«Instrumentation – Amplifiers, Filters, Stimulators and all that Other Stuff we Love to Hate Learning Objectives: Most of the industry related ...»
Instrumentation – Amplifiers, Filters, Stimulators and all that Other
Stuff we Love to Hate
Most of the industry related materials we read are exciting and fun to read. They are all about
diseases and injuries we work with every day. There are those other less desirable topics we are
required to know and understand, but are not much fun at all. This paper tackles some of those
items, from basic electronics to filters and stimulators. It is our desire to make this easy to read and understand and maybe even a little fun.
We will review:
1. Basic electronic circuits including Ohms’s law and properties of alternating current
2. Electrodes and electrode types
5. Analog to Digital conversion
6. Averaging and Signal to Noise Ratio
8. Stimulators and stimulator properties
9. Interference, and
10. Electrical safety This outline closely follows the contents of Chapter 3 of Dumitru’s book (one of our favorites), although the content is found in many sources.
Basic Electronic Circuits:
As an early grade school student we practiced rubbing a latex balloon in our hair and allowing it to stick there, then rubbing two latex balloons in our hair and observing when they would not stick to one another. Little did we know we were learning the basic electrical rule called Coulomb’s law, which states “Like charges repel and unlike charges attract.” Later the basic electrical charge was named in his honor, the Coulomb.
A circuit is an interconnection of components so that current flows in one or more closed loops.
Kirchoff’s Law states that the sum of all currents into a node equals the sum of all currents leaving a node.
With respect to DC Ohm’s Law, the formula is (E) Voltage = (I) Current x (R) Resistance. A component that has a node in common and does not connect anywhere else is a resistor in series.
The resistor in series has the same current, derivative of current and the same voltage in relation to the current. The voltage across the series then is the sum of the voltages across each. Resistors in parallel are connected end to end and have same constant voltage across the resistors.
Ohm's law states that the current (I) through a conductor between two points is directly proportional to the potential difference or voltage (E) across the two points, and inversely proportional to the resistance (R) between them.
The mathematical equation that describes this relationship is:
E I= or, E = I x R R I have always found this confusing. Here’s a way that always helped me remember and understand Ohm’s Law.
This is a water tower. Voltage is the amount of water in the tower. This is the potential difference between the positive and negative terminals. Current is the push of the electrons.
Even though you may live close to the water tower and may have great push of water into your home you can control how much water is used in the shower by opening the faucet a little or a lot. This is the resistance to the flow.
Assuming the faucet was open completely the value of water in the tower would equal the push of water, just as without resistance the voltage equals the current.
Later, in the stimulator section we will use give an everyday example of Ohm’s Law.
2 “AC” Alternating Current – Voltages or currents change polarity at certain intervals. AC current in electronics is when some voltage or current fluctuates with zero average value over a period of time as opposed to the DC which is the long term average value. AC is what is used in our homes, hospitals, etc. “DC” Direct Current, voltages or currents are a steady source flowing in one direction only. An example of direct current would be the flow of current from the negative to the positive terminal of a battery. In an AC circuit the hindrance to current flow is no longer called resistance, but rather impedance.
For electrodiagnostic testing there are two types of electrodes used, surface and intramuscular.
For a CMAP (compound muscle action potential), a recording electrode consists of two 1x1cm platinum or silver disk electrodes or the use of an adhesive electrode. Amplitude of a waveform will decrease with increasing the size of an electrode. An active G1 placed on the muscle of the nerve being tested and a reference G2 placed on a belly tendon. To record a SNAP (sensory nerve action potential) ring electrodes are placed longitudinally on the digit, along the nerve. The maximum amplitude of the response is affected by the distance between the two recording electrodes. Distances of three to four centimeters between active and reference electrodes are optimal.
Common filter settings for motor and sensory are as follows:
Low frequency (high pass) 20 Hz, High frequency (low pass) 2 or 3kHz Timebase (Sweep speed) 1 or 2 msec/div and sensitivity is 10-20 µV/div.
Low frequency (high pass) 2 Hz, High frequency (low pass) 10 kHz Timebase 2 or 5 msec/div and sensitivity is 2-5 mV/div
Input impedance in an AC circuit determines the current flow for a given alternating voltage source. Resistance that remains constant when the frequency of the voltage of the signal changes.
In an EMG study, the circuit of the needle tip and input terminal become a voltage divider with changes occurring due to the impedance. Increasing the amplifier impedance higher than electrode impedance would decrease the loss of potential. Amplifier input impedances range from 100 kilohms (kΩ) to many megohms (MΩ). The higher the input impedance of an amplifier will improve the common mode rejection ratio. The common mode rejection allows more cancellation of unwanted signals and better resolution of physiological signals. In respect to recording electrodes, the higher the input impedance, the lesser effect of electrical asymmetry.
3 Whereas, higher electrode impedance increases amplifier noise and artifact. Broken wires are a source of external interference and waveform distortion.
Sub-dermal needles are made of solid stainless steel with the lead wire made of copper and polyvinyl chloride insulation. The needle length is 12 cm and the gauge is 27. Sub-dermal needles come as a single needle or as a twisted pair, depending on the recording to achieve. The twisted needles are especially recommended for intra-operative monitoring recordings. This type of needle is useful when trying to minimize interference and artifact, due to the needle being flexible and easy to secure.
Intramuscular electrodes for routine EMG studies are either monopolar or concentric. A monopolar electrode is insulated except for the distal 0.2 mm fine point and is made of stainless steel. The wire, covered by a Teflon sleeve has an average diameter of about 0.8 mm. A surface electrode or second needle must be placed as a reference, along with the ground. A monopolar needle tends to be less stable, allowing more noise. Impedance may range from 1.3 megohms at 10Hz to 6.6 kilohms at 10 kHz (Kimura p.42). A difference between a monopolar needle and a concentric needle is how they record. A monopolar needle tends to record potentials larger and more complex, while duration and firing rate stay the same.
A concentric needle is stainless steel with a wire in the center of the shaft. The wire tends to be made of silver, platinum or nichrome, measuring approximately 0.1 mm in diameter. The external rim measures approximately 0.3 mm with the pointed tip of the needle having an area of about 150µm x 600µm. The impedance may range around 50kΩ (Kimura p. 42). A concentric needle has a smaller recording area since potentials are recorded between the wire and the shaft.
Routine monopolar and concentric needle studies have a low frequency (high pass) filter of 20 Hz, and a high frequency, low pass 10 kHz. Time base is set at 10 msec/div and sensitivity 20 to 200 µV/div.
Single fiber EMG is used to record from single muscle fiber action potentials instead of motor units. The wire is much smaller, 25µm in diameter and mounted on the side of the needle. The single fiber needles may have two or more wires exposed along the shaft serving as the leading tip. Compared to monopolar and concentric, single fiber needles will show specific asymmetries.
Single fiber EMG is best known for the use in neuromuscular junction disorders. Common filter 4 settings for single fiber needle studies are low frequency (high pass) 500 Hz or 1kHz, high frequency (low pass) 10 or 20kHz. Time base set at 0.5 to 1 msec/div and sensitivity 200 µV/div.
An amplifier is a device that increases the amplitude of a signal. In popular use, the term usually describes an electronic amplifier, in which the input “signal” is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to make the human voice louder or play recorded music. In nerve conduction studies the input “signal” is a very small physiological signal from the nerve. Motor conduction studies are measured in millivolts (mV) and sensory conduction studies are measured in microvolts (µV). In EMG, the amplifier drives the recording instrument to display the signal on a monitor for analysis.
Amplification or the display of the amplified signal on the EMG instrument allows us to place values on the output, i.e. latency and amplitude. Amplification can be expressed as Gain or Sensitivity.
Gain is a ratio of the amplifiers’ output to input signal. Sensitivity is a ratio of input voltage to screen deflection in centimeters or millimeters.
Input impedance is a very important, but a very misunderstood principle of amplifiers. We all understand that relative low impedance at the electrode sites will demonstrate a better output quality, but this would not be true if the input impedance at the amplifier was not inversely high.
This is because, as we learned by Ohm’s law, the total voltage seen on the screen is greatest when the impedance at the amplifier is significantly greater than the electrode impedance. With the impedance significantly greater at the amplifier a mathematical unity will occur thus placing more importance on the electrode impedance. This math is demonstrated nicely in Dumitru’s 2nd edition (pg. 78-79). The net result (because the unity of the input impedance) is an undesirable high impedance at the recording electrodes, which would result in smaller and distorted signals.
This could be mistaken for axonal loss. Modern instruments have input impedances in the millions of Ohm’s (MΩ).
A differential amplifier, as the name implies, magnifies the differences between the two inputs and cancels the common signals to both inputs. This happens because the second input is electronically inverted. Therefore, electrical noise from the environment should be eliminated because they would be the same to both inputs, but we know that is not the case, why? This is an example of real life as there are always differences between the two inputs and signals are never completely eliminated. Impedance differences, between the recording and reference electrodes, and using different types of electrodes (having different properties) are examples of input mismatch. These mismatched signals are presented differently to an amplifier, thus magnifying the differences instead of eliminating them.
Another confusing but important property of a differential amplifier is the Common Mode Rejection Ratio (CMRR). Simply put, this is the ratio of signals common to both inputs are amplified as compared to inputs that are different between the two inputs. This is important because external and unwanted signals, like electrical noise, are much larger than the physiological signals that we are most interested in. We want to amplify the physiological signal much more than we want to amplify the differences secondary to external causes. Modern instruments use CMRR of 100,000:1 (equal to 100dB) or higher.
Biological signals from the human body are reproduced on our EMG instruments as waveforms that can be measured on both a horizontal and vertical scale. In reality there are many different and rapidly changing waves (or subcomponents of waveforms) and are either cancelled or summated to produce these waveforms. Some components of these waveforms have higher frequencies, for instance, the risetime and summit of the peaks. Some components are composed of slower frequencies, for example, the return to baseline. Some components are not desired,
The job of filters is to include the desired frequencies that make up the important components of the waveform while excluding the undesired frequencies outside the frequencies of interest.
A high-frequency filter (also called a low-pass filter) is designed to limit the amount of highfrequencies being recorded while allowing low frequencies to pass unaffected. The lowfrequency filter (also called a high-pass filter) stops low-frequency components while allowing high-frequency components to pass.
The low-frequency filter removes or attenuates frequencies below the desired frequencies we want to record and removing these frequencies we change both measurements and shape of the waveform. When recording sensory nerve conduction studies the low filter is usually set to 20 or 30 Hz. If you raise this value, the onset latency would stay constant, but the peak latency would decrease and there would be a reduction in amplitude and area. The default low filter value for MNC is usually 2 to 3 Hz. Raising this value would have a similar effect where the onset latency would not change, but one would observe a reduction in amplitude and area.