Elemento represor E-pal.

The problem often encountered in MEG is that recordings of neuromagnetic fields are contaminated by environmental and instrumental magnetic noise sources. In the

laboratory environment where computers, monitors, amplifiers and the like are commonplace, the magnetic noise is several orders of magnitude higher than the neuromagnetic signals in which we are interested (see table 4.1). Low frequency artefacts are a particular problem because they are difficult to shield and they act to contaminate the frequency range of the neuromagnetic signals themselves. For these reasons effective shielding against magnetic noise is a vital part of MEG.

Flux Density (fT) Source

1019 Earths steady magnetic field

109 Laboratory noise

108 Urban Noise (traffic etc.)

107

Geomagnetic noise

106

Magnetised lung contaminants

105 Abdominal Currents

104

Cardiogram, oculogram

103 Epileptic and spontaneous brain activity

102

Cortical evoked activity

10 SQUID noise

1 Brainstem evoked activity

Table 4.1: Sources of magnetic noise and their amplitude (adapted from [3])

Shielded Rooms

The simplest and most effective method of reducing the effect of external magnetic noise is to place the detector system in a magnetically shielded room. At the Wellcome trust laboratory for MEG studies, this is achieved by the use of layers of µ -metal and Aluminium. µ-Metal typically has permeability —80,000 [2], and works to shield the detector system from external magnetic fields whilst the Aluminium layers, which are sandwiched between the layers of µ -metal, serve as an eddy-current and RF shield. When an external magnetic field impinges on the room it follows a path of greatest permeability, namely through the walls, around, and away from the detector system. This

shielding is most effective at high frequency and at 100 Hz, signals can be attenuated by

as much as 50 — 60 dB. However, at low frequency <0.1 Hz, this is reduced to —20 —

30 dB.

Gradiometers

The levels of attenuation provided by the shielded room are still not sufficient for making measurements with a simple SQUID magnetometer system, since the magnetic noise penetrating the shielded room is still several orders of magnitude larger than the neuromagnetic signals required. Further isolation of the neuromagnetic signals can be achieved by a simple modification to the magnetometer system. The strength of neuromagnetic fields follows approximately the inverse square law, meaning close to the field source, the field gradient is large whereas far from the source, the field gradient is much less. This fact can be exploited in MEG by measurement of the field gradient, rather than magnetic field itself. Traditionally, a small modification to the coil geometry would be made such that an additional compensation coil is introduced in series with Lp,

(see Figure 4.10) making the recorded signal effectively dB / dr . Gradiometers of this kind are physically wound coils where a single detector consists of two loops of wire wound in opposition. A time varying magnetic field passing through the detector induces oppositely directed currents, which partially cancel out. The net current measured is therefore representative of the field gradient. Physical gradiometers come in two forms: Axial gradiometers comprise two oppositely wound coils displaced from one another in the radial direction, meaning that the field gradient is also measured in the radial direction. Planar gradiometers have their coils shifted tangentially, meaning that the field gradient is measured perpendicular to the radial direction. Gradiometers allow for effective noise reduction in recorded MEG signals, however, in practice they cause the system to become less sensitive to deeper sources than to shallow sources.

In the CTF system, the idea of field gradients is taken further by the use of a novel synthetic gradiometer system. Here radial field gradients measured using physical axial gradiometers, however this information is combined with information taken from several distal reference measurements in order to allow a high order synthetic gradiometer

measurement. In order to illustrate the principle of the synthetic gradiometer system, let us first consider the simple example of the first order gradiometer synthesized from a primary magnetometer sensor at position r, and a three-component vector reference magnetometer at position r'. This is shown in Figure 4.10.

X i

Figure 4.10: Experimental set up in a first order synthetic gradiometer system.

The primary magnetometer will measure magnetic field perpendicular to the plane of the coil. Therefore, if the coil normal vector is P, the gain of that sensor ap, and the external

magnetic field B(r), then the measured field would be given by mp (r) = ap[P.B(r)]. If the

three components of the vector magnetometer are orthogonal, and have the same gain, aR,

then its output will be given by R_k (r')= a_Rk(e) where k = 1,2,3 , B_k (r') are the

three orthogonal components of magnetic field at position r' and R_k (r') form the

magnetometer output vector RH. The first order gradiometer output is given by:

g(') = mp(r)—cr (P.R(r9) [4.23]

aR

Expansion of the magnetic field using the first two terms of a Taylor series about the origin gives:

3

B(r')= B(r)+E—_, (x

k,

—x

k

)

[4.24]

k=i

axk

Where

x

k represents the three orthogonal components of

r

and likewise

x

k ' represent

the three orthogonal components of

r' .

This can be rewritten in terms of the first gradient tensor, i.e.

B. — B

x1

OB

ER

.

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