SQUID the Dewar contains liquid helium 2.2. OPM-based

SQUID is by far the most commonly used magnetometerfor MEG imaging. It consists of two superconductors separated by thininsulating layers to form two parallel Josephson junctions as shown inFigure 2. The device may be configured as a magnetometer to detect incrediblysmall magnetic fields — small enough to measure the magnetic fieldsin living organisms. Due to the extremely high sensitivity, squids have beenused to measure the magnetic fields in mouse brains to test whether there mightbe enough magnetism to attribute their navigational ability to an internalcompass. Figure2: Josephson Junction Schematic Since the invention of the first SQUID in 1964, thetechnology of SQUID has undergone a roadmap of development and now has becomemature and commercially available.

Using SQUID technology, arrays of a few hundredsof magnetic sensors are constructed to surround the whole head capturing the magneticsignals from cerebral cortex and other brain functional areas 12. SQUID-basedMEG systems have extremely high sensitivity which makes them capable to detectfetal brain signals in range of 10-14 tesla 13. One major limitation for SQUID-basedsystem is the need for liquid helium (He), which brings down the temperatureclose to absolute zero Kelvin to maintain the operation of superconductiveloops. The Dewar containing the SQUID sensors and the liquid He is formed intoa helmet shape (Figure 2) to distribute sensors around the head. The Dewarwalls of the MEG helmet are about 2cm thick to provide sufficient thermalinsulation.

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In addition, the helmet is rigid and sized for large adult heads toaccommodate the largest number of subjects. As a result, MEG measurements inindividuals with small head size, particularly children, can have manycentimeters of head-to-sensor separation. Because dipolar fields decay with theinverse cube of distance, large distances between the sensor array and brainnegate the advantage of MEG in source localization 14. Figure3: Typical SQUID-based MEG instrumentation, the Dewar contains liquid helium 2.2.

OPM-based MEG ImagingOptically pumped magnetometers (OPMs) are a potentialreplacement for low critical temperature (low-TC) SQUID sensors that requireliquid helium for maintaining operation. OPM-based MEG system was firstdemonstrated by the Romalis group and their OPM design used broadband diodelaser to pump the atomic sample 15. The focus on more recent OPM development ison modular design where optical fibers are used to bring light to the OPM. Themotivation of small modular OPMs is to reduce the sensor-to-head distance thusincrease the sensitivity to neurological signals. In OPMs an atomic gas is illuminated with light withcertain photon energy to excite electrons from singlet to triplet state whichis resonant with electronic transitions in the atom.

The OPMs used for MEGrecordings so far use electron-spin resonances in alkali atoms in the vaporphase. These atoms such as K, Rb and Cs have a single valence electron thatdetermines most of the properties of interest. Due to their electron spin andmagnetic moment, the spin processes around a magnetic field at a well-definedfrequency, the Larmor frequency. The atomic spin is illustrated in Figure 2. Photonsfrom a circularly polarized laser beam pumps the atoms while a second laserbeam probes the magnetic field-dependent spin orientation through polarizationrotation. Figure2: Electron spin and magnetic moment by laser pumping Most atomic magnetometers use a polarized alkali-metalvapour (K, Rb, Cs), and their transverse spin relaxation time is limited byspin-exchange collisions between alkali atoms, and the actual sensitivity wasestimated to be 1.8 fT Hz-1/2 with a bandwidth of about 1 Hz and ameasurement volume of 1,800 cm3 16. An extension of an OPM usingcircularly polarized laser beam was developed wherein the pump and probe laserbeams travel collinearly through the senor 17.

This operation is so callspin-exchange relaxation-free (SERF) magnetometer. The SERF magnetometeroperates at a high density of the Rb vapor to increase the frequency of atomiccollisions and near zero magnetic field to suppress the coherence throughspin-exchange collision. Under these conditions the sensitivity can be greatlyenhanced to pick up magnetic signals necessary for MEG.    2.

3 SQUID and OPM ComparisonSQUID is by probably by far the most sensitiveinstrument known to mankind. The ultimate sensitivity is reached with low temperature superconductorSQUIDs working in liquid helium at a temperature close to 0K. In theseconditions, SQUIDs present an equivalent energy sensitivity that approaches thequantum limit 18. However,the cryogenic environment to maintain SQUID operations imposes several problems:First, SQUID-based MEG systems are bulky and immobilized. Second, thermalisolation between the sensors and scalp of the subject is needed. Last, aSQUID-based sensor array is not adjustable to individual head size and shape,which further increases the average distance between sensors and scalp and thusdegrade the SNR. OPMmeasures the transmission of laser light through a vapor of spin-polarizedalkali atoms.

Incomparison with a SQUID, OPM does not require a cryogenic environment. Thedevice can be operated at room and sensors can be placed closer to the scalpsurface. Robust and small OPM sensors are also recently available that can beplaced flexibly around the head. Two important performance characteristics ofOPM sensors are sensitivity and bandwidth, and these characteristics are mainlydetermined by the spin coherence time of the polarized atoms. The theoreticalsensitivity of OPM can reach to 10-15T in the range of 10-150Hz, forhigher frequencies the performance is limited by quantum shot noise, and thisusually limits the intrinsic bandwidth to below 500Hz.

Figure 3 shows anexample of the OPM monitoring the brain signal of a patient.


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