ACQUISITION OF ELECTROPHYSIOLOGIC SIGNALS DURING MAGNETIC RESONANCE IMAGING
Acquisition of Electrophysiologic Signals During Magnetic Resonance Imaging John M. Parker BS,3 Jeffry R. Alger PhD,2 Mary A. Woo DScN,3 David Spriggs MS1 and Ronald M. Harper PhD1,3 1Departments
of Neurobiology and 2Radiological Sciences and the 3Brain Research Institute, University of California at Los Angeles, Los Angeles, CA 90095-1763 Abstract: We describe a low cost system for acquiring electrophysiological signals during magnetic resonance imaging. The system consists of high common-mode-rejection and low noise operational amplifiers, coupled by fiber optic cables to a receiver located at the periphery of the magnetic field. The system minimizes noise introduction which would contaminate image signals. Key words: Electroencephalogram; electrocardiogram; sleep; amplifier INTRODUCTION
the cable with a 1 cm PIN photodiode (UDT UV50) which provides a wide angle of acceptance. The photodiode is used in a photovoltaic mode with a Burr-Brown OPA627 amplifier — the latter used as a current-to-voltage converter with a 500 megohm resistor. Optical signal loss over 10 meters of optic fiber is negligible (2 db). The output of the receiver circuit was subjected to electronic filtering appropriate for each electrophysiologic signal, with the maximum frequency reduced at 60 Hz by 24 db. Signals were subsequently digitized at 500 Hz. The system was evaluated by acquiring electroencephalographic (EEG), electrocardiographic (ECG), and thoracic wall electromyographic (EMG) activity concurrently with spin echo magnetic resonance images. Under Ketamine and Xylazine anesthesia, an adult rabbit was instrumented with platinum needle electrodes and stainless steel screws into the skull over the orbit, and over the posterior skull to collect eye movement and EEG signals, respectively; platinum needle electrodes were placed into the intercostal musculature to acquire ECG and chest
THE HIGH STATIC AND CHANGING MAGNETIC FIELDS encountered during magnetic resonance imaging (MRI) can induce significant artifactual signals in electrophysiologic recording leads, which can saturate amplifiers or mask physiologic signals.1 We developed a low cost, optically coupled device useful for acquiring physiologic signals during MRI within sleep-waking conditions. In addition to immunity from high magnetic fields, the device minimizes interference to the image signals, and is free of magnetically attractive ferrous material. METHODS The device is enclosed within an aluminum chassis and attached with non-magnetic brass screws. The electronic components (Figure 1) consist of low noise plastic-encapsulated operational amplifiers (Burr Brown OPA627), a set of capacitive-resistance filters (bandwidth of the amplifier was .6-30 Hz, 6 db/octave) and an output light emitting diode (AND 190HAP), for each signal. The amplifier is isolated and balanced to enhance aspects of common mode rejection. A collector lens focuses the light onto a 1 mm single-strand 10 meter optical cable (Edmund Scientific, jacketed optical guide, D2536) for each output. The power supply consists of two standard 9 volt lithium batteries with conventional connectors (Part number U9VL-BP, Ultralife Batteries Inc.), which contain little ferrous material. The fiber transmits signals to a receiver located at the periphery of the magnetic field. The receiver captures the light from
Accepted for publication July 1999 Correspondence: R. M. Harper, Ph.D., Department of Neurobiology, UCLA School of Medicine, 10833 Le Conte Ave., Los Angeles, CA 90095-1763, Tel: (310) 825-5303, Fax: (310) 825-2224, E-mail:
[email protected] SLEEP, Vol. 22, No. 8, 1999
Figure 1.—The system consists of an optical driver and receiver; A Burr-Brown operational amplifier, OPA627, amplifies and sources current to the AND 190HAP light emitting diode that sends the signal over a single strand optic fiber to the photodetector (UDT PIN UV50) in the receiver. Drivers and receivers are replicated for additional channels. Generic (i.e., not precision) resistors and components are used.
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easily, as could overall EEG characteristics such as the presence of high amplitude slow waves and the occurrence of 12-14 Hz "spindle" activity (Figure 2 A). The EEG signals, because of the lower voltage, were more affected by the scanning noise than the ECG traces. Less expensive analog filters could be added to the output circuit, lessening the need for digital filters. Images acquired with and without the presence of recording amplifiers were compared and are shown in Figure 2C and 2D. No statistical differences were found between images under the two conditions. Figure 2.— A. EEG traces from an anesthetized rabbit prior to a scan (Baseline), during a spin echo scan, but before digital filtering (EEG Scan No Filter) and after 8-pole Butterworth digital filtering (EEG Scan With Filter). Bursts of EEG "spindles" (12-14 Hz, dashed lines), characteristic of anesthesia, can be observed on the Baseline trace as well as in the trace after digital filtering; B. ECG captured during a Spin Echo scan.After digital filtering, the R waves can readily be detected for rate assessment; C. T1-weighted coronal scan of a rabbit collected with the device in the scanner and recording signals; D. Scan comparable to C, acquired with the device outside the magnetic field. Although the metallic electrodes placed inside the brain cause geometric image distortion in both C and D conditions (which can be avoided with gold or carbon electrodes), the recording device does not induce noise into the image.
wall excursion. Under anesthesia, the animal was placed within a conventional head coil in a GE 3.0 Tesla scanner. These experiments conformed to the policy of the American Physiological Society. Signals were led from the electrodes to the amplifier with non-ferrous wires. A time series of ten 256 x 128 pixel T1-weighted spin echo image sets, composed of 5 coronal sections, was acquired (TR = 500, TE = 10 msec, Flip angle = 90o, FOV = 20 cm, Thickness = 3 mm, no interslice gap) with and without the electrophysiological recording device. Images were then examined using MedX software (Sensor Systems Inc., Sterling, VA). Magnetically-induced artifacts were minimized by orienting the leads for minimal pickup from the magnetic field, twisting the leads about each other to maximize common mode rejection, and minimizing the lead length.
DISCUSSION The system uses readily available components to provide low cost acquisition of electrophysiologic signals during MRI while introducing little noise to the images being collected. Although ECG recording capabilities are a common accessory for human imaging, capabilities for EEG capture are a costly, third-party upgrade. EEG signal acquisition is not readily available for high-field-strength animal scanners. Physiologic signals acquired during imaging have high frequency artifact added, but that noise could be separated with conventional filtering techniques. Many functional imaging studies require concurrent acquisition of physiologic signals to verify state, or to indicate physiologic outcomes to a particular challenge. The device readily allows such assessment. ACKNOWLEDGMENTS Supported by HL-22418. We thank Rebecca Harper for technical assistance. REFERENCES 1. Ives JR, Warach S, Schmitt F, Edelman RR, Schomer DL. Monitoring the patient's EEG during echo planar MRI. Electroenceph Clin Neurophysiol 1993;87(6):417-20.
RESULTS EEG and ECG signals during baseline conditions and during acquisition of T1-weighted scans are shown in Figure 2A and 2B. Both EEG and ECG signals were relatively unaffected by the static magnetic field during baseline recordings, even with modest subject movements from respiration. The changing magnetic field during scans induced significant transient noise on both the EEG and ECG signals. However, noise components were of significantly higher frequency from the physiological signals of interest (ECG, .5-40 Hz; EEG, .5-20 Hz) that such artifacts were readily separated post-acquisition with Butterworth digital filters. Cardiac R-R intervals could be determined SLEEP, Vol. 22, No. 8, 1999
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