SQUID magnetic fields in mouse brains to test

SQUID is by far the most commonly used magnetometer
for MEG imaging. It consists of two superconductors separated by thin
insulating layers to form two parallel Josephson junctions as shown in
Figure 2. The device may be configured as a magnetometer to detect incredibly
small magnetic fields — small enough to measure the magnetic fields
in living organisms. Due to the extremely high sensitivity, squids have been
used to measure the magnetic fields in mouse brains to test whether there might
be enough magnetism to attribute their navigational ability to an internal
compass.

Figure
2: Josephson Junction Schematic

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Since the invention of the first SQUID in 1964, the
technology of SQUID has undergone a roadmap of development and now has become
mature and commercially available. Using SQUID technology, arrays of a few hundreds
of magnetic sensors are constructed to surround the whole head capturing the magnetic
signals from cerebral cortex and other brain functional areas 12. SQUID-based
MEG systems have extremely high sensitivity which makes them capable to detect
fetal brain signals in range of 10-14 tesla 13.

One major limitation for SQUID-based
system is the need for liquid helium (He), which brings down the temperature
close to absolute zero Kelvin to maintain the operation of superconductive
loops. The Dewar containing the SQUID sensors and the liquid He is formed into
a helmet shape (Figure 2) to distribute sensors around the head. The Dewar
walls of the MEG helmet are about 2cm thick to provide sufficient thermal
insulation. In addition, the helmet is rigid and sized for large adult heads to
accommodate the largest number of subjects. As a result, MEG measurements in
individuals with small head size, particularly children, can have many
centimeters of head-to-sensor separation. Because dipolar fields decay with the
inverse cube of distance, large distances between the sensor array and brain
negate the advantage of MEG in source localization 14.

Figure
3: Typical SQUID-based MEG instrumentation, the Dewar contains liquid helium

 

2.2. OPM-based MEG Imaging

Optically pumped magnetometers (OPMs) are a potential
replacement for low critical temperature (low-TC) SQUID sensors that require
liquid helium for maintaining operation. OPM-based MEG system was first
demonstrated by the Romalis group and their OPM design used broadband diode
laser to pump the atomic sample 15. The focus on more recent OPM development is
on modular design where optical fibers are used to bring light to the OPM. The
motivation of small modular OPMs is to reduce the sensor-to-head distance thus
increase the sensitivity to neurological signals.

In OPMs an atomic gas is illuminated with light with
certain photon energy to excite electrons from singlet to triplet state which
is resonant with electronic transitions in the atom. The OPMs used for MEG
recordings so far use electron-spin resonances in alkali atoms in the vapor
phase. These atoms such as K, Rb and Cs have a single valence electron that
determines most of the properties of interest. Due to their electron spin and
magnetic moment, the spin processes around a magnetic field at a well-defined
frequency, the Larmor frequency. The atomic spin is illustrated in Figure 2. Photons
from a circularly polarized laser beam pumps the atoms while a second laser
beam probes the magnetic field-dependent spin orientation through polarization
rotation.

Figure
2: Electron spin and magnetic moment by laser pumping

Most atomic magnetometers use a polarized alkali-metal
vapour (K, Rb, Cs), and their transverse spin relaxation time is limited by
spin-exchange collisions between alkali atoms, and the actual sensitivity was
estimated to be 1.8 fT Hz-1/2 with a bandwidth of about 1 Hz and a
measurement volume of 1,800 cm3 16. An extension of an OPM using
circularly polarized laser beam was developed wherein the pump and probe laser
beams travel collinearly through the senor 17. This operation is so call
spin-exchange relaxation-free (SERF) magnetometer. The SERF magnetometer
operates at a high density of the Rb vapor to increase the frequency of atomic
collisions and near zero magnetic field to suppress the coherence through
spin-exchange collision. Under these conditions the sensitivity can be greatly
enhanced to pick up magnetic signals necessary for MEG.

 

 

 

2.3 SQUID and OPM Comparison

SQUID is by probably by far the most sensitive
instrument known to mankind. The ultimate sensitivity is reached with low temperature superconductor
SQUIDs working in liquid helium at a temperature close to 0K. In these
conditions, SQUIDs present an equivalent energy sensitivity that approaches the
quantum limit 18. However,
the cryogenic environment to maintain SQUID operations imposes several problems:
First, SQUID-based MEG systems are bulky and immobilized. Second, thermal
isolation between the sensors and scalp of the subject is needed. Last, a
SQUID-based sensor array is not adjustable to individual head size and shape,
which further increases the average distance between sensors and scalp and thus
degrade the SNR.

OPM
measures the transmission of laser light through a vapor of spin-polarized
alkali atoms. In
comparison with a SQUID, OPM does not require a cryogenic environment. The
device can be operated at room and sensors can be placed closer to the scalp
surface. Robust and small OPM sensors are also recently available that can be
placed flexibly around the head. Two important performance characteristics of
OPM sensors are sensitivity and bandwidth, and these characteristics are mainly
determined by the spin coherence time of the polarized atoms. The theoretical
sensitivity of OPM can reach to 10-15T in the range of 10-150Hz, for
higher frequencies the performance is limited by quantum shot noise, and this
usually limits the intrinsic bandwidth to below 500Hz. Figure 3 shows an
example of the OPM monitoring the brain signal of a patient.