmonics energy, resulting in a value of 0.923, whereas that of the dominant atrial frequency was equal to 0.826. It is concluded that paroxysmal and persistent AF can be discriminated from short segments with good accuracy at any time of an ambulatory recording.
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| 6. |
- Alcaraz, Raul, et al.
(författare)
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Organization tracking of long-term atrial fibrillation recordings: differences between paroxysmal and persistent episodes
- 2009
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Ingår i: [Host publication title missing]. ; 36, s. 509-512
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Konferensbidrag (refereegranskat)abstract
- In this work, a method for non-invasive assessment of AF organization has been applied to discriminating between paroxysmal and long-term persistent AF episodes. Following extraction of the atrial activity (AA) signal, the dominant atrial frequency (DAF) of the AA was computed based on a hidden Markov model. Finally, the main atrial wave (MAW) was obtained by bandpass filtering centered on the DAF, thus providing a time series suitable for AF organization estimation with sample entropy (SampEn). The performance of the method was evaluated on 24-h Holter recordings with long-term changes in AF organization. The results showed that episodes of paroxysmal AF (0.06930.0147) were consistently associated with lower SampEn than episodes with persistent AF (0.10560.0146). Moreover, 94.2% of 1-min segments with paroxysmal AF and 88.6% of 1-min segments with persistent AF could be correctly classified based on Samp- En information, thus making it possible to classify longterm recordings of patients without AF history.
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| 7. |
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| 8. |
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| 9. |
- Aunes-Jansson, Maria, et al.
(författare)
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Decrease of the atrial fibrillatory rate, increased organization of the atrial rhythm and termination of atrial fibrillation by AZD7009
- 2013
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Ingår i: Journal of Electrocardiology. - : Elsevier BV. - 1532-8430 .- 0022-0736. ; 46:1, s. 29-35
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Tidskriftsartikel (refereegranskat)abstract
- Background: The atrial fibrillatory rate (APR), on AZD7009 as compared to placebo, was investigated as a potential biomarker for electrophysiological effect in early antiarrhythmic drug development. Methods: Patients with permanent AF received infusions of AZD7009 and placebo in an exploratory two-way, single-blind, randomized cross-over study. The ECG was continuously recorded, and following QRST cancellation the APR, its standard deviation (SD), the exponential decay and the atrial electrogram amplitude were determined as 3-min averages. Results: The mean APR rapidly decreased by 43% from baseline (394 +/- 38 to 225 +/- 61 fibrillations/min, p = 0.0003) on AZD7009, but not on placebo. The SD of the AFR and the exponential decay decreased in parallel. In 2 of 8 patients, termination of AF occurred after the APR had decreased by 58% and 53%, respectively. Conclusions: The APR may potentially serve as a biomarker of electrophysiological effects in early evaluation of rhythm control agents. (C) 2013 Elsevier Inc. All rights reserved.
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| 10. |
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| 11. |
- Axmon, Joakim, et al.
(författare)
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A signal model adapted ESPRIT algorithm for joint estimation of spatial and temporal parameters in vibrational analysis of cylinders
- 2002
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Ingår i: 2002 IEEE Sensor Array and Multichannel Signal Processing Workshop Proceedings (Cat. No.02EX593). - 0780375513 ; , s. 360-364
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Konferensbidrag (refereegranskat)abstract
- This paper deals with the joint estimation of spatial mode shapes and temporal frequencies from transient vibrations measured by a uniform circular array of sensors encircling a cross-section of a cylinder. The geometry allows each 2D mode to be interpreted as two real planar waves impinging from mirrored directions with respect to the broadside of a uniform linear array. Algorithms for joint estimation tend, in the presence of noise, to produce signal roots that express planar waves impinging from only approximately mirrored directions. A recently published 2-D ESPRIT algorithm is modified to take advantage of the geometry, and to force the estimated planar waves to impinge from exactly mirrored directions. This reduces the estimation errors; the main advantage is more easily interpreted results. Memory requirements and complexity are significantly lower for the modified algorithm, since the decomposition of a large data matrix may be broken down into several small ones
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| 12. |
- Axmon, Joakim, et al.
(författare)
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Experimental study on the possibility of detecting internal decay in standing Picea abies by blind impact response analysis
- 2004
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Ingår i: Forestry. - : Oxford University Press (OUP). - 1464-3626 .- 0015-752X. ; 77:3, s. 179-192
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Tidskriftsartikel (refereegranskat)abstract
- This paper considers detection of internal decay in standing trees of species Picea abies (L.) Karst. The novel approach is based on two-dimensional spatiotemporal modal analysis of a cross-section which is excited by the hand-made impact of a hammer. An array of accelerometers is distributed around the cross-section, and the resulting impact response is analysed. The temporal frequency for a special spatial mode-shape is used for comparisons on a tree-to-tree basis. The mechanical properties of wood are inherently variable as they are for most materials of biological origin. This leads to a scatter of the analysed parameters that hinders detection of decay based on the temporal frequencies alone. Using regression analysis, we show that by incorporating the additional information on a surface wave propagation velocity, the scatter of sound trees is significantly reduced. The performance of a detector rule which incorporates the frequency and the surface wave propagation velocity is investigated and found to be better than performance reported for visual tree examination. The analyses are based on the impact responses from 94 standing trees, with 66 sound and 28 in various stages of decay. The proposed technique is yet to be considered an experimental tool. Further research, e.g. on how the mechanical properties are influenced by various environmental factors, is needed before the technique can be applied operationally.
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| 13. |
- Axmon, Joakim, et al.
(författare)
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Partial Forward-Backward Averaging for Enhanced Frequency Estimation of Real X-texture Modes
- 2005
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Ingår i: IEEE Transactions on Signal Processing. - 1053-587X. ; 53:7, s. 2550-2562
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, enhancement of the signal root estimation of a particular kind of real-valued two-dimensional (2-D) sinusoidal modes is considered. To its constitution, each mode corresponds to the superposition of two real-valued plane waves in a particular symmetry. The concept of partial forward-backward averaging, which is applicable for modes that are undamped in at least one dimension, is introduced as a means for improving the signal subspace estimate from which the signal roots are estimated. The consequences of real-valued signals for the signal root estimates are discussed in detail, and it is shown that by applying partial forward-backward averaging, the mean square errors of the estimates, and the breakdown threshold signal-to-noise ratio (SNR), are significantly reduced, compared with forward-only or conventional forward-backward (when applicable) usage of the sampled signals. The practical implication is highlighted by applying the proposed technique to modal analysis of multichannel impact responses from a tree trunk.
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| 14. |
- Axmon, Joakim, et al.
(författare)
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Partial modal analysis for health assessment of living trees
- 2001
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Ingår i: Asia-Pacific conference on non-destructive testing (APCNDT).
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Konferensbidrag (refereegranskat)abstract
- Rot in living trees cause substantial losses for the forestry industry. The common practice when evaluating forest stands for, e.g., purchase, is assessment based on visual signs. In this paper a new non-destructive assessment method based on the impact excitation method is proposed. The trunk of a living tree is excited by the impact of a hammer, and the vibrations are measured by accelerometers. Resonance frequencies, circumferential mode shapes and propagation velocity of a surface wave are analysed. A function describing the expected frequency for a sound tree is derived, and used in a detector whose performance is evaluated for 93 trees of species Norway spruce. The partial mode shape is used to ensure that the corresponding resonance frequencies are compared to each other. It is found that the detector is successful and outperforms assessments by skilled experts in forestry.
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| 15. |
- Bailon, R, et al.
(författare)
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A robust method for ECG-Based estimation of the respiratory frequency during stress testing
- 2006
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Ingår i: IEEE Transactions on Biomedical Engineering. - 1558-2531. ; 53:7, s. 1273-1285
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Tidskriftsartikel (refereegranskat)abstract
- A robust method is presented for electrocardiogram (ECG)-based estimation of the respiratory frequency during stress testing. Such ECGs contain highly nonstationary noise and exhibit changes in QRS morphology which, when combined with the dynamic nature of the respiratory frequency, make most existing methods break down. The present method exploits the oscillatory pattern of the rotation angles of the heart's electrical axis as induced by respiration. The series of rotation angles, obtained from least-squares loop alignment, is subject to power spectral analysis and estimation of the respiratory frequency. Robust techniques are introduced to handle the nonstationary properties of exercise ECGs. The method is evaluated by means of both simulated signals, and ECG/airflow signals recorded from 14 volunteers and 20 patients during stress testing. The resulting respiratory frequency estimation error is, for simulated signals, equal to 0.5% +/- 0.2%, mean SD (0.002 +/- 0.001 Hz), whereas the error between respiratory frequencies of the ECG-derived method and the airflow signals is 5.9 % +/- 4 % (0.022 +/- 0.016 Hz). The results suggest that the method is highly suitable for analysis of noisy ECG signals recorded during stress testing.
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| 16. |
- Bailon, Raquel, et al.
(författare)
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Analysis of heart rate variability during exercise stress testing using respiratory information
- 2010
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Ingår i: Biomedical Signal Processing and Control. - : Elsevier BV. - 1746-8094. ; 5:4, s. 299-310
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a novel method for the analysis of heart rate variability (HRV) during exercise stress testing enhanced with respiratory information. The instantaneous frequency and power of the low frequency (LF) and high frequency (HF) bands of the HRV are estimated by parametric decomposition of the instantaneous autocorrelation function (ACF) as a sum of damped sinusoids. The instantaneous ACF is first windowed and filtered to reduce the cross terms. The inclusion of respiratory information is proposed at different stages of the analysis, namely, the design of the filter applied to the instantaneous ACF, the parametric decomposition, and the definition of a dynamic HF band. The performance of the method is evaluated on simulated data as well as on a stress testing database. The simulation results show that the inclusion of respiratory information reduces the estimation error of the amplitude of the HF component from 3.5% to 2.4% in mean and related SD from 3.0% to 1.7% when a tuned time smoothing window is used at an SNR of 15 dB. Results from the stress testing database show that information on respiratory frequency produces HF power estimates which closely resemble those from the simulations which exhibited lower SD. The mean SD of these estimates with respect to their mean trends is reduced by 84% (from 0.74 x 10(-3) s(-2) to 0.12 x 10(-3) s(-2)). The analysis of HRV in the stress testing database reveals a significant decrease in the power of both the LF and HF components around peak stress. (C) 2010 Elsevier Ltd. All rights reserved.
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| 17. |
- Bailon, R., et al.
(författare)
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Analysis of heart rate variability using time-varying frequency bands based on the respiratory frequency
- 2007
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Ingår i: Proc. IEEE Conf. Eng. Med. Biol.
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Konferensbidrag (refereegranskat)abstract
- In this paper a methodological approach for the analysis of nonstationary heart rate variability (HRV) signals using time-varying frequency bands based on respiratory frequency is presented. Spectral analysis of HRV is accomplished by means of the Smoothed Pseudo Wigner Ville distribution. Different approaches to the definition of the low frequency (LF) and high frequency (HF) bands are considered which involve respiratory information, derived either from a respiratory signal or from the ECG itself. Results are presented which derive from recordings acquired during stress testing and induced emotion experiments.
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| 18. |
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| 19. |
- Behjat, Hamid, et al.
(författare)
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Anatomically-adapted Graph Wavelets for Improved Group-level fMRI Activation Mapping
- 2015
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Ingår i: NeuroImage. - : Elsevier BV. - 1095-9572 .- 1053-8119. ; 123:Online 07 June 2015, s. 185-199
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Tidskriftsartikel (refereegranskat)abstract
- A graph based framework for fMRI brain activation mapping is presented. The approach exploits the spectral graph wavelet transform (SGWT) for the purpose of defining an advanced multi-resolutional spatial transformation for fMRI data. The framework extends wavelet based SPM (WSPM), which is an alternative to the conventional approach of statistical parametric mapping (SPM), and is developed specifically for group-level analysis. We present a novel procedure for constructing brain graphs, with subgraphs that separately encode the structural connectivity of the cerebral and cerebellar grey matter (GM), and address the inter-subject GM variability by the use of template GM representations. Graph wavelets tailored to the convoluted boundaries of GM are then constructed as a means to implement a GM-based spatial transformation on fMRI data. The proposed approach is evaluated using real as well as semi-synthetic multi-subject data. Compared to SPM and WSPM using classical wavelets, the proposed approach shows superior type-I error control. The results on real data suggest a higher detection sensitivity as well as the capability to capture subtle, connected patterns of brain activity.
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| 20. |
- Behjat, Hamid, et al.
(författare)
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Canonical cerebellar graph wavelets and their application to fMRI activation mapping
- 2014
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Ingår i: [Host publication title missing]. - 1557-170X. ; , s. 1039-1042
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Konferensbidrag (refereegranskat)abstract
- Wavelet-based statistical parametric mapping (WSPM) is an extension of the classical approach in fMRI activation mapping that combines wavelet processing with voxel-wise statistical testing. We recently showed how WSPM, using graph wavelets tailored to the full gray-matter (GM) structure of each individual’s brain, can improve brain activity detection compared to using the classical wavelets that are only suited for the Euclidian grid. However, in order to perform analysis on a subject-invariant graph, canonical graph wavelets should be designed in normalized brain space. We here introduce an approach to define a fixed template graph of the cerebellum, an essential component of the brain, using the SUIT cerebellar template. We construct a corresponding set of canonical cerebellar graph wavelets, and adopt them in the analysis of both synthetic and real data. Compared to classical SPM, WSPM using cerebellar graph wavelets shows superior type-I error control, an empirical higher sensitivity on real data, as well as the potential to capture subtle patterns of cerebellar activity.
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| 21. |
- Behjat, Hamid, et al.
(författare)
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fMRI activation mapping using wavelet-based SPM (WSPM) integrated with gray-matter graphs
- 2014
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Konferensbidrag (refereegranskat)abstract
- In many fMRI task-evoked studies, localized brain activity can be detected by GLM fitting and statistical hypothesis testing. Statistical parametric mapping (SPM) is the classical method that requires Gaussian pre-smoothing of the data. Instead, the wavelet transform provides a compact representation of activation patterns. Wavelet based SPM (WSPM) is an extension of SPM that combines wavelet processing with voxel-wise statistical testing. However, classical wavelets used in WSPM are designed for regular Euclidean grids and thus not adapted to the convoluted nature of the cerebral cortex. We recently showed how WSPM using graph wavelets tailored to the gray-matter structure of the cortex can improve detection of brain activity in single-subject studies. Here we extend this approach to group-level analysis by modifying the design of the brain graph.
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| 22. |
- Behjat, Hamid, et al.
(författare)
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Signal-Adapted Tight Frames on Graphs
- 2016
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Ingår i: IEEE Transactions on Signal Processing. - 1053-587X. ; 64:22, s. 6017-6029
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Tidskriftsartikel (refereegranskat)abstract
- The analysis of signals on complex topologies modeled by graphs is a topic of increasing importance. Decompositions play a crucial role in the representation and processing of such information. Here, we propose a new tight frame design that is adapted to a class of signals on a graph. The construction starts from a prototype Meyer-type system of kernels with uniform subbands. The ensemble energy spectral density is then defined for a given set of signals defined on the graph. The prototype design is then warped such that the resulting subbands capture the same amount of energy for the signal class. This approach accounts at the same time for graph topology and signal features. The proposed frames are constructed for three different graph signal sets and are compared with non-signal-adapted frames. Vertex localization of a set of resulting atoms is studied. The frames are then used to decompose a set of real graph signals and are also used in a setting of signal denoising. The results illustrate the superiority of the designed signal-adapted frames, over frames blind to signal characteristics, in representing data and in denoising.
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| 23. |
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| 24. |
- Bennhagen, Rolf, et al.
(författare)
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High-frequency components in ECG analysed in guinea-pig Langendorf preparations
- 2001
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Ingår i: Clinical Physiology. - : Wiley. - 1365-2281 .- 0144-5979. ; 21:5, s. 576-583
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Tidskriftsartikel (refereegranskat)abstract
- High-frequency components in ECG during global ischaemia were studied in isolated guinea-pig hearts perfused ad modum Langendorf. Electrocardiograph recordings were carried out from the epicardial surface both in normo- and low-flow perfusion. After bandpass filtering (5-500 Hz), signal-averaging, was undertaken. The high-frequency components either increased or decreased after low-flow perfusion was instituted. Root-mean-square voltage (RMS) of the depolarization signal correlated poorly with the signal amplitude, but highly with the first and second derivative, i.e. the velocity and the acceleration of the signal. It is concluded that high-frequency components are not pathological phenomena per se, but reflect the shape of the original electrocardiographic signal.
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| 25. |
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