8/10/2023 0 Comments Referential montage definition![]() Note that negative and positive phase reversals are not the same as negative and positive discharges while negative discharges go up and positive discharges go down, negative phase reversals move toward one another and positive phase reversals move away from each other. This isn't absolute, but negative phase reversals are generally seen with epileptiform activity, while positive ones are more commonly seen with various artifacts. Negative discharges cause the surrounding tracings to point toward the electrode of maximal voltage, while positive discharges cause surrounding tracings to point away from the electrode of max voltage (an easy way to remember this: positives can fit a plus sign, and negatives can only fit a negative sign). T3-T5 and T5-O1 phase reversal means T5 has the greatest voltage of them all). With phase reversals, the middle electrode of the pair that makes the reversal is the electrode of maximal voltage (ex. That morphology is called a phase reversal, and is a key reason that bipolar montages are so popular. Notice how in this example, T4 saw the greatest voltage of the nearby discharge, and the other electrode tracings seemed to "point toward" the T4 electrode on the tracing. This same technique is used to give rise to all the tracings in all the chains. Remember that in EEG, positive values cause downward waves and negative values cause upward waves. Then we just subtract each electrode's voltage from the one in front of it to get the voltage for that tracing pair. To get the EEG tracings for this discharge all we have to do is link the electrodes in the double banana chain (the electrodes here are the temporal chain for simplicity the parasagittal and central chains are not shown). Note this is similar but not identical to ripples in a pond, because the distribution of voltages is not as symmetric as ripples. T4 is the closest and sees the majority of the voltage, and as you get farther away from the discharge the voltages seen by the electrodes diminish. There is a discharge of -50mV, but depending on its dipole and location, not every electrode will see a full -50mV. To better understand how bipolar montages lead to EEG tracings, lets take a look at the example below. For example, if Fp2 has a voltage of -50 and F8 has a voltage of -20, the Fp2-F8 tracing would show -50 - (-20) = -30mV. Because of this, in bipolar if the first electrode in the tracing line is more positive/higher than the second, you get a positive, downward deflection if the second electrode is more positive/higher, you get a negative, upward deflection. In each chain, an electrode's voltage is compared to that of the electrode behind it, so each tracing line is a pair of electrodes in which the voltage of the second electrode is subtracted from the voltage of the first. From there, you divide the head into consecutive increments of 10% and 20% to find the placement of the electrodes, as shown in the illustration below (which is not, of note, anatomically correct!). The first step in the 10-20 system setup is finding the nasion (the top of the nose bride, between the eyes) and the inion (the small bump in the middle of the back of the head). For the midline/central electrodes, instead of a number their letters are clarified with a " z." The number corresponds to the side of the brain- odds on the left, evens on the right-and the particular area of each region. The letter corresponds to its region of the brain: F for frontal region, T for temporal, P for parietal, and O for occipital (the only exception to this is that F7 and F8, while seemingly frontal, are in fact actually over the anterior temporal region). In the 10-20 system, each electrode is identified with a letter and a number. This system is so named because it splits the skull into increments of 10% or 20% to place the electrodes, ensuring that each electrode is relatively positioned to all the others and making it possible for every EEG study to be consistent despite peoples' many different head shapes and sizes. The first step to any EEG study is the placement of the electrodes, and this is most commonly done via the international, standardized 10-20 System. Now that you have a grasp of the pertinent neurophysiology that gives rise to the EEG signal, let's finally dive into the EEG itself.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |