Biomedical Engineering Reference
In-Depth Information
Transformation ratio
α
k
V
k
S
k
n
s
k
Known parameters
r
s
k
Sensor array
In liquid helium
n
c
C
i
r
c
Unknown parameters
j
Circular current
Fig. 3.23.
Verificationofcalibratedsensorparameters
Table 3.1.
Examples of calibration error
x
(mm)
y
(mm)
z
(mm)
Real value
0.000
−
75.000
0.000
Estimated value
0.181
−
75.302
0.290
n
x
n
y
n
z
Real value
1.00000
0.00000
0.00000
Estimated value
0.99999
0.00893
−
0.00177
coil position (
x
c
,y
c
,z
c
) and the coil direction
n
c
.Thenwehavecompared
this estimation with the real coil position and direction. Table 3.1 shows the
results of this procedure, illustrating that the real position and the estimated
current source position are in quite good agreement. For a circular current
source, the estimation error is
±
0
.
3 mm as shown in Table 3.1.
Alignment of the Field Distribution and Anatomical Information.
The mea-
sured biomagnetic response is not of much use unless the data is proposed
in reference to the anatomical position of the subject. In case of a magneto-
encephalography measurement, the position of the head is calibrated with
five markers, five sets of small coils, attached to the heads which is visible to
SQUID sensor array. This means that the position of the head is identified
in “the coordinate system of the SQUID sensor array”, which is calibrated
through the procedure described in the previous section. Different five mar-
kers visible to MRi (Magnetic Resonance Imaging), which contain water, are
