Eeg machine calibration

1. EEG MACHINE CALIBRATION Shehzad Hussain Technologist

2. Definition: 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.

3. Why to do calibration? 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.

4. Biocalibration 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.

5. If no or wrong calibration then: Then we can interpretate wrong and the result will be wrong. Either we make a normal EEG abnormal.

6. Conventional EEG machine

7. Schematic Diagram

8. sequential steps to check during calibration: Sufficient ink Sufficient paper Paper speed Pen alignment Pen centering Damping

9. sequential steps to check during calibration: Time constant or LFF High frequency filter (HFF) Sensitivity Step gain Amplitude linearity Noise level

10. Digital Machine

11. Analog Machine

12. Sufficient paper/Disc Space 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?

13. Consequences… Insufficient paper will lead cessation of live recording Cessation may lead to miss: any paroxysmal event a brief Interictal abnormality onset of a seizure.

14. Sufficient ink Sufficient ink should be present in the tube of writer/ printer cartridge should not be empty. Consequences: any paroxysmal event a brief Interictal abnormality onset of a seizure. Loss of paper

15. 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

16. How to check paper speed: Paper speed could be checked in two ways: 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.

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

18. Consequences of paper speed Misinterpretations of frequency Misinterpretation of waveform duration Wave morphology could change like spike could appear as sharp wave and sharp wave as slow wave.

19. How to correct paper speed 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.

20. 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 This can be done with or without calibration signal

21. Misalignment of Pens

22. Pen alignment cont…. If not aligned then: Will cause distortion of focal activity Misinterpretation of EEG

23. Consequences of Pen Alignment 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.

24. Pen centering: 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.

25. Out Pen centering in conventional EEG

26. Mechanically Out Pen centering

27. Electrical out Pen centering

28. Pen centering cont…. Types of problems: 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.

29. Pen centering cont…. 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.

30. Potentio meter

31. Pen centering cont…. Effect on EEG recording: Decrease in the high amplitude activities Pseudo asymmetry of amplitudes Decrease in the dynamic range Distortion in the channel layout

32. Correction of the Pen centering 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

33. 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.

34. Over Damping Under Damping

35. Damping Cont…. 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.

36. Damping Cont… Causes of mechanical damping Excessive / loose tightening of pen holding screw. Worn out pens Causes of electrical damping Excessive / low resistance in the GVM circuit

37. Damping Cont… 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

38. Types of Damping 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.

39. Consequences of 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.

40. 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 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.

41. Time Constant or LFF cont… It is measured in second but can be converted into frequency Formula for conversion is: f = 1/ 2  T.C

42. Time Constant or LFF cont… 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 = ?

43. Time Constant or LFF cont… 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.

44. Time Constant or LFF cont… 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.

45. Effect of Time constant on EEG… 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.

46. 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.

47. Calibration at 70 Hz:

48. High frequency filter (HFF): If not calibrated, then contact to Bio-Medical engineer to fix it as this can be the resistor dysfunction in HFF circuit

49. High frequency filter (HFF): Effects of the HFF on EEG: It drops the amplitude by 30% i.e. 3db/octave. Causes rounding of peak which may the morphology of the waveforms. Amplitude reduction which may lead to asymmetry Phase Shift will distort the focus

50. Filters: Voltage attenuation

51. 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) 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

52. Sensitivity cont… 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.

53. Consequences of Sensitivity Effects on the EEG: Misinterpretation of amplitudes. Wrong sensitivity setting in individual channels will lead to pseudo asymmetry Blocking of channels will cause difficulty in measurement of voltage and recognition of spike or sharp activity

54. Amplitude 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.

55. Amplitude linearity cont… How it can be checked Keeping sensitivity constant e.g 100uV/cm By inducing calibrator signals of 20,50 & 100uV. 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 If not controlled, then call Bio-Medical engineer

56. Cont… 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.

57. Step gain: 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.

58. Step gain cont… Effect on the EEG Misinterpretations of voltages Pseudo Asymmetry if this happen in individual channels. For correction call the bio-medical engineer

59. 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.

60. Noise level cont… How to check it Switch on the machine Disconnect the junction box to avoid any input signal Set the sensitivity to 2uV/mm Record the trace A peak to peak 2 mm deflection is acceptable.

61. Noise level cont… Effect on EEG Very low voltage EEG like ECS recording would be misinterpreted because of noise level How it is corrected If there is a problem call bio-medical engineer

62. What is this???? Square wave Calibration Sine wave Calibration

63. REFERENCES: Basic principles of digital and analog EEG,Fisch and Spehlmann’s EEG Primer