Medical Engineering & Physics

Editorial Board

2010-03-01

Motion analysis in delirium: A discrete approach in determining physical activity for the purpose of delirium motoric subtyping

Alan Godfrey, Richard Conway, Maeve Leonard, David Meagher, Gearóid M. Ólaighin — 2009-11-20

Abstract: The purpose of this study was to determine the use and feasibility of accelerometry-based monitoring and to examine a discrete multi-resolution signal analysis technique to determine motoric subtypes in patients with DSM-IV delirium. Forty consecutive patients receiving palliative care (23 male, 17 female, mean age±standard deviation 68.4±11.9 years) were assessed using 24-h accelerometer-based monitoring. The total amount of time spent per activity of sitting/lying, standing and stepping were calculated. This was achieved through the multilevel decomposition and reconstruction of the accelerometer signals by means of the discrete wavelet transform. Both the reconstructed approximations and details of the discrete transform were used for motoric subtyping. This was compared to a validated activity monitor for validation purposes. Demographic and clinical data per patient were also collected. Of the 40 patients selected for accelerometry, complete 24-h readings were available for 34 patients and analyses were confined to this group. Of the 34 patients included, 25 met criteria for DSM-IV delirium while 9 were non-delirious comparison subjects with equivalent medical diagnoses receiving treatment in the same setting. It was concluded that accelerometry-based measurement of a delirious cohort within a palliative setting is both a reliable and feasible method of continuous monitoring. Of the activities performed by the patients, periods of standing proved to be the most discriminatory in determining between each subtype.

Numerical and experimental study of blood flow through a patient-specific arteriovenous fistula used for hemodialysis

Zaher Kharboutly, Valerie Deplano, Eric Bertrand, Cecile Legallais — 2009-12-07

Abstract: Arteriovenous fistula (AVF) pathologies related to blood flow necessitate valid calculation tools for local velocity and wall shear stress determination to overcome the clinical diagnostic limits. To illustrate this issue, a reconstructed patient-specific AVF was investigated, using computational fluid dynamics (CFDs) and particle image velocimetry (PIV). The aim of this study was to validate the methodology from medical images to numerical simulations of an AVF by comparing numerical and experimental data. Two numerical grids were presented with a refinement difference of a factor of four. A mold of the same volume was created and mounted on an experimental bench with similar boundary conditions. The patient's acquired echo D006Fppler flow waveform was injected at the arterial inlet. Experimental and numerical velocity vector cartography qualitatively produced similar flow fields. Quantification with a point-to-point approach thoroughly investigated the velocity profiles using the mean difference between both results. The finest mesh generated CFD results with a mean percentage of the difference in velocity magnitude, taking the PIV as reference, did not exceed 10%. At specific zones, the coarse mesh required adaptive meshing to improve fitting with experimental data. Meshing refinement was necessary to improve velocity accuracy at wide diameters and wall shear stress at narrow diameters. Provided that these criteria were properly respected, we show through this difficult example the validity of using CFD to properly describe flow patterns in image-based reconstructed blood vessels.

Can an accelerometer enhance the utility of the Timed Up & Go Test when evaluating patients with Parkinson's disease?

2009-11-27

Abstract: Introduction: The Timed Up and Go (TUG) test is a widely used measure of mobility and fall risk in older adults and in Parkinson's disease (PD). We tested the hypothesis that body-fixed accelerometers can provide insight into TUG performance in PD patients.Methods: We examined 17 patients with PD (Hoehn and Yahr score: 2.7±0.7; ON state) and 15 age-matched healthy controls; mean ages were 66.8±5.9 years, 67.6±9.6 years, respectively. Subjects wore a 3D-accelerometer (ADXL330, Analog Devices) on the lower back while performing the TUG test. Sit-to-Stand and Stand-to-Sit times were extracted from the anterior–posterior (AP) signal. Parameters included Sit-to-Stand, Stand-to-Sit durations, amplitude range (Range) and slopes (Jerk). Acceleration median and standard deviation (SD) were also calculated.Results: Stopwatch-based TUG duration tended to be higher for the PD patients compared to the control group, although not significantly (p=0.08). In contrast, the TUG duration that was extracted from the acceleration signal was significantly (p<0.02) higher in the PD group compared to the control group. Many acceleration-parameters were also significantly different (p<0.05) between groups; most were not correlated with TUG duration.Conclusions: Accelerometer-derived parameters are sensitive to group differences, indicating that PD patients have poorer mobility during specific aspects of the TUG. In addition to test duration, these measures may serve as complementary and objective bio-markers of PD to augment the evaluation of disease progression and the response to therapeutic interventions.

A discriminant bispectrum feature for surface electromyogram signal classification

Xinpu Chen, Xiangyang Zhu, Dingguo Zhang — 2009-12-03

Abstract: This paper presents a discriminant bispectrum (DBS) feature extraction approach to surface electromyogram (sEMG) signal classification for prosthetic control. The proposed feature extraction method involves two steps: (1) the bispectrum matrix integration, and (2) the Fisher linear discriminant (FLD) projection. We compare DBS with other conventional features, such as autoregressive coefficients, root mean square, power spectral distribution and time domain statistics. First, the separability of the features is investigated by the visualization of feature distribution in the FLD subspace and quantitative measurement (Davies–Boulder clustering index). Then four linear and non-linear classifiers are used to evaluate the discriminative powers of the features in terms of classification accuracy (CA). The experimental results show that DBS has better performance than other features for identifying the motion patterns of sEMG signals, and the best CA result of DBS is 99.4%.

Analysis of unpredictable intra-QRS potentials in signal-averaged electrocardiograms using an autoregressive moving average prediction model

Chun-Cheng Lin — 2009-12-01

Abstract: Instead of extracting the abnormal intra-QRS potentials (AIQP) waveform, this study proposes the analysis of the unpredictable intra-QRS potentials (UIQP) based on an autoregressive moving average (ARMA) prediction model to detect the signals with sudden slope change within the QRS complex for the diagnosis of high-risk patients with ventricular tachycardia (VT). The UIQP is detected as the slope changes at slope discontinuities by the prediction error of the ARMA prediction model. Because of the linearity of the ARMA prediction model, the UIQP is also proportional to the amplitude of the QRS complex if the input QRS waves have the same shapes. Hence this study further defines the UIQP-to-QRS ratio to normalize the UIQP by the root-mean-square (RMS) value of the QRS complex. The study subjects were composed of 42 normal Taiwanese and 30 patients with sustained VT. The clinical results show that the UIQP-to-QRS ratios of the VT patients in leads X, Y and Z were significantly higher than those of the normal subjects. The logical combination of any 4 of the UIQP-to-QRS ratios and conventional time-domain parameters can increase the diagnosis performance of VT patients to 92.9% specificity, 93.3% sensitivity and 93.1% total prediction accuracy.

A parametric study of cylindrical pedicle screw design implications on the pullout performance using an experimentally validated finite-element model

Panagiotis E. Chatzistergos, Evangelos A. Magnissalis, Stavros K. Kourkoulis — 2009-11-30

Abstract: The present study aims to the design of a finite-element model simulating accurately the pullout behaviour of cylindrical pedicle screws and predicting their pullout force. Three commercial pedicle screws, subjected to pure pullout from synthetic bone, were studied experimentally. The results were used for the design, calibration and validation of a finite-element model. Special attention was paid to the accurate simulation of the failure inside the host material under shear. For this purpose, a bilinear cohesive zone material model was adopted, controlling the mode-II debonding of neighbouring elements in the vicinity of the screw. Comparison between experimental and numerical results proved that the implementation of this approach can significantly enhance the accuracy of the numerical simulation of a screw's mechanical behaviour under pure pullout loads. The numerical model was used for the parametric study of various factors affecting the pullout performance of a cylindrical pedicle screw. It was concluded that the major parameter influencing the pullout force is the outer radius (increasing its value by 36% increases the pullout force by 34%). The influence of the purchase length of the screw is of similar quantitative nature. The respective dependence on the thread inclination, depth and pitch was significantly weaker.

Compressive and tensile properties of articular cartilage in axial loading are modulated differently by osmotic environment

Rami K. Korhonen, Jukka S. Jurvelin — 2009-12-02

Abstract: Aims of the present study were to test the hypotheses that (1) the compressive properties of articular cartilage are affected more by changes in the medium ionic concentration than the tensile properties, (2) collagen network controls the compression–tension nonlinearity of articular cartilage, and (3) proteoglycan (PG) and collagen contents are primary determinants of the compressive and tensile properties of cartilage, respectively. These hypotheses were experimentally tested by axial compressive and tensile tests (perpendicular to the cartilage surface) of bovine articular cartilage samples immersed in 0.005M (n=6), 0.15M (n=12) and 1.0M (n=6) saline solutions. Compressive and tensile behaviour was analyzed by a nonlinear fibril-reinforced poroelastic model. Tissue PG and collagen contents were measured using Fourier transform infrared imaging spectroscopy (FT-IRIS). The compressive modulus of cartilage varied significantly (n=6, p<0.05) as the medium concentration changed. The tensile modulus changed significantly only as the medium concentration was reduced from 0.15 to 0.005M (n=6, p<0.05). The fibril-reinforced poroelastic model with stiff, nonlinear collagen fibrils predicted the experimentally measured compression–tension nonlinearity of cartilage. Tissue PG and collagen contents accounted for the compressive and tensile properties of cartilage.

Association of cardiac autonomic neuropathy with alteration of sympatho-vagal balance through heart rate variability analysis

Ahsan H. Khandoker, Herbert F. Jelinek, Toshio Moritani, Marimuthu Palaniswami — 2009-12-09

Abstract: Early sub-clinical assessment of severity of cardiac autonomic neuropathy (CAN) and intervention are of prime importance for risk stratification and early treatment in preventing sudden death due to silent myocardial infarction. The Ewing battery is currently the diagnostic tool of choice but is unable to detect sub-clinical disease and requires patient cooperation. Time and frequency domain analysis have several shortcomings including sensitivity to recording length, respiratory activity and non-stationarities in the ECG signal. An important step forward is to have a non-invasive method of detecting CAN that is robust against these shortcomings and has a higher sensitivity for the presence of both sub-clinical and overt clinical disease. This study presents a novel parameter, tone–entropy (T–E) that analyses heart rate variability (HRV) of 20min lead II ECG recordings. Tone (T) represents sympatho-vagal balance and entropy (E) the autonomic regularity activity. Thirteen normal subjects without (CAN−) and 21 with CAN (CAN+) participated in this study. Among 21 CAN+ subjects, 13 are early CAN+ (eCAN+), eight are definite CAN+ (dCAN+) according to autonomic nervous system function tests as described by Ewing. The results showed that tone was higher and the entropy was lower in the dCAN+ group (T=−0.033 to −0.010 and E=1.73–2.24) compared with the eCAN+ (T=−0.0927 to −0.0311 and E=2.0–2.65) and normal (T=−0.128 to −0.0635 and E=2.64–3.15) group. The research verified that T–E is a suitable method to determine the presence of CAN that correctly identifies experimentally induced changes in cardiac function akin to parasympathetic and sympathetic dysfunction and differentiates between stages in CAN disease progression identified using the Ewing battery.

Auditory evoked potentials for monitoring during anaesthesia: A study of data quality

S.V. Notley, S.L. Bell, D.C. Smith — 2009-12-16

Abstract: The auditory evoked potential termed the middle latency response (MLR) has been suggested as an indicator of adequacy of anaesthesia during surgery. However, the response is small and must be extracted from high levels of background noise. A key consideration in using the MLR for clinical monitoring is whether data quality is sufficient to detect small changes. The aim of this study was to investigate the quality of the MLR recorded during anaesthesia, as a rigorous analysis of data quality is lacking in many studies. MLR recordings from patients sedated in intensive care after cardiac surgery were compared to recordings from a reference group of young volunteers with normal hearing. Data quality was measured with the Fsp parameter. A bootstrap analysis was used to measure statistical response presence and to detect within-subject changes during clinical anaesthesia. Noise levels were high in the normative group probably due to myogenic and EEG activity. With 5Hz click stimulation, MLR presence in the normative group was below 30%. Response presence improved using stimulation paradigms with chirps or maximum length sequences and reached 100% with a combination of maximum length sequences and chirps. Fsp values generally improved during anaesthesia as noise levels reduced and MLR presence was 100% for MLS click stimulation. Changes in the MLR amplitude with propofol infusion rate were small. Some within-subject changes in MLR amplitude were detected using the bootstrap analysis, but 100% detection was not possible. Conclusion: Obtaining good quality MLR data in awake subjects is challenging. Data quality improves during clinical anaesthesia and with advanced stimulation methods, but reliable detection of changes in the MLR for clinical monitoring remains a challenge.

Cardiac flow component analysis

2009-12-21

Abstract: In a chamber of the heart, large-scale vortices are shown to exist as the result of the dynamic blood flow and unique morphological changes of the chamber wall. As the cardiovascular flow varies over a cardiac cycle, there is a need for a robust quantification method to analyze its vorticity and circulation. We attempt to measure vortex characteristics by means of two-dimensional vorticity maps and vortex circulation. First, we develop vortex component analysis by segmenting the vortices using an data clustering algorithm before histograms of their vorticity distribution are generated. The next stage is to generate the statistics of the vorticity maps for each phase of the cardiac cycle to allow analysis of the flow. This is followed by evaluating the circulation of each segmented vortex. The proposed approach is dedicated to examining vortices within the human heart chamber. The vorticity field can indicate the strength and number of large-scale vortices in the chamber. We provide the results of the flow analysis after vorticity map segmentation and the statistical properties that characterize the vorticity components. The success of the cardiac measurement and analysis is illustrated by a case study of the right atrium. Our investigation shows that it is possible to utilize a data clustering algorithm to segment vortices after vorticity mapping, and that the vorticity and circulation analysis of a chamber vorticity can provide new insights into the blood flow within the cardiovascular structure.

Wall shear stress variations in a 90-degree bifurcation in 3D pulsating flows

Philip Evegren, Laszlo Fuchs, Johan Revstedt — 2009-12-25

Abstract: The exact role of fluid mechanics in the patho-physiological process of atherosclerosis has been a research topic over many years, yet without clear conclusive result. One has observed that morphological manifestations of the disease are found at some well-defined locations: certain vessel bifurcations and in curvatures. The flow in these regions is characterized by unsteadiness and often separation. Currently there are no complete theories that can explain the process since the different components in the process are not fully understood. Here we carry out detailed computations of the unsteady flow in an arterial segment typical to location of early appearance of arterial lesions. We study the wall shear stress (WSS) field variations near a junction with the purpose of identifying fluid-mechanical parameters that can be related to sites of atheroslcerosis. The results show that regions associated with atherosclerosis experience highly elevated temporal- and spatial-derivatives of the WSS, also at less commonly known locations. Thus, large derivatives in time and space do not seem unique for the most common areas of atherosclerosis. Differences in WSS character between these locations are identified as differences in the time period of back flow as well as differences in the magnitude of the WSS derivatives. The data is presented in a way that facilitates understanding of the variations in WSS.

Biomechanical comparisons of different posterior instrumentation constructs after two-level ALIF: A finite element study

Chang-Yuan Fan, Ching-Chi Hsu, Ching-Kong Chao, Shang-Chih Lin, Kuo-Hua Chao — 2010-01-11

Abstract: Anterior lumbar interbody fusion (ALIF) with cylindrical cages and supplemental posterior fixation has been widely used for internal disc derangement. However, most researchers have focused on single-level ALIF. Therefore, the biomechanical performance of various fixation constructs after two-level ALIF is not well characterized. This research used three-dimensional finite element models (FEM) with a nonlinear contact analysis to evaluate the initial biomechanical behavior of five types of fixation devices after two-level ALIF (L3/L4, L4/L5) under six loading conditions. These fixation constructs included a three-level pedicle screw and rod, a two-level translaminar facet screw, a two-level transfacet pedicle screw, a bisegmental pedicle screw and rod, and a bisegmental pedicle screw and rod with cross-linking. The FEM's developed in this study demonstrate that, compared to the other four types of posterior fixation constructs analyzed, the three-level pedicle screw and rod provide the best biomechanical stability. Both two-level facet screw fixation constructs showed unfavorable loading in lateral bending. For the construct of the three-level pedicle screw and rod, the middle-segment pedicle screw should not be omitted even though a cross-link is used. The two-level ALIF models with cages and posterior fixation constructs that we developed can be used to evaluate the initial biomechanical performance of a wide variety of posterior fixation devices prior to surgery.

Dynamic finite element analysis of the aortic root from MRI-derived parameters

2010-01-11

Abstract: An understanding of aortic root biomechanics is pivotal for the optimisation of surgical procedures aimed at restoring normal root function in pathological subjects. For this purpose, computational models can provide important information, as long as they realistically capture the main anatomical and functional features of the aortic root.Here we present a novel and realistic finite element (FE) model of the physiological aortic root, which simulates its function during the entire cardiac cycle. Its geometry is based on magnetic resonance imaging (MRI) data obtained from 10 healthy subjects and accounts for the geometrical differences between the leaflet-sinus units. Morphological realism is combined with the modelling of the leaflets’ non-linear and anisotropic mechanical response, in conjunction with dynamic boundary conditions.The results show that anatomical differences between leaflet-sinus units cause differences in stress and strain patterns. These are notably higher for the leaflets and smaller for the sinuses. For the maximum transvalvular pressure value, maximum principal stresses on the leaflets are equal to 759, 613 and 603kPa on the non-coronary, right and left leaflet, respectively. For the maximum aortic pressure, average maximum principal stresses values are equal to 118, 112 and 111kPa on the right, non-coronary and left sinus, respectively.Although liable of further improvements, the model seems to reliably reproduce the behaviour of the real aortic root: the model's leaflet stretches, leaflet coaptation lengths and commissure motions, as well as the timings of aortic leaflet closures and openings, all matched with the experimental findings reported in the literature.

Biomechanical properties of the costovertebral joint

Sonia Duprey, Damien Subit, Hervé Guillemot, Richard W. Kent — 2009-12-25

Abstract: Proper modeling of the human trunk requires a quantitative assessment of the stiffness of the costovertebral joints.Twelve samples (adjacent thoracic vertebrae and one rib segment) were harvested from three subjects. The ribs were loaded in the cranial–caudal direction, the ventral–dorsal direction and in torsion around the cervical rib axis. The force applied to and the displacement of the loading point on the rib were measured and used to determine the moment–angle responses. Characteristic average curves and boundary curves containing the dataset were developed.The torsion response presented a range of motion—defined as the change in the angle for an applied moment varying from −0.1 to 0.1Nm—of 16.9±6.8° which is more than three times the range in cranial–caudal flexion and five times the range in ventral–dorsal flexion. Statistical tests showed a significant difference between these ranges of motion. Significant inter-subject variability was observed for the cranial–caudal flexion (p<0.05) while no intra-subject variability appeared. The characteristic moment–angle responses of the joints were well represented by third order polynomials (R2>0.9).This study expands and supplements the limited data available in the literature. Furthermore, it provides biomechanical data (closed-form moment–angle functions) that can be directly integrated into spine-ribcage models.

Calendar

2010-03-01