qEEG intro
Quantitative EEG (QEEG) provides a compressed visual summary of conventional digital EEG recordings, allowing several hours of electroencephalographic data to be displayed on a single screen. It is generated by mathematically analyzing various comonents of the EEG signal, including spectral power, amplitude, frequency distribution, rhythmicity, and interhemispheric symmetry. These measurements are transformed into graphical trend displays.
A QEEG study typically consists of multiple trend panels, each highlighting a different feature of the underlying EEG activity. For instance, amplitude-integrated EEG (aEEG) depicts changes in signal amplitude over time. The specific QEEG trends and their clinical applications are discussed in greater detail in the following sections.
By condensing large amounts of EEG data into easily interpretable trends, QEEG facilitates the detection and quantification of seizures, assessment of treatment response, adjustment of anesthetic therapy, and identification of evolving cerebral abnormalities such as focal dysfunction or ischemia. For EEG interpreters, these displays can substantially improve the efficiency of reviewing long-term recordings. Nevertheless, QEEG should be viewed as a complementary tool rather than a replacement for conventional EEG interpretation, and important findings should always be verified using the raw EEG data.
The figure below illustrates the difference between conventional EEG and QEEG displays. The panel on the left presents a standard EEG recording, showing only a brief segment of activity at one time. The panel on the right displays a QEEG representation summarizing approximately two hours of EEG data. Depending on the clinical application, the time scale can be adjusted to display anywhere from several minutes to many hours of recording. A useful analogy is that conventional EEG allows examination of individual trees, whereas QEEG provides a broader perspective of the entire forest, making long-term trends and patterns more readily apparent.
Commonly used QEEG Trends:
- Power color density spectrogram
- Asymmetry spectrogram
- Rhythmicity Spectrogram
- Amplitude integrated EEG
- Suppression Ratio
- Envelope trend
- Asymmetry Indices
- Relative Alpha Variability (Ischemia Detection)
- Alpha Delta Ratio (Ischemia Detection)