Philips Electronics India Limited is the leading Healthcare Company today. It strives to improve the quality of people’s lives by focussing on their health and well-being. As a company it is essentially divided into Patient Care and Clinical Informatics (PCCI), Imaging and Ultrasound. PCCI as a modality includes patient monitors, diagnostic ECG which features HOLTER, stress test etc, anaesthesia machines, ventilators, telemetry, foetal monitoring systems, and cardiac resuscitation systems (CRS) like defibrillators and automatic external defibrillators (AED). Imaging on the other hand can be classified into MRI (magnetic resonance imaging), CT (computed tomography), digital X-rays, cardio-vascular catheterization (cath lab), nuclear medicine and ultrasound.
PATIENT MONITORING SYSTEMS
Patient monitoring is vital to care in operating and emergency rooms, intensive care and critical care units. Additionally, it is invaluable for recovery rooms, radiology, cath labs, ambulatory, home and sleep-screening applications. Philips has been the pioneer in patient monitors. It has three ranges of products in the market namely IntelliVue, SureSigns and Goldway. IntelliVue Patient Monitor allows a clear view of patient information with its 20 configurable screens and a clinical decision support system (CDSS). It collects and combines physiological data to provide a clear picture of patient status. Its various models include MP 2, MP 5, MP 20 Jr, MP 20, MP 30, MP 40, MP 50, MP 60, MP 70, MP 80 and MP 90 etc.
A patient monitor is an instrument that allows real-time monitoring of several parameters of the patient simultaneously. These parameters include ECG (electrocardiogram), respiration rate, non-invasive blood pressure (NIBP), invasive blood pressure (IBP), temperature, SP02, cardiac output (CO), CO2 and much more. It also has several other special parameters such as EEG (electroencephalogram), BISx (bi-spectral index), PiCCO (pulse contour invasive continuous cardiac output), spirometry, and EASI 5-lead ECG and pulse- pressure variation. Typical monitors alarm in the case of patient or equipment problems and offer limited data storage and retrieval (trending). All patient monitors interface to the Philips Clinical Network and other equipments.
Apart from monitoring a variety of parameters useful to the doctors, PHILIPS patient monitors are extremely user-friendly. They are incorporated with advanced algorithms like ST/AR for arrhythmias, FAST SP02 and 12 XL interpretations. Its CDSS provides Neonatal Event Review (NER), OxyCRG, ST map, Horizon View and Advanced Event Surveillance (AES). Its noticeable features include 20 configurable screens, provision for touch screen, PS/2 trackball, mouse or keyboard.
Components of a patient monitor:
1. Power supply
2. Battery board
3. Main board
4. Recorder board
5. Touch panel
6. I/O board
7. System interface board
8. MMS CPU
Working of patient monitor:
The AC/DC converter transforms the AC power coming from the power plug into 14 V/80W DC source and isolates the monitoring system from the AC power mains. The 14V is distributed via power bus and supplies power to all the components of the system. The MMS and MMS extension requires 48V DC power which is created by an isolating DC/DC converter. The CPU is supplied with 3.3 V and 5 V DC power while the LEDs are supplied with 12V DC power. The multi-measurement server (MMS) consists of various modules which measure the parameters discussed above. These modules are nothing but instrumentation amplifiers with post processing circuits wherever required. The various I/O boards could include PS/2, parallel printer, USB, remote devices and BISx interface.
I ] FEATURES:
Touchscreen User Interface:
Most range of IntelliVue monitors now has resistive touch technology. The protective layer prevents damage from fingernails or sharp objects. This feature provides easy access since the entire screen is active. Layers of complicated menus can be avoided. It is extremely useful in infection control since it can be operated with a disposable stylus. As a result, no bacteria are transferred from gloves to the monitor. Additionally, IntelliVue monitors have no fans or filters unlike other products and thus no danger of spread of bacteria. A remote monitor outside the isolation chamber is possible using VGA display and PS/2 controller so that nurses can view monitor without entering the cubicle. These screens can be cleaned with 70 % isopropyl alcohol. Every patient monitor integrates the display and processing unit into a single package. A monitor with a single integrated multi-measurement server (MMS) can be connected to additional building blocks to form a monitoring system with a large number of measurements, additional interface capabilities and slave display. These elements cooperate as a single integrated real-time measurement system.
Philips patient monitors identify 22 classes of arrhythmia. ST/AR pacemaker analysis identifies and alarms on pacemaker failure. This analysis requires only two active leads (ECG) for full accuracy. It also facilitates auto switching of active leads on lead failure. Neonatal arrhythmia detection is also possible with 3 lead ECG. Most other patient monitors in the market require 4 specific ECG leads 1,2,3,5 for full accuracy. If any of these leads fail, the monitor cannot identify the arrhythmia properly. Also, as opposed to 22 classes identified by IntelliVue, most other patient monitors can identify only 14 classes with no alarming on pacemaker failure.
This special feature is especially helpful in the ICU, OR, CCU, ER and the NICU. Horizon View displays real- time data, trends and intended goal of therapy. This helps clinicians see the effectiveness or failure of the therapy.in the ER and ICU, Horizon View helps keep track of the changes in the condition of the patient. It also allows monitoring of anaesthetic status (BISx) in the OT along with ventilation warning and goal directed therapy in the ICU.
This feature of the PMS is helpful in the ICU, OR, CCU and the ER. ST has 12 numbers that change every minute. Therefore, ST map shows a simple graphic explanation of the cardiac condition. It is based on Eithoven’s triangle and relavant ST leads are grouped automatically. In the ER, it allows the monitoring of chest pain patients. It is useful for thrombolytic therapy in the CCU and for tracking intra-operative ST changes in the OR. It also provides monitoring of the effects of stenting in the Cathlab.
Advanced Event Surveillance (AES):
It has special use in the ICU, OR, CCU, ER and the NICU. AES looks for changing parameters simultaneously. Rules can include rate of change of trends, events that trigger off alarms (yellow or red status). It is useful for the detection of sepsis, whether therapy targets have been reached, and multiple organ failure etc.
Philips invented OxyCRG which stands for Oxy CardioRespiroGram. It shows the heart rate trend, oxygen saturation trend and compressed respiration waveform. It allows neonate diagnosis and differentiate conditions known as central and obstructive apnea in prematures. If arterial blood pressure (ABP) is added, doctors can see if the apnea has caused a drop in blood pressure. If the EEG is added, it can give information on whether the apnea was caused by a seizure. To conclude, OxyCRG together with NER (neonatal event review) allows the full documentation of events.
Neonatal Event Review:
NER provides a snapshot review of the OxyCRG display whenever a neonatal event occurs like bradycardia, bradypnea and desaturation. IntelliVue MP60 provides storage of upto 50 events while MP50 and below allows 25 event storage. Thus NER provides a count of how many episodes occur and how critical they are.
II ] MODULES:
Non-invasive blood pressure:
The conventional method for non-invasive blood pressure measurement (NIBP) was the Korotkoff method. However, modern day patient monitoring systems now use the oscillometric method for measurement.
In the oscillometric method, blood pressure is measured as oscillations superimposed on cuff pressure. The cuff, inflated around the patient’s limb, senses arterial pulses as oscillations whose amplitude changes as the cuff is deflated. The initial oscillations are roughly equivalent to systolic pressure; the larger oscillations represent mean pressure; and the diastolic pressure is then derived from the mean pressure.
To take the measurement, the cuff is placed around the patient’s limb and inflated until the artery is occluded, at a point just above systolic pressure. Blood movement ceases when cuff pressure is greater than arterial pressure. As cuff deflation commences, blood begins to flow through the artery at the systolic pressure, and the resulting pulsations are detected as oscillations. The cuff continues to deflate, and the amplitude of arterial oscillations increases until it reaches a maximum value, which represents mean arterial pressure. As cuff deflation continues, arterial pressure drops due to decreased resistance from the cuff pressure. Diastolic pressure is measured when the oscillations begin diminishing rapidly.
Three different methods can be used to obtain the measurements: • Manual: For each request, one measurement of systolic, diastolic, and mean pressures is taken. • Auto: Repeated measurements of the three values are taken at timed intervals specified by the user. • Stat: Measurements of the three values are taken immediately and repeatedly over a period of five minutes. This method uses a faster measurement procedure but produces a less accurate reading.
PiCCO stands for Pulse Invasive Contour Continuous Cardiac Output. This module can be used in the ICU, OR, CCU, NICU etc. It provides beat to beat cardiac output, continuous systemic vascular resistance (SVR), intrathoracic blood volume (ITBV) and extra vascular lung water (EVLW). By calculating these values, doctors can identify the amount of blood loss and the correct volume of fluid. The patient requires a central venous line ideally the internal jugular or subclavian vein, and an arterial catheter with a thermistor is placed in one of the larger systemic arteries, e.g. the femoral or brachial artery. PiCCO system works on the principle that a known volume of thermal indicator (ice-cold saline) is injected into a central vein.
The injectate rapidly disperses volumetrically and thermally within the pulmonary and cardiac volumes. This volume of distribution is termed the intrathoracic volume. When the thermal signal reaches the arterial thermistor, a temperature difference is detected and a dissipation curve is generated. The area under the curve is inversely proportional to the cardiac output. By calculations, an estimate of both intrathoracic blood volume (ITBV) and EVLW can also be made. ITBV can be derived from global end diastolic volume (GEDV) determined by thermodilution measurement. PiCCO uses only CVP line and temporal artery line and hence is less invasive. These catheters can be used for a period as long as 8-14 days. It is also less expensive than other methods of continuous cardiac output monitoring.
| ITTV = CO * MTtIntra thoracic volume determined from the mean transit time| | PTV = CO * DstPulmonary Thermal Volume determined from the exponential downslope time| | GEDV = ITTV – PTVGlobal end diastolic volume.Can also be re-arranged as GEDV = CO * (MTt-DSt)| | ITBV = 1.25 * GEDVIntra thoracic blood volume (total cardiopulmonary intra vascular fluid volume)Can also be re-arranged as ITBV = 1.25 * CO * (MTt-DSt)| | EVLW = ITTV – ITBVExtra vascular Lung Water = Intra thoracic blood volume. Can also be re-arranged as EVLW = ITTV – 1.25 * GEDV| Cardiac Output (CO):
The simplest technique of cardiac output measurement is the thermodilution method. The thermodilution method is based on the principle that the flow rate of an unknown quantity of liquid can be determined by adding a known quantity of indicator and measuring its concentration downstream. Thermodilution involves injecting a cooled solution at a known temperature into the heart. The injectate solution mixes with and cools the warmer surrounding blood. When the blood leaves the heart, the temperature as a function of time indicates the rate of blood flow.
The measurement is taken with a pulmonary artery (PA) Swan Ganz catheter. The catheter is inserted into the heart, with the proximal lumen opening positioned in the right atrium for introducing the injectate, and the thermistor, used for monitoring temperature, positioned in the pulmonary artery. Positioning of the catheter is gauged by using pressure measurements and X-rays of the catheter location. A small amount of thermal indicator is introduced into the right atrium. The indicator mixes with the blood in the right ventricle and approaches temperature equilibrium. When the diluted blood reaches the pulmonary artery, the thermistor measures the decrease in blood temperature over time. The temperature of the injectate solution can be measured either indirectly or directly. Indirectly, the temperature of the cooling bath is measured. Direct measurement uses a flow-through temperature probe to measure the injectate temperature as it is being introduced into the catheter. The probe is positioned where the syringe injects the indicator into the proximal lumen port. The time/temperature curve resulting from the measurement resembles a bell-shaped curve, except that it has an exponential decay. The data are integrated to calculate the area beneath the curve. By convention, the y-axis of the graph shows decreasing temperature.
Stewart Hamilton’s Equation
BISx stands for bi-spectral index and can be used in the ICU, OR, CCU and the ER. The Bispectral Index (BIS) is a continuous processed EEG parameter that provides a measure of the state of the brain during the administration of anesthetics and sedatives. BIS was designed to correlate with “hypnotic” clinical endpoints (sedation, lack of awareness and memory) and to track changes in the effects of anesthetics on the brain. 3rd generation BIS technology is currently being used in the OR and ICU. However, most other companies still have the 1st generation BIS technology in place.
There are 4 sticker electrodes used for this purpose. Gel is used to provide good electrical contact with the patient. The 1st is placed at the center of the forehead. The 2nd is placed next to it automatically. 3rd electrode is placed at the same level as the eyelashes midway of the eye and the hairline. The 4th electrode gets placed between the 2nd and 3rd automatically. These electrodes measure the neuroactivity of the brain i.e. EEG signals and convey it to the mainline. This type of monitoring is especially useful for elderly patients or pediatrics with faint or delicate systems as well as during lengthy surgical procedures.
COMPONENTS OF ANAESTHESIA
This module is generally used in the ICU, OR, CCU, ER and NICU. It is used for monitoring shunt flow (mixing of blood between the aorta and the pulmonary artery), detecting gangrene (by monitoring the saturation and perfusion in a damaged limb versus baseline) and in monitoring the saturation and perfusion in a re-attached or transplanted limb. It uses FAST algorithm, is motion proof and low perfusion capable. The module is a plug and play with no prior setup requirements and enables continuous display. Measurement of Spo2 is done using pulse oximetry. Pulse oximetry is based on the principle that red blood cells absorb different amounts of light depending on the amount of oxygen they contain.
When light is transmitted through body tissue such as a finger, it is absorbed differently by skin pigments, tissue, cartilage, bone, arterial blood, and venous blood. Most of these substances absorb light at a constant rate. The blood in the arteries and arterioles, however, is pulsatile. As the blood vessels expand and contract, the length of the light path is altered, affecting its absorption. Because the only significant variable is due to pulsing blood, the ratio of HbO2 to Hb can be measured in the pulsatile part of the signal to reveal the oxygen saturation of the arterial flow. The measurement is derived using two wavelengths of light — one in the red region and one in the infrared – to measure maximum and minimum absorption differences between the two molecules. This is done by the help of a probe, usually attached to a ﬁnger or the earlobe.
The SpO2 measurement is taken by means of an optical cuff that is placed on the patient’s fingertip. From the transmitting side of the transducer, red and infrared light is scattered through the capillary bed and detected by a photo-diode on the receiving side. The measurement is independent of skin pigmentation, tissue absorption, and other constants. The resulting measurement is plotted as a plethysmogram. The waveform is proportional to the blood volume changes, the pulse rate, and the relative perfusion of the skin and transducer.
The technique is unreliable if peripheral perfusion is poor and may produce erroneous results in the presence of nail polish, excessive movement or high ambient light. In general, arterial oxygenation is satisfactory if SpO2is greater than 95%.
The temperature measurement used by Philips’ patient monitoring systems is based on a thermistor whose resistance is inversely proportional to its temperature. By measuring the thermistor’s resistance, its temperature can be calculated. The resistance of the thermistor is measured by passing a current through it and measuring the voltage developed across it.
The delta temperature measurement reflects two different temperature pro bevalues and calculates the difference in temperature between the two measurements. A temperature delta from different sites can be diagnostic of an altered physiologic state. Temperature can be measured by a variety of temperature probes designed for use with different anatomic sites. The choice of site is determined by the type of information needed by the clinician.
Invasive blood pressure measurement:
This measurement is especially useful in the ICU and the OT. When measuring invasive blood pressure, the force of movement of the blood in the patient’s systemic arterial system is transported by a fluid column in the pressure line to the transducer. This pressure causes an electrical signal to be generated which is then amplified to display the pressure wave and the numerics for the systolic, diastolic, and mean pressure values.
An invasive blood pressure measurement is collected through a pressure transducer that is connected to a pressure line by means of a catheter which is invasively placed in the patient’s blood stream. Blood pressure is depicted as a pressure wave with the numerics for systolic, diastolic, and mean pressure values. The blood pressure shows the cycles of contraction and release within the heart and the resultant pressure that is generated to move the blood through the vessels.
Pulse Pressure Variation:
It is generally used in the ICU, OR and the CCU. It helps doctors decide whether fluid administration is feasible when the patient has low blood pressure or cardiac output. Measurement of these indicators can predict an increase in cardiac output induced by volume expansion before volume expansion is actually performed. It is calculated using variations in the arterial blood pressure (ABP) during ventilation. This module is again a plug and play device and can be used whenever any arterial pressure is available (ABP, ART, VAP). Hence a PiCCO catheter is not required. Capnography :
Capnography is a simple method of monitoring the concentration or partial pressure of carbon dioxide (CO2) in the respiratory gases. This method of monitoring directly shows the elimination of CO2 by the lungs and indirectly reflects the production of CO2 by tissues and CO2 circulatory transport to the lungs. It is a non-invasive and accurate method. Several procedures are available for monitoring airway CO2. The first procedure is by using a side stream sample measured through a rapidly responding infrared CO2 analyser or measured through a mass spectrometer. The second procedure is direct measurement of CO2 values through an infrared analyser at the end of the endotracheal tube.
The ETCO2 (End Tidal Carbon Dioxide) measurement for Carbon Dioxide uses a technique based on the absorption of infrared radiation by certain gases. Infrared light is absorbed by C02. The amount of absorption varies according to the CO2 concentration in the gas mixture. By using an infrared detector to measure the absorption, the CO2 concentration in a gas can be derived. CO2 respiratory gas measurements are evaluated as gas passes through the airway adapter on the patient’s intubation system. Respiratory CO2 gas readings are depicted as a real-time CO2 waveform together with numerics for ETCO2, Airway Respiration Rate (AWRR), and Inspired Minimum Carbon Dioxide (IMCO2). During calibration, the value for instantaneous CO2 is also obtained.
The electroencephalogram (EEG) is the measurement of the electrical activity of the brain. This activity creates an electrical signal that, depending on the person’s state of consciousness or well-being, produces characteristic waveforms. The waves seen are generated from electrical potentials located on the surface of the brain. These potentials are recorded by placing electrodes on the scalp.
It gives us information regarding the blood flow to the brain during cardiac and vascular surgery. It also enables coma monitoring and prognosis. EEG allows one to monitor the effects of barbiturate sedatives on epileptics and
is useful for detection seizures in neonates.
Percentage of total power in each frequency band:
• Alpha – Alpha waves represent the EEG in a normal awake adult. Their frequency is defined as 8-13 Hz. • Beta – Beta waves represent the EEG during periods of conscious effort. Their frequency is defined as 13 to 30 Hz • Theta – Theta waves are usually seen in the presence of pathology. Their frequency is defined as 4 to 8 Hz • Delta – Delta waves are seen in the presence of pathology, coma and certain stages of anesthesia. Their frequency is defined as 0.5 to 4 Hz.
The International 10/20 System of Electrode Placement is a procedure for the measured location of equally spaced electrodes on the scalp. This system is based on the relationship between the cortical areas of the brain and the location of the EEG electrodes that reside directly above them.
Traditionally there are 21 electrode locations in the 10/20 system. All of these are needed for diagnostic recordings; however in EEG monitoring a sub-set of these electrodes may be used to determine the neurological status of the patient. Each location is marked with a unique letter/number combination. The letters correspond to the cortical areas of the brain where they are located. The numbers are odd for the left positions, even for the right positions, and increase as they move away from the midline (which is designated as Z for zero). When using an EEG monitor locating the proper place for electrodes can be simplified. The electrode pair can be marked using landmarks on the head such as the hairline, ears and top of the head to approximate the 10/20 locations. A pattern of electrodes on the head and the channels they are connected to is called a montage. There are many different combinations of electrode pairs.
The skin surface electrocardiogram (ECG) measures the electrical activity of the patient’s heart, or myocardium. This information indicates the condition of the heart’s electrical conduction system. The electrical cells of the heart generate, conduct, and coordinate electrical impulses that cause the heart’s mechanical cells to contract. As these impulses move through the various parts of the heart’s conduction system, small electrical currents also move toward the body’s surface. The measurement represents the changing electrical potentials and their progression through the heart. The electrical signals are detected by electrodes placed on various areas of the patient’s trunk and limbs. The information carried by the signals varies according to where and how the electrodes are placed.
An ECG is recorded using lead cables connected to a monitoring device. ECG leads are single electrodes, or arrays of electrodes, placed at specific anatomical positions that detect the electrical voltage of a specific cardiac vector. Each lead monitors the heart’s electrical activity from a different perspective. Unipolar leads detect signals moving from the heart to the skin’s surface; bipolar leads detect the surface electrical activity moving from one electrode to another. The EASI 12-lead ECG uses a special lead positioning that detects electrical activity, which does not correspond directly to the standard ECG vectors, but from which all 12 of the standard vectors can be derived.
Lead cable sets are available with various numbers of lead wires; common types are three, five, and twelve lead sets. The end of each wire is attached to an electrode and is color-coded to facilitate anatomical placement. Electrodes can be placed in many different arrangements, depending on factors such as clinical application, type of patient, and suspected diagnosis.
Limb Leads Three bipolar limb leads are called leads I, II, and III. Electrodes are placed on the patient’s right arm, left arm, and left leg, forming a pattern known as Einthoven’s triangle. For convenience, the electrodes can also be placed on the patient’s trunk near the shoulders and hip. The unipolar leads are called aVR, aVL, and aVF; they can be monitored by the same three electrodes. When selected on the monitor, these leads measure the current from the heart out to the specified limb.
Chest Leads Chest leads, also called precordial leads, are unipolar. Leads labelled V with a number or letter designation can be placed around the entire circumference of the chest, as well as the back. The most common clinical objective in matching V leads is ischemia detection.
Modified Chest Lead (MCL1) is a bipolar chest lead in which two electrodes are placed over the chest
Electrodes for five-lead sets can be placed in various positions. The right leg (RL) lead serves as a ground.
Diagnostic sonography (ultrasonography) is an ultrasound-based diagnostic imaging technique used to visualize subcutaneous body structures including tendons, muscles, joints, vessels and internal organs for possible pathology or lesions. Obstetric sonography is commonly used during pregnancy and is widely recognized by the public.
In physics, the term “ultrasound” applies to all acoustic energy (longitudinal, mechanical wave) with a frequency above the audible range of human hearing. The audible range of sound is 20 hertz-20 kilohertz. Ultrasound is frequency greater than 20 kilohertz.
Typical diagnostic sonographic scanners operate in the frequency range of 2 to 18 megahertz. It requires a medium for transmission and travels at a speed of 1540 m/s in blood or tissues. The choice of frequency is a trade-off between spatial resolution of the image and imaging depth: lower frequencies produce less resolution but image deeper into the body.
It is possible to perform both diagnosis and therapeutic procedures, using ultrasound to guide interventional procedures (for instance biopsies or
drainage of fluid collections). Sonographers are medical professionals who perform scans for diagnostic purposes.
Sonography is effective for imaging soft tissues of the body. Superficial structures such as muscles, tendons, testes, breast and the neonatal brain are imaged at a higher frequency (7-18 MHz), which provides better axial and lateral resolution. Deeper structures such as liver and kidney are imaged at a lower frequency 1-6 MHz with lower axial and lateral resolution but greater penetration.
The Doppler Effect (or Doppler shift), is a change in frequency of a wave (or other periodic event) for an observer moving relative to its source. The received frequency is higher during the approach, it is identical at the instant of passing by, and it is lower during the recession. When the source of the waves is moving toward the observer, each successive wave crest is emitted from a position closer to the observer than the previous wave. Therefore the time between the arrivals of successive wave crests at the observer is reduced, causing an increase in the frequency. Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. Ths Doppler shift is given by the equation:
Ft=transmitted Doppler frequency
V=velocity of blood flow
Theta=angle between flow of blood and ultrasound beam
C= speed of sound in tissue
Sonography employs the Doppler Effect to assess whether blood is moving towards or away from the probe, and its relative velocity. By calculating the frequency shift of a particular sample volume, for example flow in an artery or a jet of blood flow over a heart valve, its speed and direction can be determined and visualised. This is particularly useful in cardiovascular studies (sonography of the vascular system and heart) and essential in many areas such as determining reverse blood flow in the liver vasculature in portal hypertension. The Doppler information is displayed graphically using spectral Doppler, or as an image using color Doppler (directional Doppler) or power Doppler (non-directional Doppler).
The two main types of Doppler echocardiographic system differ in transducer design and operating features, signal processing procedures and in the types of information provided. 1. CONTINUOUS DOPPLER:
Continuous wave (CW) Doppler is the older and electronically simpler of the two kinds. As the name implies, CW Doppler involves continuous generation of ultrasound waves coupled with continuous ultrasound reception. A two crystal transducer accomplishes this dual function with one crystal devoted to each function. The main advantage of CW Doppler is its ability to measure high blood velocities accurately. CW Doppler can accurately record the highest velocities in any valvular and congenital heart disease. It is also of considerable importance for the quantitative evaluation of abnormal flows.
The main disadvantage of CW Doppler is its lack of selectivity or depth discrimination. Since CW Doppler is constantly transmitting and receiving from two different transducer heads (crystals) there is no provision for imaging or range gating to allow selective placing of a given Doppler sample volume in space. The absence of anatomic information during CW examination may lead to interpretive difficulties, particularly if more than one heart chamber or blood vessel lies in the path of the ultrasound beam.
2. PULSED WAVE DOPPLER:
Most modern sonographic machines use pulsed Doppler to measure velocity. Pulsed wave machines transmit and receive series of pulses. The frequency shift of each pulse is ignored; however the relative phase changes of the pulses are used to obtain the frequency shift (since frequency is the rate of change of phase). The major advantages of pulsed Doppler over continuous wave is that distance information is obtained (the time between the transmitted and received pulses can be converted into a distance with knowledge of the speed of sound) and gain correction is applied. The disadvantage of pulsed Doppler is that the measurements can suffer from aliasing.
When pulses are transmitted at a given sampling frequency (known as the pulse repetition frequency), the maximum Doppler frequency that can be measured unambiguously is half the pulse repetition frequency. If the blood velocity and beam/flow angle being measured combine to give a frequency value greater than half of the pulse repetition frequency, ambiguity in the Doppler signal occurs. This ambiguity is known as aliasing. Low pulse repetition frequencies are employed to examine low velocities (e.g. venous flow). Aliasing will occur if low pulse repetition frequencies or velocity scales are used and high velocities are encountered .Conversely, if a high pulse repetition frequency is used to examine high velocities, low velocities may not be identified.
Ultrasound transducers convert electric signals into ultrasound energy that is transmitted into tissues and the reflected ultrasound energy back into an electric signal. Ultrasound transducers consist of the following:
1. Piezoelectric crystal
2. Live electrode
3. Ground electrode
4. Backing block
5. Acoustic insulator
6. Plastic housing
7. Insulated cover
The most important component is a thin piezoelectric crystal element located near the face of the transducer. The front and back faces of the crystal are coated with a thin conducting film to ensure good contact with the two electrodes that will supply the electric field used to strain the crystal i.e. deformity caused in the crystal when a voltage is applied to it. The surfaces of the electrodes are coated with gold or silver electrodes. The outside electrode is grounded to protect the patient from electrical shock and its outside surface is coated with a watertight electrical insulator. The inside electrode abuts against a thick backing block that absorbs sound waves that travel back into the transducer. The housing is usually a strong plastic. An acoustic insulator of rubber or cork prevents the sound waves from passing into the housing. Strong, short electrical pulses from the ultrasound machine make the transducer ring at the desired frequency. The sound is focused either by the shape of the transducer, a lens in front of the transducer, or a complex set of control pulses from the ultrasound scanner machine. This focusing produces an arc-shaped sound wave from the face of the transducer. The wave travels into the body and comes into focus at a desired depth.
New technology transducers use phased array techniques to enable the sonographic machine to change the direction and depth of focus. Materials on the face of the transducer enable the sound to be transmitted efficiently into the body (usually seeming to be a rubbery coating, a form of impedance matching). In addition, a water-based gel is placed between the patient’s skin and the probe to ensure good electrical contact.
The sound wave is partially reflected from the layers between different tissues. Specifically, sound is reflected anywhere there are density changes in the body: e.g. blood cells in blood plasma, small structures in organs, etc. Some of the reflections return to the transducer.
The return of the sound wave to the transducer results in the same process that it took to send the sound wave, except in reverse. The return sound wave vibrates the transducer; the transducer turns the vibrations into electrical pulses that travel to the ultrasonic scanner where they are processed and transformed into a digital image.
1. Linear array probe: Used in abdominal organs (extrathoracic), superficial structures (skin, mammary glands).
2. Curved array probe: Used in abdominal ultrasonography and pregnancy diagnosis.
3. Sector array probe: Used in echocardiography, intra pelvic and intra thoracic organs, brains, eyes.
1. A-mode: A-mode is the simplest type of ultrasound. A single transducer scans a line through the body with the echoes plotted on screen as a function of depth. Therapeutic ultrasound aimed at a specific tumor or calculus is also A-mode, to allow for pinpoint accurate focus of the destructive wave energy.
2. B-mode: In B-mode ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on screen.
3. M-mode: M stands for motion. In m-mode a rapid sequence of B-mode scans whose images follow each other in sequence on screen enables doctors to see and measure range of motion, as the organ boundaries that produce reflections move relative to the probe.
4. Doppler mode: This mode makes use of the Doppler Effect in measuring and visualizing blood flow. a. Color doppler: Velocity information is presented as a color coded overlay on top of a B-mode image
b. Continuous doppler: Doppler information is sampled along a line through the body, and all velocities detected at each time point is presented (on a time line).
c. Pulsed wave (PW) doppler: Doppler information is sampled from only a small sample volume (defined in 2Dimage), and presented on a timeline.
d. Duplex: a common name for the simultaneous presentation of 2D and (usually) PW doppler information. (Using modern ultrasound machines color doppler is almost always also used, hence the alternative name Triplex.
The system control panel contains the following keys, rotary controls, and slide controls:
2D : To exit the current imaging mode or application and return to 2D Mode.
2D GAIN: To adjust the gain, or overall brightness, of a 2D or MMode image.
Acquire: In live imaging, Stress Echocardiography, and Quick Review playback, it is used to start and stop the acquisition of a loop. When an image is frozen it is used to acquire a frame.
Angio: To enter Color Power Angio or to add angio information to the reference image in PW Doppler or CW Doppler.
Angle: In PW Doppler, an arrow, called the angle-to-flow arrow, appears on the imaging screen along with the Doppler cursor line to position the angle-to-flow arrow parallel to and in the same direction as the blood flow.
Baseline: In PW and CW Doppler, used to adjust the zero baseline position in the Doppler spectral trace.
Body Mark: To display the body marker soft keys and choose a body marker to place on the imaging screen.
Color: To enter Color Mode or to add color information to the reference image in PW Doppler, CW Doppler, or MMode.
CW: In a cardiac preset, used to display the CW spectral trace. In a noncardiac preset, it is used to position the CW reference line.
Del: To delete a selected label. If no label is selected, all of the labels are deleted. Also used to delete a selected measurement. If no measurementis selected, all of the measurements are deleted.
Depth: To increase or decrease the distance from the face of the transducer to the deepest point in the displayed image.
Doppler Gain: In PW and CW Doppler, it is used to adjust the brightness of the spectral display.
Enter: To click an item or choose a menu option.
Focus: To move the location of the focal zone or focal zones, the area or areas where the image is most clearly focused.
Freeze: To freeze a live image and initiate Quick Review, which allows you to scroll through the frames using the trackball.
Fusion: To cycle through fusion frequency settings available for the selected transducer and the mode.
Keyboard: To type information into fields and to type labels, titles.
Label: To display the Label soft keys and choose a label to place on the imaging screen.
Left: In live imaging, it is used to enter Dual Imaging. The live image is on the left, and the frozen image appears on the right.
LGCs: Moving the slide controls up or down adjusts the amplification of a returning 2D signal.
Measure: To display the measurement soft keys and to start an unlabelled measurement. A caliper appears on the image.
Mic: To turn the microphone on and off when recording a voice annotation during a VCR recording.
MMode: In a cardiac preset, it is used to display the MMode trace. In a non-cardiac preset, it is used to enter MMode Preview.
Option Keys (1, 2, 3, and 4): The option keys are labeled 1, 2, 3, and 4. Before using 3D Mode, Panoramic Imaging, Stress Echocardiography, or Tissue Doppler, you need to assign one of the option keys to the mode or application in the Options setup window.
Patient: To open the Patient Identification window in which you can create a new patient study, edit information about the current patient, or restart a patient study that was started earlier the same day.
Power: To vary the acoustic power transmitted for the current mode.
Probe: To activate the next connected transducer going from top to bottom. The name of the current transducer appears on the right side of the imaging screen.
PW: To enter PW Doppler Preview, so that you can position the Doppler sample volume gate.
Report: To open the report for the current study.
Review: To open Image Review for the current study.
Right: In live imaging, it is used to enter Dual Imaging. The live image is on the right, and the frozen image appears on the left.
Scale: In Color Power Angio, Color Mode, or CW or PW Spectral Doppler, it is used to display higher or lower velocities and frequencies The Scale setting changes the pulse repetition frequency (PRF).
Spectral: In CW Preview or PW Preview, it is used to display the CW or the PW Doppler spectral trace, respectively.
TGCs: Moving the TGC (Time Gain Compensation) slide controls to the right or left adjusts the amplification of the returning 2D signals at a specific image depth.
THI: To enter Tissue Harmonic Imaging.
Volume: To adjust the volume of the speaker for CW and PW Doppler and for VCR playback.
Zoom: To place a zoom box on an image and magnify the area in the zoom box.