Projects:2014S1-19 Analysis Of Heart Sound Signals using the Wavelet Transform
Auscultation with a stethoscope is one of the most important tools in diagnosing the heart diseases. However diagnosing the heart sounds through auscultation is subjective since it depends largely on hearing ability and skill of physicians. Thus, our research is to develop a standard heart sounds diagnosing fools which is able to recognize the characteristic of the heart sounds and able to classify the heart sounds automatically.
Modelling of heart sound
Heart sounds provides information about people's physical conditions. The first heart sounds (S1) occurs at the beginning of ventricular contraction during the closure of the mitral and tricuspid valves. It indicates the beginning of ventricular systole. The second heart sound (S2)marks the end of ventricular systole and the begining of ventricular relaxation, following the closure of the aortic and the pulmonary valves. The identification of systole and diastoel is important in determing other heart sound and murmurs. Heart murmurs also may be present in the phonocardiogram (PCG), are generally assocated with abnormal function of the cardiac valve, except for the innocent murmurs which may occur during systole in young people with normal hearts. The murmur can be classified as systolic murmurs and diastolic murmurs according to theri production phase.
As the PCG signals are complex signals that are very difficult to analyze visually, they can be transformed to frequency domain to analyze. However heart sound signals are non-stationary signals whcih means the time components are important as well, therefore FFT cannot indicates some sudden situations. The above figures illustrate that FFT cannot indicate the time components, therefore there are not too many differences between normal heart sounds and abnormal heart sounds in frequency domain.
The STFT is obtained from the usual FT by multiplying the time signal x(t) by an appropriate sliding time window w(t). It provides some information about time and frequencies that a signal event occurs. However, one can only obtain this information with limited precision, which is determined by the size of the window,also, it has strict limitations on time-frequency resolution.
The wavelet transform is also used to analyze the heart sound in time and frequency domains. It uses narrow windows when observing the high frequency and automatically uses wide window when observing the low frequency. This particular property of wavelet generates good time resolution at high frequency and good frequency resolution at low frequency.
Mr XiaoBo Xue Ms Jie Ren Ms Yue Liang