Cours 1 : bases techniques

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1 Cours 1 : bases techniques
Electromyographie Cours 1 : bases techniques Cours 2 : EMG au repos Cours 3 : EMG lors de la contraction volontaire Anjali Nandedkar – François Wang

2 Lors de la contraction musculaire volontaire, l’EMG étudie la traduction électrique de l’activation des motoneurones spinaux Motoneurone spinal Axone moteur Terminaison axonale Fibre musculaire EMG The controlled activation, de-activation and co-ordination of muscles to generate the necessary force and displacement are accomplished by the structural organization of the muscle fibers and neurons in to so called motor units (MUs). A motor unit consists of all muscle fibers innervated by one motor neuron. The motor neuron, upon activation, produces a nerve action potential that propagates through the axons and its terminal branches. At the motor endplate, also called the neuromuscular junction or synapse, the action potential is transmitted from the nerve terminal to the muscle fiber by release of acetylcholine. The muscle fiber depolarizes and generates an action potential that propagates from the endplate to both tendons. The AP initiates a chain of reactions that leads to the contraction of the muscle fiber. Thus every time the motor neuron discharges, all muscle fibers in the MU respond by producing an electrical output, which is recorded as the electromyogram; and a mechanical output, which is a twitch force and displacement. Plaque motrice Force Le PUM précède de 30 à 100 ms le sommet du twitch L’unité motrice

3 EMG/biopsie électrophysiologique
L’EMG par électrode aiguille évalue de façon indirecte la structure des unités motrices EMG PUMs de grande amplitude, de durée augmentée, pulsant à fréquence élevée et réduits en nombre Biopsie musculaire Groupement de fibres The EMG examination compliments the muscle biopsy studies. The biopsy allows one to directly observe the organization and even the microscopic details of the muscle fibers. The needle EMG analysis is an indirect approach to the assessment of motor unit structures. Therefore, it is necessary to understand the relationship between the EMG measurements and their generators. We have used computer simulations to explain the EMG findings in “anatomic” terms. We call this approach the “electrophysiologic biopsy”. This is demonstrated through the sketches of motor unit structure for some EMG recordings. 2mV/D ; 10 ms/D EMG/biopsie électrophysiologique

4 1. Activité d’insertion = irritation des membranes musculaires
Gain/sensibilité : 50 µV/D Durée du balayage : 100 ms Vitesse de balayage : 10 ms/D The examination can be divided in to 5 parts. First, the needle is inserted in to the tested muscle and moved to different sites. The needle movement irritates the muscle fiber membranes and the resulting potentials are called the “insertional activity”. EMG de routine en 5 étapes

5 EMG de routine en 5 étapes
2. Activité spontanée Gain/sensibilité : 200 µV/D Durée du balayage : 100 ms Vitesse de balayage : 10 ms/D Types: Pointes positives Fibrillations DRC Myotonie Fasciculations …… In the second step, the needle is maintained fixed within the muscle and the patient is instructed to completely relax the tested muscle. In pathologic muscles, one records potentials generated in spontaneous fashion. These potentials have characteristic waveforms and discharge patterns that are used to characterize them, e.g. positive sharp waves, fibrillations, complex repetitive discharge, etc. This is called “spontaneous activity”. Some forms of spontaneous activity can be triggered by needle movement and can be seen as part of insertional activity also. EMG de routine en 5 étapes

6 3. PUM (bref temps de montée/son aigu)
Gain/sensibilité : 200 µV/D Durée du balayage : 100 ms Vitesse de balayage : 10 ms/D Paramètres d’analyse: Amplitude Durée Morphologie ……. In the third step, the motor unit action potential waveform is assessed. The patient contracts the muscle minimally and the position of the needle is adjusted to record potentials with a short rise time. This is facilitated by listening to the sound of the EMG. Potentials with a short rise time have a very “sharp” and “crisp” sound. They also have higher amplitude. The amplitude, duration and waveform of MUAPs is characterized. EMG de routine en 5 étapes

7 EMG de routine en 5 étapes
4. Tracé d’interférence Gain/sensibilité : 500 µV/D Durée du balayage : 500 ms Vitesse de balayage : 50 ms/D Paramètres d’analyse: Nombre de PUMs Ordre de recrutement Fréquence de décharge ……. In the fourth step, the patient is asked to gradually increase the force of contraction to reach maximal effort. The resulting signal is complex and contains discharges of several different motor unit potentials. It is called the interference pattern or the recruitment pattern. The signals are assessed for the number, orderly recruitment and firing rates of the motor units. EMG de routine en 5 étapes

8 L’EMG de routine reste une évaluation subjective,
visuelle et auditive, des signaux électriques musculaires 5. Évaluation des résultats et décision de la suite de l’examen EMG quantifiée Autre muscle Conduction nerveuse ……. In the fifth step, the clinician re-assesses the results of the tested muscle and previous studies to define the next step in electrodiagnostic testing. Needle EMG may be performed in additional muscles to demonstrate the pattern of abnormalities. Many laboratories have their favorite muscles to test based on the referring diagnosis. The routine needle EMG examination is usually performed by observing the EMG signals on the display screen and by listening to their sound. It is quite subjective. EMG de routine en 5 étapes

9 2. Faire des mesures + calculs statistiques (moyennes)
Evaluation objective 1. Enregistrer des signaux EMG suivant un protocole standard (20 PUMs, température, type d’aiguille) 2. Faire des mesures + calculs statistiques (moyennes) 3. Comparer les résultats à des valeurs de référence Problèmes 1. Ca prend du temps 2. Etablir ses propres valeurs de référence 3. Expertise, entrainement, matériel 4. Pas prévu par notre nomenclature In contrast, quantitative analysis offers an objective method of signal analysis. This involves three steps. First the signals are recorded using standardized settings of the instrument. In additional, the physical variables such as recording electrode, temperature also need to be controlled. A sample of 20 signals is generally considered adequate to permit statistical analysis. Secondly, the signals are quantified by measuring its features such as amplitude. Finally, the individual measurements or their statistics, i.e. mean values, are compared against suitable reference values. Quantitative analysis has always been time considered time consuming. As example, analysis of 20 MUAPs used to requite over 30 minutes. With access to computers, this time has decreased significantly. Some methods require special software for making the measurements and analysis. Finally, there has been a paucity of reference values. Hence, quantitative analysis is not performed in most EMG laboratories. Even in those laboratories with necessary skills, expertise and equipment for quantitative analysis, the methods are used in a selective fashion. Methods such as single fiber EMG are more sensitive than other methods. Other methods may not be sensitive, but can be useful to assess the progression of the disease, e.g. Macro EMG. As a result, many consider quantitative analysis as something remote from the routine EMG examination. This is a myth. EMG quantifiée (Buchthal, > 50 ans)

10 Moyenne ± DS, méthode des percentiles : neurographie > EMG
Anomalies infracliniques chez certains sujets inclus dans le groupe contrôle Des patients sains (< 5%) peuvent avoir des paramètres hors des limites de la “normale” Moyenne ± DS, méthode des percentiles : neurographie > EMG Outliers We prefer the term “reference” values instead of the “normal” values for two reasons. Many healthy subjects who participate in the study are considered “normal” based on the subject’s assurances and the clinicians impression. It is not unusual to find sub-clinical abnormalities in some subjects. The inclusion or exclusion of those studies poses a great dilemma. Secondly, the limits are usually defined as 2 – 3 standard deviations about the mean value. Assuming a Gaussian distribution of the measurements, there is a slight change (usually < 5%) that a “normal” subject will have measurements outside the “normal” limit. Valeurs de référence plutôt que valeurs normales

11 Ne pas surinterpréter les données de l’EMG
Un tracé anormal  muscle pathologique Pour un paramètre EMG donné, 10% des valeurs individuelles pouvent être hors limites The same strategy based on mean values is still used in many different electrophysiologic tests. While this approach works quite satisfactorily in nerve conduction studies, the routine needle EMG presents several difficulties. In contrast to quantitative analysis, the clinician does not make detailed measurements of each signal. One usually recognizes signals that are “obviously abnormal”. This may be in the form of a spontaneous discharge, or a very complex motor unit potential, or presence of just one or two fast firing large motor unit action potentials at maximal efforts. When such signals are found just once or twice, it is considered a chance phenomenon. However, when obvious abnormalities are found in several different sites in the tested muscle, the clinician has sufficient confidence in considering the muscle abnormal. 2mV/D ; 10 ms/D Outliers (Stålberg)

12 Particulièrement utile : EMG de fibre unique, Macro-EMG, techniques quantifiées d’analyse des PUMs, analyse turn/amplitude des tracés d’interférence Gain de temps : l’analyse est pathologique dès que la limite de 10% est franchie (ex. 3/20 PUMs hors limites) Ne peut pas être utilisé dans un suivi EMG pour apprécier l’évolution des anomalies Prof. Stalberg and colleagues described the above strategy as the “outlier” principle. They have defined the reference limits for the mean values of the needle EMG measurements and also for the individual signal values. A study consisting of 20 recordings is considered normal when the mean values are within the reference range and more than 90% of individual measurements are also “normal”. In most studies, the mean values are individual measurements demonstrate a parallel change, i.e. both are increased or both are reduced. However, in some instances some of the individual values are increased while others are reduced. The mean values remain normal. This poses a challenge to the interpretation of the data. Based on computer simulations, we will demonstrate how some of these findings can be explained without invoking the “mixed” or “neuro-myopathy” concept. The strategy based on mean and outlier values has been used successfully in single fiber EMG, Macro EMG, interference pattern analysis and motor unit potential analysis. The “outlier” approach is especially time saving. In a normal muscle, 2 of 20 measurements are allowed to be outside the reference range. When a third abnormal recording is made, the test has reached the diagnostic significance and the study in that muscle can be terminated. This may occur within the first few sites of recordings and within a minute or two. In this manner it is no longer necessary to spend several minutes to study individual muscle. The “outlier” approach can demonstrate abnormalities from a very small sample of measurements. This has inherently a higher variance. Therefore, the technique is not suitable in assessment of serial investigations. When “objective follow up” studies are planned, one must use a large sample of recordings, usually 20+ for each muscle, to assess any electrophysiologic changes.  In “objective & interactive EMG” approach we use the principles and techniques of quantitative analysis to better recognize, isolate and document the “outlier” signals as shown earlier. This will be the theme of signal analysis in this tutorial. Outliers (Stålberg)

13 Outliers (Stålberg) # PUMs “Normal” # PUMs # PUMs Amplitude Amplitude
Limites de la moyenne Limites des valeurs individuelles Outliers (Stålberg) CASA, 2001

14 Facteurs physiologiques et biologiques
Muscle Durée moyenne : 9.4 ms pour 1IO, 11.4 ms pour Tib Ant Amplitude moyenne : 752 µV pour 1IO, 436 µV pour Biceps Fréquence de décharge plus élevée pour les muscles oculaires Age Sujets âgés ont des signaux plus amples Type d’aiguille Aiguille monopolaire => potentiels plus polyphasiques et plus grands en amplitude . Filtres, Temperature (amplitude), ….. Finally, the reference values are affected by many factors: physiologic and technical. As example, the motor unit action potential duration values are lower in small muscles and higher in large force generating muscles such as biceps. In contrast the amplitudes are often high in small muscle like first dorsal interosseus. The motor unit firing rate is higher in ocular muscles compared to limb muscles. The measurements are known to change with age. Older subjects tend to have higher signal amplitude on needle EMG. The signals recorded by monopolar needle usually have higher amplitude and more complex waveforms compared to the concentric needle. The signals are affected by instrument settings like low and high pass filters. The temperature affects the signal amplitude. It is important to recognize these differences in the needle EMG examination. Hence, it is often said that one must develop his/her own reference values for different EMG procedures. This is easier said than done. Facteurs physiologiques et biologiques CASA, 2001

15 Utilisation des valeurs de référence publées par d’autres =>
Idéalement, chaque laboratoire devrait constituer ses propres valeurs de référence Utilisation des valeurs de référence publées par d’autres => - technique absolument identique - vérifier la validité de ces normes chez quelques sujets sains In quantitative analysis, one should try to mimic the recording procedure that was used in defining reference values. Furthermore, one should also test a few “healthy” subjects to ensure that the published reference range is “reproducible” in the hands of the electromyographer. Valeurs de référence CASA, 2001

16 Pour simplifier, bruit”thermique” + interférences = BRUIT
EMG implique la détection et l’enregistrement de potentiels de très faible amplitude dans un environnement riche en bruit parasite Bruit “thermique” des composants électroniques : résistances, transistors…. Interférences liées à des radiations électromagnétiques externes : transmissions TV et radio, champs électromagnétiques des lignes électriques Pour simplifier, bruit”thermique” + interférences = BRUIT An electromyograph is a sophisticated electronic instrument whose primary task is to record and display very small amplitude neurophysiologic potentials in an environment with high ambient noise. In electronic devices “noise” refers to the internal “thermal” noise generated by the various components such as the resistors, transistors, etc. “Interference” refers to the external electromagnetic radiation that is recorded by the device, e.g. radio & TV transmissions, power line electromagnetic fields. For simplicity, we will refer to both phenomenon as noise. As the technology evolved, the commercially available EMG systems have assume more responsibilities: automated measurements, automated analysis, data storage, report generation, etc. Electromyographe

17 Fils & Câbles Pré-amplificateur (analogique) Amplificateur (analogique) Filtres Electrodes Digitalisation du signal (échantillonnage) Imprimante Sauvegarde des données ….. Représentation graphique et sonore Traitement numérique du signal (filtres digitaux) et analyses This illustration shows the signal flow in a typical modern digital EMG system. The electrodes are used to register the neurophysiologic potentials. The leads and cables bring it to the amplifier input. The amplifier may be divided in to “pre-amplifier” and “main amplifier” sections. For the sake of simplicity, we will treat these sections as one entity. The amplifiers are analog devices and may contain some signal conditioning circuits called filters. Once the signals are amplified, they are fed in to a device called “analog to digital” converter. This converts the EMG signal into a record of voltage values at different times. Electromyographe moderne

18 Electromyographe moderne
Echantillonnage DS Temps Voltage (ms) (µV) …. Signal analogique TNS The digitized record is the foundation of the EMG instruments. These records can be analyzed using the “digital signal processing” (DSP) systems. This includes filtering the signals to get rid of noise and interference. These are called the digital filters. One can also write applications to process the signals, enhance the signal to noise ratio, make automated measurements, etc. Analyse Electromyographe moderne

19 Les SIGNAUX et le BRUIT sont affectés par toutes les composantes de
Les SIGNAUX et le BRUIT sont affectés par toutes les composantes de l’électromyographe et toutes les étapes du traitement du signal Il faut atténuer le BRUIT sans distorsion du SIGNAL Il faut donc augmenter le rapport SIGNAL/BRUIT At each stage in this process, the signal and noise are affected. Hence it is important to understand their important characteristics and specifications. The goal should be to attenuate noise without distorting the signal. This is best accomplished by minimizing the ambient noise. Reduced noise with high amplitude signal will increase the signal to noise ratio. A good recording has high signal amplitude and low noise. Rapport signal/bruit

20 PUM capté par aiguille concentrique est plus ample et plus bref que par électrode de surface
(Guihéneuc) Électrodes d’enregistrement

21 Électrode aiguille concentrique
150 µm Cannule (référence) Surface elliptique d’enregistrement (active) 15° Canule métallique (0,45 mm de diamètre) Fil platine, nickel-chrome, acier inox (0,15 mm de diamètre) The concentric needle consists of a metal wire inserted in to a hollow metal cylinder called cannula. The tip of this assembly is ground at 15 degrees angle to expose an elliptical recording surface called the core. The core is the active recording surface of the needle and the cannula serves as the reference recording electrode. An additional ground electrode is also necessary for high quality EMG recordings. + électrode terre Électrode aiguille concentrique

22 Électrode aiguille concentrique
Amplificateur Active Référence Terre Unité motrice Spike (pointu et son aigu) Signal EMG Le territoire d’enregistrement est hémisphérique (Spike) 2 mm The needle core is surrounded by the metal cannula that acts as a “shield”. Hence the core can record sharp action potentials from only those muscle fibers that are in front of the core. Such potentials are rich in high frequency components, e.g. fibrillations, spike component of the motor unit action potential. Therefore, the recording territory of this needle was considered to be hemispherical.

23 Électrode aiguille concentrique
Amplificateur Active Référence Terre Unité motrice Début et fin du potentiel (faible amplitude et son grave) Signal EMG Le territoire d’enregistrement est sphérique (PUM) Recent experiments have demonstrated that the core can register low frequency signals from muscle fibers that are behind the cannula. This represents the initial and terminal components of the motor unit action potential.

24 Électrode aiguille concentrique
It is important to recognize that the concentric needles are commercially available in two difference sizes. The so-called “facial” needle uses a smaller diameter cannula and a smaller wire for the core. Hence patients find it more comfortable. It is often used to record from the small facial muscles. It is also used in small muscles of the hand and foot. Because of the smaller recording surface (0.02 mm2), this needle also gives higher amplitude for EMG signals. We performed quantitative analysis using these needles in the same muscle of normal subjects and found a difference in amplitude, but not in duration. To our knowledge, the reference values for the concentric EMG analysis have been developed using the regular CN needle with 0.07 mm2 recording surface. Électrode aiguille concentrique

25 Électrode aiguille concentrique
It is important to recognize that the concentric needles are commercially available in two difference sizes. The so-called “facial” needle uses a smaller diameter cannula and a smaller wire for the core. Hence patients find it more comfortable. It is often used to record from the small facial muscles. It is also used in small muscles of the hand and foot. Because of the smaller recording surface (0.02 mm2), this needle also gives higher amplitude for EMG signals. We performed quantitative analysis using these needles in the same muscle of normal subjects and found a difference in amplitude, but not in duration. To our knowledge, the reference values for the concentric EMG analysis have been developed using the regular CN needle with 0.07 mm2 recording surface. (Guihéneuc) Électrode aiguille concentrique

26 VRU : volume de recueil utile (2 mm de diamètre) 20-50 UM 20-30 fm/UM
It is important to recognize that the concentric needles are commercially available in two difference sizes. The so-called “facial” needle uses a smaller diameter cannula and a smaller wire for the core. Hence patients find it more comfortable. It is often used to record from the small facial muscles. It is also used in small muscles of the hand and foot. Because of the smaller recording surface (0.02 mm2), this needle also gives higher amplitude for EMG signals. We performed quantitative analysis using these needles in the same muscle of normal subjects and found a difference in amplitude, but not in duration. To our knowledge, the reference values for the concentric EMG analysis have been developed using the regular CN needle with 0.07 mm2 recording surface. (Guihéneuc) Électrode aiguille concentrique

27 Souvent utilisées pour la face et les extrémités
Plus fines Moins douloureuses Souvent utilisées pour la face et les extrémités Plus petite surface d’enregistrement (0,02 mm2) donne des signaux de plus grande amplitude Pas de normes disponibles It is important to recognize that the concentric needles are commercially available in two difference sizes. The so-called “facial” needle uses a smaller diameter cannula and a smaller wire for the core. Hence patients find it more comfortable. It is often used to record from the small facial muscles. It is also used in small muscles of the hand and foot. Because of the smaller recording surface (0.02 mm2), this needle also gives higher amplitude for EMG signals. We performed quantitative analysis using these needles in the same muscle of normal subjects and found a difference in amplitude, but not in duration. To our knowledge, the reference values for the concentric EMG analysis have been developed using the regular CN needle with 0.07 mm2 recording surface. Électrode aiguille concentrique « faciale »

28 - + Z V Source de tension avec une impédance en série
Z = Impedance + Source de tension avec une impédance en série L’impédance représente la “qualité” du contact entre la surface d’enregistrement et les générateurs électrophysiologiques L’impédance doit être basse pour avoir un bon rapport SIGNAL/BRUIT une impédance élevée donne plus de BRUIT V - Regardless of the shape and construction, all electrodes record the electrophysiologic potential and the ambient noise. The needle can be modeled as a voltage source in series with an impedance. The impedance represents the “quality” of contact between the recording surface and the electrophysiologic generators. Low impedance implies a good contact. This gives potentials of high amplitude while reducing the amplitude of noise and interference. High electrode impedance gives more noise and interference. Modèle électrique d’une aiguille

29 Électrode aiguille concentrique
L’active et la référence étant en contact avec les fluides biologiques, l’impédance est faible => bon rapport SIGNAL/BRUIT Amplificateur Active Référence Terre Unité motrice Signal EMG In needle EMG, the recording tip is surrounded by highly conductive body fluids. This gives the desired low impedance. Électrode aiguille concentrique

30 Électrode aiguille monopolaire
Amplificateur Active Signal EMG Référence Terre Impédance - faible pour l’active - élevée pour la référence (électrode de surface cutanée) Le déséquilibre d’impédance donne plus de BRUIT By virtue of construction, the concentric needle core and cannula are bathed in body fluids and have low impedance. In monopolar needle EMG, the reference electrode is placed on skin surface. This has higher impedance compared to the intramuscular monopolar needle tip. This imbalance gives slightly higher noise in MNEMG. Unité motrice Électrode aiguille monopolaire

31 Aiguille concentrique défectueuse
EXTRA INTRA Aiguille concentrique défectueuse

32 Un câble est un ensemble de fils, 3 en EMG : Active, Référence, Terre
Des fils métalliques connectent l’électrode aiguille au système d’amplification Un câble est un ensemble de fils, 3 en EMG : Active, Référence, Terre Chaque fil se comporte comme une antenne pour les radiations électromagnétiques Plus le fil ou câble est long et plus il captera du BRUIT Les câbles blindés captent moins de BRUIT A lead connects the recording electrode to the amplifier input. A collection of leads makes a cable. Needle EMG recording requires three leads: active reference and ground. A lead is essentially a piece of wire, just like an antenna. Hence it also acts as a recording device for the ambient electromagnetic radiation. The signals recorded by the lead appear as noise in the EMG recordings. Just as a longer antenna picks more signal for better reception; a longer lead will pick up more noise. Hence the lead should be as short as possible. Shielding the leads will reduce the noise recorded by them. Fils et câbles

33 Lampes à incandescence plutôt que fluorescentes
Débrancher les équipements électriques non nécessaires Eloigner les émetteurs radio et TV Tenir éloigner les câbles des prises de secteur et des ordinateurs Trouver la meilleure place dans le local Utiliser une cage de Faraday Utiliser un amplificateur différentiel (cf plus loin) Référence et active dans un même câble (aiguille concentrique >< aiguille monopolaire) The noise recorded by the lead is best minimized by reducing the ambient electromagnetic radiation. This is accomplished by several simple steps. Fluorescent lights give out much more radiation than incandescent variety. Hence it is a good practice to turn off the tube-lights and provide illumination using incandescent lamps. Power off the unnecessary electrically powered instruments. They emit electromagnetic radiation. Better yet, unplug their power cord from the wall outlet. Even when the instrument is not powered on, one of its power cable leads is still connected to the main supply. It behaves as a transmitting antenna and adds noise to the EMG recordings. The cables can truly behave as a “conventional” antenna and pick up radio and television transmissions. Many have experienced this type of interference. When it is low in amplitude, one may not hear it, but it can be seen as high frequency noise along the baseline. There are three strategies. First, identify the broadcast frequency and inform the instrument manufacturer of that setting. It may be possible to make modifications to the amplifier to attenuate that band of frequencies. Secondly, position the instrument such that it’s back is facing the broadcasting station. The third solution is to reduce the high frequency setting of the bandpass filter of the amplifier, e.g. from 10 kHz to 3 kHz. This reduces the “demodulation” of the radio transmission, It also attenuates the signal amplitude and increases its risetime. This is not the most desirable solution. Keep the leads away from sources of electromagnetic radiation, i.e. other power cords, computer monitors, etc. The ambient noise in the EMG examination room can vary significantly at different positions of the instrument within the room. This is easily investigated by moving the system to different locations, before choosing the location that gives the least noise. In extreme cases, one may have to prepare the so-called “Faraday cage” to keep off the external radiation. This is a very costly proposal requiring alterations to the wall of the room. Réduire les interférences

34 Conducteur cassé ou connection endommagée Survient progressivement
Pas visible (enveloppe plastique) BRUIT inhabituel en fonction de la position du câble REMPLACER A lead will fail due to repetitive use. The failure may occur by break in the conductor or by deterioration of the solder between the conductor and the end connection. The failure occurs slowly over a modest period of time. It is often difficult to recognize the failure due to the plastic sheath over these parts. Transient noise affected by adjustments to the cable position usually suggests a failing cable. The failure of specific lead (active, reference or ground) can take time to identify. My suggestion is to discard faulty cable or lead immediately after cutting it with scissors (just like a credit card). It is not a bad idea to replace the cables after every few months. Câble défectueux

35 Amplificateur Sortie - Fort signal EMG - Faible BRUIT
Rapport SIGNAL/BRUIT élevé Amplificateur Entrée - Faible signal EMG - BRUIT élevé Faible rapport SIGNAL/BRUIT This is perhaps the most critical component of the electromyography. The recording electrode registers low amplitude neurophysiologic potentials in an environment with high ambient noise. At the input of the amplifier, the signal to noise ratio is poor. At the output of the amplifier, we desire high signal amplitude and a reduced noise, i.e. increased signal to noise ratio. This is accomplished by using a so-called “differential amplifier”. Amplificateur

36 Amplification de la différence entre l’active et la référence
Terre VA VR 2000 µV Sortie = 0 V VR = 0 50 µV Sortie = 0.5 V VA – VR = 0 Let us consider that a monopolar electrode is recording a fibrillation potential of 50 microV amplitude. The reference electrode registers no electrical activity. The difference between these two potentials is amplified and the signal at the amplifier output has the same shape but of 0.5 volt amplitude At the same time there is a 2 mV amplitude noise at the amplifier inputs. However, this noise is identical at both inputs. Their difference is zero and hence we do not see the noise at the amplifier output. In this fashion we have managed to selectively amplify the EMG signal while attenuating the noise Amplificateur différentiel

37 Amplificateur différentiel
50 µV Gain différentiel = V / 50 µV = 10,000 Sortie = 0.5 V Active VA VR = 0 Terre Référence 2000 µV Gain commun = µV / 2000 µV = 1 Active Sortie = 2000 µV VA The real amplifiers are not so perfect in attenuating the noise. Signals common to the active and reference inputs are also amplified, but with a much lower gain. In this illustration, the signal common to active and reference input is the noise of 2 mV amplitude. This is often called the common mode signal. The amplifier output is the same signal with same amplitude. This gives it a “common mode gain” of one. In contrast, the differential gain of the amplifier is 10,000 in this illustration. A good amplifier should have a high differential gain and a low common mode gain. These two characteristics are incorporated in to a single measure of performance by computing the differential to common mode gains. The result is called the “common mode rejection ratio (CMRR)”. Engineers often express this value in decibels by using the formula CMRR (db) = 20 log (CMRR). Modern EMG instruments have a CMRR greater than dB. 2000 µV VR Terre Référence “common mode rejection ratio (CMRR)” > db Amplificateur différentiel

38 Signal Filtres Signal BRUIT Types : analogique (circuit de résistances et de capacités), digital (algorithme de l’ordinateur) Filtres affectent le signal et le BRUIT - utiliser les réglages standards - comprendre les effets d’un réglage non-standard The purpose of a filter is to stop the unwanted noise and interference, and let pass the signal itself. Two types of filters are used in the EMG systems. The analog filter is a hardware circuit consisting of resistors, capacitors, etc. A digital filter is a signal processing algorithm on the computer. The advantage of a digital filter is that it can be “updated” by software. For practical purposes, we need not make distinction between these two types. Perhaps the most commonly used filter is the “tone” control on an audio CD or tape player. One reduces the “treble” to reduce the hiss, or reduces the “base” to cut down the drum line. The same principle is also used in the EMG signal processing. One can judiciously use filters to eliminate noise and interference, and to highlight specific characteristics of the EMG potential. It is important to realize that filters do affect the EMG signals, and as far as possible one should not deviate from their standard settings. When using nonstandard settings, it is important to understand the signal distortion by the filters. Filtres

39 Filtre passe-haut ou filtre basse fréquence
Freq = 200 Hz VIN Filtre VOUT Freq = 50 Hz VIN Filtre VOUT = 0 VOUT / VIN We will treat the filter as a “black box”. A sinusoidal signal is connected to the input of the filter and the ratio of output to input signal amplitude is measured. This is the gain of the filter, and it is less than or equal to one. By plotting the gain against the input signal frequency, the filter characteristics are defined. In this example, the gain is equal to 1 for all frequencies below 100 Hz. For higher input frequency the gain is zero. In other words, the signals of frequency less than 100 Hz pass through the circuit. Higher frequency signals are stopped and are not seen at the output. Because low frequency signals “pass” through the circuit to its output, this type of a filter is called a “low pass” filter. The frequency where the transition between “pass” and “stop” occurs is called the “cutoff frequency”. 1 Fréquence (Hz) 100 (cutoff) Filtre passe-haut ou filtre basse fréquence

40 Filtre passe-bas ou filtre haute fréquence
Freq = 500 Hz VIN Filtre VOUT = 0 Freq = 100 Hz VOUT VIN Filtre VOUT / VIN The “high pass” filter attenuates low frequency signals, while high frequency signals pass without attenuation. Practically all electronic amplifier systems use a combination of a “low pass” and a “high pass” filters. The result is so called a bandpass filter. This filter is characterized by two frequencies, the low setting (fL), and the high setting (fH). Note that the low frequency setting corresponds to the cutoff frequency of the high pass filter circuit. Similarly the fH corresponds to the low pass filter cutoff setting. This is often confusing in the initial learning stages. 1 Fréquence (Hz) 200 Filtre passe-bas ou filtre haute fréquence

41 Fréquences du BRUIT 20 µV 20 ms 20 µV 5 ms Signal filtré 20 µV 20 ms
Bases fréquences 20 µV 20 ms Hautes fréquences 20 µV 5 ms It is relatively simple to understand the effect of filters using sinusoids. However, an EMG signal is much different the sinusoids. How can we recognize the frequency composition of a signal? Slow fluctuations about the baseline represent low frequency. These are better seen on a slower sweep (i.e. 20 ms/div or more) or by the “bouncy” baseline that moves up and down the screen. Rapid voltage change about the baseline gives high frequencies. At a display gain of 10 or 20 uV/division, one can see many peaks and baseline crossings. The baseline also appears thick by visual inspection. The high frequency noise is usually heard as a “hiss” on the audio monitor. Signal filtré 20 µV 20 ms Fréquences du BRUIT

42 Fréquences du signal EMG
Basse fréquence Hautes fréquence In the EMG potentials, the spike component gives the high frequencies. Signals with many peaks, phases and short rise time are rich in high frequencies. In contrast, low frequencies are generated by slowly changing components seen at the beginning and end of the motor unit potentials. A potential with long rise time also has primarily low frequencies. The needle EMG signals are recorded with a low frequency setting of Hz and a high frequency setting of 10 kHz. Let us look at change in the EMG properties by changing these settings. Fréquences du signal EMG

43 Parfois une bonne solution pour étudier l’activité de repos aux SI
fL = 10 Hz fL = 100 Hz When the low frequency setting is increased, e.g. from 10 Hz to 100 Hz, the slow changing potentials are filtered out. This change also acts like a “clamp” on the baseline. It does not allow the signal to deviate from the baseline. When deviations do occur, it make the signal return quickly back to the baseline. In effect, increasing the low frequency setting stabilizes the baseline. This is a great trick in the assessment of the spontaneous activity in hostile environment, e.g. intensive care unit. Increasing the low frequency setting also reduces the signal duration. The amplitude and phases may also reduce when the low frequency setting is significantly increased, e.g. 500 Hz, and the MUP may resemble a fibrillation. 50 µV 100 ms Augmentation du filtre passe-haut

44 Augmentation du filtre passe-haut : 0, 200, 1000 et 2000 Hz
When the low frequency setting is increased, e.g. from 10 Hz to 100 Hz, the slow changing potentials are filtered out. This change also acts like a “clamp” on the baseline. It does not allow the signal to deviate from the baseline. When deviations do occur, it make the signal return quickly back to the baseline. In effect, increasing the low frequency setting stabilizes the baseline. This is a great trick in the assessment of the spontaneous activity in hostile environment, e.g. intensive care unit. Increasing the low frequency setting also reduces the signal duration. The amplitude and phases may also reduce when the low frequency setting is significantly increased, e.g. 500 Hz, and the MUP may resemble a fibrillation. Augmentation du filtre passe-haut : 0, 200, 1000 et 2000 Hz

45 Réduction du filtre passe-bas
Risetime = 260 µs fH = 10,000 Hz 340 µs 3000 Hz 1000 Hz 500 µs When the high frequency setting is reduced, e.g. from 10 kHz to 3 kHz or lower, the fast changing components of the signal are filtered out. We will not see them and the signals appear “dull” instead of “spiky”. The number of peaks is reduced. The peaks appear rounded instead of being sharp. The rise time of the potential is increased. The EMG sounds “dull” on the audio monitor. If there was any high frequency noise, often heard as a hiss, that to is eliminated and not heard on the monitor. The signal amplitude is reduced. 500 Hz 700 µs Réduction du filtre passe-bas

46 Filtre Notch (50 Hz) Fréquence = fN Notch Filter ( fN ) VIN VOUT
VOUT / VIN There is a lot of electromagnetic radiation from the power cords of different electrical devices in the laboratory and the neighboring area. Despite the differential amplification, it may not be adequately suppressed. Therefore, EMG systems offer a filter that “stops” or “attenuates” a very narrow band of frequencies. Other frequencies are unaffected. In the filter characteristics, it looks like a notch in the pass band. Hence this is called the “notch filter”. Its attenuation frequency is matched to the power line frequency. This is quite useful in a hostile environment, e.g. in the ICU. The power line interference is recognized quite easily. If the sweep duration is 100 ms, each sweep contains 6 cycles of a sinusoid for 60 Hz interference. A 50 Hz interference will give 5 cycles. Use notch filter to suppress it. 1 fN Fréquence (Hz) Filtre Notch (50 Hz)

47 Réglages standards et non-standards
- Basse fréquence 10 – 20 Hz - Haute fréquence 10 kHz Ligne de base instable (SI ou activités de repos) - Basse fréquence 100 – 200 Hz Jiggle (stabilité des PUMs) - Basse fréquence 500 – 2000 Hz Analysie quantifiée des PUMs - Basse fréquence 3 –10 Hz In our laboratory, the default filter settings are 10 Hz to 10 kHz. We often set the low frequency at 100 Hz to assess spontaneous activity. We think that it gives a much better baseline. In assessment of motor unit potentials we often increase the low frequency setting to even 500 Hz to assess the so-called “stability” of the motor unit potentials. Having examined these characteristics, we always return to the default settings of 10 Hz – 10 kHz. In quantitative analysis of motor unit potentials, we often use 2-3 Hz setting for the low frequency. The notch filter is used when necessary. Réglages standards et non-standards

48 Echantillonnage Tous les échantillons sont reliés entre eux pour construire le signal EMG digitalisé Permet de sauver les traces et d’y retourner ultérieurement, de les geler sur l’écran, de faire des mesures automatiques et des analyses complexes This is the most critical component of the digital EMG instruments. The analog to digital converter, called the ADC, measures the EMG signal amplitude at regular intervals. Each measurement is called a sample. These samples are connected by straight lines to construct the digitized EMG signal. The samples of an EMG recording can be saved as on the computer as a file. It allows the user to review the signals in an “off-line” fashion, freeze the trace on the screen, make automated measurements, perform complex analysis that could not be performed manually, etc. Indeed, the digitized EMG record offers many benefits that were not available from the previous generation analog EMG systems. At the same time, one must be aware of some technical problems that can arise in digital EMG systems. Failure to recognize them may result in some incorrect inferences about the underlying pathology. Digitalisation

49 Intervalle d’échantillonnage
The time interval between the samples is the “sampling interval”. Reciprocal of sampling interval is the sampling frequency. In this illustration, there is a good concordance between the analog and digital versions of the EMG. Let us now double the sampling interval. The EMG record contains only half the samples compared to the earlier record. The digital EMG signal looks quite different from the analog signal. Specifically, the amplitude is reduced, the rise time is increased and sharp peaks appear smooth and some peaks are missing. This type of distortion, called aliasing, would clearly affect our EMG signal interpretation. Taux d’échantillonnage = 1 / intervalle d’échantillonnage Digitalisation

50 Digitalisation Analog Digital
In this illustration, there is a good concordance between the analog and digital versions of the EMG. Analog Digital Digitalisation

51 Intervalle d’échantillonnage doublé => aliasing
Let us now double the sampling interval. The EMG record contains only half the samples compared to the earlier record. The digital EMG signal looks quite different from the analog signal. Specifically, the amplitude is reduced, the rise time is increased and sharp peaks appear smooth and some peaks are missing. This type of distortion, called aliasing, would clearly affect our EMG signal interpretation. Intervalle d’échantillonnage doublé => aliasing

52 Fréquence d’échantillonnage > 2 X la plus haute fréquence à mesurer
Théorème de Nyquist Fréquence d’échantillonnage > 2 X la plus haute fréquence à mesurer Ingénieurs Fréquence d’échantillonnage > 3- 4 X la plus haute fréquence à mesurer Attention !!!! Enregistrement EMG de haute qualité => fréquence d’échantillonnage > 2-3 X 10 KHz (filtre passe-bas) L’échantillonnage ne dépend plus du hardware, mais du logiciel EMG. The take home message is that we must have lot of samples in the EMG record. The signal processing theory provides a rule called “Nyquist Theorem” which states that the sampling frequency must be at least twice the highest signal frequency component. Engineers prefer to use even higher sampling rates. From EMG perspective, the high frequency of the bandpass filter is set to 10 kHz. It implies the sampling rate for EMG should be at least 20 kHz, preferably more than 30 kHz. Fréquence d’échantillonnage

53 Potentiels manquants Amplitude réduite Morphologie déformée Aliasing
Let us look at some examples of “aliasing” in needle EMG recordings. The first illustration shows an EMG recording that contains spontaneous activity in the form of fibrillations and positive sharp waves. The top tracing was obtained using a high sampling rate of 50 kHz. The lower trace used a sampling rate of only 5 kHz. Visual assessment shows missing spikes and some potentials with reduced amplitudes in the bottom trace. In this particular example, one may argue that we still have sufficient number of abnormal potentials. But others may interpret small amplitude potentials as indicators of an old disease process and large amplitude potentials as evidence of a recent phenomenon. Clearly the signal interpretation will be affected by the sampling rate. Aliasing

54 Technologie ou pathologie
PUM normal, pas d’aliasing PUM normal, aliasing This illustration contains MUP discharges from a normal muscle in the top and middle trace. The sampling rate is 50 kHz for the top trace and shows minimal difference in MUP amplitude from one discharge to another. The same recording at 3.3 kHz sampling rate shows significant amplitude variation. This is the “hallmark” of abnormal neuromuscular transmission, and it is seen in patients with neuromuscular junction diseases, neuropathy and in some patients with myopathy. The bottom trace shows such a recording from a patient with neuropathy when the sampling rate was 50 kHz. PUM anormal, pas d’aliasing Technologie ou pathologie

55 Aliasing ou pas ? Filtre passe-bas 10 kHz.
PUM normal (risetime < 500 µs) Balayage 1 seconde (100 ms/division) Si l’amplitude du PUM reste constante => pas d’aliasing The amplitude variation is difficult to assess when the sampling rate is unknown. Most equipment manufacturers do use chips that permit sampling rate up to 200 kHz or even more. However, the actual rate is defined by the SOFTWARE, and it is usually not fully specified. Furthermore, the sampling rate may change when the sweep duration is changed. Hence, aliasing can occur on commercially available EMG systems. This can be tested quite easily as follows: Set the sweep duration to a low value, e.g. 20 ms (or 2 ms/div). Record a sharp MUP with risetime of less than 500 µs. Note the signal amplitude and its variation from one discharge to another. This can be facilitated by using an amplitude trigger delay line and superimposing the MUP discharges. A slight variation is expected due to noise, interference, baseline shift, etc. Next, change the sweep duration to 1 second and display signals in a free running mode (i.e. turn off the amplitude trigger). If the MUP potential is unchanged and does not vary, there is no aliasing in the EMG system. Amplitude variation suggests aliasing, most probably related to the sweep duration. I would advice one not to use long sweep durations in such systems. Furthermore, any amplitude assessments must be performed with caution. Aliasing ou pas ?

56 Résolution Nombre de bits = N 10 16 Nombre de pas = 2N 1024 65536
Gamme d’amplification = A 100 mV 100 mV Résolution = A/2N 100 µV 1.5 µV If a ramp like signal is sampled by an ADC, the digital output looks like a “stair case”. The ADC can resolve amplitude change only in “quantal” fashion. The amplitude corresponding to the size of the step is called the “least bit resolution”. Clearly, this amplitude should be low for high fidelity recordings. The resolution is defined by the “number of bits” in the ADC. These days, most commercially available systems use a 16+ bit ADC. Hence amplitude resolution is no longer an issue in EMG systems. Résolution

57 The amplitude trigger and delay line are often used in quantitative EMG studies. However, they can also be of great value in the routine needle EMG examination. When the trigger is turned ON, a horizontal bar or line is shown on the screen. It represents an amplitude level. If the EMG signal amplitude is less than the trigger amplitude level, no EMG signal is displayed. When the signal exceeds the amplitude level, a “trigger” is found. One sweep of EMG signal is acquired and displayed on the screen. Another sweep will be drawn when the next trigger occurs, and so on. This mechanism allows one to see the signal after the trigger point, but not before it. Trigger d’amplitude

58 Trigger d’amplitude + ligne de retard
The delay line delays the signal by user defined time. When the delayed signals are shown on screen, we can recognize the entire EMG waveform. Trigger d’amplitude + ligne de retard

59 Réglage du niveau du trigger
When the trigger level is low, all discharging motor unit potentials (MUPs) can exceed the trigger level. When the trigger level is raised, the smaller amplitude MUPs can not trigger the display. When the level is set so that only one MUP exceeds it, the EMG display is updated only by discharges of that MUP. The MUP discharges will appear time locked on the screen. Other EMG activity appears randomly on the sweeps. This makes it very easy to assess the triggering MUP waveform and its measurements. Without the trigger, it would be quite difficult to assess the MUP stability and linked potentials. Réglage du niveau du trigger

60 Réglage du niveau du trigger
When the trigger level is raised, the smaller amplitude MUPs can not trigger the display. Réglage du niveau du trigger

61 When the level is set so that only one MUP exceeds it, the EMG display is updated only by discharges of that MUP. The MUP discharges will appear time locked on the screen. Other EMG activity appears randomly on the sweeps. This makes it very easy to assess the triggering MUP waveform and its measurements. Without the trigger, it would be quite difficult to assess the MUP stability and linked potentials. Analyse de PUM

62 It is obvious that the amplitude trigger can isolate only the largest amplitude MUP in the EMG signal. A “peak” or “window” trigger is also available on some systems. In this mode, two amplitude levels are defined by the operator. When EMG signal crosses the lower level, but not the higher level, a trigger is found. In this example, the smaller MUP fulfils the trigger criteria. The larger MUP crosses both amplitude levels and fails to trigger the sweep. Thus the waveform of the smaller amplitude MUP is identified. Needless to say, this type of trigger requires extensive manipulation. It is rarely used in the routine EMG studies. Trigger-fenêtre

63 Signal moyenné Trigger d’amplitude et ligne de retard
Signal déclenché par le trigger The signal averager is used in conjunction with amplitude triggered delay line to extract MUPs from background activity. The trigger time-locks the discharges of the MUP on the sweep while the background activity is random. The acquired sweeps are added to a buffer, and the buffer contents are divided by the number of sweeps to obtain the average. This is identical to the averaging technique used to improve the signal to noise ratio in the sensory nerve conduction studies. The positive potentials in the background cancel the negative potentials when the sweeps are summated. This reduces the noise while the MUP remains unaffected. PUM moyenné Signal moyenné

64 Le BRUIT diminue avec la racine carrée de n
10 Le BRUIT diminue avec la racine carrée de n 25 50 100 Averaging should not be used as a substitute for poor signal quality. When averaging the noise decreases quite rapidly in the first few sweeps. The noise reduces much less as more sweeps are acquired. If the noise has gaussian distribution the noise reduces with the square root of the number of sweeps. In this illustration, observe the gradual decline in noise with the number of sweeps. The noise amplitude decreased much less from 100th to the 200th sweep. 202 5 ms/div, 10 µV/div Le moyennage et faible rapport SIGNAL/BRUIT

65 La médiane n’est pas affectée par un spike sporadique de grande taille
Amplitude # 1 82 µV # 2 -1 µV # 3 0 µV Moyenne 27 µV New automated methods use the “Median averaging” to improve the signal to noise ratio. This computation is performed after the triggered sweeps have been acquired. In this illustration, the computations are described from 3 sweeps. At the location of the vertical line, the signal amplitude is 82, -1 and 0 microvolts. Their average is 21 and quite different from the expected value of zero. For median average, the sample values are arranged in ascending (or descending) order. The central value is the median. Note that the median is not affected by occasional large spikes. Médiane 0 µV La médiane n’est pas affectée par un spike sporadique de grande taille Valeur moyenne ou médiane

66 Permet d’évaluer l’amplitude
Sensibilité Gain µV/division Gain = 100 µV/div Amplitude = 3 divisions Amplitude = 3 x 100 = 300 µV To facilitate measurements, the trace display area is divided in a rectangular grid. The vertical divisions are used for amplitude assessment. The amplitude corresponding to one division of vertical deflection is called the “Sensitivity” or the “Display gain”. The signal deflection is measured in the number of vertical divisions. This is multiplied by the display gain to obtain the amplitude value. In this example, the MUP peak to peak deflection is 3 divisions. The display gain is 100 µV/division. Multiplying these two numbers we estimate the MUP amplitude being 300 µV. Gain / sensibilité

67 Permet d’évaluer la durée
Durée du balayage (ms) Vitesse de balayage (ms/div) = Durée du balayage/nombre de div Durée du balayage = 100 ms Vitesse de balayage = 10 ms/div Largeur du PUM = 1,5 div Durée du PUM = 1.5 x 10 = 15 ms The horizonatal divisions of the grid are useful to assess the time related measurements. The sweep duration refers to the time of the entire sweep on display. In this illustration, the sweep duration is 100 msec. There are 10 horizontal divisions. Thus the time corresponding to one horizontal division is 10 msec, and it is called the sweep speed. Sweep speed (ms/division) = Sweep Duration (ms) / Number of horizontal divisions. The time interval between the beginning and end point of the displayed MUP is about 1.5 divisions. Multiplying by the sweep speed, we estimate the MUP duration to be 15 milliseconds. Balayage

68 Fréquence de décharge 1 2 3 4 IDI
IDI Fréquence de décharge (Hz ) = 1 / intervalle inter-décharge (secondes) = / intervalle inter-décarge (ms) Durée de balayage = 500 ms Vitesse de balayage = 50 ms/div Intervalle inter-décharge = 2div = 100 ms Fréquence de décharge = 10 Hz Time measurements are necessary for estimating the firing rate of EMG potentials. In this illustration, the sweep duration is 500 ms, and it corresponds to a sweep speed of 50 ms/div. The second and third MUP discharges are separated by 2 divisions. Multiplying these two numbers, we get an inter-discharge interval of 100 ms. The firing rate (also called the firing frequency) is the reciprocal of inter-discharge interval, when latter is measured in seconds. In EMG one uses milliseconds for time measurements. Therefore the firing frequency is obtained by the formula Firing Rate (Hz) = 1000 / IDI (ms) In this example the firing rate is 10 Hz. Fréquence de décharge

69 The rate computations from the inter-discharge interval represent the instantaneous firing rate of the potential. This varies from one discharge to another. We often like to get a rough estimate of the firing rate. This is easily achieved by counting the number of potentials in one sweep and scaling the number for 1 second epoch. In this example of a complex repetitive discharge, the potential occurs 7 times in 100 msecs. Therefore in 1000 msec, or 1 second epoch, we should see 70 occurences. The firing rate is 70 Hz. Durée de balayage = 100 ms. => 7 décharges Durée de balayage = 1000 ms => 70 décharges Fréquence = 70 Hz Fréquence de décharge

70 Fréquence de décharge (balayage = 100 ms, mode raster)
In the raster mode, the firing rate can be estimated from the pattern of discharges seen on successive sweeps when the sweep duration is 100 ms. If the discharge frequency is less than 10 Hz, the time interval between two MUP discharges will be more than 100 msec, or one sweep. Hence MUPs appear to shift right on successive sweeps. In some sweeps, the MUP may not be seen at all. When the discharge frequency is roughly 10 Hz, the interdischarge interval will be 100 msec, i.e. 1 sweep. The successive MUP discharges will fall roughly below each other. When the discharge frequency is roughly 10 Hz, the interdischarge interval will be 100 msec, i.e. 1 sweep. The successive MUP discharges will fall roughly below each other. IDI = 100 ms Fréquence = 10 Hz IDI > 100 ms Fréquence < 10 Hz Fréquence de décharge (balayage = 100 ms, mode raster)

71 Fréquence de décharge (balayage = 100 ms, mode raster)
When the firing rate is more than 10 Hz, the interdischarge interval is less than 100 mesc, or one sweep. Hence the discharges appear to shift left on consecutive sweeps. On some sweeps, the MUP may be seen twice. Higher discharge rates will give proportionally more discharges on each sweep. This giant MUP is discharging at Hz and gives 3-4 discharges on each sweep. Such patterns can be recognized instantaneously, making it easy to assess the firing frequencies of the EMG potentials. IDI < 100 ms Fréquence > 10 Hz Fréquence Hz Fréquence de décharge (balayage = 100 ms, mode raster)