THE MEG SYSTEM


The UUMSI MEG system is a "Vectorview" from Elekta Neuromag Oy in Helsinki (Finland). It allows for simultaneous measurement of 306 magnetoencephalography (MEG) and 64 electroencephalography (EEG) channels. Human subjects can be scanned in fully supine and seated positions. The MEG system is in a magnetically shielded room made from Imedco (Switzerland).

Sensor Array

  • Field distribution by 510 coils at distinct locations. Coils configured into 306 MEG channels.
  • The coil configuration optimally combines the focal sensitivity of 204 planar gradiometers and the widespread sensitivity of 102 magnetometers.
  • Accurate geometry and excellent balance of the sensors provided by thin-film technology.
  • Interference compensation by on-line Signal Space Projection (SSP) using the sensor array as a reference; involves no increase of white noise.
  • Whole-head design with realistic shape based on standard (EN960:1994).


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Data Acquisition

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  • Up to 370 channels can be sampled simultaneously. Distributed, upgradable architecture based on multiple parallel real-time computers.
  • Digital signal processors to ensure precise and identical filtering and sampling of all channels
  • Resolution of conversion: 24 bits/sample, samples are stored on disk as 16-bit words.
  • Sampling rate for 306 MEG and 64 EEG channels: 5kHz.
  • Fully integrated EEG systemwith 60 single ended and 4 differential channels.
  • DC and AC measurement for MEG and EEG.
  • 16 digital trigger lines, 8 auxiliary analog input channels.

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Cryogenic System

  • Low boil-off rate in both measurement positions enabling a refill interval of 7 days.
  • Safety exhaust line to ensure patient safety.
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Patient Positioning System

  • Measurement positions: fully supine and seated signals.
  • Mobile patient bed with adjustable upper bed.
  • Mobile patient chair. Rigid gantry minimizes vibration artifacts; head is supported by the helmet itself to gain minimal distance between functional brain areas and the sensors.
  • Optional pediatric chair insert.

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Head Phantom

  • 32 current dipoles down to 34 mm from phantom origin and 4 magnetic dipoles.

System Noise

  • Low noise gradiometers: white noise typically 3 fT/(cm√Hz)
  • Low noise magnetometers: white noise typically 3 fT/(√Hz)
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Magnetically Shielded Room

A magnetically shielded room (MSR) is required for medical applications of MEG where the equipment's highly sophisticated sensors detect and measure biomagnetic signals which emanate from the human brain. The MSR is able to shield these sensors from the site's ambient magnetic and electromagnetic noise which could mask the signals or make them less reliable.

The MSR is analogous to a vault, designed by using layers of non-ferrous metals and special alloys that attenuate (absorb) the large magnetic and electric stray fields which may originate from a multitude of outside sources.

The MSR door is hinged to swing outward. The door consists of two independent leaves which are mechanically connected. The 2000 mm (6'-6") high and 1000 mm (3'-3") wide clear opening with a nearly flush threshold provides easy access for wheelchairs, gurneys and dewars. A pneumatically assisted lock/unlock mechanism makes operation easy.

Imedco's MSR is designed to guarantee a specific attenuation or shielding factor, which is always within the published requirements of the medical equipment supplier.


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Analysis Software

UUMSI offers several different software packages for MEG/EEG data analysis and MRI integration:

  • Elekta Neuromag software package for data aquisition and analysis (HP Unix platform, soon to be changed to Linux)

Elekta Neuromag Software Package

Data Acquisition

  • Control and display of both MEG, EEG, and trigger channels on acquisition workstation.
  • Data acquired continuously with external events marked on digital trigger lines.
  • Real-time selective averaging with adjustable on-line display. Supports up to 20 response categories.
  • Real-time artifact rejection for MEG/EEG, and EOG/EMG/ECG channels.
  • Configurable sets of 32 MEG/EEG channels can be monitored continuously.
  • Measurement positions: fully supine and continuous recording of head position signals.

Data Plotting

  • Display and plotting of evoked-response data or segments of continuous data.
  • Multiple data sets can be overlaid and displayed simultaneously. Mathematical operations between data sets supported by source estimation.
  • Both spherically symmetric and boundary element conductor models supported.
  • Single moving dipole and time-varying multiple dipole analysis.
  • Display and forward modeling of EEG signals in 1- or 3-layer spherically symmetric conductor model.
  • Simulation of signals produced by multiple time-varying dipole sources signal processing.
  • Several abstraction levels including windowing interface, functionality accessible through commands, and full programmability in LISP.
  • Extensive set of mathematical functions including FFT, power- and cross-spectrum estimates, template matching, matrix operations, off-line averaging, and slow wave and spike analysis.
  • Tools available to build up virtual channels, e.g. for single trial analysis or for virtual depth electrode purposes.

MRI Integration and Report Composer

  • MRI dipole overlay and fast browsing through the interpolated images with zooming and panning.
  • MRI segmentation for 3-D surface rendering and for boundary element conductor models.
  • Electronic 'canvas' for transporting graphics and illustrations from other software modules using drag-and-drop technique. Supports several different layouts and commonly used export data formats.
  • MRI import/export functions using DICOM 3.0
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