Digital EEG Systems: Recording & Notation

• Digital EEG systems offer advantages over analogue systems, including higher fidelity recording, post-processing, automated detection, and efficient storage
• Digital EEG allows control over montage, filters, gain, and scaling, enabling EEG/video recordings to stream to a central server with HIPPA compliance

Patient Information

• Electronically recorded data should include patient name, birth date, test date, and relevant identification numbers
• The EEG report ideally merges with the EEG signal after physician review, permitting correction of identifying information post-recording

Information Notation

• Calibration signals should be recorded at the beginning of each recording
• The time of day should is recorded with EEG data with other information to locate stored events
• Technologist comments and event codes should be included, added even after completion
• Codes can represent eye activity, hyperventilation, photic stimulation, and patient state, and include free text
• Automatic recording provisions should include impedance values, sampling frequency, filter settings, gain, montage selections, and amplifier settings
• Any changes made during the recording period must be immediately notated with the EEG data

Recording

• EEG data acquisition should occur at a minimum sampling rate of 256 samples per second to be greater than three times the high-frequency filter setting, assumed at 70 Hz
• Higher rates (512 Hz) are preferable to prevent aliasing, especially with high-resolution screens
• Higher EEG sampling rates are preferred in automated detection algorithms
• Digitization should use a resolution of at least 16 bits per sample, including any sign bit; 24-bit resolution is best practice
• Should allow for 0.05 microvolt resolution down to 0.05 µV while recording potentials up to ± 1.638 mV
• Interchannel crosstalk should be less than 1%
• Common mode rejection ratio should be at least 90 dB for each channel
• Additional noise in the recording should be less than 1µV peak-to-peak at 0.5-100 Hz
• Ideally, video recording would be synchronized with the EEG to help facilitate review, especially for identification of artifacts and clinical events

Recording Storage

• Nonerasable and erasable optical storage devices can be used for outpatient routine, 1-hour, 6-hour video, and ambulatory EEG recordings
• Digital EEG systems should employ widely-supported types of optical storage media (e.g. CD-ROM, DVD-ROM, or Blue Ray)
• Optical storage devices not recommended as optical discs can be easily damaged and may disappear over the next few years.
• USB flash drives and network access servers are inexpensive, reliable, and practical
• Inpatient long-term recordings lasting 24+ hours, storage on a digital server system is recommended to facilitate backup and remote review by hospital IT
• Business grade server storage solutions for video-EEG minimizes data loss via data storage redundancy and regular data back-up
• Server storage enables HIPAA compliance by recording a full audit trail of patient record access
• EEG recording formatted to store EEG signal data, technologist comments, and event codes
• Manufacturers should provide a method for outputting copies of the recording with patient identifiers removed for research
• EEG recording systems should be able to input/output in publicly-available formats (EDF/EDF-plus)
• Manufacturers should guarantee newer reader systems can still read older data recorded by the equipment, even when those data are no longer compatible with current reading systems
• EEG laboratories should remain aware and convert older recordings when nearing technical obsolescence
• Manufacturers should inform customers about the need to convert data files to maintain legibility
• Note is made of the existence of state and local hospital statutes governing medical records in each of the individual states, as well as existence of local or hospital statutes regarding EEG record storage.
• These govern the duration of storage, and in some instances they may also dictate whether magnetic or optical storage is allowed

Display

• Recording systems should review recorded EEG data on a computer screen with good temporal and spatial resolution
• A standard horizontal scaling should allow 1 second to occupy 25-35 mm, with a minimum resolution of 128 data points/second with standard screen size of ten seconds
• Scaling differing from standard by a factor of 2, e.g., 7.5, 15, 30, or 60 mm/sec, should also be available.
• A standard vertical scaling with a minimum spacing of 10 mm per channel should be used for displays of up to 21 channels
• Horizontal and vertical scales should be indicated on the display
• Important considerations are the maximum number of channels and seconds displayed on a single screen with standard scaling
• Post hoc digital filtering should also be available on individual channels
• System should allow simultaneous display of multiple EEG segments for visual comparison within or obtained on different days
• Although many digital EEG review systems may display results of EEG processing, the reviewer should always have access to view the raw EEG data
• Montages for review should align with standard laboratory practices and ACNS recommendations using bipolar & referential reconstruction techniques
• Systems should display channel (montage) designations, gain/filter settings, technologist comments, and event markers, along with raw/transformed EEG data
• A time stamp on each screen or page of EEG data is essential