Digital Signal Processing in Biomedical Engineering
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Digital Signal Processing is concerned with the representation of signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing is a branch of the science of the signal processing. The other branch of the signal processing is Analog Signal Processing.
DSP includes the areas of signal processing like: audio and speech signal processing, sonar and radar signal processing, sensor array processing, spectral estimation, statistical signal processing, digital image processing, signal processing for communications, control of systems, biomedical signal processing, seismic data processing, etc.
DSP is one of the most important areas of study in the fields of communication, electronics instrumentation, research and analysis and many more fields. The main reason is that the signals need to be processed so that the information that they contain can be displayed, analysed, or converted to another type of signal that may be of use. Before we take any usual signal of our day to day life, that analog signal must be converted into the digital signal so that it can be fed to the electronic circuits. Electronic circuits can input the signal in form of only binary digits, I.e. 1&0. And analog signal has a definite value at each and every point. As such, we have to take up the sampling procedure and convert it to a digital signal as:
This process is known as sampling. Sampling is the base point of digital signal processing. An analog signal can never be processed without sampling.
Converters such as an Analog-to-Digital converter then take the real-world signal and turn it into the digital format of 1's and 0's. From here, the DSP takes over by capturing the digitized information and processing it. It then feeds the digitized information back for use in the real world. It does this in one of two ways, either digitally or in an analog format by going through a Digital-to-Analog converter. All of this occurs at very high speeds.
DSP is the mathematics, the algorithms, and the techniques used to manipulate these signals after they have been converted into a digital form. This includes a wide variety of goals, such as: enhancement of visual images, recognition and generation of speech, compression of data for storage and transmission, etc.
We have to choose the domain in which to process a signal by making an informed guess as to which domain best represents the essential characteristics of the signal. A sequence of samples from a measuring device produces a time or spatial domain representation, whereas a discrete Fourier transform produces the frequency domain information that is the frequency spectrum. Autocorrelation is defined as the cross-correlation of the signal with itself over varying intervals of time or space.
BIOMEDICAL SIGNAL PROCESSING
Biomedical signals are the recording of the observations of physiological activities of organisms, ranging from gene and protein sequences, to neural and cardiac rhythms, to tissue and organ images. It is the clinical study of the internal body metabolisms, diagnosis of ailments, and detection of diseases using the electronic instrumentation. Biomedical signal processing aims at extracting significant information from biomedical signals. With the aid of biomedical signal processing, biologists can discover new biology and physicians can monitor distinct illnesses.
Digital signal processing came into the field of the biomedical signal processing with the advent of the use of advanced electronic instruments in the biomedical field. Various scientists invented many instruments that detected the biological diagnostic results from the biological organisms. BMP (biomedical signal processing) has enabled the people from the medical field to enable them to ease off their burdens of life support in a very healthy manner. While these techniques are well established, the field of Biomedical signal processing continues to expand thanks to the development of various novel biomedical instruments.
BMP is prevalent in the following fields:
- MRI scan,
- X-Ray Scans,
- PET- Positron Emission Tomography
- Electrocardiography (ECG)
- cellular motion tracking,
- laparoscopic scanning,
- Computerised Tomography (CT) scans,
- Ultra sound.
- Nuclear Medicine Imaging.
- Gene detection.
- Electronic microbial detection.
- Biorhythm analysis.
- DNA analysis and comparison
- Biochemical synthesis analysis.
- RNA fingerprinting.
- Life Support Systems.
These are some of the many fields of the biomedical sphere where the signal processing is used. The sphere is increasing incredibly day by day. And the advancement in each of these fields is also at an incredible rate. These systems have revolutionised the medical sphere by leaps and bounds. These systems have made life easy and increased the life expectancy of people by decades.
A brief explanation of the main ones of these systems is given in the following pages.
The electrocardiography is better known as an ECG. This technique is used to record the electrical impulses which immediately precede the contractions of the heart muscle. This method causes no discomfort to a patient and is often used for diagnosing heart disorders such as coronary heart disease, pericarditis or inflammation of the membrane around the heart, cardiomyopathy or heart muscle disease arrhythmia and coronary thrombosis.
How an ECG works:
When cell membranes in the heart depolarise, voltages change and currents flow. Because a human can be regarded as a volume conductor, changes in potential are transmitted throughout the body, and can be measured. When the heart depolarises, it is convenient to represent the electrical activity as a vector between two point charges.
An ECG is recorded by placing electrodes on the surface of the skin.
The ECG measures the electrical activity of the heart. This electrical activity controls the heartbeat. Special cells called pacemakers release bursts of electrical energy which travel through the heart muscle, causing it to contract and relax. There are valve movements between the two cycles. As a result of the electric pulses, the electrodes record some charge. This charge is displayed over a graph paper.
The ECG works in the following steps:
Data is acquired from the electrodes of the ECG. The ECG machine then reads the direction of the flow of the charge in the human body after the amplification.
The heart gives different direction of flow of electric impulse charges during its rhythm. As such, by the detection of the direction of the flow of charges, the rhythmic movement of the hearth can be detected. There are some set parameters for the normal heart rhythms. If there is any abnormality, then it can be judged by comparing with normal values.
Generally, the result of an ECG is obtained on a graph paper as:
This signal is in the form of a wave on the graph paper. This signal can be read in the form of values on the graph.
Advantages- was the first and currently the most effective way of the analysis of the rhythm of the heart.
Disadvantages- External interference of even a very small charge may adversely effect the output.
MRI SCANS: Magnetic resonance imaging is a medical imaging technique used in radiology to visualize detailed internal structures. The good contrast it provides between the different soft tissues of the body make it especially useful in brain, muscles, heart, and cancer compared with other medical imaging techniques such as computed tomography (CT) or X-rays. MRI is a fairly new technique that has been used since the beginning of the 1980s.
The MRI scan uses magnetic and radio waves, meaning that there is no exposure to X-rays or any other damaging forms of radiation.
The patient lies inside a large, cylinder-shaped magnet. Radio waves 10,000 to 30,000 times stronger than the magnetic field of the earth are then sent through the body. This affects the body's atoms, forcing the nuclei into a different position. As they move back into place they send out radio waves of their own. The scanner picks up these signals and a computer turns them into a picture. These pictures are based on the location and strength of the incoming signals.
Our body consists mainly of water, and water contains hydrogen atoms. For this reason, the nucleus of the hydrogen atom is often used to create an MRI scan in the manner described above.
Using an MRI scanner, it is possible to make pictures of almost all the tissue in the body. The tissue that has the least hydrogen atoms (such as bones) turns out dark, while the tissue that has many hydrogen atoms (such as fatty tissue) looks much brighter. By changing the timing of the radio wave pulses it is possible to gain information about the different types of tissues that are present.
PROCESSING OF SIGNALS IN AN MRI:
It is this relationship between field-strength and frequency that allows the use of nuclear magnetic resonance for imaging. Additional magnetic fields are applied during the scan to make the magnetic field strength depend on the position within the patient, in turn making the frequency of the released photons dependent on position in a predictable manner. Position information can then be recovered from the resulting signal by the use of a Fourier transform. These fields are created by passing electric currents through specially-wound solenoids, known as gradient coils. Since these coils are within the bore of the scanner, there are large forces between them and the main field coils, producing most of the noise that is heard during operation. Without efforts to dampen this noise, it can approach 130 decibels (dB) with strong fields.
An image can be constructed because the protons in different tissues return to their equilibrium state at different rates, which is a difference that can be detected. Five different tissue variables ââ‚¬" spin density, T1 and T2 relaxation times and flow and spectral shifts can be used to construct images. By changing the parameters on the scanner, this effect is used to create contrast between different types of body tissue or between other properties, as in fMRI and diffusion MRI.
A typical example of an MRI signal in RAW form is as under:
Health care professionals use MRI scans to diagnose a variety of conditions, from torn ligaments to tumours. MRIs are very useful for examining the brain and spinal cord.
Advantages- Not harmful like CT scans and X-Ray scans.
Disadvantages- Costly, and sometimes may require fasting by the patient before diagnosis. Else, it may give a huge error in the observations.
An ultrasound scan is a painless test that uses sound waves to create images of organs and structures inside your body. It is a very commonly used test. As it uses sound waves and not radiation, it is thought to be harmless. It is cyclic sound pressure with a frequency greater than the upper limit of human hearing.
Ultrasound scan is also called Medical Sonography or Ultrasonography. It is used to visualize muscles, tendons, and many internal organs, to capture their size, structure and any pathological lacerations with real time tomographic images. The technology is relatively inexpensive and portable, especially when compared with other techniques, such as magnetic resonance imaging (MRI) and computed tomography (CT). Ultrasound is also used to visualize fetuses during routine and emergency prenatal care.
Ultrasound travels freely through fluid and soft tissues. However, ultrasound is reflected back (it bounces back as 'echoes') when it hits a more solid or denser surface. For example, the ultrasound will travel freely through blood in a heart chamber. But, when it hits a solid valve, a lot of the ultrasound echoes back. Another example is that when ultrasound travels though bile in a gallbladder it will echo back strongly if it hits a solid gallstone.
So, as ultrasound 'hits' different structures in the body of different density, it sends back echoes of varying strength. These echoes are received by the receiving end of the sonograph. These signals are very weak signals. These are amplified and then processed.
The signals received after the amplification are as given above. These signals are still not perfect for signal processing. They require sampling and filtering. Filtering is required so that the output formation on the CRT screen is clear and easy to read. Signal tracing is applied for the purpose of sorting out the signals. It can be shown as:
These signals then are processed to make a 2-D image of the part being diagnosed on the CRT screen. Nowadays, the Ultrasound scans have started using LCD screens too.
This is the general output type signal of the ultrasound scans.
OTHER BIOMEDICAL SIGNAL PROCESSING AREAS:
Gene detectors: these are the microprocessor chips which are often placed in the bodies of organisms, generally animals. These have different functions. They are controlled with wireless radio frequency modulated signals. These are generally used to study the effect of various genes on the body, the different ailments in them and the metabolic action of the body.
These chips record the signal through the spinal impulses or the electrolytic properties of the body fluids. Then process the signal, transfer it to the receiver unit, where the analog signal is processed and the required output is visualised.
This is generally used in studying the psychiatric behaviour of a human being or an animal. This technique is similar to ECG in terms of operation. The difference is that this maps the brain of the individual. It uses the electrical signals of the nerve impulses so as to track the nervous rythms of the brain.
This technique is quite successful in lie detector tests, depression detection tests, hypertension detection tests.
From the above mentioned content, it has been made clear that the DSP is the most important part of the biomedical signal processing. The principle key factor of the working of these instrumentation systems is the signal processing. It is the signal processing that enables to change one form of signal to other. As such, it can be concluded that the Biomedical Signal processing has been the key factor in the detection, research and analysis fields of the Biomedics.
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