Semi-automatic algorithm for construction of the left ventricular area variation curve over a complete cardiac cycle.

BACKGROUND: Two-dimensional echocardiography (2D-echo) allows the evaluation of cardiac structures and their movements. A wide range of clinical diagnoses are based on the performance of the left ventricle. The evaluation of myocardial function is typically performed by manual segmentation of the ventricular cavity in a series of dynamic images. This process is laborious and operator dependent. The automatic segmentation of the left ventricle in 4-chamber long-axis images during diastole is troublesome, because of the opening of the mitral valve. METHODS: This work presents a method for segmentation of the left ventricle in dynamic 2D-echo 4-chamber long-axis images over the complete cardiac cycle. The proposed algorithm is based on classic image processing techniques, including time-averaging and wavelet-based denoising, edge enhancement filtering, morphological operations, homotopy modification, and watershed segmentation. The proposed method is semi-automatic, requiring a single user intervention for identification of the position of the mitral valve in the first temporal frame of the video sequence. Image segmentation is performed on a set of dynamic 2D-echo images collected from an examination covering two consecutive cardiac cycles. RESULTS: The proposed method is demonstrated and evaluated on twelve healthy volunteers. The results are quantitatively evaluated using four different metrics, in a comparison with contours manually segmented by a specialist, and with four alternative methods from the literature. The method's intra- and inter-operator variabilities are also evaluated. CONCLUSIONS: The proposed method allows the automatic construction of the area variation curve of the left ventricle corresponding to a complete cardiac cycle. This may potentially be used for the identification of several clinical parameters, including the area variation fraction. This parameter could potentially be used for evaluating the global systolic function of the left ventricle.

Effects of the time response of the temperature sensor on thermodilution measurements.

Thermodilution is widely used to measure cardiac output, ejection fraction and end diastolic volume. Even though the method is based on dynamic temperature measurements, little attention has been paid to the characterization of the dynamic behavior of the temperature sensor and to its influence on the accuracy of the method. This paper presents several theoretical and empirical results related to the thermodilution method. The results show that, at flow velocities above 0.2 m s(-1), the response of temperature sensors embedded in Swan-Ganz catheters can be accurately described by a convolution operation between the true temperature of the blood and the impulse response of the sensor. The model developed is used to assess the influence of the probe response on the measurement of cardiac output, and this study leads us to the conclusion that the probe response can cause errors in the cardiac output measurement, but this error is usually small (2% in cases with a high degree of arrhythmia). The results show that these small errors appear during arrhythmias that affect the R-R interval and when the real temperature distribution at the pulmonary artery does not possess a shape with perfect temperature plateaux.

The behavior of action potential conduction velocity on isokinetic knee extension tests.

The present study investigates the behavior of action potential conduction velocity (CV) on each repetition of an isokinetic test set and on each set as a whole. A total of seven healthy men (27.7 ± 2.8 yrs, 1.74 ± 0.06 m, and 79.6 ± 11.0 Kg) performed 3 (three) sets of 10 (ten) maximal concentric repetitions of dominant knee extension at 60°/s on an isokinetic dynamometer, with 1 minute of rest interval between the sets. The surface electromyographic (SEMG) signals were recorded from the vastus lateralis muscle during the exercises. CV was estimated with a spectral matching method which requires the SEMG acquisition technique based on a flexible linear array of electrodes (here used with 8 electrodes and 5 mm inter-electrode distance). With the view to minimize the factors other than fatigue that also influence the CV behavior, only the extension phase of the isokinetic exercise repetition was considered for measurements. Results showed that CV usually increases during a single repetition whereas it has a decreasing tendency along the isokinetic set seen as a whole.

Isokinetic work-to-surface electromyographic signal energy ratios as a muscular fatigue indicator.

Efficiency of muscular work is usually measured as the relationship between work load and maximum exercise duration. The present study analyzes the efficiency feature as a ratio between mechanical work (WK) and the energy (E) of the surface electromyographic signal (SEMG). This relation (WK/E(SEMG)) was compared with the most common electromyographic descriptors and its behavior was observed during muscle fatigue. A total of sixteen healthy men (26.8 +/- 4.7 yrs, 175.7 +/- 4.7 cm, and 79.2 +/- 9.4 kg) performed three sets of ten maximal concentric repetitions of dominant knee extension at 60 degrees /s on an isokinetic dynamometer, with 1 minute of rest interval between the sets. The SEMG signals were recorded during the exercises. With the view to minimize the factors other than fatigue that also influence the SEMG descriptors behavior, the only isokinetic repetition phase considered for measurements was the load range. Statistical analyses showed significant correlations between WK/E(SEMG) and the traditional electromyographic fatigue indicators.

Two-dimensional compression of surface electromyographic signals using column-correlation sorting and image encoders.

We present a new preprocessing technique for two-dimensional compression of surface electromyographic (S-EMG) signals, based on correlation sorting. We show that the JPEG2000 coding system (originally designed for compression of still images) and the H.264/AVC encoder (video compression algorithm operating in intraframe mode) can be used for compression of S-EMG signals. We compare the performance of these two off-the-shelf image compression algorithms for S-EMG compression, with and without the proposed preprocessing step. Compression of both isotonic and isometric contraction S-EMG signals is evaluated. The proposed methods were compared with other S-EMG compression algorithms from the literature.

On the use of motion-based frame rejection in temporal averaging denoising for segmentation of echocardiographic image sequences.

We have recently introduced an algorithm for semi-automatic segmentation of the left ventricular wall in short-axis echocardiographic images (EMBC 30:218-221). In its preprocessing stage, the algorithm uses temporal averaging for image denoising. Motion estimation is used to detect and reject frames that do not correlate well with the set of images being averaged. However, the process of estimating motion vectors is computationally intense, which increases the algorithm's computation time. In this work, we evaluate the viability of replacing the motion estimation stage with less computationally intense approaches. Two alternative techniques are evaluated. The ventricular contours obtained from each of the three algorithm variants were quantitatively and qualitatively compared with contours manually-segmented by a specialist. We show that it is possible to reduce the algorithm's computational load without significantly reducing the segmentation quality. The proposed algorithms are also compared with three other techniques from the literature.

Surface EMG and spatial resolution analysis with estimation of electromyographic descriptors.

Spatial filtering has become a common way to improve the resolution of surface electromyographic signals (SEMG) when used in connection with electrode arrays. The goal of this study is to observe the behavior of S-EMG amplitude and spectral descriptors when signals are submitted to a longitudinal quadruple differentiating spatial filter. Signals were acquired at 20% and 60% of the maximum voluntary contraction using a linear array of eight surface electrodes in order to understand the impact of the filtering technique in the S-EMG variables during fatiguing and non-fatiguing contractions. The final results show that the filtering procedure yields better selectivity, suggesting that single motor units can be better observed if spatial filters and measurement configurations with smaller pick-up areas are used. During fatiguing contractions, however, further analysis is needed.