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Prospects for Minimally Invasive Long Term Monitoring of Brain Function

By Sydney S. Cash, MD, PhD

Two devices are being developed to allow improved long-term non-invasive or minimally invasive EEG monitoring. The devices use iPod Touch or iPhones to transmit the data to secure server.

Abstract
Electroencephalography (EEG) remains the mainstay test for the diagnosis and treatment of patients with epilepsy as well as many other neurological disorders and has increasing use in the development of brain computer interfaces. Unfortunately, there has been remarkable little overall change in EEG systems. As a result, we are currently employing systems which can only be used for long term recordings (days to weeks) within the hospital environment or even shorter (hours to days) in the home environment. This leaves a tremendous gap in possible diagnostic and therapeutic utilization of EEG. Here we discuss novel technical approaches toward devices that can be used non-invasively or minimally invasively to record EEG in the home environment. These include a behind-the-ear EEG recording device that uses an iPhone or iPod Touch to continuously upload the patient’s data to a secure server. We also discuss a subdermal implanted system currently in development which can act as the cerebral equivalent of a cardiac loop monitor and record neural activity over weeks, months or indefinitely. Such systems can tremendously extend our ability for diagnosis and treatment of neurological diseases including neuroprosthetic / brain-computer interface applications.

Introduction to EEG systems and capabilities
Electroencephalography (EEG), the recording of the brain derived electrical potentials at different points at the scalp, remains the mainstay test for the diagnosis and treatment of patients with epilepsy. It also has widespread use in the diagnosis of other neurological conditions, utility in cognitive studies as well as application in neuroeconomics and elsewhere. One of the most actively expanding areas for utilization of EEG signals includes brain-computer interfaces and neuroprosthetics. Unfortunately, there has been remarkable little overall change in EEG systems over the past few decades. Clinically, systems are currently designed to handle 10’s to 256 channels for long term recordings (days to weeks) within the hospital environment. Some systems include smaller recorders which can be used in the home environment for 1-3 days. The majority of clinical systems rely on metal electrodes (e.g. Au or Ag-AgCl) which require conductive paste and adhesive to stay in place. Various cap systems also exist for use in the clinic or laboratory environment. More recently, several commercial entities have introduced low channel count EEG systems to the market (i.e. Cognionics, Emotiv) for gaming, neuroeconomics and other purposes. The quality and capability of these systems have yet to be fully described. At this point, however, no system – either clinically employed or otherwise available – is able to provide low cost, cosmetically satisfactory, technically robust long term recordings of brain activity in a minimally invasive fashion. Yet just such a system is sorely needed.

The need for long term EEG recording systems
Many neurological diseases are intermittent by their very nature or may have waxing and waning characteristics. This is true for movement disorders such as Parkinson’s disease, dementias including Alzheimer’s and, perhaps most prototypical, epilepsy which is characterized by occasional seizures which are brief but entirely disruptive.

Neuropsychiatric diseases such as depressive disorders, anxiety disorders and PTSD can also have waxing and waning or paroxysmal symptoms. Furthermore, rapid advances in the domain of brain computer interfaces have led to impressive progress in being able to decode neural signals. Yet, for all of these potential applications, there is no system capable of continuous, long term recording which meets criteria for a low cost, widely applicable, cosmetically acceptable and minimally invasive approach. This leaves a tremendous gap in possible diagnostic and therapeutic utilization of EEG.

Toward Long Term Minimally Invasive Brain Activity Monitoring
Here we discuss novel technical approaches toward devices that can be used non-invasively or minimally invasively to record EEG in the home environment. These include a behind-the-ear EEG recording device that uses an iPhone or iPod Touch to continuously upload the patient’s data to a secure server. This device not only gives the doctors access to the EEG data in real time but can be easily removed and re-applied by the patient at any time, thus reducing the interference with quality of life. The core of the system is a simple recorder built with off-the-shelf components with 256 Hz sampling rate and 12 bit ADC and uses Bluetooth communication protocols to send data to an iPod touch which then sends the data to a secure server. Early testing in the hospital environment has demonstrated high fidelity recordings comparable to simultaneously recorded data using standard clinical grade EEG machines (Do Valle et al, 2014). The primary next phase of development is transitioning to home testing

This system is designed for relatively low channel count non-invasive applications covering weeks of recordings – most probably in the setting of diagnostic uncertainty or sub-chronic management of medications or the like. The limit in this setting is the electrode system which rests on the skin and is therefore cosmetically sub-optimal and also requires significant maintenance. To overcome this limitation we are also developing subdermal implanted systems which can act as the cerebral equivalent of a cardiac loop monitor and record neural activity over weeks, months or even more. Such a system can be used for diagnosis, neuroprosthetic / brain-computer interface applications and beyond. Building on experience with the external system described above, we have designed a low-power recoding system that can be enclosed in hermetic packaging and placed, along with custom designed electrodes, beneath the scalp. The intended procedure for implanting the system is simple, quick and does not require a hospitalization.

Remaining challenges in Developing Minimally Invasive Brain Activity Monitoring
Many of the engineering challenges that persist are not necessarily tremendous. These mostly revolve around continuing improvement of the electronics packages to achieve smaller scale, longer battery life / decreased power consumption and increased data handling capabilities. Each of these represents a significant hurdle but also dovetails with similar problems within many medical devices as well as in the larger sphere of commercial and telecommunications electronics as a whole. As a result, continued improvement is expected in general as well as specific applications.

Perhaps the largest barrier is inherent in the signal of interest. The most direct method for measurement of brain activity remains the EEG. This is, at its essence, a measure of voltage changes and is therefore referential by nature and also greatly altered by the different resistances of the tissues it passes through. For extracranial measurements, as suggested here, this includes at least the meninges surrounding the brain and the skull itself. These distortions in the signal and the nature of the signal raise a number of problems for recordings in general. Not least, are effective electrodes that can be used inside or, more importantly, outside the skin. Advances in electrode design including active electrodes and dry electrodes may overcome some of these issues. Novel approaches including capacitive measurements or measurements of magnetic activity may be other avenues which overcome some of these issues.

In any case, an accelerating pace of advances in engineering new methods for recording neural activity or its surrogates may make traditional EEG recording approaches obsolete in the near future and unsure in a new era in home monitoring and continuous diagnosis and treatment of neuronal health.

Acknowledgements
Work discussed in this summary was carried out in collaboration with Charlie Sodini and Bruno Do Valle (both at MIT).

For Further Reading

B.G. Do Valle, S.S Cash, C.G. Sodini, “Wireless Behind-the-Ear EEG Recording Device with Wireless Interface to a Mobile Device (iPhone/iPod Touch),” Engineering in Medicine and Biology Society (EMBC), 2014 Annual International Conference of the IEEE. Submitted for publication.


Contributor

Sydney S. CashSydney S. Cash received his MD and PHD from Columbia University in New York City. He is now an Associate Professor in the Epilepsy Division of the Neurology Department at Massachusetts General Hospital and at Harvard University. He is also a member of the BrainGate clinical trial team, Co-director of the Department of Neurology NeuroTechnology Trials Unit and clinical trials director of the New England Pediatric Device Consortium. Current research in Dr. Cash’s lab is, broadly speaking, dedicated to trying to understand normal and abnormal brain activity, particularly oscillations, using multi-modal and multi-scalar approaches. This includes a focus on the development of novel neurotechnological approaches to help diagnose and treat common and devastating neurological diseases. Read more

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May 2014 Contributors

brucehechtBruce Hecht received the M.A.Sc. and B.A.Sc. degrees in Electrical Engineering from the University of Waterloo, Ontario, Canada. Originally from Montreal, Quebec, he joined Analog Devices in 1994, where he is currently with the Worldwide Quality Systems Engineering Group in Wilmington, MA, USA. His interests are in design of all kinds of electronic systems for medical, automotive, industrial, consumer, and communications systems. Read more

Sydney S. CashSydney S. Cash received his MD and PHD from Columbia University in New York City. He is now an Associate Professor in the Epilepsy Division of the Neurology Department at Massachusetts General Hospital and at Harvard University. He is also a member of the BrainGate clinical trial team, Co-director of the Department of Neurology NeuroTechnology Trials Unit and clinical trials director of the New England Pediatric Device Consortium. Current research in Dr. Cash's lab is, broadly speaking, dedicated to trying to understand normal and abnormal brain activity, particularly oscillations, using multi-modal and multi-scalar approaches. This includes a focus on the development of novel neurotechnological approaches to help diagnose and treat common and devastating neurological diseases. Read more

Michael FilbinMichael Filbin, MD, is an emergency physician at Massachusetts General Hospital and Assistant Professor at Harvard Medical School. Dr. Filbin attended medical school at Baylor College of Medicine and did his residency training in the Harvard-Affiliated Emergency Medicine Residency (HAEMR) program. Dr. Filbin's research interest is in human clinical trials of septic shock with a particular focus on early identification and intervention. Read more

Omer T. InanOmer T. Inan is an Assistant Professor of Electrical and Computer Engineering at the Georgia Institute of Technology, where he researches physiological and biomedical sensing and monitoring. He received his PhD in Electrical Engineering from Stanford University. Read more