Atila Yilmaz

Design and implementation of a digital ambulatory ECG recorder based on flash MultiMediaCard memory

Long time digital data recordings in ambulatory devices have become possible today with the presence of high capacity flash memories. The aim of this study in general is to highlight some improvements on the performance of ambulatory ECG monitoring system recording clinically significant digital ECG signals on a flash card over a long period of time. The system is also compatible with the commercially available general purpose flash card readers and commonly used operating systems and softwares to access ECG data stored on the card. The device is able to record ECG waveforms on flash MultiMediaCard over one day through up to 3-channels

Comparative analysis on wavelet-based detection of finite duration low-amplitude signals related to ventricular late potentials.

Ventricular late potentials (VLPs) are considered as a noninvasive marker of patients with myocardial infarction, who are prone to the development of ventricular tachycardia. This paper investigates the effects of variations in physical properties of myocardial infarcts in terms of their effects on the parametric variations in VLP analysis. A sufficiently large set of signals underlining the behavior of physical parameters was employed to represent the effect of physical size, position, orientation and type of infarct. The approximated signals are variations from real electrocardiography signals by adding potentials representing late potentials based on duration, frequency, amplitude and position. The aim is not to exactly model VLP but rather to generate an approximate set of signals to examine the performance of the standard methods for different possibilities in infarct dynamics. We investigate some of the detection approaches together with their related assumptions, and try to pinpoint the drawbacks and inaccuracies of these methods and also their assumptions. The three widely accepted criteria--QRS duration, root-mean-square and duration of the signal at the end of QRS for VLP detection--were used in the investigation. Results from the application of these parameters to the set of signals are presented. In addition we investigate the physical nature of an infarct and list a number of possible reasons that might be the cause of a low success rate for the detection of additive potentials. To improve the performance of the common methods, two more wavelet transform parameters are added to those of the standard methods. The method derived from this analysis is presented as an alternative means for the detection of late signals named as delayed potentials, a more general class that includes VLP as a subset.

Elliptic cylinder geometry for distinguishability analysis in impedance tomography

Electrical impedance tomography (EIT) is a technique that computes the cross-sectional impedance distribution within the body by using current and voltage measurements made on the body surface. It has been reported that the image reconstruction is distorted considerably when the boundary shape is considered to be more elliptical than circular as a more realistic shape for the measurement boundary. This paper describes an alternative framework for determining the distinguishability region with a finite measurement precision for different conductivity distributions in a body modeled by elliptic cylinder geometry. The distinguishable regions are compared in terms of modeling error for predefined inhomogeneities with elliptical and circular approaches for a noncircular measurement boundary at the body surface. Since most objects investigated by EIT are noncircular in shape, the analytical solution for the forward problem for the elliptical cross section approach is shown to be useful in order to reach a better assessment of the distinguishability region defined in a noncircular boundary. This paper is concentrated on centered elliptic inhomogeneity in the elliptical boundary and an analytic solution for this type of forward problem. The distinguishability performance of elliptical cross section with cosine injected current patterns is examined for different parameters of elliptical geometry.

A hip simulator hardware for designing and testing knee prosthesis

In this study, a hip simulator mechanism have been presented for designing and testing knee prosthesis. The main reason to design this simulator mechanism is to generate the periodic move of above knee activity by using a basic control unit and electromechanic hardware. This platform will help us to assess the design outcomes of the prosthesis for specifically above knee amputees. The mechanic basis of this simulator has been obtained from the modified simple card drill machine. The system includes two alternative current motors which provide two dimensional movements for gait simulation, a rotary encoder and a pulse counter for each motor control unit, a reductor used for decreasing the speed of the electrical motor to appropriate levels, and drivers for controlling the motors. The system is completed by a software for real-time control. Thus, computer controlled above knee activity can be produced for assembled foot or knee prostheses.

Home recording for pre-phase sleep apnea diagnosis by Holter recorder using MMC memory

Long time data recordings by ambulatory devices became common today with widespread use of high capacity memories. In this study, this technology is used on home recording device especially designed for the pre-phase sleep apnea analysis at home before having certain diagnosis by proper polysomnographic examination in a hospital. Designing a portable recording system for clinically significant signals and analyzing an associated apnea detection algorithm are two significant and complementary studies carried out and reported in this paper. According to our aim, the Holter device discussed here is capable of recordings from many channels simultaneously but specifically for this study we consider just three signals namely ECG, breathing activity and oxygen saturation which are all significant for apnea detection in polysomnographic diagnostic phase. In apnea detection part we have initially studied the algorithm based on Hilbert Transform of ECG signal. The study also includes the comparison of three selected QRS detection algorithms (digital filter based, differentiation based and higher order statistics based) for ECG processing required for the Hilbert transform based apnea detection algorithm. Differentiation based QRS algorithm is preferred over other two due to its better performance on noisy ECG signals.

Analysis of direct motion measurement system for design of above knee prosthesis

In this study, motion measurement systems (MMS) are designed and analyzed in the laboratory for the necessary use in the development of any electronic above knee prosthesis (AKP). A low cost 2D image based MMS is one of the designs in order to analyze gait dynamics and provide reliable reference base for the comparison with direct MMSs which are also designed subsequently. Three direct MMSs using several combinations with accelerometers and/or gyroscopes are configured in the experimental set up and the records are compared to those obtained from image based MMS. The virtual sensor method showed the best accuracy and the approach using the combination of gyroscope and accelerometer had a lower accuracy while the calculations based on single accelerometers provide the worst performance in the relative knee angle estimations. It is recommended that MMSs employing virtual sensor and assembly of accelerometer and gyroscope are more reliable platforms for AKP designs.

Application of neural based estimation algorithm for gait phases of above knee prosthesis

In this study, two gait phase estimation methods which utilize a rule based quantization and an artificial neural network model respectively are developed and applied for the microcontroller based semi-active knee prosthesis in order to respond user demands and adapt environmental conditions. In this context, an experimental environment in which gait data collected synchronously from both inertial and image based measurement systems has been set up. The inertial measurement system that incorporates MEM accelerometers and gyroscopes is used to perform direct motion measurement through the microcontroller, while the image based measurement system is employed for producing the verification data and assessing the success of the prosthesis. Embedded algorithms dynamically normalize the input data prior to gait phase estimation. The real time analyses of two methods revealed that embedded ANN based approach performs slightly better in comparison with the rule based algorithm and has advantage of being easily-scalable, thus able to accommodate additional input parameters considering the microcontroller constraints.

Design of semi active knee joint with magnetorheological (MR) damper

In this study, electronic above knee prosthesis using magnetorheological (MR) cylinder is developed. The prosthesis structure having a proper knee mechanism is used for MR cylinder damping produced for experimental studies. Movement of knee joint originated from hip is damped with these cylinders and through a control algorithm running on microcontroller the system achieves the motion closer to the natural movement of leg. The performance of MR cylinder in the prosthesis are examined while a healthy subject wearing a custom designed socket mounted to prosthesis walks interchangeably at predetermined speed. According to the results obtained by the image based Motion Analysis System (MAS) the walk with the prosthesis using the MR damper is closer to the natural gait in terms of gait cycle duration and phases. On the other hand, in terms of the required maximum knee angle, the prosthesis with pneumatic damper performs better when the natural gait cycle is considered.

Simulations of knee angle control in dynamical Gait model for above knee prosthesis

Electronic above knee prosthesises need control methods and stratagies in order to conrol gait in an adaptive manner to user and enviroment. In this study, by using Proportional-Derivative (PD) and Computed Torque (CT) methods, knee control is analyzed. During gait, by modelling the movement of gait as two joint robot arm, dynamic equations of movement can be obtained. By applying artificial torques to dynamic equations, the results of simulation of gait without control is observed. Based on the problems observed, together with PD and CT methods, simulations are carried out for knee joint control. It is observed that the results of the simulations of controlled gait phases of gait dynamic model is close to the real data and follow the reference signal.

Design of portable holter recorder with MMC memory for prephase sleep apnea diagnosis

Long time digital data recordings in portable devices have become possible today with widespread usage of high capacity memories. The aim of this study is to obtain a better performance in the sleep apnea analysis by recording portable EKG recording systems as well as different clinically significant signals on flash cards for a long time. Thus in the first stage a Holter Recorder Device is designed to record ECG and respiratory signals for the diagnostic of apnea. The device is designed to handle more than two channels simultaneously. In the second phase of the study, an apnea detection algorithm using Hilbert Transform was chosen and implemented as an example among other algorithms using only ECG signals. Based on the comparison of three selected algorithms (digital filter based, differentiation based and higher order statistics based) to be used for the QRS detection for any apnea detection algorithm, differentiation based algorithm is preferred due to its better performance.