Date post: | 16-Jul-2015 |
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Shehzad Hussain
Technologist
Calibration is a procedure to ensure through the checks that the EEG machine is fit for the purpose.
It is set of sequential steps to test, compare and adjust the measuring instrument before recording.
Ensure that the instrument is fit for the purpose
Helps interpreter in correct measurements
Lets interpreter know the minor deviations while reporting
Helps to compare the recording with previous.
Bio calibration assesses the response of the amplifiers, filters and the recording apparatus on a complex biological signal. Electrode FP1 and O2 are connected to all amplifier inputs. The recordings for all channels should be identical.
SIGNIFICANCE OF BIOLOGICAL CALIBRATION:
It is a montage because a fixed pattern is set in which throughout activity of 18 channels is obtained.
It reassures the machine calibration through amplifiers.
Then we can interpretate wrong and the result will
be wrong.
Either we make a normal EEG abnormal.
Conventional EEG machine
Schematic Diagram
1. Sufficient ink
2. Sufficient paper
3. Paper speed
4. Pen alignment
5. Pen centering
6. Damping
7) Time constant or LFF
8) High frequency filter (HFF)
9) Sensitivity
10) Step gain
11) Amplitude linearity
12) Noise level
Digital Machine
Analog Machine
Ensure that as per plan of the recording their is more than enough paper/Disc space.
Before taking printout sufficient paper is present in the printer.
What’s wrong if not checked?
Insufficient paper will lead cessation of live recording
Cessation may lead to miss:
1. any paroxysmal event
2. a brief Interictal abnormality
3. onset of a seizure.
Sufficient ink should be present in the tube
of writer/ printer cartridge should not be
empty.
Consequences:
1. any paroxysmal event
2. a brief Interictal abnormality
3. onset of a seizure.
4. Loss of paper
It is the speed at which the paper roles off through the pen writer.
Standard paper speed is 3cm/sec or 30millimeter/second
Standard paper size is 30 cm/fold Slow Paper Speed: Used with time constant to make the slowing
prominent. Like 15mm/sec. Fast paper Speed: It is used to check the onset of an event. 60mm/sec
Paper speed could be checked in two ways:
1. Manually by;
by measuring the distance simultaneously running the paper and monitoring the time lapsed.
Distance/time lapsed= paper speed.
2. Automatically by;
By using the time marker of the machine.
Distance=27cm, Time =30seconds P.S=27cm/30sec= 0.9cm/sec or 9mm/sec
Misinterpretations of frequency
Misinterpretation of waveform duration
Wave morphology could change like spike could appear as sharp wave and sharp wave as slow wave.
In digital EEG machine internal setting of machine is difficult, interfere can cause other consequences. So….
If paper speed is wrong then inform the biomedical engineer to correct it.
All pens should start and stop simultaneously in a perpendicular line.
Could be checked by drawing a straight line perpendicular to the paper drive by using set squares
This can be done with or without calibration signal
Misalignment of Pens
If not aligned then:
1. Will cause distortion of focal activity
2. Misinterpretation of EEG
Causes of misalignment:
1. More/ less Bent of stylus nib
2. Worn out stylus nib (Nib can be bent inward or outward).
Corrections of Pen alignment:
Simply change the stylus, all pens should be change simultaneously.
All pens should be in the center ( at Zero line) of the pen writer/galvano meter.
How it is checked
There should be no deflection off the base line while starting the machine or switch on and off the amplifier with no calibration signal.
Out Pen centering in conventional EEG
Mechanically Out Pen centering
Electrical out Pen centering
Types of problems:
1. Mechanical pen centering
Apparently pens are not in the center of the
galvano meter.
1. 2. Electrical pen centering
While giving the electrical signal to the pens
and simultaneously on and off the switch of
galvano meter. Ideally there should no
deflection.
Mechanical pen centering problem means that the pen are not staying at zero line apparently when the machine is switched on.
Electrical pen centering problem means that the pens are deviating from base (zero) line when amplifier switched on or off without any calibration signal while the machine is switched on.
Potentio meter
Effect on EEG recording:
1. Decrease in the high amplitude activities
2. Pseudo asymmetry of amplitudes
3. Decrease in the dynamic range
4. Distortion in the channel layout
1. Correct the mechanical pen centering by Allen Wrench-key, pull out the pen from its base form the galvanometer and fix it on the zero line.
2. Correct the electrical pen centering by switch on and off the amplifier and adjusting the potentiometer. There should be no deflection off the zero line.
3. Again adjusting the mechanical pen centering
It is the pressure of the pens on the paper
This could be
Mechanical pressure on pens called mechanical damping
Electrical damping undesirable resistance in the GVM circuit called electrical damping.
Standard damping is 2.5 gram.
Over Damping
Under Damping
There should be no rounding or overshoot of peak while the TC is set to 0.1second and HFF is set to 70 Hz as an standard parameters.
Causes of mechanical damping
1. Excessive / loose tightening of pen holding screw.
2. Worn out pens
Causes of electrical damping
1. Excessive / low resistance in the GVM circuit
How these can be corrected;
Mechanical damping should be considered first.
By using pen pressure tool and adjusting the central pen holding screw to get critical damping
By changing pen if worn out
By adjusting pen damping potentiometer to get critical damping
1. Over Damping (under shoot)
Excessive pressure on pens will result in
over damping or Under shoot.
Decrease in voltages measurement b/c of
rounding of peak.
2. Under Damping (over shoot)
low pressure on pens will result under
damping or over shoot.
Increase in voltage measurement b/c of
overshoot at peak.
Undershoot result in rounding of peak of waveform
pseudo amplitude reduction.
Overshoot will result in pseudo voltage increase.
Pens can collide with each other and can break.
Both will result in waveform distortion.
It is the low frequency filter/High pass filter
One of the most important component of the circuit which can make EEG normal and abnormal.
Defined as the time content is the time required for the pen to fall 63% from the peak of the deflection produced.
T.C depends upon the size of the capacitor.
T.C effects the decaying signal of the Calibration signal.
It is measured in second but can be converted into frequency
Formula for conversion is:
f = 1/ 2 T.C
Record a known calibration signal
Ideally at standard parameters setting of:
Sen = 7 V/ mm
Input voltage = 50 V
Paper speed = 30 mm / sec
TC = ?
Measure 1/3 ( 37%) of the total pen deflection from the base line and mark it
Draw a base line crossing the initial deflection till is merge back to the baseline
Draw a parallel line to the baseline at the mark mentioned above to get crossing at decaying curve.
Draw a perpendicular using set square from this point to the baseline
Measure the distance from the initial deflection to this bisecting perpendicular
Divide this distance with paper speed to get TC.
Effects on EEG / usage
It attenuates the low frequencies.
Could be used with appropriate paper speed to show focal abnormalities prominent.
This can be used with extreme care as in the case of sweating artifacts b/c it can make an EEG normal or abnormal.
Also called Low pass filter.
It mainly effects the rise time in calibration signal.
In EEG we are interested in the high frequency of 70 Hz.
It allows low frequencies to pass and stop higher frequencies.
If not calibrated, then contact to Bio-Medical engineer to fix it as this can be the resistor dysfunction in HFF circuit
Effects of the HFF on EEG:
1. It drops the amplitude by 30% i.e. 3db/octave.
2. Causes rounding of peak which may the morphology of the waveforms.
3. Amplitude reduction which may lead to asymmetry
4. Phase Shift will distort the focus
Voltage attenuation
This is the magnitude of input voltage requires to produce a standard pen deflection i.e. (known input voltage to produce known pen deflection)
Can be checked by inducing a known input calibration voltage (50 V) and measuring the pen deflection under standard parameters i.e. HFF 70Hz, LFF 0.1 Sec.
Sensitivity=input voltage/Pen deflection=50uV/7.1mm
= _____V / mm
All channels should have the same sensitivity as standard
If input voltage is change the output pen deflection should be change in direct proportion.
sensitivity is adjusted to avoid cutting of the waveforms.
Effects on the EEG:
1. Misinterpretation of amplitudes.
2. Wrong sensitivity setting in individual channels will lead to pseudo asymmetry
3. Blocking of channels will cause difficulty in measurement of voltage and recognition of spike or sharp activity
A recording channel is said to be linear if the pen deflection is proportional to the amplitude of the input signal applied.
If input voltage is doubled the pen deflection should be doubled in all channels.
How it can be checked1. Keeping sensitivity constant e.g 100uV/cm2. By inducing calibrator signals of 20,50 & 100uV.3. Measuring the calibration signal. P.D=100 micro volt/10 micro volt =1mm P.D=100 micro volt/20 micro volt =5mm P.D=100 micro volt/50 micro volt =2mm
1. If not controlled, then call Bio-Medical engineer
Characteristics:
Every system has an amplitude level beyond which non-linear distortion occurs.
The range over which the system is linear is called dynamic range.
Non-linearity can occur at very low pen deflection because of pressure or default in the writer system causing small amplitude calibration signal (1 or 2 mm) to be rounded.
This is the ratio of signal voltage obtained at the output of amplifier to signal voltage applied at the input.
For example if an amplifier is set to give a voltage of 10V for an input voltage of 10uV then the gain is said to be 1 million.
In contrast to sensitivity, gain is predefined
it increases with increasing amplification.
Gain is not as important than sensitivity because gain is not measured. It is prebuilt in the system.
Effect on the EEG
1. Misinterpretations of voltages
2. Pseudo Asymmetry if this happen in individual channels.
3. For correction call the bio-medical engineer
All electrical circuit has small vibration in the components of the circuit if current pass through them because of electron movements
This give a small output even if there is no external signal
This is called noise level of the measuring unit.
How to check it
1. Switch on the machine
2. Disconnect the junction box to avoid any input signal
3. Set the sensitivity to 2uV/mm
4. Record the trace
5. A peak to peak 2 mm deflection is acceptable.
Effect on EEG
1. Very low voltage EEG like ECS recording would be misinterpreted because of noise level
2. How it is corrected
If there is a problem call bio-medical engineer
What is this????
Square wave
Calibration
Sine wave
Calibration
Basic principles of digital and analog EEG,Fisch and Spehlmann’s EEG Primer