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The advantages of digital over analog recording techniques
Electroencephalography and clinical Neurophysiology 106 (1998) 113–117
The advantages of digital over analog recording techniques Barbara E. Swartz* VA Medical Center, West Los Angeles, 11301 Wilshire Boulevard (W127B), Department of Neurology, UCLA, Los Angeles, CA 90073, USA Accepted for publication: 3 October 1997
Abstract In the past few years digital technology has brought EEG and evoked potentials into emergency rooms, intensive care units and operating rooms with a variety of automatic data trending. Networking of these systems makes access to clinical neurophysiologists nearly immediate. Digital EEG has made montage reformatting and quantitation of parameters readily available. Increased spatial and temporal resolution is available with routine EEG, and combined topographic and frequency mapping of a given potential, spike or seizure focus is possible. In the future, these and other features such as dipole mapping and cognitive EP analysis will be available on a routine basis. 1998 Elsevier Science Ireland Ltd. Keywords: Digital; EEG; Electroencephalography
1. Introduction Computers have become a common component of our everyday activities. Their application to clinical neurophysiology has revolutionized the ease of many interpretive techniques, although few new basic concepts have been added. This is no small contribution, however, since most clinical neurophysiologists face increasing workload and time burdens. Several authors have summarized the contribution of digital encephalography (EEG) to the neurophysiologist’s diagnostic armamentarium (Gotman, 1990; Quinonez, 1998). This present article reviews this contribution with respect to both routine diagnostic and long-term monitoring, and attempts to highlight some new applications that may become routine at a later date. Blum (1998) elaborates on the potential pitfalls of digital techniques. Other papers in this special issue discuss some of the exciting research applications of digital EEG, evoked potentials (EPs), and cognitive evoked potentials.
2. Digital versus analog eeg in routine studies In Table 1, a comparison of digital and conventional EEG is shown. Montage reformatting has been available for many years with video EEG monitoring systems and is now widely available on digital EEG systems. This means * Tel.: +1 310 2683017; fax: +1 310 2684936.
0013-4694/98/$19.00 1998 Elsevier Science Ireland Ltd. All rights reserved PII S0013-4694 (97 )0 0113-2
that the technician can choose one montage to run a subject at, and then the interpreter can choose alternative montages to read from. The selection of the recording montage is theoretically of no technical concern because the reference, usually, but not always, Cz, is canceled out (see below). This ability allows the reader to see the true extent of field, accurately localize phase reversals, and readily compare the appearance of an event with multiple montage selections. The reader can then adjust filters, gain, and chart speed to enhance event appearance. The result is a greater burden placed on the reader to ‘customize’ the EEG tracing. The technician is responsible for an artifactfree record with good notation, as always, but can be freed from the difficult task of choosing the best montage for a given finding. What is being seen today is a melding of the previous technology of EPs and video EEG recording with that of the routine EEG laboratory. Analog EEG will undoubtedly remain a part of clinical neurophysiology for another 10 years, given the resilience and low prices of the analog machines and the relative costs of upgrading. However, the field emerging is revitalized by the technological shift.
3. Interpretation of EEG: digital versus analog It has been noted that interpretation of a digital EEG may take longer, as the reader reformats to his/her satisfaction. Whether it improves the reader’s ability to accurately inter-
EEG 31
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B.E. Swartz / Electroencephalography and clinical Neurophysiology 106 (1998) 113–117
pret the EEG has not been unequivocally determined. However, one study looked at the interpretation of 3 blinded readers given: (1) a routine paper EEG, (2) a digital EEG of the same event but without the ability to reformat, and (3) a digital EEG with reformatting (Levy et al., 1994). They found that the interrater agreement was fair for digital EEG without reformatting, good for paper EEG, and excellent for reformatted digital EEGs. This study suggested that readers who do not reformat may be less accurate than they were with paper, although the authors attributed the disagreements to small screen size. The greatest agreement on whether an abnormality was focal or generalized was achieved with the reformatted digital records. Another interesting finding was that 25% of EEGs interpreted as abnormal on the paper trace were called normal using reformatted digital EEG. Therefore, there is some evidence that digital EEG with reformatting can lead to improved interpretation.
4. Topographic mapping The ability to produce a topographic map of the spatial extent of various frequency bands is standard on most digital EEG systems, having been introduced by Duffy et al. (1979). The technique is also easily applied to epileptogenic spikes. While topographic mapping requires careful attention to artifact rejection, its use has disseminated the appreciation of EEG to non-neurophysiologists, such as surgeons and psychiatrists. Statistical comparisons of topographic maps can result in a tendency to overinterpret rare benign variants as pathological. Nevertheless, use of topographic mapping and statistical norms by trained neurophysiologists can improve communication with non-specialists. Functional mapping is also possible but remains largely a research tool (Grillon and Buschbaum, 1986; Gevins, 1998; Halgren, 1998). Topographic mapping has recently been applied to ictal recordings (Ives et al., 1993). In this study, individual wavelets of an ictal discharge were marked and averaged. The marking was done by hand, but an optimization software package could be developed to automate this procedure. In conjunction with the mapping, a computer program that splayed out the 10–20 system from different angles was applied. It is of special interest in this report that a bipolar circular recording montage was used. Cz amplitude, which normally serves as the reference electrode, was arbitrarily set to ‘0’ and the bipolar pairs were then converted to a referential display. An amplitude normalization applied to the referential display allowed for comparison of the threedimensional maps generated between or among patients with an error rate of ,5%. The authors pointed out that the bipolar recording technique was chosen because the amplifiers were smaller, low-powered, battery operated, and could be placed directly on the patient’s head in the hospital or at home.
5. Digital EEG in the intensive care unit (ICU) and operating room The use of computers in the operating room and ICU significantly decreases the personnel and time required for EEG monitoring since direct transfer of data to the clinical neurophysiology laboratory is possible (Quinonez, 1998). For this application monitoring systems that operate in real-time should be sought as the storage capacity for replay is also key. The increased storage and computational requirements make this equipment fairly expensive unless only a few channels are involved for such monitoring. Spectral analysis, spectral edge frequency (SEF) and dipole mapping programs are available, but sometimes at additional cost to the basic digital EEG package. These special functions are usually made available at a ‘reader station’. Some systems display the power spectrum directly for readout by nurses, anesthesiologists, etc. Trained neurophysiologists should be available to review the raw data when unusual trends occur. These systems all do spectral analysis, but derived variables such as SEF and trending with total or alpha power should be available. Analysis of change relative to the subject’s individual baseline is most reliable (Nuwer et al., 1993). It may also be advisable to have the ability to set alarms which will alert the user to significant changes in SEF or power in the ICU arena. Less computation can be done with online systems because it is impractical to make the analysis time greater than the collection time. Off-line analysis requires storage of the EEG in computer memory. The size of the memory is determined by the number of digitizing steps and sampling rate. Formerly this limited the amount of EEG that could be stored to the hard disk (1 h of 16 channels at 200 samples/s and a 12 bit converter requires 23 megabytes). Direct recording to laser disk is now possible, and the size of hard disks has increased, so either option is now practical for storing many hours of EEG.
6. Evoked potentials The ideas of sampling, averaging, and aliasing were first applied to EPs, since they are not visible without averaging unless recorded from the surface of the brain (Jasper, 1954). Because the principles of analog-to-digital conversion and sampling are the same, most digital EEG systems can be purchased with EP software as an option on the same system. The sampling rate of these systems should be at least 200 Hz with a notch filter available to reduce EMG activity as needed. The analog-to-digital converter should be at least 10 bit (for a range of −500 to +500 mV at 1 mV steps). Twelve bit converters are also common, but the need for higher precision is unlikely (Gotman, 1990). Spatial and topographic maps can be plotted of EPs as well as EEGs.
B.E. Swartz / Electroencephalography and clinical Neurophysiology 106 (1998) 113–117
7. Sampling the head or brain Another advantage of digital systems is the density of data that can be recorded. Routine analog EEG systems employed anywhere from 8–25 channels. Some laboratories used 32 channels, but the large size and weight of such machines limited their popularity. Digital EEG systems start at 25 channels, and can go as high as the laboratory can afford. Telemetry video EEG was limited to 12–14 channels by the band width available for use. Digital technology through cable hook-up has made available 128 channels, with no theoretical limitation. The availability of increased numbers of channels has made it possible for the American Board of Clinical Neurophysiology to recommend that routine recordings should use the 10–10 system (American Electroencephalographic Society, 1994; Eber-
115
sole and Pacia, 1996) as opposed to the 10–20 system (Jasper, 1958). The greater the number of channels, the greater the spatial sensitivity, as discussed by Gevins (1998), so we should anticipate continued increases in the numbers of recording channels in routine use.
8. Topographic maps of seizure foci Standard topographic mapping gives the spatial extent of an event being analyzed. Ebersole and colleagues have shown that spike voltage topographic maps (dipole analysis) can identify two types of frontotemporal foci. (Ebersole, 1991; Ebersole and Wade, 1991). Combinations of temporal and spatial maps may further refine event analysis (Lopes da Silva, 1987). This technique has recently been applied to
Table 1 A comparison of the qualities of digital and analog EEG recordings Function
Digital
Analog
Montage selection
Mathematical reconstruction of any montage at any time is possible One can record with ‘wide open’ filters and high gain at any speed, then view at different filters, sensitivity and speed if necessary Events are marked and can be retrieved individually or grouped for later printout by events or by montage ‘Best’ montage for viewing a given event can be selected in retrospect, including use of an average reference Settings are automatically recorded
Multiple montages must be recorded sequentially in time Prolonged recording necessary to see different filter and gain settings, e.g. ECS recordings Events are easily recognized by marks on paper but comparing them is cumbersome as with multiple spike amplitudes and fields Highly trained technician must recognize event, select appropriate montage and hope to catch the event again Technician must be diligent in recording parameters High-volume teaching files must be collected to demonstrate many principles and variants of EEG. An interpretation is given with little opportunity of the referring physician to view the results personally. Figures or slides with many channels are difficult to clearly reproduce Visual analyses are qualitative and subjective. Less experienced readers may miss subtle changes Subtle seizures may be unrecognized by staff and patient, increasing duration of monitoring. EEG changes may be difficult to identify without reformatting Cable sway can become a significant source of record artifact. High-voltage activity at the reference may not be completely subtracted out with intracranial recordings 20–60 min
Sensitivity, filter, and time base adjustments
Event retrieval
Error reduction
Teaching
EEG analysis
Seizure analysis
Machine artifacts
Test time Data storage and retrieval
Instrument size
There is easy retrieval of events, demonstration of effects of changes in amplification, filters, time base, and selection of events to print out. Computer display methods ‘demystify’ EEG for untrained personnel and allows ease of selection of slides or figures for publication Dipole source localization, statistical analysis, sleep analysis, and trend analysis is possible Event detection improves sensitivity, synchronous EEG video improves diagnosis, cross-correlation analysis enhances localization of abnormal discharges No pen alignment problems or curvilinear pen effects exist. Digital filters provide clean cut-off without time constant problems 20–60 min. Digitization does not eliminate the need for an adequate initial sampling Space requirements roughly equivalent to microfilm, but easier to read. Some optical disks are quite expensive; a universal format should be demanded of companies making the systems, and rewritable disks should be available Recording systems may be more lightweight but if combined with large hard drive, printer, and reader station, total space is comparable to or greater than conventional EEG instrumentation
Paper requires excess storage space. Reading large sheets of paper is easier than reading a small computer monitor
Number of channels limited by increasing weight of systems
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B.E. Swartz / Electroencephalography and clinical Neurophysiology 106 (1998) 113–117
ictal recordings, where the determination of propagation versus source of ictal activity becomes critical. Baumgartner et al. (1995) applied a similar technique to surface recordings of interictal spikes. They also found two dipole sources for frontotemporal spikes: a medial temporal one, probably related to a large mesiobasal temporal area, and a second one reflecting lateral temporal activity. The distance between the two sources averaged 45 mm, but this represented only the center of activity (Cooper et al., 1965). The large extent of the mesial source of such spikes has been discussed by Marks et al. (1992), who performed simultaneous surface, sphenoidal, and depth recordings. By avoiding spike averaging, Baumgartner was able to determine a 40 ms delay between the mesial source and the lateral source in the combined data of 4 subjects. This agreed with the results of Sutherling and Barth (1989) using MEG. The dipole modeling of Ebersole (1991) and Scherg and Ebersole (1993) also found mesial to lateral propagation. However, these are somewhat in conflict with those of Marks et al. (1992), who found propagation in either direction in 3 of their subjects. The latter is the only direct study; the others rely on models with some inherent inaccuracies (see Koles, 1998). A recent report suggested that dipole modeling may reduce the need for depth recordings in temporal lobe epilepsy (Boon et al., 1997). In the study of Du¨mplemann et al. (1994), depth recordings were evaluated, and it was found that analyzing both the spatial and temporal spreads of ictal activity simultaneously decreased the ambiguity of defining the seizure focus in extratemporal seizures. While exciting, these approaches demand further evaluation before being used in clinical practice.
9. Ictal activity analysis: technical considerations While most video EEG recording systems allow for a sampling frequency of 200 Hz (high-frequency cut-off of 100 Hz), some investigators have recorded frequencies up to or higher than 100 Hz at the onset of seizures recorded intracranially (Allen et al., 1992; Fisher et al., 1992; Arroyo et al., 1994). Therefore, new equipment designs should incorporate the availability to record at very fast frequencies. The recording of DC potential shifts has been very important for the understanding of ictal development and propagation in basic research studies done in animals (Speckmann and Elger, 1987). This technique has also been applied to human recordings using digital recordings with a low-frequency filter (LFF) setting of 0.016 Hz. Using this setting, Ikeda et al. (1996) found a more restricted potential 1–10 s prior to the EEG change seen using a 1.5 Hz LFF setting. Again, standard video EEG equipment does not typically include the option of variable LFFs but should be encouraged to do so. If intracranial voltages exceed the amplifier’s dynamic range, the subtraction algorithm may not completely eliminate contamination at the
reference. Thus, it is frequently advisable to first use a midline extracranial reference until the seizure laterality is known, and then an intracranial reference contralateral to the focus (Morris and Luders, 1985). Detection of spike or seizure events has been greatly enhanced by software that annotates the recording at sharp or rhythmic EEG changes. These programs are continually being upgraded by techniques such as training the computer to ‘recognize’ a particular ictal pattern by either template matching (Stellar systems) or frequency filters (Koles, pers. commun.).
10. Summary Computerized or digital EEG has many advantages over conventional EEG. It can and will provide for the rapid transfer of techniques from the research to the clinical laboratory. This has already been seen with Fourier analysis, frequency, and amplitude topographic maps. Dipole analysis, temporal mapping, DC recordings, coherence studies and source derivations may soon follow suit. Moreover, even as addition of multiple functions drives the price of EEG machines up, so the rapid advances in computer technology and the emergence of many digital EEG companies continue to drive prices downwards. Thus, we can look forward to continued enrichment in the clinical neurophysiology laboratory by enhanced computational abilities.
References Allen, P.J., Fish, D.R. and Smith, D.J.M. Very high-frequency activity during SEEG suppression in frontal lobe epilepsy. Electroenceph. clin. Neurophysiol., 1992, 82: 155–159. American Electroencephalographic Society. Guidelines for standard electrode position nomenclature: guideline thirteen. J. Clin. Neurophysiol., 1994, 11(1): 111–113. Arroyo, S., Lesser, R.P., Fisher, R.S., Vining, E.P., Krauss, G.L., BandeenRoche, K., Hart, J., Gordon, B., Uematsu, S. and Webber, R. Clinical and electroencephalographic evidence for sites of origin of seizures with diffuse electrodecremental pattern. Epilepsia, 1994, 35 (5): 974– 987. Baumgartner, C., Lindinger, G., Ebner, A., Sull, S., Serles, W., Olbrich, A., Lurger, S., Czech, T., Burgess, R. and Luders, H. Propagation of interictal epileptic activity in temporal lobe epilepsy. Neurology, 1995, 45: 118–122. Blum, D.E. Computer-based electroencephalography: technical basics, basis for new applications, and potential pitfalls. Electroenceph. clin. Neurophysiol., 1998, 106: 118–126. Boon, P., D’Hawe, M., Adam, C., Vonck, K., Boulac, M., Van der Kerckhove, T.I., De Reuck, J. Dipole modeling in epilepsy surgery candidates. Epilepsia, 1997, 38(2): 208–218.. Cooper, R., Winter, A.L., Crow, H.J. and Walter, W.G. Comparison of subcortical, cortical and scalp activity using chronically indwelling electrodes in man. Electroenceph. clin Neurophysiol., 1965, 18: 217– 228. Duffy, F.H., Burchfield, J.L. and Lombroso, C.T. Brain electrical activity mapping (BEAM): a method for extending the clinical use of EEG and evoked potential data. Ann. Neurol., 1979, 5: 309–321.
B.E. Swartz / Electroencephalography and clinical Neurophysiology 106 (1998) 113–117 Du¨mplemann, M., Hufnagel, A., Burr, W. and Elger, C. Determination of the epileptogenic zone by the use of spike clustering and spike-amplitude-latency tomograms. Epilepsia, 1994, 58: 28. Ebersole, J.S. EEG dipole modeling in complex partial epilepsy. Brain Topogr., 1991, 4: 113–123. Ebersole, J.S. and Pacia, S.V. Localization of temporal lobe foci by ictal EEG patterns. Epilepsia, 1996, 37 (4): 386–399. Ebersole, J.S. and Wade, P.B. Spike voltage topography identifies two types of frontotemporal epileptic foci. Neurology, 1991, 41: 1425–1433. Fisher, R.S., Webber, W.R.S., Lesser, R.P., Arroyo, S. and Uematsu, S. High-frequency EEG activity at the start of seizures. J. Clin. Neurophysiol., 1992, 9: 441–448. Gevins, A. The future of electroencephalography in assessing neurocognitive functioning. Electroenceph. clin. Neurophysiol., 1998, 106: 165– 172. Gotman, J. The use of computers in analysis and display of eeg and evoked potentials. In: D.D. Daly and T.A. Pedley (Eds.), Current Practice of Clinical Electroencephalography, 2nd edn. Raven Press, New York, 1990, pp. 51–84. Grillon, C. and Buschbaum, M.S. Computed EEG topography of response to visual and auditory stimuli. Electroenceph. clin. Neurophysiol., 1986, 63: 42–53. Halgren, E., Marinkovic, K. and Chauvel, P. Generators of the late cognitive potentials in auditory and visual oddball tasks. Electroenceph. clin. Neurophysiol., 1998, 106: 156–164. Ikeda, A., Terada, K., Mikuni, N., Burgess, R., Youssef, C., Taki, W., Hamano, T., Kimura, J., Luders, H.O. and Shibasaki, H. Subdural recording of ictal DC shifts in neocortical seizures in humans. Epilepsia, 1996, 37 (7): 662–674. Ives, J.R., Ichihashi, K., Gruber, L.J., Mainwaring, N.R., Blume, H.W. and Schomer, D.L. New topographic mapping of temporal lobe seizures. Epilepsia, 1993, 34 (5): 890–896. Jasper, H.H. Electrocorticography. In: W. Penfield and H.H. Jasper (Eds.), Epilepsy and the Functional Anatomy of the Human Brain. Little, Brown, Boston, 1954, pp. 692–738.
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Jasper, H.H. Report of the committee on methods of clinical examination in electroencephalography. Electroenceph. clin. Neurophysiol., 1958, 10: 370–375. Koles, Z.J. Trends in EEG source localization. Electroenceph. clin. Neurophysiol., 1998, 106: 127–137. Levy, S.R., Berg, A.T., Testa, F.M., Novotny, E.J. and Chiappa, K.H. Comparison of digital and conventional EEG interpretation. Epilepsia, 1994, 35 (Suppl. 8): 127. Lopes da Silva, F. EEG analysis: theory and practice. In: E. Niedermeyer and F. Lopes da Silva (Eds.), Electroencephalography: Basic Principles Clinical Applications and Related Fields. Urban & Schwarzenberg, 1987, pp. 871–898. Marks, D.A., Katz, A., Booke, J., Spencer, D.D. and Spencer, S.S. Comparison and correlation of surface and sphenoidal electrodes with simultaneous intracranial recording: an interictal study. Electroenceph. clin. Neurophysiol., 1992, 82: 23–29. Morris, H.H. and Luders, H. Electrodes. In: J. Gotman, J.R. Ivers and P. Gloor (Eds.), Long-term Monitoring in Epilepsy. Electroenceph. clin. Neurophysiol., 1985, Suppl. 37: 3–26. Nuwer, M.R., Daube, J., Fischer, C., Schramm, J. and Yingling, C.D. Neuromonitoring during surgery. Report of an IFCN committee. Electroenceph. clin. Neurophysiol., 1993, 87: 263–276. Quinonez, D. Common applications of electrophysiology (EEG) in the past and today: the technologist’s view. Electroenceph. clin. Neurophysiol., 1998, 106: 108–112. Scherg, M. and Ebersole, J.S. Models of brain sources. Brain Topogr., 1993, 5: 419–423. Speckmann, E.J. and Elger, C.E. Introduction to the neurophysiological basis of the EEG and DC potentials. In: E. Niedermeyer and F. Lopes da Silva (Eds.), Electroencephalography: Basic Principles, Clinical Applications and Related Fields. Urban & Schwarzenberg, 1987, pp. 1–14. Sutherling, W.W. and Barth, D.S. Neocortical propagation in temporal lobe spike foci on magnetoencephalography and electroencephalography. Ann Neurol., 1989, 25: 373–381.
The advantages of digital over analog recording techniques Barbara E. Swartz* VA Medical Center, West Los Angeles, 11301 Wilshire Boulevard (W127B), Department of Neurology, UCLA, Los Angeles, CA 90073, USA Accepted for publication: 3 October 1997
Abstract In the past few years digital technology has brought EEG and evoked potentials into emergency rooms, intensive care units and operating rooms with a variety of automatic data trending. Networking of these systems makes access to clinical neurophysiologists nearly immediate. Digital EEG has made montage reformatting and quantitation of parameters readily available. Increased spatial and temporal resolution is available with routine EEG, and combined topographic and frequency mapping of a given potential, spike or seizure focus is possible. In the future, these and other features such as dipole mapping and cognitive EP analysis will be available on a routine basis. 1998 Elsevier Science Ireland Ltd. Keywords: Digital; EEG; Electroencephalography
1. Introduction Computers have become a common component of our everyday activities. Their application to clinical neurophysiology has revolutionized the ease of many interpretive techniques, although few new basic concepts have been added. This is no small contribution, however, since most clinical neurophysiologists face increasing workload and time burdens. Several authors have summarized the contribution of digital encephalography (EEG) to the neurophysiologist’s diagnostic armamentarium (Gotman, 1990; Quinonez, 1998). This present article reviews this contribution with respect to both routine diagnostic and long-term monitoring, and attempts to highlight some new applications that may become routine at a later date. Blum (1998) elaborates on the potential pitfalls of digital techniques. Other papers in this special issue discuss some of the exciting research applications of digital EEG, evoked potentials (EPs), and cognitive evoked potentials.
2. Digital versus analog eeg in routine studies In Table 1, a comparison of digital and conventional EEG is shown. Montage reformatting has been available for many years with video EEG monitoring systems and is now widely available on digital EEG systems. This means * Tel.: +1 310 2683017; fax: +1 310 2684936.
0013-4694/98/$19.00 1998 Elsevier Science Ireland Ltd. All rights reserved PII S0013-4694 (97 )0 0113-2
that the technician can choose one montage to run a subject at, and then the interpreter can choose alternative montages to read from. The selection of the recording montage is theoretically of no technical concern because the reference, usually, but not always, Cz, is canceled out (see below). This ability allows the reader to see the true extent of field, accurately localize phase reversals, and readily compare the appearance of an event with multiple montage selections. The reader can then adjust filters, gain, and chart speed to enhance event appearance. The result is a greater burden placed on the reader to ‘customize’ the EEG tracing. The technician is responsible for an artifactfree record with good notation, as always, but can be freed from the difficult task of choosing the best montage for a given finding. What is being seen today is a melding of the previous technology of EPs and video EEG recording with that of the routine EEG laboratory. Analog EEG will undoubtedly remain a part of clinical neurophysiology for another 10 years, given the resilience and low prices of the analog machines and the relative costs of upgrading. However, the field emerging is revitalized by the technological shift.
3. Interpretation of EEG: digital versus analog It has been noted that interpretation of a digital EEG may take longer, as the reader reformats to his/her satisfaction. Whether it improves the reader’s ability to accurately inter-
EEG 31
114
B.E. Swartz / Electroencephalography and clinical Neurophysiology 106 (1998) 113–117
pret the EEG has not been unequivocally determined. However, one study looked at the interpretation of 3 blinded readers given: (1) a routine paper EEG, (2) a digital EEG of the same event but without the ability to reformat, and (3) a digital EEG with reformatting (Levy et al., 1994). They found that the interrater agreement was fair for digital EEG without reformatting, good for paper EEG, and excellent for reformatted digital EEGs. This study suggested that readers who do not reformat may be less accurate than they were with paper, although the authors attributed the disagreements to small screen size. The greatest agreement on whether an abnormality was focal or generalized was achieved with the reformatted digital records. Another interesting finding was that 25% of EEGs interpreted as abnormal on the paper trace were called normal using reformatted digital EEG. Therefore, there is some evidence that digital EEG with reformatting can lead to improved interpretation.
4. Topographic mapping The ability to produce a topographic map of the spatial extent of various frequency bands is standard on most digital EEG systems, having been introduced by Duffy et al. (1979). The technique is also easily applied to epileptogenic spikes. While topographic mapping requires careful attention to artifact rejection, its use has disseminated the appreciation of EEG to non-neurophysiologists, such as surgeons and psychiatrists. Statistical comparisons of topographic maps can result in a tendency to overinterpret rare benign variants as pathological. Nevertheless, use of topographic mapping and statistical norms by trained neurophysiologists can improve communication with non-specialists. Functional mapping is also possible but remains largely a research tool (Grillon and Buschbaum, 1986; Gevins, 1998; Halgren, 1998). Topographic mapping has recently been applied to ictal recordings (Ives et al., 1993). In this study, individual wavelets of an ictal discharge were marked and averaged. The marking was done by hand, but an optimization software package could be developed to automate this procedure. In conjunction with the mapping, a computer program that splayed out the 10–20 system from different angles was applied. It is of special interest in this report that a bipolar circular recording montage was used. Cz amplitude, which normally serves as the reference electrode, was arbitrarily set to ‘0’ and the bipolar pairs were then converted to a referential display. An amplitude normalization applied to the referential display allowed for comparison of the threedimensional maps generated between or among patients with an error rate of ,5%. The authors pointed out that the bipolar recording technique was chosen because the amplifiers were smaller, low-powered, battery operated, and could be placed directly on the patient’s head in the hospital or at home.
5. Digital EEG in the intensive care unit (ICU) and operating room The use of computers in the operating room and ICU significantly decreases the personnel and time required for EEG monitoring since direct transfer of data to the clinical neurophysiology laboratory is possible (Quinonez, 1998). For this application monitoring systems that operate in real-time should be sought as the storage capacity for replay is also key. The increased storage and computational requirements make this equipment fairly expensive unless only a few channels are involved for such monitoring. Spectral analysis, spectral edge frequency (SEF) and dipole mapping programs are available, but sometimes at additional cost to the basic digital EEG package. These special functions are usually made available at a ‘reader station’. Some systems display the power spectrum directly for readout by nurses, anesthesiologists, etc. Trained neurophysiologists should be available to review the raw data when unusual trends occur. These systems all do spectral analysis, but derived variables such as SEF and trending with total or alpha power should be available. Analysis of change relative to the subject’s individual baseline is most reliable (Nuwer et al., 1993). It may also be advisable to have the ability to set alarms which will alert the user to significant changes in SEF or power in the ICU arena. Less computation can be done with online systems because it is impractical to make the analysis time greater than the collection time. Off-line analysis requires storage of the EEG in computer memory. The size of the memory is determined by the number of digitizing steps and sampling rate. Formerly this limited the amount of EEG that could be stored to the hard disk (1 h of 16 channels at 200 samples/s and a 12 bit converter requires 23 megabytes). Direct recording to laser disk is now possible, and the size of hard disks has increased, so either option is now practical for storing many hours of EEG.
6. Evoked potentials The ideas of sampling, averaging, and aliasing were first applied to EPs, since they are not visible without averaging unless recorded from the surface of the brain (Jasper, 1954). Because the principles of analog-to-digital conversion and sampling are the same, most digital EEG systems can be purchased with EP software as an option on the same system. The sampling rate of these systems should be at least 200 Hz with a notch filter available to reduce EMG activity as needed. The analog-to-digital converter should be at least 10 bit (for a range of −500 to +500 mV at 1 mV steps). Twelve bit converters are also common, but the need for higher precision is unlikely (Gotman, 1990). Spatial and topographic maps can be plotted of EPs as well as EEGs.
B.E. Swartz / Electroencephalography and clinical Neurophysiology 106 (1998) 113–117
7. Sampling the head or brain Another advantage of digital systems is the density of data that can be recorded. Routine analog EEG systems employed anywhere from 8–25 channels. Some laboratories used 32 channels, but the large size and weight of such machines limited their popularity. Digital EEG systems start at 25 channels, and can go as high as the laboratory can afford. Telemetry video EEG was limited to 12–14 channels by the band width available for use. Digital technology through cable hook-up has made available 128 channels, with no theoretical limitation. The availability of increased numbers of channels has made it possible for the American Board of Clinical Neurophysiology to recommend that routine recordings should use the 10–10 system (American Electroencephalographic Society, 1994; Eber-
115
sole and Pacia, 1996) as opposed to the 10–20 system (Jasper, 1958). The greater the number of channels, the greater the spatial sensitivity, as discussed by Gevins (1998), so we should anticipate continued increases in the numbers of recording channels in routine use.
8. Topographic maps of seizure foci Standard topographic mapping gives the spatial extent of an event being analyzed. Ebersole and colleagues have shown that spike voltage topographic maps (dipole analysis) can identify two types of frontotemporal foci. (Ebersole, 1991; Ebersole and Wade, 1991). Combinations of temporal and spatial maps may further refine event analysis (Lopes da Silva, 1987). This technique has recently been applied to
Table 1 A comparison of the qualities of digital and analog EEG recordings Function
Digital
Analog
Montage selection
Mathematical reconstruction of any montage at any time is possible One can record with ‘wide open’ filters and high gain at any speed, then view at different filters, sensitivity and speed if necessary Events are marked and can be retrieved individually or grouped for later printout by events or by montage ‘Best’ montage for viewing a given event can be selected in retrospect, including use of an average reference Settings are automatically recorded
Multiple montages must be recorded sequentially in time Prolonged recording necessary to see different filter and gain settings, e.g. ECS recordings Events are easily recognized by marks on paper but comparing them is cumbersome as with multiple spike amplitudes and fields Highly trained technician must recognize event, select appropriate montage and hope to catch the event again Technician must be diligent in recording parameters High-volume teaching files must be collected to demonstrate many principles and variants of EEG. An interpretation is given with little opportunity of the referring physician to view the results personally. Figures or slides with many channels are difficult to clearly reproduce Visual analyses are qualitative and subjective. Less experienced readers may miss subtle changes Subtle seizures may be unrecognized by staff and patient, increasing duration of monitoring. EEG changes may be difficult to identify without reformatting Cable sway can become a significant source of record artifact. High-voltage activity at the reference may not be completely subtracted out with intracranial recordings 20–60 min
Sensitivity, filter, and time base adjustments
Event retrieval
Error reduction
Teaching
EEG analysis
Seizure analysis
Machine artifacts
Test time Data storage and retrieval
Instrument size
There is easy retrieval of events, demonstration of effects of changes in amplification, filters, time base, and selection of events to print out. Computer display methods ‘demystify’ EEG for untrained personnel and allows ease of selection of slides or figures for publication Dipole source localization, statistical analysis, sleep analysis, and trend analysis is possible Event detection improves sensitivity, synchronous EEG video improves diagnosis, cross-correlation analysis enhances localization of abnormal discharges No pen alignment problems or curvilinear pen effects exist. Digital filters provide clean cut-off without time constant problems 20–60 min. Digitization does not eliminate the need for an adequate initial sampling Space requirements roughly equivalent to microfilm, but easier to read. Some optical disks are quite expensive; a universal format should be demanded of companies making the systems, and rewritable disks should be available Recording systems may be more lightweight but if combined with large hard drive, printer, and reader station, total space is comparable to or greater than conventional EEG instrumentation
Paper requires excess storage space. Reading large sheets of paper is easier than reading a small computer monitor
Number of channels limited by increasing weight of systems
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ictal recordings, where the determination of propagation versus source of ictal activity becomes critical. Baumgartner et al. (1995) applied a similar technique to surface recordings of interictal spikes. They also found two dipole sources for frontotemporal spikes: a medial temporal one, probably related to a large mesiobasal temporal area, and a second one reflecting lateral temporal activity. The distance between the two sources averaged 45 mm, but this represented only the center of activity (Cooper et al., 1965). The large extent of the mesial source of such spikes has been discussed by Marks et al. (1992), who performed simultaneous surface, sphenoidal, and depth recordings. By avoiding spike averaging, Baumgartner was able to determine a 40 ms delay between the mesial source and the lateral source in the combined data of 4 subjects. This agreed with the results of Sutherling and Barth (1989) using MEG. The dipole modeling of Ebersole (1991) and Scherg and Ebersole (1993) also found mesial to lateral propagation. However, these are somewhat in conflict with those of Marks et al. (1992), who found propagation in either direction in 3 of their subjects. The latter is the only direct study; the others rely on models with some inherent inaccuracies (see Koles, 1998). A recent report suggested that dipole modeling may reduce the need for depth recordings in temporal lobe epilepsy (Boon et al., 1997). In the study of Du¨mplemann et al. (1994), depth recordings were evaluated, and it was found that analyzing both the spatial and temporal spreads of ictal activity simultaneously decreased the ambiguity of defining the seizure focus in extratemporal seizures. While exciting, these approaches demand further evaluation before being used in clinical practice.
9. Ictal activity analysis: technical considerations While most video EEG recording systems allow for a sampling frequency of 200 Hz (high-frequency cut-off of 100 Hz), some investigators have recorded frequencies up to or higher than 100 Hz at the onset of seizures recorded intracranially (Allen et al., 1992; Fisher et al., 1992; Arroyo et al., 1994). Therefore, new equipment designs should incorporate the availability to record at very fast frequencies. The recording of DC potential shifts has been very important for the understanding of ictal development and propagation in basic research studies done in animals (Speckmann and Elger, 1987). This technique has also been applied to human recordings using digital recordings with a low-frequency filter (LFF) setting of 0.016 Hz. Using this setting, Ikeda et al. (1996) found a more restricted potential 1–10 s prior to the EEG change seen using a 1.5 Hz LFF setting. Again, standard video EEG equipment does not typically include the option of variable LFFs but should be encouraged to do so. If intracranial voltages exceed the amplifier’s dynamic range, the subtraction algorithm may not completely eliminate contamination at the
reference. Thus, it is frequently advisable to first use a midline extracranial reference until the seizure laterality is known, and then an intracranial reference contralateral to the focus (Morris and Luders, 1985). Detection of spike or seizure events has been greatly enhanced by software that annotates the recording at sharp or rhythmic EEG changes. These programs are continually being upgraded by techniques such as training the computer to ‘recognize’ a particular ictal pattern by either template matching (Stellar systems) or frequency filters (Koles, pers. commun.).
10. Summary Computerized or digital EEG has many advantages over conventional EEG. It can and will provide for the rapid transfer of techniques from the research to the clinical laboratory. This has already been seen with Fourier analysis, frequency, and amplitude topographic maps. Dipole analysis, temporal mapping, DC recordings, coherence studies and source derivations may soon follow suit. Moreover, even as addition of multiple functions drives the price of EEG machines up, so the rapid advances in computer technology and the emergence of many digital EEG companies continue to drive prices downwards. Thus, we can look forward to continued enrichment in the clinical neurophysiology laboratory by enhanced computational abilities.
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