Biomedical Instrumentation Questions & Answers

Q1. What are the problems encountered in biomedical measurements?

The problems encountered in biomedical measurements are:

• Variability of variables – the measurements taken under fixed set of conditions at one time may not be the same under the same conditions at another time. Therefore, physiological variables can never have definite values but these can be represented by some probabilistic distributions.
• Inaccessibility of variables – in a living system, it is impossible to gain access to many variables such as dynamic neurochemical activities, activities in the brain, etc. For this reasons, indirect measurements of these variables have to be made.
• Poor interrelationship knowledge – better understanding of interrelationship of physiological variables is unavailable. That is why, physiological measurement with large tolerances are often accepted by the physicians where indirect measurements for inaccessible variables are used.
• Interaction among physiological signals – given that many physiological signals are being generated by the body, and these have some degree of interaction which present a problem in measurement.
• Artifacts –any outside signal may affect the variable being measured by the biomedical instrumentation system.
• Effect of transducer – any measurement is affected in some way by the presence of the transducer in the biomedical instrumentation system.

Q2. What are the key requirements for specifying biomedical instrumentation systems?

They key specifications to consider when specifying biomedical instruments include:

• Sensitivity – this is the measure of the change in the output of an instrument for a change in the measured variable. The sensitivity determines the minimum variation that the instrument can accurately read. The requirement of sensitivity varies from one application to another hence sensitivity of the biomedical instrument has to be specified.
• Range – the range of an instrument specifies the lowest and highest readings it can measure.
• Linearity – this is the measure of the proportionality between the actual value of a variable being measured and the output of the instrument over its operating range. It is desirable to have linear scale as much as possible over the entire range of measurements otherwise this linear range should be specified.
• Hysteresis – this is the difference in readings when an instrument approaches a signal from opposite directions i.e. if an instrument reads a midscale value going from zero, it can give a different reading from the value after making a full-scale reading. This is may be due to stresses induced into the material of the instrument changing its shape in going from the zero to full-scale deflection. Hysteresis range should be specified in which measurements differ depending upon whether the measurement is taken is ascending or descending order.
• Accuracy – the accuracy of an instrument is the difference between the indicated value and the actual value. Accuracy varies from one instrument system to another. The instrumentation system has to be selected as per the accuracy necessity for the measurement.
• Frequency response – an instrument has different responses depending upon frequency and its measurements depend on the frequency range. Therefore the frequency response must be specified in which the instrument can accurately measure.
• Isolation – the instrumentation system should not produce a direct electrical connection between the subject and the ground. The requirement for electrical isolation or other measures should be specified clearly.
• Signal-to-noise ratio – the signal-to-noise ratio should be as high as possible. It is necessary to specify the environment in which the instrumentation system can work properly to give high signal-to-noise ratio.
• Stability – the biomedical measurement system must be capable to achieve a steady-state condition after each reading. The range in which the instrumentation system has stability has to be specified.
• Simplicity – the biomedical instrumentation system should be simple to operate and the procedure for the measurement must be specified.

Q3. What are the diagnoses made from Electroencephalogram (EEG)?

The brain cells are referred to as neurons. The recorded representation of bioelectric potentials generated by the activity of the brain (neuronal activity) is called the Electroencephalogram (EEG). The EEG waveform is very complex and much more difficult to recognize than the ECG. The waveform varies immensely with the positioning of the measuring electrodes on the surface of the scalp.

A typical example of EEG sample is illustrated in the figures below:

The frequency of the EEG is found to be affected by the mental activity of a person. An alert and wide awake person usually displays unsynchronized high frequency EEG while a person activity in sleep or having epileptic seizures often produces a large amount of rhythmic activity having low frequency in the range of 8 to 13 Hz. The frequency of EEG is classified into five bands for analysis purposes as:

1. Delta waves δ – 0.5 to 4 Hz
2. Theta ϴ – 4 to 8 Hz
3. Alpha α – 8 to 13 Hz
4. Beta β – 13 to 22 Hz
5. Gamma γ – 22 to 30 Hz

The waveforms may be further characterized and described as per table 1.

Table 1 EEG for different mental activities

EEG is a common method used for diagnosis of tumour, stroke and other local brain disorders. Since EEG is a measure of brain waves, it is readily available test that provides evidence of how the brain functions over a time. EEG is employed in the evaluation of brain disorders. Most commonly, it is used to show the type and location of the activity of the brain during a seizure. It is also used to evaluate people who have problems with brain functioning. These problems might include coma, confusion, and tumours, long time difficulties with thinking or memory or weakening of specific parts of the brain due to strokes. An EEG is also used to determine whether the person has brain death. It may be employed to establish if a person on life support equipment has no chance of recovery.

Q4. Describe how Electromyogram (EMG) is used in medical diagnosis?

The recorded representation of bioelectric potentials generated by muscle activity is called Electromyogram (EMG). These biopotentials may be measured at the surface of the body near a muscle of interest or directly from the muscle by inserting a needle electrode. The typical example of EMG waveform is illustrated below:

The potentials in EMG waveform range from 20 μV to 50 mV depending on the amount of muscle activity. The action potential in the waveform lasts for only few milliseconds. The duration of the waveform is in the range of 2 to 15 milliseconds. The frequency of the waveform ranges from 10 Hz to 2 kHz.

EMG is used to diagnose diseases that can be classified into three categories:

1. Neuropathies
2. Neuromuscular junction diseases
3. Myopathies

Neuropathic disease has the following important EMG characteristics:

• An action potential amplitude having value twice of normal amplitude. This happens with the increased number of fibres per motor due to reinnervation of denervated fibres.
• An increase in the duration of the action potentials.
• A decrease in the number motor units in the muscles.

Myopathy disease has the following important EMG characteristics:

• A decrease in the duration of the action potentials.
• A reduction in the area to amplitude ratio of the action potentials.
• A decrease in the number of motor units in the muscles.

Applications of EMG

EMG is used as:

• Diagnostic tool to identify neuromuscular diseases.
• As an interface device to control robots or electric wheelchair.
• Unnoticed speech recognition by observing EMG activity.
• A tool to control the flight systems (man-made interface).

Q5. Describe the types of electrodes used in biomedical measurements?

There are there basic types of biopotential electrodes namely:

1. Microelectrodes – these electrodes are designed to measure bioelectric potentials near or within a single cell.
2. Skin surface electrodes – these are designed to measure ECG, EEG and EMG potentials from the surface of the skin. Therefore, these types of Bioelectrodes are least traumatic.
3. Needle electrodes – these are designed to penetrate the skin so that it can record EEG potentials from a particular region of the brain or EMG potentials from a specific group of muscles.

The Bioelectrodes classified above, are used to measure the biopotential from the metal-electrolyte interface. That is, the electrode potential is developed across the metal-electrolyte interface which is proportional to the exchange of ions between the metal and the electrolyte of the body.

Microelectrodes

These electrodes are much smaller in cross-sectional area as compared to the size of the cell in which they are to be inserted so that their penetration doesn’t damage the cell. A cell is rarely larger than 500 microns. Hence, these microelectrodes should have tip dimension of about 5 microns and the tip should be strong enough to penetrate the cell without damage.

Microelectrodes can be of two types:

1. Metal
2. Micropipette

Metal microelectrodes are formed from a fine needle of a suitable metal. Then the needle is coated almost to the tip with an insulating material.

Micropipette microelectrode is a microcapillary made of glass which is filled with an electrolyte as shown below:

The metal microelectrodes are used in direct contact with cell and they have lower resistance but these electrodes tend to develop unstable electrode offset potentials. The micropipette microelectrodes have dual interface. One interface is formed by a metal wire in contact with the electrolyte solution filled in the micropipette while the other interface is formed between the electrolyte inside the micropipette and the fluids inside or immediately outside the cell. The micropipette microelectrodes tend to develop stable electrode offset potentials and they are preferred where steady-state potentials measurements are required. However metal electrodes have the following advantages:

• Lower impedance.
• Repeatable and reproducible performance.
• Infinite shelf life.
• Easy cleaning and maintenance.

Body surface electrodes

The body surface electrodes are designed and used to measure bioelectric potentials from the surface of the body. They are available in many sizes and forms. They are used to sense ECG, EEG, and EMG potentials.  The larger electrodes are usually used for sensing of ECG potentials as these measurements do not depend on the specific localization of electrodes. But, for sensing of EEG and EMG potentials, smaller electrodes are used as sensing for them depends upon the location of electrodes or measurement. Metal plate and suction cup type electrodes are body surface electrodes but they have a common problem i.e. the possibility of slippage or movement. These electrodes are sensitive to movements thereby producing wrong measurements on shifting. To avoid this problem, the floating electrodes are used. The principle of the floating electrode is to eliminate the movement artifacts (false signals) by avoiding the direct contact of the metal electrode with the skin. The contact between the metal electrode and skin is maintained by the electrolyte paste or jelly as illustrated below:

Needle electrodes

These electrodes are designed to penetrate the skin surface of the body to some depth to record EEG potentials of a region of the brain or EMG potentials of a muscle. The electrodes have to be sharp and small like subdermal needles which help them to easily penetrate the scalp for measuring EEG potentials.

They are required to penetrate up to some surface at certain depth of the skin which is parallel to the surface of the brain or muscle.

Q6. Explain the method used to measure heart sounds

The beating of the heart and the pumping of the blood is associated with the generation of sounds. The technique of listening to sounds produced by the heart and blood vessels is called auscultation. The physicians are trained to diagnose the heart disorders by listening to these sounds. Stethoscope is the device used to listen to these sounds. The ‘’lub-dub’’ are two distinct sounds that are hearable by the help of stethoscope with each heartbeat. The ‘’lub’’ is produced when the atrioventricular valves close and prevent any reverse flow of the blood from ventricles to atria. The ‘’lub’’ is also called the first heart sound is produced by the closing of the pulmonary and aortic valves. The ‘’dub’’ is also called the second heart sound and it occurs about the time of the end of T wave of the ECG. The third sound is sometimes produced after the second sound by the rushing of the blood from atria to ventricles.

A graphic record of heart sound is called phonocardiogram. The transducer that is used for the phonocardiogram is a microphone having the necessary frequency response generally ranging from 5 Hz to 1000 Hz. An amplifier with similar characteristics is used with suitable low-pass filters to block noise signals.

Q7. What are the special characteristics of blood flow?

The special characteristics of blood flow are:

• The Reynolds number of the blood is high (range 10,000) which results into a larger entry length. The entry length is the length in the vessel in which the 99% of the velocity profile is achieved. In most cases in the blood vessels, the fully developed flow is never achieved as the blood start branching before this stage is attained.

• The blood flows through the blood vessels which have different properties. The reason for the different properties is that the blood vessels are formed of different substances such as elastic, collagen and smooth muscles.
• The blood vessels have to bend into unusual curvature in the blood circulating system which leads to secondary flow in some cases.
• Unusual large bifurcation or branching of the blood vessels. There are millions of blood vessels in the blood circulating system.
• The turbulent flow may arise due to stenosis or obstruction in the circulating system. It may also arise due to defective valves.
• The blood vessels, especially veins can contract or enlarge. The veins can therefore enlarge and act as reservoir to store more blood.
• The blood has unusual pulsating nature, which arises from rhythmic action of the heart. The pressure and volume increase during systole which gradually decreases during diastole.
• Lastly, the blood has unusual fluid properties. The blood has millions of different corpuscles suspended in the plasma. These corpuscles can deform when they are required to pass through the blood vessels having diameter smaller than their own.

Q8. What is mean arterial pressure (MAP)?

The mean arterial pressure is given by the relation as shown below:

It is generally accepted that MAP is a direct indication of the blood pressure available for tissue perfusion and a continuously increasing or decreasing MAP beyond certain limits is dangerous. There are automatic devices designed to give alarms whenever the blood pressure of a patient crosses the laid down limits of MAP.

Q9. What is electrocardiograph (ECG) and what are the normal values of ECG parameters?

ECG is a graphic recording or displaying of the time-variant voltage produced by the heart during the cardiac cycle as illustrated below:

The waves are designed as follows:

1. P wave
2. QRS wave
3. T wave

The above waves are associated with:

1. Polarization, depolarization and repolarization of the muscles.
2. Contraction of atria and ventricles.
3. Opening and closing of the heart valves.

The P wave represents the depolarization of atria. The QRS wave represents combined effects of the repolarization of the atria and depolarization of the ventricles. T wave represents the repolarization of ventricles. The P-Q interval represents the time interval during which the excitation wave is delayed at AV node.

The normal values of ECG parameters are:

1. Amplitude: P wave = 0.26 mV, R wave = 1.50 mV, Q wave = 25% of R wave and T wave = 0.1 to 0.5 mV.
2. Duration: PR wave interval = 0.1 to 0.25 sec, Q-T interval = 0.34 to 0.45 sec, P wave interval = 0.1 sec and QRS interval = 0.08 sec.

Q10. Why is a differential amplifier preferred for use as a biomedical measurement amplifier instead of electronic amplifiers?

Differential amplifier is preferred over electronic amplifiers because of the following reasons:

• Its ability to reject common-mode interferences which are can be problematic especially when dealing with small bioelectric potentials as with biomedical measurements.
• As a direct coupled amplifier, it has good stability and versatility.
• It has high stability because it can be insensitive to temperature changes; which is often the source of excessive drift in other configurations.

Related: Biomedical Instrumentation Multiple Choice Questions & Answers (MCQ1)