Note: Optical receiver system for 152-channel magnetoencephalography

Note: Optical receiver system for 152-channel magnetoencephalography Jin-Mok Kim, Hyukchan Kwon, Kwon-kyu Yu, Yong-Ho Lee, and Kiwoong Kim Citation: Review of Scientific Instruments 85, 116105 (2014); doi: 10.1063/1.4902335 View online: http://dx.doi.org/10.1063/1.4902335 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High-T c superconducting quantum interference device recordings of spontaneous brain activity: Towards high-T c magnetoencephalography Appl. Phys. Lett. 100, 132601 (2012); 10.1063/1.3698152 Note: Unshielded bilateral magnetoencephalography system using two-dimensional gradiometers Rev. Sci. Instrum. 81, 096103 (2010); 10.1063/1.3482154 High bandwidth data recording systems for pulsed power and laser produced plasma experiments Rev. Sci. Instrum. 77, 10F504 (2006); 10.1063/1.2219417 Analog fiber optic transmission link Rev. Sci. Instrum. 72, 3687 (2001); 10.1063/1.1396664 Time-to-space conversion of Tbits/s optical pulses using a self-organized quantum-well material Appl. Phys. Lett. 77, 3487 (2000); 10.1063/1.1328365

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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 116105 (2014)

Note: Optical receiver system for 152-channel magnetoencephalography Jin-Mok Kim, Hyukchan Kwon, Kwon-kyu Yu, Yong-Ho Lee, and Kiwoong Kim Center for Biosignals, Korea Research Institute of Standards and Science, Daejeon 305-600, South Korea

(Received 3 October 2014; accepted 9 November 2014; published online 21 November 2014) An optical receiver system composing 13 serial data restore/synchronizer modules and a single module combiner converted optical 32-bit serial data into 32-bit synchronous parallel data for a computer to acquire 152-channel magnetoencephalography (MEG) signals. A serial data restore/synchronizer module identified 32-bit channel-voltage bits from 48-bit streaming serial data, and then consecutively reproduced 13 times of 32-bit serial data, acting in a synchronous clock. After selecting a single among 13 reproduced data in each module, a module combiner converted it into 32-bit parallel data, which were carried to 32-port digital input board in a computer. When the receiver system together with optical transmitters were applied to 152-channel superconducting quantum interference √ device sensors, this MEG system maintained a field noise level of 3 fT/ Hz @ 100 Hz at a sample rate of 1 kSample/s per channel. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4902335] A magnetoencephalography (MEG) system with superconducting quantum interference device (SQUID) sensors has been applied to diagnose brain diseases in hospitals, to research legal recognition in cognitive science, and to measure precise biomagnetism.1–4 A SQUID MEG system, measuring precise magnetic signals from a human brain, demands a magnetically shielded room (MSR) and a filter system to reduce environmental magnetic or electric noises, and to improve readout signals detected by SQUID sensors.5, 6 When a SQUID MEG system operates together with other electronic devices such as electroencephalography (EEG), visual or acoustic stimulus generator, etc., inside MSR, it may be interfered from these devices because of their ground-loop effect and electric/magnetic interference.7, 8 An optical transmitter module, directly transferring SQUID readout outputs to the outside of MSR by optical cables, eliminates the outlet wires effect and the hardware filter system, but also improves the trouble of installation when an MEG system is assembled together with other electric/magnetic devices in MSR. The transmitter modules release 32-bit readout serial data with slightly-various transfer rates because the optical transmitter modules were regulated by their own master clock frequencies.9 The various master clocks in the modules avoid the distortions of SQUID operation that are easily influenced when the same master clocks are multiplied. However, an optical receiver system has to synchronize these serial data of all transmitter modules in order to send them regularly into an acquisition computer. An optical receiver system has been developed to acquire SQUID readout data from unsynchronized optical transmitters, which consisted of 13 serial data restore/synchronizer modules and a single module combiner. A block diagram of optical receiver system, including optical transmitters and digital input/output (DIO) board, was shown in Fig. 1. A serial data restore/synchronizer module extracted 32-bit readout serial data from 48-bit streaming data of an optical transmitter, and then reproduced the identical serial data 13 times that were synchronized to a reference sample rate of 2048 Sample/s. A module combiner selected a single among 13

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reproduced synchronous serial data in a serial data restore/ synchronizer module, and converted a single of serial data into 32-bit synchronous parallel data. In order to array 152 SQUID readout data in an acquisition computer, a data arranger software piled up 152 data and lined up them in order of channel number. The data arranger software provided 152 SQUID readouts with a voltage of 24-bit resolution and 1024 Sample/s per channel. The optical receiver system together with optical transmitter modules were applied to 152 double relaxation oscillation SQUID (DROS) sensors for measuring human MEG.10 The procedure, operating a transfer rate of 2 kSample/s per channel in an optical transmitter module, was shown in Fig. 2, along with the sequences of analog switch, analogto-digital converter (ADC) conversion, and data discharge time.9 An optical transmitter sent out 48-bit serial data continuously through an optical cable, which data were a single package of serial data: a channel number of 8 bits, a voltage of 24 bits, and a blank of 16 bits. As a single data package flowed with a speed of around 30 μs, the transmitter sent out 2 kSample/s per channel. The transmitter modules for SQUID readouts were regulated by various master clock frequencies to avoid SQUID sensor distortion caused by the influence of multiplied high-frequency noises.11 These modules chose an interval frequency of 5 kHz, comparing to those of other modules. Although SQUID sensors detected the beat-frequency signals of 5 √ kHz as noise signals, their noise levels appeared below 5 fT/ Hz, which were easily cut-off by a simple resistance-capacitance (RC) low-pass filter. The master clock frequencies of 13 transmitter modules had a frequency range of 15.7036–15.7586 MHz for 12 modules of SQUID readouts and EEG outputs, and 15.7286 MHz for the accessory module that did not influence SQUID sensors because it located outside MSR. A serial data restore module converted 48-bit serial data into 32-bit parallel data, on identifying 32-bit channelvoltage serial data after receiving 48-bit serial data that were transferred through an optical cable. A simplified structure for a serial data restore was shown in Fig. 3, where sig-

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FIG. 1. Structure of optical receiver system.

nal extractor, channel identifier, serial/parallel converter were assembled. After separating a data signal and a clock signal from optical serial data, a signal extractor generated a load signal when clock signals were countered to 48 clocks. These three signals were carried to 32-bit serial-parallel converter. In the serial/parallel converter, serial data signals were shifted and registered by clocks and reform clocks so that 32-bit data among 48-bit data were charged with 32-bit parallel data by a load signal, where other 16-bit data among 48-bit data were discarded. A data synchronizer module, transforming 32-bit parallel data into the reproduced serial data that synchronized with a common clock, was shown in Fig. 4. A 32-bit serial/parallel converter provided 32-bit parallel data with Serial data-1, Clock-1, and Load-1 from a serial data restore. A 32-bit parallel/serial converter changed 32-bit parallel data into 13 reproduced serial data that was synchronized with the reference common signals of Serial-Out1N, Clock-N, and Load-N.

FIG. 2. Operating procedure in optical transmitter module for a transfer rate of 2 kSample/s.

FIG. 3. Serial data restore for identification of 32-bit channel-voltage data.

A progressive load signal generator delivered a new load signal of Load-N to the parallel/serial converter for avoiding the collision between Load-1 and Load-N. A module combiner sent 13 channels data by scanning 13 data restore/synchronizer modules once, on selecting a single among 13 reproduced synchronous serial data. A simplified structure for a module combiner was shown in Fig. 5. A 13:1 Mux (multiplex) scanned the serial data of 13 modules in turn by which a single scan was fulfilled. A 32-bit serial/parallel converter turned the selected serial data into 32-bit parallel data. A reference master clock frequency of Clock-N was 13.6315 MHz for supporting a module combiner and data restore/synchronizer modules. A 32-clock counter provided a common load signal Load-N of 425.984 kHz for loading the serial or parallel data of converters. A 4-bit counter generated

FIG. 4. Serial data synchronizer module for reproducing synchronous data.

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FIG. 5. Simplified structure of module combiner and DIO board.

4-bit parallel data to regulate 13:1 Mux after receiving LoadN signal. A data arranger software in a computer collected 32-bit parallel data from DIO board to array them with a single data package of 208 channels in order of channel number. The various transfer rates of optical transmitter modules for avoiding the distortion of SQUID characteristics, induced duplicated channel data in a single data package. These duplicated data occurred every time when the transfer rates did not match with a reference sample rate of 2048 Sample/s. When a transfer rate was 2052 Sample/s, the duplicated data were generated with a rate of 4 Sample/s. A data arranger prevented the duplicated data with deleting a half of 2048 data skipping one among 2048 data for a second, even though a sample rate was reduced into 1024 Sample/s. For the measurement of human MEG, 152-channel MEG system was constructed with 152 DROS sensors, SQUID readouts, and transmission system that was assembled with 10 optical transmitter modules, optical receiver system, and data levels of 152 DROS sensors were arranger software.12 Noise √ measured with about 3 fT/ Hz at 100 Hz on a sample rate

Rev. Sci. Instrum. 85, 116105 (2014)

FIG. 7. MEG patterns of an epileptic measured by 152-channel SQUID MEG system, where signals inner an oval line expressed epileptic evoked peaks, and channel #1 did not work.

of 1024 Sample/s. A noise level of channel #14 among√152 channels was √ shown in Fig. 6, where it was 2.5 μVrms / Hz or 2.8 fT/ Hz at 100 Hz, which was measured for 5 s.9, 13 A signal map of 152 channels detected by this MEG system was shown in Fig. 7, which was a brain signal pattern of an epileptic recorded for 1 s without any software filter. In summary, an optical receiver system consisting of serial data restore/synchronizer modules, a module combiner, and a data arranger software, achieved to acquire SQUID readout voltages when optical transmitter modules sent out their serial data with various transfer rates to avoid a distortion of SQUID characteristics. When the optical receiver system together with optical transmitter modules connected to 152 SQUID √ sensors, this MEG system maintained a noise level of 3 fT/ Hz @ 100 Hz with a sample rate of 1024 Sample/s, which provided sufficient biomagnetic measurement in human MEG. This optical transmission system improved ground loop effect and devices interference when a SQUID MEG system operated together with other electronic devices like EEG system or stimulus generators inside a magnetically shielded room. 1 N.

FIG. 6. Voltage noise level of sensor #14 in 152 DROS sensors.

Nowak, Biomagnetic Instrumentation Magnetism in Medicine (WileyVCH, Berlin, 1998). 2 J. Gross, S. Baillet et al., NeuroImage 65, 349–363 (2013). 3 F. E. Smith, P. Langley et al., Europace 8, 887–893 (2006). 4 M. Hämäläinen, R. Hari et al., Rev. Mod. Phys. 65, 413–497 (1993). 5 D. Drung, in SQUID Sensors: Fundamentals, Fabrication and Applications, edited by H. Weinstock (Kluwer, Dordrecht, 1996). 6 R. L. Fagaly, Rev. Sci. Instrum. 77, 101101 (2006). 7 J. Clark and A. I. Braginski, The SQUID Handbook (Wiley-VCH, 2006), Vol. II. 8 P. Baess, A. Zhdanov et al., Front. Hum. Neurosci. 6, 83 (2012). 9 J. M. Kim, H. Kwon et al., Rev. Sci. Instrum. 84, 125109 (2013). 10 D. J. Adelerhof, H. Nijstad et al., J. Appl. Phys. 76, 3875–86 (1994). 11 T. W. Kang, Y. H. Lee et al., IEEE Trans. Electromagn. Compat. 51, 151– 156 (2009). 12 J. M. Kim, Y. H. Lee et al., Appl. Supercond. 17, 13–19 (2007). 13 Y. H. Lee, K. K. Yu et al., Supercond. Sci. Technol. 22, 114003 (2009).

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