Magnetic Resonance Imaging (MRI) Scan Test

MRI Introduction:

Magnetic Resonance imaging or Nuclear Magnetic Resonance (NMR) is a medical imaging technique which is commonly used in radiology to visualize the internal structures and functions of the body. An MRI scan is a technique that uses magnetism, radio waves and computers to produce images of body structures. Unlike other imaging techniques it uses a powerful magnetic field to align the nuclear magnetization of hydrogen atoms in water in the body. It provides high quality 2-D or 3-D images of organs without using X-rays or any other radiations.

Magnetic Resonance Imaging Scan Test

The magnet creates a strong magnetic field that aligns the protons of hydrogen atom, which are then exposed to a beam of radio waves. The technique is based on the principle that when the hydrogen atoms of water molecule are subjected to a strong magnetic field hydrogen atoms release protons. That is when a person goes inside the powerful magnetic field of the scanner, the magnetic moments of these protons align with the direction of field. The magnet is the largest and most expensive component of the scanner. Different magnets such as permanent magnet, resistive electromagnet, superconducting electromagnet etc can be used. Magnetic field strength is also an important factor because as the magnetic fields increases the signal-to-noise ratio also increase which results in higher resolution of the image. An RF electric field is then turned on which causes the protons to change their alignment relative to the field. Again when this field is turned off the protons return to the original magnetization alignment. By applying additional magnetic field during scanning, the position of protons in the body can be determined and it allows an image of the body to be created on the computer. MRI can be particularly used in detecting tumors in tissues because the protons in different tissues return to their equilibrium state at different rates. By manipulating the return rate, the affected tissues can be distinguished. An MRI scan is a pinless radiology technique that avoids the terror of X-ray radiation exposure.

The MRI scanner cutaway is shown.

The different components of the MRI system are briefly explained below.

1. Magnet: In MRI system there is a magnet which is provided for a strong uniform steady magnetic field. Now superconducting magnets are used commonly. The superconducting magnets can provide better Signal-to-noise ratio and better image quality than the output obtained through conventional magnets.

2. RF coils: Different RF coils used are
1) Surface coil. 2) Paired saddle coil. 3) Helmholtz pair coil. 4) bird cage coil

Surface coils are the simplest which are commonly used for spines, shoulders and other relatively small body parts.

Paired saddle coils are commonly used for imaging of knee. These are also used for the xand y gradient coils.

The Helmholtz pair coils consist of two circular coils parallel to each other. They are commonly used as the z gradient coils in MRI scanners.

The fourth type bird cage coil can provide the best homogeneity of all the RF coils. The name implies that it has the appearance of a bird cage. This coil is commonly used as the transceiver coil for the imaging of head.

3. Gradient coils: Gradient coils are of so importance as they are used to produce deliberate variations in the main magnetic field. There are usually three sets of gradient coils one for each direction. We have to vary the magnetic field because it permits the localization of image slices, phase encoding, frequency encoding etc. Changing magnetic field induce electrical currents in the conductor.

The block diagram of MRI hardware is shown below.

Advantages and Disadvantages of CT Scan

Advantages of CT Scan:

1. The main advantage of CAT is that it completely eliminates the superposition of images of interest.
2. Because of the high contrast resolution of CT, differences between tissues can be distinguished.
3. Images in the axial coronal planes can be done which is known as multiplanar imaging.
4. The improved resolution of CAT has led to deep researches on body parts. CT is considered as the popular high resolution imaging technique.

Disadvantages and challenges with CT Scan: 

1. During CAT procedure, many patients may experience severe allergic reactions .
2. The contrast agents used in CAT system can lead to chronic problems such as kidney damage.
3. The main issue with CAT is to reduce the radiation dose during the procedure. But as it may affect the image quality usually we have to use high radiation doses which can affect the patient.
4. In CAT as radiation dose increases the risk of radiation induced cancer may also increase,

Applications of CT Scan:

1. CT scanning of head is used to detect conditions such as blood clot or bleeding within the brain stroke, brain tumors, malformations of skull etc.
2. CAT of head can be used to plan radiation therapy for cancer of brain.
3. It can be used to detect chronic changes in the internal parts of lungs.
4. Imaging of coronary arteries can be done with the help of high speed high resolution multi-slice CT.
5. Diagnosis of abdominal diseases can be used to determine the different stages of cancer.
6. Another advantage of CT is that it can be used to image highly complex fractures. It is because of its ability to reconstruct the area of interest in multiple planes.
7. CT angiography of chest is becoming a primary method for detecting aortic dissection.

Computerized Axial Tomography How it Works

Computerized Axial Tomography How it Works:

Computerized Axial Tomography (CAT) is the medical imaging method employing tomography created by computer processing. It is also known as computed tomography. In CT imaging X-rays are used in conjunction with computing algorithms to image the body. In computed tomography a volume of data is produced. This data can be processed and analyzed using different techniques and the various bodily structures can be demonstrated based on their ability to block X-rays. Tomography is derived from Greek words namely ‘tomos’ meaning 'slice' and ‘graphen' meaning ‘to write'. So by using CAT technique we can produce clear 2-D or 3-D cross sectional images of deep internal organs.

Scanning components and principle of operation

CT scanning can be the best choice in diagnosing some urgent conditions such as clots in the arteries of lungs, tearing of aortic wall, appendicitis, obstructing kidney stones etc. ln CAT otherwise called as Computerized Axial Tomography X-ray slice data is generated using an X-ray source that rotates around the object. X-ray sensors are placed on the opposite side of the circle from the X-ray source. The earliest sensors were the scintillation detectors with photomultiplier tubes. But nowadays we use scintillation systems based on photodiodes instead of photo multipliers and modern scintillation materials with more desirable characteristics. In Computerized Axial Tomography, a computer generated cross-sectional image (tomogram) is produced with the effect of an X-ray generator and X-ray detector in a ring shaped apparatus. The tomographic reconstruction is done using proper computational algorithms. The main scanning components are illustrated in the block in figure. It is relatively a new method of forming images from X-rays. Here measurements are taken by passing X-rays through the body and contain information from almost all parts of body. Many data scans are taken progressively from the object and they are combined together by the mathematical procedures known as tomographic reconstruction. The data are arranged in a memory, and each data point is convolved with its neighbors according with a seed algorithm using Fast Fourier Transform techniques. This data can then be displayed, photographed, or used as input for further processing, such as multi-planer reconstruction.
CAT scanner block diagram
1. X-ray source: The X-ray generating tube generates and direct-rays towards the subject. During, examination) a low dose of X-ray beam passes through the body around 360 degrees in a rotating table. In CAT, measurements are taken from the transmitted X-rays through the body. These contain information on all parts of the body in the path of X-ray beam. The radiation dose for a particular study depends on many factors such as volume scanned, number and types of scan sequences, the desired resolution of image and the image quality. The intensity of X-rays can be regulated by controlling then anode voltage, beam current etc.

2. Rotary table: The subject is placed in a rotary table. During the process, the tissues absorb small amount of radiation depending on the intensity of X-ray beam. There are two types of CT scanners. They are body scanners and head scanners. As the name indicates, the body scanners scan the whole body while the head scanners scan only the head. CT exposes the patient to more ionizing radiation than a radiograph. The main issue with CT examinations is how to reduce the radiation dose during examinations. We can reduce the radiation dose up to an extent but reducing the radiation dose can affect the image quality. But several methods can be used to reduce the radiation dose. One such method is using software technology. Here the software works as a fitter that reduces random noise.

3. X-ray detector: The X-ray detectors are placed in a ring shaped apparatus which rotate around the patient. They detect the X-ray beam intensity of each tissue and feed it to a computer. Actually the rays coming out of the detector are converted into electronic signals due to light sensitive nature of the detectors. Multidirectional scanning detectors are used to obtain more accurate tomogram. Detectors such as Spiral multi-detector can used which utilizes 8, 16 or 64 detectors during continuous motion of patient through the radiation beam to obtain much clearer images. MDCT (Multi Detector CT) scanners give high resolution and image quality. Nowadays faster scanning times and improved resolution have increased the accuracy of CT scanning.

4. Reconstruction of tomogram : CT produces a group of data which can be manipulated and processes to demonstrate various bodily structures based on their ability to block the X-ray beam. It is called ‘windowing’ technique. The reconstruction of tomogram is done by using a suitable computational algorithm using a computer. By using a computer the image can be produced in a television screen. This is called tomogram and it can provide a very accurate cross sectional view of any area of the body. To reconstruct the image, a number of mathematical operations have to be done and for this we use different computational tools.

Multiplaner reconstruction (MPR) is the simplest tomographic reconstruction method. Digital geometry imaging systems are used to generate a three-dimensional image of the inside of an object. Modern software allows reconstruction of the tomogram in many planes so that any plane can be selected to display an anatomical structure. This may be useful for visualizing the structure of extremely small elements of body such as bronchi.

Block Diagram of X-ray Machine

The block diagram of X-ray machine is shown in figure below. The function of each block of an X-ray machine is also explained below.

block diagram of x-ray machine
1. Multitap ac transformer 

We use a multi-tap ac transformer in order to select taps to compensate for incoming line variations. The number of outputs is referred to as 'taps' and it may range from 2 outputs to many outputs depending on the type of multi-tap transformer used. The advantage of multi- tap transformer is that it has different taps in different voltages. So we can select a higher voltage tap or lower voltage tap depending on the intensity of X-ray exposure needed. These also permit the operator to choose voltages for specific applications.

2. X-ray tube filament transformer:

This transformer transforms the ac line to supply power for heating the cathode filament. This power can be selected by taps to change the filament heat which in turn change the X-ray tube current and total energy delivered to the patient.

3. X-ray tube high voltage transformer and bridge rectifier 

This block together transforms the ac line to supply the high dc voltage for accelerating the electrons from cathode to anode. The high dc voltage is selected by taps.

4.Timing circuit  

Timing circuit is used to control the turn-on, turn-off and length of X-ray exposure delivered to the patient. It consists of an electronic counter that applies high voltage to the X-ray tube anode for short periods of time.

Advantages of X-rays in medicine:

1. X-ray can be used to produce an image of any body parts.
2. It is also available as a portable unit which can be used in hospitals widely and X-rays can be taken anywhere even in bedside.
3. It is less costly when compared to other imaging models like MRI scan.
4. It can produce fast results.
5. It is a comparatively easy technique.

Applications of X-rays in medicine: 

1. X-ray machines are used in healthcare for Visualizing bone structures and other dense tissues such as tumors.
2. The two main fields which use X-ray machines are radiography and dentistry.
3. Radiography is used for fast and highly penetrating images.
4. By using X-rays cancer cells can be treated in radiotherapy.

X-Ray Tube Working Principle

Radiology - Basic X-Ray Machine

It is a special branch that deals with the study and application of imaging technology using X-ray radiations or such other radiation devices for the purpose of obtaining visual information to diagnosing and treatment of diseases.  X-ray machines are devices that generate exceedingly high frequency high energy electromagnetic waves that penetrate the body during medical procedures to provide visual information. The x-ray tube working principle and diagram is shown below.

Generation of X-rays in X-ray tubes.

An X-ray generator is a device used to generate X-rays .An X-ray imaging system consists of an X-ray source or generator (X-ray tube) and an image detection system. The X-ray tube (high vacuum diode) operates by emitting electrons from a heated cathode tungsten filament toward a rotating high voltage anode disc. The point where the electrons (beam) strike the target is called the focal spot. At the focal -spot X-ray photons are directed at all directions. X-rays arise from the target disc at right angles and are focused by a collimator. For more viewing contrast we use, photomultipliers. The images are received and viewed on a photographic plate. Here light and dark areas on the film represent high and low tissue penetration. The basic schematic of an X-ray tube is shown below.
X-ray machines work by applying controlled voltage and current to the X-ray tube. So the beam intensity of X-rays can be controlled by controlling voltage or current. The beam is projected on the object. Some of the beams will pass through the object and some are absorbed. The resulting pattern of radiation is detected in a photographic film as told earlier. In an X-ray tube, the rotating anode is used to overcome the overheat problem. Also the anode is made of tungsten alloy which helps in avoiding over heat.

Advantages and Disadvantages of Ultrasonography

Advantages of Ultrasonography

1. It has no long term side effects and comfortable
2. It images the muscles, bone surfaces, soft tissues etc well
3. By using ultrasonic system, the physician can easily diagnose the various diseases.
4. Relatively inexpensive when compared with systems such as magnetic resonance imaging.
5. It shows the structure of organs.
6. Equipment is readily available and comparatively 'flexible.
7. Examinations can be performed at the bedside by using small and easily carried scanners.

Disadvantage of Ultrasonography

1. The performance of sonographic system degrades when there is gas between the transducer and the organ of interest It is due to the change in acoustic impedance.
2. The method of ultrasonography is operator dependent. A high level skill and experience - is needed for better imaging.
3. There may difficulties in imaging structures deeply in the body especially in abnormally fat patients.
4. Once an image has been acquired, there is no exact way to tell which part of the body was imaged.
5. A major disadvantage is that sonographic devices have the trouble of penetrating the bone.

Applications of Ultrasonography

1. Medical ultrasonography is used in the study of many different systems such as cardiology, ophthalmology, neurology etc.
2. Sonography is highly effective in imaging soft tissues of the body.
3. Ultrasound may be used to clean teeth in dental hygiene.
4. By using Focused Ultrasound Surgery (FUS) we can treat for tumors and cysts. Here focused ultrasound is used to generate highly localized heating.
5. Focused ultrasound can also be used to break up kidney stones.
6. Low intensity ultrasound exposure can help in stimulating bone-growth.
7. High Intensity Focused Ultrasound (HIFU) is used for the treatment of disease: such as cancer.

Ultrasonic Blood Pressure Measurement

Ultrasonic Blood Pressure Measurement:

Ultrasonic techniques can he used to measure arterial blood pressure indirectly with the same method as used for flow detection. Here piezoelectric crystals are placed between the patients arm and a blood pressure cuff. The ultrasonic generator generates pulses and these are made to fall on the bronchial artery. Due to the effect of pulses, the blood flow varies. This causes Doppler shift. The Doppler shift is measured by the ultrasonic circuits.

To measure Blood pressure, first the bronchial artery is occluded and since there is no relative velocity between the transmitter and the receiver this time, the Doppler shift is zero. But as the Blood pressure rises so that the arterial pressure is able to overcome the occlusion, a frequency shift is produced (200-500Hz range). This is proportional to systolic pressure. Then as the arterial pressure reaches the diastolic pressure, the frequency shift due to Doppler effect is around 25-100 Hz. So by comparing the frequency shift caused due to each heart beat the blood pressure can be measured by comparing the frequency shifts obtained each time with the standard reference values.

Ultrasonic Flow Meter Diagram

Transcutaneous flow detectors are designed to use the Doppler effect to detect the flow of blood in arteries close to the surface of the body. Here we use two piezoelectric crystals. One for the transmit function and the other for reception. These two piezoelectric crystals are placed in a plastic housing and positioned over a peripheral artery. The schematic ultrasonic flow meter diagram is shown in figure.
ultrasonic flow meter diagram
An oscillator of desired frequency (say 10 Mhz) is used to produce an alternating signal which is to be applied to the input transducer. The oscillations cause the piezoelectric crystal to generate pulses. Suppose the blood is moving at the transmitter piezoelectric crystal (input) with a certain flow rate. The pulses cause the velocity (flow rate) of blood to change when it reaches the receiver piezoelectric crystal which is placed near the transmitter, due to change in velocity between transmitter and receiver piezoelectric crystals. Doppler effect will come into play. So there will be a drift in frequency at the receiver side. The moving blood produces a shift that is proportional to the blood flow rate. A frequency shift of approximately 200 Hz at 10 Mhz corresponds to a blood velocity of approximately 6 cm/s. By exactly calculating the frequency shift, we can indirectly convert it to the flow rate of blood. The signal at the output of the receiver crystal is amplified and then fed to the detector.

Ultrasound Scan Modes

There are mainly four modes of sonography

M - mode (T-M mode)
Doppler mode

1. A- scan mode:

 The A-mode ultrasound is used to judge the depth of an organ or otherwise estimate the dimensions or an organ. A-mode scan is the simplest and oldest scanning mode. In A-scan mode a single transducer fires sound pulses into the tissue and the echoes are analyzed. The received sound impulse is processed and it appear as a vertical reflection of a point. In the display it look like spikes of different amplitudes. Strong echoes are marked with greater amplitude and weaker echoes are marked with less amplitude in an oscilloscope. Hence it is also called Amplitude (A) mode. The A-mode ultrasound could be used to identify structures normally located in the midline of brain.
2. B-scan mode:

B-scan mode is called so because here the stronger echoes are marked with greater brightness. Also it is a two dimensional imaging system. Here the transducer is scanned back and forth to create a 2-dimensional view. Here sweeping a narrow ultrasound beam through the transmitting pulses and detecting echoes along closely spaced scan lines produces the B-scan images. Ultrasound beam is swept repeatedly to generate a series of individual 2D images that show motion.
This is another modified B-scan mode called modified 2-dimensional B-scanning. Here the transducer is at fixed position and it produces pulses. These pulses are directed to concerned body organ by two-dimensional scanning scheme.

We can also create a transducer array to create 2-dimensional view. Here different transducer arrays are used and each transducer can produce pulse in a particular direction. The transducer is selected by a rotary switch. Each transducer produces separate echoes and these echoes are united together to form a 2-dimensional view.

3. M-mode (T-M mode) scan:

Here M-mode scan means motion scan. In T-M mode scanning a sequence of B-mode scans are moved rapidly to enable to measure the range of motion. Hence it is also called Time-Motion mode (T-M). Here each instantaneous position in the scan produces depth information on one axis and time information on the other axis.

4. Doppler mode:

This is a special scanning mode to measure blood flow with the aid of Doppler effect. Doppler effect is the change of frequency that occurs when the transmitter and the receiver move relative to each other,
The frequency shift is given by F = 2v Cos Ø
Where Ø is the angle between the transmitter and the receiver, v is relative velocity.

Ultrasound Transducer Working

Ultrasound means acoustical waves with a frequency above human hearing(20KHz). Ultrasonography is an ultrasound based diagnostic imaging technique. It is mainly used to visualize internal body organs. Sonography is effective for imaging soft body tissues. Structures such as muscles, tendons etc are imaged at a low frequency.

Ultrasound Transducer Working: 

In ultrasound transducers mainly two types of conversions takes place
1) Conversion of ac oscillations into acoustic oscillations
2) Conversion of acoustical oscillations back into electrical vibrations

A creation of an image from sound is done in three steps. It is shown in figure below.

Producing a sound wave: 

A sound wave is typically produced by a piezoelectric transducer. A piezoelectric element produces a voltage across their two surfaces when deformed. So when the crystal is at rest, no voltage is produced. If the crystal is deformed to the right voltage changes to one polarity and if it is deformed to the left it changes to another polarity.

The sound wave is typically produced by a piezoelectric transducer. Strong electrical pulses from the ultrasound machine make the transducer oscillate at desired frequency. The sound is focused to the concerned body part either by the shape of the transducer or by the process called beamforming. This focusing helps the wave travel into the body and focus at a desired depth. The sound wave is partially reflected from the layers between different tissues. Some of the reflections return to the transducer.
Receiving the echoes:

The returning reflections make the transducer to vibrate and these vibrations are reconverted into electrical pulses. The pulses are passed through an ultrasonic scanner and they are processed and transformed into digital images.

Forming the images:

The ultrasonic scanner determines how long it took the echoes to return back to the transducer after the sound wave was sent. It will also determine the strength of the echo. The time taken by the echo to travel back to the probe is measured and it is used to calculate the depth of the tissue interface causing the echo. After this reception of echoes the ultrasonic scanner produces a digital image.

ECG Machine Block diagram and working

ECG Machine Block diagram and working:

Normally we use the above set up for the measurement and plotting of ECG. The main blocks of an ECG machine and the function of each block is explained below.

1. Electrodes: We know the ECG electrodes mainly used for the pickup of ECG are five in number. By placing these electrodes at appropriate parts of body.

2. Lead selector:  As told earlier each pair of lead conveys certain information. So for the appropriate waveform or view we have to select an appropriate lead pair. The lead pair can be selected by a lead selector switch which can be switched to different lead pairs according to the type of waveform needed.

3. Pre-amplifier: The ECG signal is having very weak amplitude levels. So it is necessary that for proper analysis and plotting purpose , the waveform is to be amplified. The pre-amplifier used here will be an operational amplifier or instrumentation amplifier with high gain. They have High CMRR and extremely high input impedance.

4. Driver:  We use a driver motor of suitable specification to drive the paper roller. Normally the ECG waveform is to be plotted on a moving chart paper to find out the irregularities (if there is ) in the P,Q,R,S,T and U regions of the ECG waveform .So the paper movement and the moving speed can be controlled by the driver motor, which supplies the trigger the roller.

5. PMMC Galvanometer: PMMC (Permanent Magnet Moving Coil) galvanometer is a special type of device, where the deflection of the coil depends on the amplitude and the polarity of the signal applied to its input. The writing tip of the hot tip pen is connected to the chart paper. So the pen will be at rest in the center of its travel when no current flows in the coil. So the direction of deflection in the coil and the amount of deflections is determined by the amplitude and polarity of the ECG waveform.

6. Hot - tip stylus and stylus heater:  In most common ECG recording techniques, we use hot tip stylus for thermal writing. It is because, we normally use thermal recorders for the plotting of waveforms. The stylus is kept hot always by the stylus heater power supply. The writing tip is a stylus heated by a resistance wire.

7. Recorders : As discussed now, usually we use thermal recorders for the representation of ECG waveform. The paper used in thermal recorders is of special material which turns black when heated. The hot tip of the stylus will turn the white paper black whenever it touches. The tip of the stylus moves in accordance with the movement of the coil which in turn is proportional to the amplitude and polarity of the ECG waveform. Since the tip of the stylus is in contact with the thermal chart recorder, a clear representation of the ECG waveform is obtained.

ECG Measurement System


There are two types of leads used for the ECG measurement System. They are called unipolar and bipolar leads. The unipolar electrode have only one true pole(positive pole).The bipolar leads have one positive and one negative pole. In the standard ECG recording, there are five electrodes connected to the patient. They are

I) Right Arm (RA).
2) Left Arm (LA).
3) Left Leg (LL).
4) Right Leg (RL).
5) Chest( C).

The standard lead I is the signal between the Right Arm ( RA) electrode (negative) and the Left Arm(LA) electrode(positive).Standard lead II is the signal between the negative Right Arm (RA) electrode and the positive Left Leg(LL) electrode. Lead III is the signal between the negative LA electrode (Left Arm) and the positive Left Leg (LL) electrode.(Refer figure) Three augmented limb leads are also shown in the figure .These are 'aVR', ‘a VL' and 'a VF'.

• The lead 'a VR' or augmented vector right has a positive electrode on the right arm. The negative electrode is the combination of left arm electrode and left leg electrode which augments (increases the strength) the signal strength of positive electrode on the right arm.Hence this electrode is called augmented vector right.

• Lead 'a VL' or augmented vector left has a positive electrode on the left arm. Here the negative electrode is the combination of right arm electrode and the left leg electrode which augments the signal strength of positive electrode on the left arm.

• Lead 'a VF' or augmented vector foot has the positive electrode on the left leg. The negative electrode is a combination of the right arm electrode and the left arm electrode which augments the signal of the positive electrode on the left leg.
The different electrodes are connected to the inputs of differential amplifier through a lead selector switch. The recordings obtained by placing different pair of electrodes results in different waveform shapes and amplitudes. These different waveforms are called views. An important point about the lead placement to he noted is that each lead conveys a certain amount of information that is not available in the other leads. The physicians are expertise in diagnosing the type and site of heart disease by examining the ECG waveforms or views. Interpretation of ECG relies on the fact that different leads can give different views from different angles. This has the benefit that the leads which are showing problems can be used to guess which region attic heart is affected. Each of 12 views created by the ECG is shown in a short segment in the printed ECG report generated by the ECG machine. Often a longer reading of one oldie 12 views is printed out in a rhythm strip.


As told earlier, we normally use three standard leads for ECG measurement System. During his work with ECG from these three standard leads Einthoven put forward a new postulation. He studied the relationship between these electrodes, forming a triangle where the heart electrically constitutes the null point. The relationship between the standard leads Einthoven triangle. The points of this triangle represent the electrode positions for the three limb leads. He proved that at any instant of the cardiac cycle, the frontal plane representation of the electrical axis of the heart is a two dimensional vector. Einthoven assumed that the heart is near the center of an equilateral triangle.( refer figure)
Einthoven's law states that the potential differences between the bipolar leads measured simultaneously will , at any given moment, have the values II = I + III. The above said Einthoven's rule is a relationship between the amplitude of the QRS complexes in each lead.

Schematic representation of a normal ECG waveform is shown in figure. A typical ECG representation of the normal cardiac cycle consists of a P- wave, a QRS complex and a T- wave. A small U-wave is also visible in most ECGs. Each segment of the ECG waveform is discussed below. The below given explanation is related to the standard ECG waveform. The shape, polarity etc may vary with the location of measuring electrode and the cardiac condition of patient.

Baseline: The horizontal segment of the waveform before the P- wave is called baseline or iso-potential line. The baseline voltage of the electrocardiogram is also knows as the isc electric line. Typically the basic line is measured as the portion of the tracing following the T-wave and preceding the next P- wave.

P- wave: Simply the P- wave represents the depolarization of the atrial muscles. During normal atrial depolarization, the main electrical vector is directed from the SA node towards the (AV node and spreads from the right atrium to the left atrium. This results in P- wave.

QRS Complex: The QRS complex is a recording of a single heart beat on the ECG that corresponds to the depolarization of both ventricles. So it is a combined result of re-polarization of atria and depolarization of ventricles.

PR interval: PR interval is measured from the beginning of the P wave to the beginning of QRS complex.

T - waveThe T-wave represents the recovery or repolarization of ventricles. The interval from the beginning of QRS complex to the apex of T- wave often referred to as absolute refractory period. The second half of the T- wave is called relative refractory period.

U - wave: Actually U- wave is the result of after potentials of the ventricular muscles. The U- wave is not seen in all ECG waveforms. It is typically small.

Integrated Injection Logic Operation

Operating Principles of I2L gates

Integrated Injection Logic Operation is explained as follows. To understand the working of a typical I2L gate, consider Fig. 3.46, which consists of five transistors, connected as shown. It can be seen that the base current IB1 of T1 is derived from +VCC through the input terminal B. The base-bias current IB3 of transistor T3 and the collector current IC1 of T1 are obtained from the collector current of transistor T2 whose injector (emitter) also is connected to +VCC.

If IB1 is sufficient, the base-to-emitter voltage VBE1 of T1 will become the saturation base-emitter voltage VBES1 (= 0.8 volt), and in this condition, its collector-to-emitter voltage VCE1 will become saturation base-emitter voltage VCES1 (= 0.2 volt). So, we find that T1 is in the ON-state and its output is at logic-0 level. Since VCES1 = 0.2 volt, the collector current IC2 of T2 will flow through the collector of T1 as IC1, and VBE3 of T3 = 0.2 V = VCES1. This means that at this moment, T3 is OFF and its collector-emitter voltage VCE3 = 0.8 volt.
Now, let the input voltage Vi = 0 V. This makes transistor T1 to be in OFF-state, and IC2 to flow through the base of T2. Thus, VBE3 = VBES3 = 0.8 V. This makes VCE1 = VBES3 = logic 1, and VCE3 = VCES3 = logic 0. Thus, the transistors perform inversion operation, with logic 0 = VCES = 0.2 V, and logic 1 = VBES = 0.8 V. The voltage swing of this gate, therefore, is

Vswing=VBES – VCES = 0.8 – 0.2 = 0.6 volt                                     
It can be seen that the logic swing of I2L gates is very low and this is a major defect of these gates.

I2L Gates for Performing Single Function

Figure 3.47 shows a typical I2L gate which performs the single function of logical inversion (or NOTing). In this case, MOS transistor N1 performs the inversion of input A to produce the output A'. MOS transistor N2 performs inversion of this output and produces A again. Thus the circuit shown in Fig. 3.47 performs the operation of double inversion.

I2L Gates for Performing Multiple Functions

Figure 3.48 shows a multi-function I2L gate that can perform the operations of  NOT, AND, NAND, and NOR. Notice that only transistors are used. However, an external resistor R may be used to derive the currents required for all the PNP transistors from a single +VCC supply. Notice that the PNP transistors are not shown in Fig. 3.48. 
Transistors T1 and T2 invert inputs A and B to produce A' and B′, respectively. One collector of T1 and one collector of T2 are now tied together to perform ANDing to produce the NAND output of A'B′. Using De Morgan, we find that this operation is also equivalent to the NOR output (A+B).. The other collectors are used to drive respectively, transistors T3 and T4, which now invert A' and B′ to give the AND output AB. T5 inverts AB to give (AB). We notice that all logical functions can be obtained from a single I2L chip. Since only transistors are used the packing density of I2L gate is seen to be comparable to that of the MOS logic; but the voltage swing is only 0.8 − 0.2 = 0.6 volt. Due to this small swing in the output voltage, the I2L technology has not progressed very much, and is not in much use now.

Integrated Injection Logic Circuit Diagram

We have seen that TTL technology using BJTs is not suitable for high-density VLSI circuits. This is due to the extensive use of large chip-area eating passive components such as resistors, inductors, and capacitors in the circuits to be integrated. Integrated-injection logic family (I2L) is a technology that makes use of only PNP and NPN bipolar junction transistors; use of resistors, capacitors, and inductors are completely avoided in this technology. Also, no isolation is required between individual transistors. With these specialties, it can be seen that the packing density of I2L gates has considerably increased. The basic device in this technology is the multi-collector structure shown in Fig. 3.45(a). This is similar to the multi-emitter TTL structure.

The device is manufactured with the substrate being heavily doped to form an N++ region. Over this, an N epitaxial region is grown, which forms the emitter. We diffuse two P+ regions into this N epitaxial region, one small and one quite large to accommodate as many collectors as required. The smaller P+ region forms the injector terminal. We find that the structure shown in Fig. 3.45(a) consists of a vertical multi-collector NPN transistor T1 driven by the lateral PNP transistor T2.

Figure 3.45(b) shows the circuit details of the I2L gate whose structural form is presented in Fig. 3.45(a). As can be seen from Fig. 3.45(b), the injector terminal is the emitter of the PNP transistor whose base is the N epitaxial layer, which also is the emitter of the NPN transistor. The collector of the lateral transistor is the larger P+ region. The collectors of the NPN transistor are designated as C1 and C2, respectively. Its base is designated as B in Fig. 3.45(b). The N++ terminal (not shown) forms the PNP base and NPN emitter shorted together to earth. Integrated Injection Logic Circuit Diagram is shown below

BICMOS Inverter Circuit Diagram

BICMOS logic circuits are made by combining the CMOS and bipolar IC technologies. These ICs combine the advantages of BJT and CMOS transistors in them. We know that the speed of BJT is very high compared to CMOS. However, power dissipation in CMOS is extremely low compared to BJT. By combining such advantages, we construct the BICMOS circuits.
Figure 3.43 shows one configuration of the BICMOS inverter, and Fig. 3.43 shows its modified version. In Fig. 3.43, we see that MOS transistors T3 and T4 form the CMOS inverter logic circuit. We find that T3 and T4 are driven separately from +VDD//VCC rail. With input voltage Vi = 0, the PMOS will conduct and the NMOS will remain OFF. This drives a current through the base of the bipolar junction transistor T1 and turns it ON. This in turn charges the parasitic load capacitance CL at a very fast rate. Thus the output voltage Vo rises very fast, which is usually much faster than the charging of CL by an MOS transistor by itself.
Now, let Vi = +VDD (≡ logic 1). Then T4 will turn ON and T3 will turn OFF; this drives T2 into the ON-state. Since T3 is OFF, T1 will also remain OFF. In this condition, CL discharges very fast through T2ON. Thus the charging and discharging of CL is through BJTs and hence very fast.
The circuit shown in Fig. 3.43 has two major defects. The first of these is that whenever the input is at logic 1, since T3 is ON, there will be a continuous path from +VDD to ground. As a result, steady power drain will occur in this case. This is totally against the advantages of CMOS gates. The second defect is that there is no discharge path for the base currents of T1 and T2. This will therefore reduce the speed of the circuit.
To overcome these problems, we modify the circuit, as shown in Fig. 3.44. In this case, NMOS transistor T4 has its drain connected to the output terminal rather than to +VDD. As before, when T3 is turned ON, T1 is also turned ON. Now, when T4 is turned ON, we find T2 also to turn ON.. However, since the collector and base of T2 are shorted together through T4ON, the output voltage Vo will now be equal VBES, the saturation base-emitter voltage of T2 (= 0.8 V). Thus in this case, the output swing is between VCC and VBES­.
Features of BICMOS gates:

1.    This has the advantages of both the BJTs and CMOS gates.
2.    The power driver (BJT amplifier) in the output stage is capable of driving large loads.
3.    The circuit, because of its CMOS input transistors, has high input impedance.
4.    The output impedance of the circuit is low.
5.    The noise margin is high because of the CMOS input stage.
6.    The supply voltage VDD is 5 V.
7.    The chip area is small.

Pass Transistor Logic Working

Transmission gates can be used to construct logic gates, since they can also be used as digital switches. In this mode of operation, the gates act as pass-transistors and the logic using pass-transistors as logic elements is called as pass-transistor logicPass Transistor Logic Working is given below.

Pass Transistor Logic for XOR Gate

Figure 3.41 shows a PTL EXOR gate. The working of the circuit may be explained as follows. As shown, the inputs to the pass-transistor gates (PTGs) 1 and 2 are A and A', respectively, and the control-gate voltages are B and B', respectively.

      Let initially A = B = 0. Then A' = B' = 1; this makes TG1 enabled and TG2 disabled. Under these conditions input A (= 0) gets transmitted through TG1 to the output, which makes the output Z = 0. Similarly, if A = B = 1, then A' = B' = 0 and TG2 becomes enabled with TG1 disabled. Under this situation, A' (= 0) gets transmitted through TG2 to make the output Z = 0 again.

      Now, let A = 1 and B = 0. Then A' = 0 and B' = 1. This makes TG1 enabled and TG2 disabled, and A (= 1) gets transmitted through TG1 to make Z= 1. Similarly, when A = 0 and B = 1, TG1 is disabled and TG2 is enabled. Then B (= 1) gets transmitted through TG2 making the output Z= 1 again. It can be seen that the four operations explained above suggest that Fig. 3.41 is an EXOR gate.

We know that a conventional CMOS EXOR gate requires twelve transistors for its implementation. However, the TG logic EXOR gate requires only eight transistors (four for the two inverters to form the A' and B' inputs, and four for the two TGs). Hence, pass-transistor logic gates are much smaller in chip area than their CMOS counterparts.
Pass Transistor Logic for Multiplexer

Figure 3.42 shows a 2 ×1 multiplexer using TGs. Here, the inputs A and B are applied to TG1 and TG2, respectively. The control inputs are C and C', respectively, as shown. Let C = 0, then C' = 1 and TG1 is enabled now; this makes A to be selected to Z. The output for this condition is Z = AC'. Next consider C = 1 and C' = 0, then we find that TG2 is enabled and B gets selected to the output. The output for this condition is Z = BC. The combined output now becomes Z = AC' + BC. The output expression suggests that the circuit shown in Fig. 3.42 acts as a 2x1 multiplexer. The 2x1 mux using pass transistor logic is shown below.

CMOS Transmission Gate Working

To study pass-transistor logic gates, we first discuss the CMOS Transmission Gate. The basic CMOS TG and its symbol are shown in Fig. 3.39(a) and (b), respectively. In Fig. 3.39(a), we find that the source terminals of a PMOS transistor and an NMOS transistor are tied together to form a single source of the complementary structure. Similarly, the drains are tied to form a single drain. However, the respective gates are left as independent. We will later find that these are the control terminals. The symbol shown in Fig. 3.39(b) indicates the bidirectional nature of TG.
Principles of Working of the CMOS TG

The CMOS Transmission Gate Working is explained as follows. Figure 3.40 shows a CMOS Transmission Gate in which GP (control terminal C′ of PMOS), is connected to +VDD. and GN (control terminal C of NMOS) is connected to –VSS, as shown. A positive voltage of value +VDD at the N substrate of a PMOS will keep it in the OFF-state. Similarly, a negative voltage of value –VSS at the P substrate of an NMOS will keep it in the OFF-state.
Now, to turn the transistors on, we apply control voltages that will neutralize effects of the supply voltages. Thus, we apply V′C = –VDD (≡ logic 0) to the PMOS transistor and VC = +VSS (≡ logic 1) to the NMOS transistor. These voltages cancel the effects of the supply voltages +VDD and –VSS, respectively, and the TG gets turned-on. The input voltage Vi will now flow through the TG to the load resistor RL. The advantages of the CMOS TG are:
1.    They are bidirectional, i.e., signal can flow in through the gate in either direction.
2.    The ON-resistance (resistance of the circuit when it is turned on) of the gate is low.
3.    The ON-resistance remains more or less constant. This is because, when the PMOS is ON, the NMOS is OFF, and vice versa. Therefore, the resistance of the parallel combination of the ON and OFF resistances is in effect the resistance of the ON-device only. Hence, it is almost a constant.

Pass Transistor Configuration

A series pass-transistor (or simply, pass-transistor) is a transistor configuration that is connected in series with the input and output sections of a circuit, as shown in Fig.3.37. In this diagram, we have shown an NMOS transistor in the pass-transistor mode. But, any transistor (BJT or FET) connected in this manner is called a pass-transistor. Figure 3.38 shows a pass-transistor structure using CMOS transistors. This structure is also known as transmission gate (TG). In this mode of operation, the structure acts like an analog switch. But the same structure can be used in the construction various logic gates. Such gates are called as pass-transistor logic (PTL) gates. It can be seen that TG is a bidirectional switch and can conduct in both directions.