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