Biomedical Engineering Reference
In-Depth Information
The Design of the Magnetically-Shielded Room.
The shielding factor is used
as the measure that shows the e
ciency of the magnetically shielded room
(MSR). The shielding factor
S
is shown as the ratio of the magnetic field
outside,
H
e
, and the magnetic field inside,
H
i
,oftheMSR:
S
=
H
e
H
i
.
(3.57)
Supposing that the MSR is spherical, with outside diameter
D
, thickness
t
, and relative permeability of the material
μ
, the shielding factor is simply
shown by
S
=
4
3
μt
D
.
(3.58)
The higher the permeability of the material and the thicker the shell of MSR,
the higher is the expected shielding factor.
The permalloy that is used as the shielding material is heavy and expen-
sive. Therefore, to reduce the use of the shielding material, the MSR is made
with a multiple shell structure. However, the structure becomes complicated
with an increasing number of shell layers and becomes uneconomical. For
most practical MSRs, a two- or three-layer structure is adopted.
The shielding effect against the alternating magnetic field depends on the
material and the frequency of the field change. To shield a slowly varying
field generated by cars and trains, a material having an especially high initial
permeability must be chosen. Against an electromagnetic wave of high fre-
quency, a shield must be made of a good-conductivity metal such as copper
or aluminum. Because the nature of a superconductor is to reject a magnetic
force line, if this is used for a shielding material, we can ideally shield the sta-
tic magnetic field from the high-frequency field. However, since the shielding
material must be cooled below its superconducting transition temperature,
this method is not economical and is not of practical use.
Most of the magnetically shielded rooms for biomagnetic field measure-
ment consist of a box of side 2-3 m to house the primary equipment and the
bed inside. The wall consists of two layers of permalloy plate and a single
layer of copper or aluminum plate. The common shielding factor of these
MSRs is approximately 1000 at 1 Hz. The range of magnetic field noises in
the MSR at various sites is shown in Fig. 3.20, with the white noise of SQUID
sensors.
As a special example, there is a soccer ball shaped MSR [17] that has a
shielding factor of 100 000 and four layers.
Active Shielding.
By combining compensation coils with the ferromagnetic
shield, attempts have been made to improve the characteristics of the shiel-
ding in the low-frequency area. This method is called “active shielding” and
is a way of making the magnetic noise zero by adding an artificial magnetic
field that has the same strength in the direction opposite to the detected
magnetic noise.
