Biomedical Engineering Reference
In-Depth Information
B(t)
V(t)
C
R
S
Output
Integrater
k
turns
Fig. 3.3.
the configuration of a search coil
vector at the search coil, the value measured by the magnetometer is the
inner product of two vectors
B
(
t
)and
N
:
B
(
t
)=
B
(
t
)
ยท
N
.
(3.9)
Most of the magnetometers describe later, such as the search coil, the
fluxgate magnetometer, and the SQUID magnetometer, are needed to mea-
sure a component of the vector magnetic field as an inner product of two
vectors
. As important information for further analysis, not only
the sensitivity of the magnetometer, but also the position and direction of
the sensor should be recorded.
B
(
t
)and
N
3.1.8
The Proton Magnetometer and Other Magnetometers
On the other hand, a proton magnetometer measures the direct value of the
intensity of the magnetic field; in other words, the absolute value of the vector
magnetic field. The proton magnetometer uses the phenomenon of nuclear
magnetic resonance (NMR). A proton in water has spin, and if the proton
is superposed on an artificial dc magnetic field, the spin shifts to a certain
angle, which is determined by the summation of the ambient and the artificial
magnetic fields. If we erase the artificial magnetic field stepwise, the shifted
spin comes back to the initial state: this is called a relaxation process. During
the process, the proton emits an electromagnetic wave whose frequency is
proportional to the intensity of the ambient magnetic field. Therefore, if we
count the wave of the relaxation process, we could calculate the intensity of
the magnetic field. This is called a proton magnetometer. The concept of the
configuration of the proton magnetometer is shown in Fig. 3.4.
There are many magnetic measurement devices: the compass magneto-
meter, the fluxgate magnetometer (Fig. 3.5), the optically pumped magne-
tometer, the Hall effect magnetometer, and the SQUID magnetometer. The
