Chapter A phonocardiogram or PCG is a graphical

Chapter 1
Introduction
1.1 Overview of PCG signals
A phonocardiogram or PCG is a graphical interpretation of the cardiac sounds produced by the heart. Phonocardiography is the study of the sounds acquired using a standard tool such as an electronic stethoscope. Phonocardiograms (or heart sounds) acquired during cardiac auscultation contain bio-acoustic information related to the proper operation of the heart.
Stethoscope was first invented by R.T.H. Laënnec 12 in the year 1819. A perforated wooden cylinder was used to transmit the human heart sound from the patient’s chest to the physician’s ear. This new device helped Laënnec to diagnose diseases such as tuberculosis at an earlier stage than was previously possible. Later, this monaural stethoscope was modified into the binaural type stethoscope consisting of two flexible rubber tubes that attaches the chest piece to spring-connected metal tubes with earpieces.
The present day stethoscope uses a dual bell-shaped, open-ended chest piece, which transmits low-pitched sounds well, and the flat diaphragm based chest piece that detects sounds of higher frequency. Both types of chest piece are arranged so that they can be rapidly interchanged by turning a valve.

Heart sounds gives medical information regarding the health of the human beings. It also serves many purposes. It has recently been used as a biometric tool, for education purpose, for long term monitoring of patient health and telemetry. Heart sound is usually considered as a valid biomedical tool alternative to finger print or face recognition tool. Also various types of phonocardiogram instruments are currently in use for long time monitoring of patients’ health. Continuous time monitoring of the health of a patient helps the doctors to understand the cause of the disease better in real time and updates the doctor with changes.

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Heart Sounds

Fig. 1.1 shows PCG signal with heart sound components S1, S2, S3 and S4.
First Heart Sound (S1): The first heart sound (S1) is caused by the closure of the mitral and tricuspid valves at the start of ventricular systole. The mitral component (M1) occurs slightly before the tricuspid component (T1) usually 20-30ms after M1 sound 15. The S1 is a low-pitch sound with longer duration 14. The intensity of the M1 sound is much higher than the T1 sound intensity due to the abrupt rise in left ventricular pressure. The first sound must be evaluated with its quality, intensity, and degree of splitting 13- 18. A decrease in the intensity of S1 is associated with myocardial depression, ventricular septal defect, and acute aortic regurgitation. The splitting of S1 sound is usually 60ms in patients with right bundle branch block (RBBB), or ventricular tachycardia or premature ventricular contraction (PVC) 14.

Second Heart Sound (S2): Usually at the end of the systole, the second heart sound (S2) is caused by the closure of the aortic and pulmonic valves. The S2 is shorter and slightly higher pitched sound. The S2 sound has frequency components in the range 10-400 Hz and the duration in the range 50-150ms 9. The S2 is composed of aortic closure sound (A2) and the pulmonic closure sound (P2). They last for less than 50ms 9. The delay between the closure of the aortic and pulmonic valves results in a split S2 sound. The sound S2 is evaluated based on the presence and degree of respiratory splitting and the relative intensities of A2 and P2. The amplitude and frequencies of A2 sound is slightly higher than the P2 sound 3. The sound interval at split widens on inspiration and narrows on expiration. The time interval between the A2 and P2 components is an indicator of aortic blood pressure 8. The various pathologies related to split S2 are pulmonic stenosis, RBBB, left bundle branch block (LBBB), atrial septal defect and right ventricular failure. In normal cases, the S1-S2 interval (systole) is shorter than the S2-S1 interval (diastole) 30- 40.

Third Heart Sound (S3): During diastolic period after 100-150ms of the S2 the third heart sound (S3) is produced by the sudden deceleration of blood flow within the ventricles. The S3 includes significant low amplitude-frequency components as compared to the first and second heart sounds. The S3 has 30-90 Hz frequency components with sound duration of 70 ± 15ms during diastole period 43, 44. S3 often occurs in patients with impaired myocardial reserve 16. The clinical studies show that the S3 can provide clinical information about hemodynamic and systolic dysfunction, and evaluation of patients with congestive heart failure 41- 48. The auscultation of S3 in adults is abnormal related with heart failure.

Fourth Heart Sound (S4): The fourth heart sound (S4) is caused by the contraction of atria resulting in forcing of blood into the distended ventricle. The S4 is a low-pitched sound that occurs before the first heart sound. The S4 sound arises from low-frequency vibrations with frequency of 20 to 30Hz. It is present in patients with diminished left ventricular compliance 13- 18. The clinical studies show that the diastolic heart sounds combining with electrocardiogram (ECG) may improve the non-invasive diagnosis of myocardial ischemia.

Heart Murmurs and Other Pathological Sounds

Murmurs are caused by turbulence flow of blood or vibration in tissues. Valvular dysfunctions results in pathological murmurs. Murmurs may be systolic, diastolic or continuous during every systole and diastole. Heart murmurs are organised into various categories by the timing (early, mid, late, or pan), intensity, duration, pitch (low, medium, or high), quality (blowing, harsh, rumbling or musical), and shape configuration crescendo (increasing intensity), decrescendo (decreasing intensity), crescendo-decrescendo (increasing then immediate decreasing intensity) 6, 14- 18. The pitch and intensity depends on the velocity of blood flow that produces the murmur. The timing of a murmur helps in accurate diagnosis of diseases.

The systolic murmurs can be separated into groups namely the early systolic murmur of acute mitral regurgitation and tricuspid regurgitation; the mid-systolic murmurs of aortic stenosis (AS), pulmonic stenosis (PS), hypertrophic obstructive cardiomyopathy (HOCM) and atrial septal defects (ASD); the late-systolic murmurs of mitral valve prolapse (MVP); and the holosystolic (or pansystolic) murmurs of the mitral regurgitation (MR), tricuspid regurgitation (TR), and ventricular septal defects (VSD). The diastolic murmurs include groups such as aortic and pulmonic regurgitation (early diastolic), and mitral or tricuspid stenosis (mid-late diastolic). The murmur of a patent ductus arteriosus (PDA) and systemic arterio-venous fistulae (AVF) is continuous throughout systole and diastole 11- 17, 49- 68. Fig. 1.2 shows the different types of systolic and diastolic heart murmurs. The click and snap sounds are associated with valves opening of the semilunar valves and the mitral and tricuspid valves. The clicks and snaps are associated with distinctive features of some heart defects.

Clinical PCG Parameters for CVD Diagnosis
In clinical studies, the specific heart sound indexes are measured for evaluating heart functions of subjects, maternal, foetal and infants with various physiological and pathological conditions. The heart sound parameters are: the cardiac contractility change trend (CCCT) (the increase of the S1 amplitude after exercising with respect to the S1 amplitude recorded at rest) 10; the amplitude of S1 29; the ratio of S1 amplitude to S2 amplitude (S1/S2) 10, 28; the ratio of the amplitude of tricuspid sound to the amplitude of the mitral sound (T1/M1); the ratio of S3 amplitude to S2 amplitude (S3/S2); the ratio of diastolic to systolic duration (D/S) 25, 10, 27; S1 localization 7; the duration, energy of instantaneous frequencies (EIFs) and splits of the aortic (A2) and pulmonic (P2) valve components 1, 3, the heart rate (HR) 25, 27; the duration and frequencies of S3 and S4 sounds, and the timing (location), configuration (shape), loudness (intensity), spectral content, duration of murmurs. Although most modern digital stethoscope can amplify, play, display and record heart sound signals in real time, automatic and quantitative measurement of heart sound parameters is very important for accurate and effective diagnosis of various cardiac diseases and disorders.

Challenges of Automated Diagnosis
The major challenge faced during the recording of the heart sounds is that the problem of acoustic noise component hinders the detection of the milder heart sounds. So separation of noise from heart sound becomes important. Separation of noise makes the heart sound robust and suitable for further processing. Pre-processing of heart sounds is used to assess the signal quality in terms of performance metrics such as Signal-to-Noise Ratio (SNR) and Segmental-Signal-to-Noise Ratio (SSNR). Pre-processing removes baseline changes and high frequency noises. Pre-processing can be used to extract relevant features.

Identification of heart sound components in noise free heart sound is important to understand the normality of the heart as well as working of individual valves in the heart. The process of identification of heart sound involves segmentation of the heart sound, prominently normal S1-S2 sounds, murmurs, clicks, and snaps. Segmentation is used to delineate the start and the end of each phase of the heart beat-S1, systole, S2, diastole.

Once the heart sound is segmented, the sound can further be classified as normal or abnormal sound by means of the classification process by identifying features in heart sound. Classification is done by comparing the various features of heart sound with that of the sounds in the reference database. In the process, we map the features for each segmented phase of the beat to the unknown phase or sound or the entire recording of the pathology.

Organisation of the Thesis
Chapter 1 describes the introductory concepts of phonocardiogram signal, their advantages in clinical decision analysis along with the problem statement and proposed solution. Chapter 1 also describes the heart sounds both normal and pathological in detail. The chapter discusses the clinical parameters for CVD diagnosis. The challenges for automated diagnosis is also described here in this chapter.

Chapter 2 describes the literature survey with focus on existing methods of heart sound analysis with respect to Pre-processing, segmentation and Classification of heart sounds. This section also focusses on the database used for heart sound analysis. Gaps and Limitations of the existing methods are also discussed here. The chapter also includes identification of the problem and objectives of the proposed work. There is a special focus on the methodology used in the proposed work.

Chapter 3 discusses the popular heart sound segmentation algorithms namely Homomorphic Filtering Segmentation and Segmentation using Mel-Scaled Wavelet Transform. The chapter proposes a new method of Heart Sound Segmentation using the Event Synchronous Method. There is a focus on the comparison between the proposed and existing methods in literature in terms of the obtained results.

Chapter 4 describes the de-noising procedure for PCG signals using time frequency techniques. There is a special focus on the time frequency block threshold method. A comparative study with wavelet de-noising and Time frequency soft threshold and overlapping group shrinkage algorithm is also described in this chapter.

Chapter 5 describes the extraction of loudness features and unsupervised classification of heart sounds with the state of the art classifiers namely K-Means, Fuzzy C-Means and GMM classifiers.

Chapter 6 describes the conclusion and the future direction of the thesis.

CHAPTER bulk material termed as the ‘matrix’ and

CHAPTER 1
INTRODUCTION

1.1 Composite Materials

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A composite material is one which is composed of at least two phases combined together to produce new material properties that are different to the properties of those elements on their own. Practically, most composites composed of a bulk material termed as the ‘matrix’ and reinforcement added essentially to increase the overall strength and stiffness of the matrix.

Metal matrix composite (MMC) is engineered combination of the metal (Matrix) and hard particle/ceramic (Reinforcement) to get modified properties. Metal matrix composites increasingly found their application in aerospace and automotive sectors because of their properties like high strength, high stiffness, low density, wear resistance, elastic modulus, and tensile strength. Most commonly used metal matrices are Aluminum, Magnesium, Titanium and their alloys. The reasons for using these matrices are less weight, low cost and availability. Similarly reinforcements are also classified as fibers, whiskers and particulates. Particulates are extensively used in weight critical application in aerospace, automotive parts fabrication. Hybrid metal matrixes are engineering material that constitutes two or more reinforcement in order to obtain the combined advantage compare to individual constituent.

Particulate reinforced Aluminium matrix composites (PRAMC) have received considerable observation in twenty first century because of their high specific strength and good wear resistance. The PRAMCs were usually produced by Casting technique and Powder metallurgy.

In recent years many research have been investigated by combining Aluminium matrix with SiC, Al2O3, TiC reinforcements to gain better understanding of the mechanical behavior of these composites and their excellent wear resistance.

1.2 Classification of Composites:
The composites can be classified as two types on the basis of matrix constituent and reinforcement form.
The first level of classification with respect to the matrix constituent includes:
1. Polymer Matrix Composites
2. Metal Matrix Composites
3. Ceramic Matrix Composites
The second level of classification with respect to the reinforcement form includes:
1. Fiber reinforced composites
2. Laminar composites
3. Particulate composites.

1.2.1 Classification Based on Matrices
The first level of classification is usually made with respect to the matrix constituent. The figure 1.1 indicates classification of MMC’s based on matrices.

Fig 1.1: Classification of MMC’s based on matrices

Polymer Matrix Composites (PMC’s): Polymer based composites are also known as Fiber reinforced polymers (FRP). These material use polymer based resin as the matrix and a different types of fibers such as glass, carbon as the reinforcement.
Metal Matrix Composites (MMC’s): Metal matrix composites are increasingly found in the aerospace and automotive industry. These materials use a metal such as Aluminium as the matrix and reinforce it with fiber, particulates or whiskers such as Silicon carbide, Boron carbide etc to get tailored properties.

Ceramic Matrix Composites (CMC’s): Ceramic matrix composites are used in very high temperature environment. These materials use a ceramic as the matrix and reinforce it with short fibers or whiskers such as those made from silicon carbide and boron nitride.

1.2.2 Classification Based on Reinforcements
The figure 1.2 indicates classification of MMC’s based on Reinforcements.

Fig 1.2: Classification of MMC’s based on reinforcements

1.3 Metal Matrix Composites (MMC):
Metal Matrix Composites (MMCs) provide remarkably enhanced properties over regular conventional materials, such as good strength, weight savings and stiffness. Metal matrix composites are used in a wide range of high performance applications today. Most of their current applications are in aviation, ground transportation, electronics and sports industries. The applications of metal matrix composites in aeronautics have been established in the aero-structural, aero-propulsion and subsystem categories. The Aluminium alloys are very attractive because of their low cost and light weight and can be heat treated to fairly high-strength levels. Also Aluminium is one of the most easily fabricated of the high-performance materials, which usually correlates directly with lower costs.
1.3.1 Advantages of MMC’s
• Higher temperature capability.
• Fire resistance.
• Higher transverse stiffness and strength.
• No moisture absorption.
• Higher electrical and thermal conductivities.
• Better radiation resistance.
1.3.2 Disadvantages of MMC’s
• Higher cost of some material systems.
• Relatively immature technology.
• Complex fabrication methods for fiber-reinforced systems (except for casting).
• Limited service experience.

1.3.3 Applications of Metal Matrix Composites

Space: The space shuttle uses Boron/Aluminium tubes to support its fuel usage frame. In addition to decreasing the mass of the space shuttle by more than 145 kg, Boron/ Aluminium also reduced the thermal insulation requirements because of its low thermal conductivity.

Military: Precision components of missile guidance systems require dimensional stability that is the geometries of the composites cannot alter during use. MMC’s such as SiC composites satisfies this requirement because they have high micro yield strength. In addition, the volume fraction of reinforcements can be varied to have a suitable co-efficient of thermal expansion suitable with other parts of the system.

Transportation: MMC’s are finding use now in automotive engines that are lighter than their metal counter parts. Also, because of their high strength and low weight, MMC’s are the material of choice for gas turbine engines.

1.4 Fabrication Process:
1.4.1 Stir Casting Method
Stir casting is an economical liquid phase method of composite materials fabrication. Here a dispersed phase (like short fibers, ceramic particles) is mixed with a molten metal by means of mechanical stirring. Then the liquid composite material is poured into an appropriate crucible and cast by conventional metal forming technologies. However, major challenge in the liquid phase processing is to achieve uniform distribution of reinforcement and to obtain strong interfacial bonding between the reinforcement and the matrix. The stir casting set up is shown in figure 1.3.

Fig 1.3 Stir casting

1.4.2 Characterization of Stir Casting

1. Percentage of dispersed phase is limited (usually not more than 30% by volume)
2. Dispersed phase distribution is not perfectly homogeneous throughout the matrix.
• Gravity segregation of the dispersed phase can occur due to a difference in the densities of the dispersed and matrix phase.
• Clusters of the dispersed particles (fibers) may form.
3. The technology is relatively simple and low cost.

1.5 Aluminium Alloys:
Aluminium alloy is a composition consisting mainly of aluminum to which other elements have been added. The typical alloying elements are Magnesium, Manganese, Copper, Silicon and Zinc. There are two primary classifications namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable.

About eighty-five percentage of Aluminium is used for wrought products, for example foils, rolled plate and extrusions. Cast Aluminium alloys yield cost-effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys. Alloys comprised mostly of Aluminium plays crucial role in aerospace manufacturing since the introduction of metal skinned aircraft.

Aluminium alloys are extensively used in engineering structures and components where light weight or corrosion resistance is required. Wrought Aluminium alloys are used in the shaping processes: stamping, rolling, forging, extrusion, pressing. Cast Aluminium alloys comes after sand casting, permanent mould casting, die casting, investment casting, centrifugal casting, squeeze casting and continuous casting.

1.5.1 Cast Aluminium Alloys

Aluminium and its alloys are used in a variety of cast and wrought form and conditions of heat treatment. Forgings, sections, extrusions, sheets, plate, strip, foils and wire are some of the examples of wrought form while castings are available as sand, pressure and gravity die-castings e.g. Al-Si and Al-Mg alloys. The designation of Cast Aluminium alloy is shown in Table 1.1.
Table 1.1: Designation of Cast Aluminium alloys

Alloy Designation Details
1XX.X 99% pure Aluminium
2XX.X Cu containing alloy
3XX.X Si, Cu/Mg containing alloy
4XX.X Si containing alloy
5XX.X Mg containing alloy
7XX.X Zn containing alloy
8XX.X Tin containing alloy
6XX.X Unused series

1.5.2 Wrought Aluminium Alloys

To meet various requirements, Aluminium is alloyed with Copper, Manganese, Magnesium, Zinc and Silicon as major alloying elements. A four-digit numerical designation system is used to identify wrought Aluminium alloys. As shown in table 1.2. below, the first digit of the four-digit designation indicates the group.

Table 1.2: Designation of Wrought Aluminium alloys

Alloy designation Details
1XXX 99% pure Aluminium
2XXX Cu containing alloy
3XXX Mn containing alloy
4XXX Si containing alloy
5XXX Mg containing alloy
6XXX Mg and Si containing alloy
7XXX Zn containing alloy
8XXX Other alloys

1.5.3 Designation of Aluminium Alloys

The Aluminium Association of America has classified the wrought Aluminium alloys according to a four-digit system. The classification is adopted by the International Alloy Development System (IADS). Table 1.1 & 1.2 gives the basis of designation of wrought and cast Aluminium alloys in the four-digit system. The first digit identifies the alloy type the second digit shows the specific alloy modification. The last two digits indicate the specific Aluminium alloy group.

x

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