Tuesday, October 23, 2007
Digital EEG instruments
A digital EEG system converts the waveform into a series of numerical values. This process is known as Analogue-to-Digital conversion (ADC).
A digital EEG system converts the waveform into a series of numerical values. This process is known as Analogue-to-Digital conversion (ADC).
The values can be stored in the computer memory, manipulated and then redisplayed as waveforms on a computer screen. The rate at which the waveform data is sampled in order to convert it into a numerical format is known as the sampling rate.
The sampling rate is usually expressed in Hz, for example 240 Hz is 240 times per second. The minimum acceptable sampling rate is 2.5 times greater than the highest frequency of interest but most digital EEG systems will sample at 240 Hz.
The sampling rate is usually expressed in Hz, for example 240 Hz is 240 times per second. The minimum acceptable sampling rate is 2.5 times greater than the highest frequency of interest but most digital EEG systems will sample at 240 Hz.
Some recordings which involve recording activity directly from the brain surface, may have activity of a higher frequency, for example 200 Hz. Therefore some digital EEG systems will have optional sampling rates of 480 Hz available.
Sampling at rates lower than this will mean that when the signal is converted back to analogue form, it will not resemble the original waveform A second factor that affects the accuracy of the waveform is sampling skew. Sampling skew occurs when all channels are not sampled simultaneously. Many digital EEG systems sample channel 1 first, then sample channel 2, then channel 3 etc. The time lag between sampling of each channel is known as sampling skew. To reduce the sampling skew, some digital systems use burst mode sampling. This increases the speed between successive channels sampling in order to reduce the amount of sampling skew.
A third factor that affects the accuracy of digital EEG waveforms is the display. The accuracy of a monitor display depends on the number of points or pixels that are available. The number of pixels available is referred to as the screen resolution. Screen resolution is described in numbers that represent the pixels available in the horizontal and vertical axis.
A VGA display has a resolution of 640 x 480 pixels while a monitor with a Super VGA display will have a screen resolution of around 1024 x 768 pixels. A typical page of EEG contains 10 seconds of data. A digital EEG system, sampling at rates of 240 Hz will need to display 2400 samples horizontally for each recording channel. The highest screen resolutions available today do not have enough pixels to match the number of data samples. Systems that draw every other sample or every third sample in order to match the screen resolution will have the effect of reducing the sampling rate and displaying incomplete data. An accurate digital system will draw two data samples per screen pixel. This means that all data points can be displayed and sampling rates will not be decreased.
EEG signals that have been digitised can be manipulated to change the montage ‘on-line’ at the time of recording or ‘off-line’ after the recording is completed. This ‘remontaging’ is accomplished by recording all EEG channels with a common reference electrode. Regardless of the montage used to display the data while it is being recorded, data is stored into the computer memory in common reference mode. This allows the data to be displayed using different montages at a later time. Since digital systems store the analogue signal as numerical values, remontaging is a simple subtraction process which results in cancellation of the common reference.
An example is shown in the next figure. The reference electrode A1 is common to both channels on input 2. It has the identical value in each channel. Remontaging these two channels together into one new channel is by subtraction which mathematically will cancel the value at the reference electrode. The resulting channel will therefore display the potential difference between F3 (input) 1 and F4 (input 2).
Sampling at rates lower than this will mean that when the signal is converted back to analogue form, it will not resemble the original waveform A second factor that affects the accuracy of the waveform is sampling skew. Sampling skew occurs when all channels are not sampled simultaneously. Many digital EEG systems sample channel 1 first, then sample channel 2, then channel 3 etc. The time lag between sampling of each channel is known as sampling skew. To reduce the sampling skew, some digital systems use burst mode sampling. This increases the speed between successive channels sampling in order to reduce the amount of sampling skew.
A third factor that affects the accuracy of digital EEG waveforms is the display. The accuracy of a monitor display depends on the number of points or pixels that are available. The number of pixels available is referred to as the screen resolution. Screen resolution is described in numbers that represent the pixels available in the horizontal and vertical axis.
A VGA display has a resolution of 640 x 480 pixels while a monitor with a Super VGA display will have a screen resolution of around 1024 x 768 pixels. A typical page of EEG contains 10 seconds of data. A digital EEG system, sampling at rates of 240 Hz will need to display 2400 samples horizontally for each recording channel. The highest screen resolutions available today do not have enough pixels to match the number of data samples. Systems that draw every other sample or every third sample in order to match the screen resolution will have the effect of reducing the sampling rate and displaying incomplete data. An accurate digital system will draw two data samples per screen pixel. This means that all data points can be displayed and sampling rates will not be decreased.
EEG signals that have been digitised can be manipulated to change the montage ‘on-line’ at the time of recording or ‘off-line’ after the recording is completed. This ‘remontaging’ is accomplished by recording all EEG channels with a common reference electrode. Regardless of the montage used to display the data while it is being recorded, data is stored into the computer memory in common reference mode. This allows the data to be displayed using different montages at a later time. Since digital systems store the analogue signal as numerical values, remontaging is a simple subtraction process which results in cancellation of the common reference.
An example is shown in the next figure. The reference electrode A1 is common to both channels on input 2. It has the identical value in each channel. Remontaging these two channels together into one new channel is by subtraction which mathematically will cancel the value at the reference electrode. The resulting channel will therefore display the potential difference between F3 (input) 1 and F4 (input 2).
# posted by Dr. Ulman @ 10:37 PM 
