Artifacts in eeg final

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Artifacts in eeg final

  1. 1. Introduction: • Unwanted electrical activity arising from different sources, other than cerebral activity • EEG -highly sensitive recording device, easily interrupted by other electrical activity of very high voltages • Some readily distinguished, others closely resemble cerebral activity .
  2. 2. Principles used to discriminate artifacts from EEG signals: • Physiological activity has a logical topographic field of distribution with an expected fall off of voltage potentials. • Artifacts have an illogical distribution that defies the principles of localization
  3. 3. Classification: • Physiological : from patient’s own physiological generator sources other than the brain • Extraphysiological : Externally generated e.g. instrumental & environmental
  4. 4. Physiological artifacts: • Cardiogenic-electrical mechanical • Muscle activity – Eye movement – surface EMG – Movement glossokinetic respiration photomyoclonic • Skin artifacts perspiration salt bridges • Eye movements Blink flutter Lateral gaze Lateral rectus spike Slow eye movements Electroretinogram REM sleep
  5. 5. Eye movement artifacts: • most common, due to cornea-retinal potentials. The cornea is positive to the retina by 50-100 mv • Fp1, Fp2, F7, F8 , amplitude is approx 50- 200uv • frontopolar rhythmic slow wave activity - simulating brain activity • Types of eye movement artifacts – Eye open, eye closure and blink – Eyelid flutter – Eyeball movement (lateral, vertical and oblique eye movements
  6. 6. Eye opening and eye closure
  7. 7. Blink Eye blink is a “U” shape transient potential which appears in frontopolar electrodes which are closer to the eyes.
  8. 8. Eyelid flutter Fine eyelid movements may produce rhythmic 4-8 Hz activity in the frontal leads (disappear on fixation)
  9. 9. Lateral eyeball movements
  10. 10. Elimination: • close the eyes or eye fixation • close his eyes with his fingers or place a soft cloth or a cotton pads over the closed eyelids • Monitoring eye movements by placing extraocular electrodes will easily help to distinguish from frontal slow waves
  11. 11. Muscle (EMG) artifacts: • EMG artifacts are due to the muscle contraction superimposed upon the EEG activity • The motor unit potentials (MUPs) arising from the scalp muscles may cause misinterpretation by resembling spike or cortical β-activity
  12. 12. Lateral rectus spikes:
  13. 13. Frontalis:
  14. 14. Temporalis: Clenching and grinding teeth
  15. 15. Swallowing: Swallowing of saliva usually produces a short burst activity
  16. 16. Chewing:
  17. 17. Sniffling artifact: Small EMG component with a slow wave
  18. 18. Facial myokymia:
  19. 19. Glossokinetic artifact: Movement of the tongue during speaking may produce synchronous intermittent rhythmic EEG activity (2-6 Hz) because the mass of the tongue acts as a charged body with a negative tip
  20. 20. Glossokinetic artifact:
  21. 21. Tremor:
  22. 22. Differentiation between EMG artifacts & the cortical spikes: • EMG potentials are of extremely short duration (2-20 msec) and spiky even on increasing paper speed to 60 mm/s • On basis of morphology and frequency • During sleep EMG activity is reduced whereas cortical spikes increase • Document using extra muscle electrodes
  23. 23. Reduction/elimination: • Frontalis - close eyelids lightly and relax • Temporalis - opening the mouth • Use of relaxation techniques (reassuring, comforting the patient),Massaging the particular muscle groups • Occipitalis - putting the pillow under the neck • As a last resort change the high frequency filter to 35/15 Hz
  24. 24. Cardiogenic artifacts: • EP arising in the cardiac muscles, high in amplitude • Spread to scalp by volume conduction • Prominent in babies, obese, short – neck, cardiomegaly • Referential montage picks up ECG artifact due to large interelectrode distances and close proximity of ear electrodes to the heart • Equipotential on the scalp - not picked up in bipolar montage
  25. 25. Types: • ECG • Pulse • Pacemaker
  26. 26. ECG Coming at regular intervals synchronising with ECG
  27. 27. ECG: Coming at regular intervals synchronising with ECG
  28. 28. Pulse
  29. 29. Pulse:
  30. 30. Elimination: • Changing the head position relative to the thorax • Lower the electrode resistance (A1 and A2) or use body earth (A1+A2) • Change reference to Cz • Shift the electrode a little - eliminate pulse wave
  31. 31. Sweat artifact: • Warm and humid recording room • Anxiety and emotional tension • Autonomic dysfunction • Electrical potentials due to sweat glands • Reduction in the skin resistance (GSR) • NaCl and lactic acid of sweat may produce large base line sways
  32. 32. Sweat: These are the slow waves of very low frequency (1/3-1/2 Hz)
  33. 33. Salt bridge: Electrolyte in sweat interact with electrode gel / paste to produce a salt bridge
  34. 34. Elimination: • Proper air conditioning • Reducing emotional stress • Sponging the patient’s face & forehead with alcohol/spirit • Setting the short time constant (LFF) if all the efforts fail
  35. 35. Movement artifacts: • Movement of head, body and limbs produce irregular high voltage potentials • Movement of the head during HV or respiration can produce occipital electrode artifacts • Movement associated with seizure may obscure cerebral activity • Artifacts may occur in essential tremors, Parkinson disease and other movement disorders
  36. 36. Head movement: Movement between the head and the pillow
  37. 37. Movement associated with seizure: Contraction of skeletal muscles before the seizure activity starts
  38. 38. Movement associated with seizure: Difficult to determine the origin of the seizure activity
  39. 39. Patting artifacts: Mother patting baby
  40. 40. Respiration artifacts: More commonly seen during HV as slow wave activity
  41. 41. Reduction/elimination: • Solicit the co-operation of the patient to be calm and still • Changing the position or turning the patient over will diminish tremor artifacts • Placing a roll of towel or firm material under the neck may do away with respiratory artifacts
  42. 42. Extra physiological artifacts: – Electrode –electrode pop, contact –Technical-lead movement, placement –Instrumental –AC 60hz artifact ventilator, circulatory pumps –Iv fluids – Environmental-mobile, TV, radio, electromagnetic
  43. 43. • The scalp-electrode junction is an important and delicate link in the whole system • Electrode artifacts occur due to the change in the resistance or electrode potential between the scalp and the electrode • Poor contact, improper electrolyte application, broken lead, poor contact at the junction box, dry out of electrode paste Electrode artifacts:
  44. 44. Electrode pop: A sudden change in the electrical potential between the electrode and scalp gives rise to the ‘pop’
  45. 45. Electrode artifact: Loose contact of the electrode with the scalp
  46. 46. Electrode movement artifact: This is movement of electrode on the scalp, because of excessive jelly
  47. 47. Recognition & differentiation from cerebral activity: • Electrode artifact is strictly confined to one lead or channels having input from this electrode, where as cerebral activity would usually spread to the adjacent electrodes • In a bipolar montage, the artifacts show mirror images (like phase reversals) in two adjacent channels • Wave morphology may change from time to time Electrode artifacts:
  48. 48. Elimination • Applying proper electrolyte • Check electrode impedance & continuity • Change the electrode • If it still present change the position at a distance of 0.5-1cm Electrode artifacts:
  49. 49. • due to the malfunctioning of certain parts of the machine -Pen deflection -Amplifier -Galvanometer -Faulty ground -Photic stimulator Instrumental:
  50. 50. • Non-symmetrical electrode placement • Erroneous settings of different channels • Unequal electrode impedances result in loss of common mode rejection and recording of AC artifact Technical artifacts:
  51. 51. Pen deflection:
  52. 52. A1- disconnected:
  53. 53. Photic:
  54. 54. Interchanging electrode placement: Interchanging the Fp2 electrode with O2
  55. 55. AC artifact:60 hz artifact Very high electrode impedance of more than 25 kΩ
  56. 56. • troublesome as they cannot be easily controlled by the technologist • Movement of other persons • Types of environmental artifacts: – Electromagnetic Interference – Electrostatic Induction – Radio frequency Interference • As technology expands – new artifacts appear – Mobile phone artifacts Environmental artifacts:
  57. 57. Electromagnetic interference: • Current flowing through an electrical conductor (main power supply), produces a magnetic lines of force around the conductor • The greater the current, the stronger the field • Occurs between mains and patient or electrode attached to the scalp or junction box • Common sources – power transformers – Electric motors in lifts, fans, air-conditioners • AC interference (result of capacitance or inductive effects on wires leading from the scalp to the junction box)
  58. 58. Electrostatic Induction: • Opposite polarities of two charged bodies produces electrostatic induction when negative electric charge is applied • Presence of several instruments adjacent to the EEG machine (like fluorescent light, ventilators etc.) • Movement of any charged body near the patient (like plastic, rubber, synthetic fibres etc) • Often the technologist himself is the recipient of this discharge • Low electrical resistance between patient and the environment
  59. 59. Static artifact: Produced by placing a charged synthetic paper near the patient
  60. 60. Static artifact: walking in to collect a report
  61. 61. Electrostatic artifact: Startle or clap response can also cause static artifact
  62. 62. AC Interference
  63. 63. Cell phone artifact:
  64. 64. Minimisation/removal of environmental artifacts: • Keeping electrode leads close together. • Move away from source (at least 10 feet) (since electrostatic induction is inversely proportional to the distance between patient and source) • Changing the orientation of the patient/source parallel to the electromagnetic lines of force • Locate the EEG lab at least 50 feet away from AC feeders, transformers etc. • One earthing point common to all equipment • Shielding the source of artifact and machine with earthed metal shield
  65. 65. Artifacts are also useful: • Slow lateral eye movements and disappearance or reduction of EMG are a valuable sign of drowsiness/sleep • Eye open and eye closure artifacts record the precise point where the patient opened or closed his eyes • Short bursts of EMG activity may mark the occurrence of myoclonic jerks
  66. 66. Environment for artifact free recording • Quiet atmosphere • Comfortable bed • No synthetic carpeting • Convenient control of lighting • Proper earthing / shielding • Different power supply to equipment and air conditioner • Comfortable temperature and humidity levels to avoid sweating, tension, restlessness
  67. 67. Thank you

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