The use of the Hilbert transform in ECG signal analysis

This paper presents a new robust algorithm for QRS detection using the first differential of the ECG signal and its Hilbert transformed data to locate the R wave peaks in the ECG waveform. Using this method, the differentiation of R waves from large, peaked T and P waves is achieved with a high degree of accuracy. In addition, problems with baseline drift, motion artifacts and muscular noise are minimised. The performance of the algorithm was tested using standard ECG waveform records from the MIT-BITH Arrhythmia database. An average detection rate of 99.87%, a sensitivity (Se) of 99.94% and a positive prediction (+P) of 99.93% have been achieved against study records from the MIT-BITH Arrhythmia database. A detection error rate of less than 0.8% was achieved in every study case. The reliability of the proposed detector compares very favorably with published results for other QRS detectors.

Haemodynamic effects of increasing angle of head up tilt

OBJECTIVE

To investigate the haemodynamic effects of varying the angle of head up tilt.

METHODS

20 healthy subjects (12 female, eight male; mean (SD) age 33.6 (8.4) years) underwent head up tilt for five minutes to each of four angles of tilt in random order, with a five minute rest period at the horizontal between each angle. Forearm blood flow was measured using intermittent occlusion mercury strain gauge plethysmography at two and five minutes. Subjects underwent continuous monitoring of heart rate, systolic blood pressure (SBP), and diastolic blood pressure (DBP) by Finapres and cardiac output and stroke volume by impedance cardiography. Each variable was measured at two and five minutes, averaged over the period of blood flow measurement.

RESULTS

Every haemodynamic variable at each angle was significantly different from supine values. Head up tilt produced progressive increases in heart rate (11-21%), SBP (12-21%), and DBP (20-33%) with increasing tilt angle. However, although 45 degrees produced significantly less haemodynamic effect, there were no significant differences for angles between 60 degrees and 90 degrees. Cardiac output fell on head up tilt by 17-20% and stroke volume by 28-34%, but increasing tilt angle produced no significant additional reduction in cardiac output and stroke volume because of increases in heart rate and vasoconstriction.

CONCLUSIONS

Angles < 60 degrees produce significantly less haemodynamic effects than steeper angles. Increasing tilt angle beyond 60 degrees produces no apparent additional effect on cardiac output or sympathetic tone. Increasing tilt angle beyond 60 degrees confers no additional orthostatic stress and may not affect the sensitivity and specificity of head up tilt testing as previously thought. Sixty degrees of tilt is a more practical angle for support of a syncopal patient and is recommended.

The application of local grey level histograms to organelle classification in histological images

The performance of a histogram matching algorithm was evaluated for the classification of organelles in histological images. The test set used for this investigation comprised several digital images of sheep pituitary gland captured using a CCD camera linked to a Tunnelling Electron Microscope. Results show that the normalized image histogram is a good feature to choose as the basis of object recognition in biological images.

Definition and application of a fourier domain texture measure: applications to histological image segmentation

A texture measure is defined for the purpose of two-dimensional histological image classification, based upon a novel exploitation of the modulus of the Fourier transform. Its implementation in the form of an algorithm is presented and discussed, and employed to classify a set of histological images. The results, which were obtained by the analysis and classification of different mammalian tissue types, compare favourably with established texture recognition techniques. Furthermore, the results suggest that the method has a comparable performance to other techniques at reduced storage and time costs for images of a high grey level resolution.

Radiographic image analysis in the study of bone morphology

Early changes in bone morphology, arising from such conditions as rheumatoid arthritis and osteoarthritis, are difficult to detect and even more difficult to quantify when estimates are based upon visual examination of conventional radiographs. In this paper, we describe in detail the software operations of a computer-based image analysis system that will readily detect small changes in periarticular bone contours. Detailed studies have shown that the system compares favourably with clinical evaluations, and that the results so obtained are both reliable and reproducible.

Measurement of radiographic changes occurring in rheumatoid arthritis by image analysis techniques

We have applied image analysis techniques to serial measurements of bone contour in standard radiographs of single small joints of the hands in subjects with rheumatoid arthritis (RA) and controls. Adequate reproducibility was shown in 20 controls radiographed twice over a six month period. The technique showed significant changes in a proximal interphalangeal joint of 13 of 15 patients with RA studied over periods of three to 10 months. In further serial studies in selected small joints of RA hands significant changes could be shown as early as four months. These results justify further development of these techniques to allow their full scale evaluation in multiple joints in patients with RA receiving long term drug therapy.

High-speed spline fitting, with application to boundary tracing in low-contrast digital images

The practical implementation of a rapid Catmull-Rom (cardinal) spline is described, and its advantages with respect to speed and ease of use are discussed. Consideration is also given to interactive tensioning of the curve, and how the spline may be used to contour the boundary in digital images prior to segmentation. For this purpose the spline control points are positioned by the user and automatically optimized by means of a local, two dimensional edge detector.