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Neurophysiological Signals in IONM Part II
- 1. IONM Part III Anurag Tewari MD 1
- 2. AnuragTewariMD Anurag Tewari MD AMPLIFIER Anurag Tewari MD 2 Medsurant Health Academy
- 3. AnuragTewariMD Anurag Tewari MD AMPLIFICATION • The amplifier not only determines the morphology and polarity of its output signal, it also amplifies the signal so that it can be • FILTERED • DIGITIZED • DISPLAYED • The amplifier in an IONM machine must be able to handle amplification of a wide variety of neurologic signals Anurag Tewari MD 3 Medsurant Health Academy
- 4. AnuragTewariMD Anurag Tewari MD AMPLIFICATION • EP signals are in the order of 0.1 – 5 μV • EEG signals are in the 10 to 100 μV range and • EMG signals are in the millivolt range Amplifier has to be • SENSITIVE to record very tiny signals • VERSATILE to amplify a wide range of signal types Anurag Tewari MD 4 Medsurant Health Academy
- 5. AnuragTewariMD Anurag Tewari MD GAIN & SENSITIVITY • Gain describes how much our equipment amplifies our signal before we see it on the screen. • This is different than display gain and is NOT assigned any units. • Power Gain • Voltage Gain •P=V2/R Anurag Tewari MD 5 Medsurant Health Academy
- 6. AnuragTewariMD Anurag Tewari MD GAIN & SENSITIVITY • Sensitivity: Sensitivity or display gain on EP machines is a measure of • INPUT VOLTAGE PER DIVISION on the screen • The display screen is divided into equal intervals called divisions Sensitivity = Input voltage / vertical deflection (in divisions or mm) Anurag Tewari MD 6 Medsurant Health Academy
- 7. AnuragTewariMD Anurag Tewari MDDECIBEL-POWER GAIN AMPLIFIER • Amplifier is an electronic device for increasing the amplitude of electrical signals Anurag Tewari MD 7 Medsurant Health Academy
- 8. AnuragTewariMD Anurag Tewari MDDECIBEL-POWER GAIN AMPLIFIER • The power gain (Ap) of an Amplifier is the logarithmic ratio between • Input Power (Pi) and • output power (Po) Anurag Tewari MD 8 Medsurant Health Academy
- 9. AnuragTewariMD Anurag Tewari MDDECIBEL-POWER GAIN AMPLIFIER Anurag Tewari MD 9 Medsurant Health Academy
- 10. AnuragTewariMD Anurag Tewari MDDECIBEL-POWER GAIN AMPLIFIER Anurag Tewari MD 10 Thus a 3 dB GAIN causes a DOUBLING of power Medsurant Health Academy
- 11. AnuragTewariMD Anurag Tewari MDDECIBEL-POWER GAIN AMPLIFIER Decibel – Attenuation: A reduction of one hundred times is expressed as -20dB Anurag Tewari MD 11 Medsurant Health Academy
- 12. AnuragTewariMD Anurag Tewari MD OPERATIONAL AMPLIFIER •The operational amplifier (op-amp) is the basic building block of the amplification system in IONM machine •It consists of • two input poles and • an output pole Anurag Tewari MD 12 OUTPUT INPUT INPUT Medsurant Health Academy
- 13. AnuragTewariMD Anurag Tewari MD OPERATIONAL AMPLIFIER • Is a DIFFERENTIAL AMPLIFIER, • means that its output depends on the nature of both of its inputs • The input marked (–), is the inverting pole, also known as • grid 1 (G1) • the negative input, or the • active input • The other pole, (+), is the noninverting pole, also known as • grid 2 (G2) • the positive input, or • the reference input Anurag Tewari MD 13 INVERTING POLE NON-INVERTING POLE Medsurant Health Academy
- 14. AnuragTewariMD Anurag Tewari MD OPERATIONAL AMPLIFIER • The output signal consists of the difference between the input signals at the two poles • Signals introduced at the inverting pole are inverted at the output • Signals introduced at the noninverting pole have the same polarity at the output Anurag Tewari MD 14 NO output signalSame signal introduced at both poles Medsurant Health Academy
- 15. AnuragTewariMD Anurag Tewari MD OPERATIONAL AMPLIFIER • If an upgoing signal is placed at the inverting pole and no signal at the noninverting pole • If an upgoing signal is introduced at the noninverting pole and no signal at the inverting pole, the resulting output will be an upgoing signal Anurag Tewari MD 15 Output will be a DOWN-going signal Output will be a UP-going signal Medsurant Health Academy
- 16. AnuragTewariMD Anurag Tewari MD OPERATIONAL AMPLIFIER Anurag Tewari MD 16 Medsurant Health Academy
- 17. AnuragTewariMD Anurag Tewari MD OPERATIONAL AMPLIFIER • When NEAR-FIELD potentials are recorded • The electrode over the generator site is placed in the inverting pole of the op-amp • The input to the noninverting pole is placed at a site away from the generator, where no activity will be recorded Anurag Tewari MD 17 Medsurant Health Academy
- 18. AnuragTewariMD Anurag Tewari MD OPERATIONAL AMPLIFIER • Electrically negative signals introduced into the op-amp are displayed as an upgoing signal at the output • This is why the normal convention for most IONM modalities is “NEGATIVE UP” • One exception to this is the BAEP Anurag Tewari MD 18 Medsurant Health Academy
- 19. AnuragTewariMD Anurag Tewari MD OPERATIONAL AMPLIFIER To be classified as an operational amplifier, the circuit must have three important characteristics •Very HIGH gain •Very HIGH input impedance •Very LOW output impedance Anurag Tewari MD 19 Medsurant Health Academy
- 20. AnuragTewariMD Anurag Tewari MD COMMON MODE-REJECTION • Common mode rejection: elimination of signals that are common to both inputs of a differential amplifier • The benefit is that the far-field distant signals such as noise artifacts will be rejected if they are common to both recording electrodes • The closer the electrodes are to each other, the more alike their input signals will be and the more far-field rejection you get The danger of this is if your recording electrodes are too close together for the signal you are trying to measure, you will reject much of the signal Anurag Tewari MD 20 Medsurant Health Academy
- 21. AnuragTewariMD Anurag Tewari MD COMMON MODE-REJECTION • DISCRIMINATION is the ability of an amplifier to pass differential- mode signals and to reject common-mode signals • It is measured as a common mode rejection ratio or CMRR • Higher the common mode-rejection ratio, the more efficient the amplification system is at discerning the differences between signals introduced at the input poles Anurag Tewari MD 21 Medsurant Health Academy
- 22. AnuragTewariMD Anurag Tewari MD COMMON MODE-REJECTION • In an ideal op-amp, when the same signal is introduced at both input poles, there should be absolutely no signal at the output pole • In practice however, the op-amp’s ability to do this is imperfect • There is always a stray signal that is seen at the output when a common signal is introduced at the input Anurag Tewari MD 22 Medsurant Health Academy
- 23. AnuragTewariMD Anurag Tewari MD COMMON MODE-REJECTION • The ratio is as follows: For every increase in 20dB the CMRR increases 10X • Guidelines require EP amplifiers to have a minimum CMRR = 10,000:1 or 80 dB Anurag Tewari MD 23 Medsurant Health Academy
- 24. AnuragTewariMD Anurag Tewari MD COMMON MODE-REJECTION •Manufacturers offer amplifiers with common-mode rejection •The common-mode rejection ratio given by manufacturers refers to an ideal situation that is rarely attainable •Thus, the practical obtainable common-mode rejection ratio is lower than that given in the specifications for any amplifier Anurag Tewari MD 24 Medsurant Health Academy
- 25. AnuragTewariMD Anurag Tewari MD INPUT IMPEDANCE • Impedance as measure ACROSS THE INPUTS of a differential amplifier • It is NOT the same as electrode to skin impedance • We want impedance to be very high so that no current flows through the recording electrodes, influencing the potentials being measured. • High input impedance is important, as it ensures that most of the recorded signal voltage will be across the inputs of the amplifier • The ACNS guidelines state that neuro-diagnostic equipment’s amplification system should have an input impedance of at least 100 Mega ohms • Most of the commercially available IONM machines meet this specification Anurag Tewari MD 25 Medsurant Health Academy
- 26. AnuragTewariMD Anurag Tewari MD HOW DOES INPUT IMPEDANCE WORK? • The impedance of the patient and input impedance of the amplifier are in series • Since resistors in series act as a voltage dividers, the voltage recorded across the patient and the voltage recorded across the amplifier are proportional to their respective impedances Anurag Tewari MD 26 Medsurant Health Academy
- 27. AnuragTewariMD Anurag Tewari MD HOW DOES INPUT IMPEDANCE WORK? • Assume that the impedance of the electrodes on the patient is 1 Kohm and the input impedance across the amplifier is 100 MEGA Ohm • That means that the portion of the total signal voltage across the amplifier is 100,000 times greater than that across the patient Anurag Tewari MD 27 Medsurant Health Academy
- 28. AnuragTewariMD Anurag Tewari MD HOW DOES INPUT IMPEDANCE WORK? • For all intents and purposes, all of the signal voltage is across the amplifier; only a minuscule amount (1 x 10–5) is across the patient electrode • Therefore the output signal is a true representation of the input signal Anurag Tewari MD 28 Medsurant Health Academy
- 29. AnuragTewariMD Anurag Tewari MD HOW DOES INPUT IMPEDANCE WORK? • Voltage Divider Rule: • The voltage recorded across the patient and across the amplifier is proportional to their impedance • Vamp is 100,000 greater than Vpatient Anurag Tewari MD 29 Medsurant Health Academy
- 30. AnuragTewariMD Anurag Tewari MD Resources Anurag Tewari MD 30 What is an operational amplifier? Inverting op-amp circuit Non-inverting op-amp circuit Medsurant Health Academy