EEG calibration and its steps

EEG calibration are some steps which was followed by technologists in conventional EEG times. they are the steps to check the machines integrity either its properly working or not before the procedure began.it is very important to calibrate machine before the initiation of the test to interpret false results.

1. PRESENTATION ON : CALIBRATION AND ITS SEQUENTIAL STEPS PRESENTER :FAIZAN ABDULLAH DESIGNATION :TRAINEE TECHNOLOGISTS NEUROPHYSIOLOGY INSTITUTE:THE AGHA KHAN UNIVERSITY HOSPITAL DATE: 20/08/20

2. Content 1.Introduction to calibration 2.Biocalibration 3. Sufficient ink 4. Sufficient paper 5. Paper speed 6. Pen alignment 7. Pen centering 8. Damping 9. Time constant or LFF 10. High frequency filter (HFF) 11. Sensitivity 12. Step gain 13. Amplitude linearity 14. Noise level

3. INTRODUCTION TO CALIBRATION Calibration are the steps of checks to make sure the machine is properly working or not. What are consequences is calibration not done properly ? You can harm the patient You can make misinterpretation or false diagnosis we make a normal EEG abnormal.

4. BIOCALIBRATION Bio calibration assesses the response of the amplifiers, filters and the recording biological signal. Electrode FP1 and O2 are connected to all amplifier inputs you can make montage of FP2 and O1. The recordings for all channels should be identical. It is a montage because a fixed pattern is set in which throughout activity of 18 channels is obtained.

5. THERE ARE 12 STEPS TO CHECK DURING CALIBARTION 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

6. Sufficient ink Sufficient ink should be present in the tube of writer/ printer cartridge should not be empty. What are the if the ink is not properly checked Consequences ? 1.any paroxysmal event 2.a brief Interictal abnormality 3.onset of a seizure. 4. Loss of paper

7. Sufficient paper 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 if you don’t check the quantity of paper that you have? 1. You can miss any paroxysmal event 2. You can miss a brief Interictal abnormality 3. You can miss onset of a seizure.

8. Paper speed 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

9. How to check paper speed 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.

10. Distance=27cm, Time =30seconds P.S=27cm/30sec= 0.9cm/sec or 9mm/sec

11. Pen alignment 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  If not aligned then: Will cause distortion of focal activity Misinterpretation of EEG

12. Misalignment of Pens

13. What if pens are not properly aligned?  Causes of misalignment: More/ less Bent of stylus nib 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.

14. Pen centering All pen s should b e i n th e center ( at Zero line) of the pen writer/ galvano meter.

15. Out Pen centering in conventional EEG

16. Mechanical out pen centering

17. Types of problems: 1. Mechanical pen centering Apparently pens are not in the center of the galvano meter. 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.

18. 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. Effect on EEG recording: Decrease in the high amplitude activities. Pseudo asymmetry of amplitudes

19. Electrical out pen centering

20.  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

21. Galvanometer

22. Allen wrench

23. Damping 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.

24. Over Damping Under Damping

25. Types of dumping 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. 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.

26. What if we don’t properly check damping? 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.

27. Time constant or LFF 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 contant is the time required for the pen to fall 63% from the peak of the deflection produced. 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 >2x3.14=6.28 >1/6.28=0.1592 >0.1592x0.1=0.01592 >0.01592x100=1.592 f=1.592

28. 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=0.1sec Hff =70HZ Lff =1.6HZ

29. Effects on EEG Effects on EEG / usage It attenuates the low frequencies. It can cut down slow frequencies of need

30. High frequency filter (HFF) 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.

31. Effects of the HFF on EEG: . It drops the amplitude by 30% . . Causes rounding of peak which may the morphology of the waveforms. . Amplitude reduction which may lead to asymmetry

32. Sensitivity This is the magnitude of input voltage requires to produce a standard pen deflection i.e. (known input voltage to produce known pen deflection). All channels should have the same sensitivity as standard. It is the ratio of; S=V/H

33.  Effects on the EEG: 1. Misinterpretation of amplitudes. 2. Wrong sensitivity setting in individual channels will lead to pseudo asymmetry

34. Amplitudes linearity 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.

35. How it can be checked Keeping sensitivity constant e.g 7uV/mm By inducing calibrator signals of 20,50 & 100uV. Measuring the calibration signal. P.D=7uv/mm/50uv =7.1mm H=v/s

36. Step gain This is the ratio of signal voltage obtained at the output of amplifier to signal voltage applied at the input. it increases with increasing amplification. Gain is not as important than sensitivity because gain is not measured. It is prebuilt in the system. GAIN=OUTPUT/INPUT

37. Noise level 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.

38. References Basic principles of digital and analog EEG,Fisch and Spehlmann’s EEG Primer Cnp peer lecture