Biomedical Engineering Reference
In-Depth Information
lines are negative values. L and R represent the left and right hemispheres,
respectively. The local minimum and part of the local maximum are observed
on the left hemisphere. From these spatial data and the calibration data
for markers on the head, equivalent current dipoles are estimated using a
spherical model and displayed as a white dot superimposed on the MRI image
(lower three diagrams). The white line stretching from the white dot shows
the direction and the amplitude of the estimated ECD. The position of the
ECD is on the posterior part of the central sulcus of the brain. The result is
in good agreement with anatomical knowledge. The intensity of the ECD is
also calculated to be 9.75 nAm. The reproducibility of an equivalent current
dipole model for measurements on actual patients/subjects is also good and
the spread is in a box of 1-2 mm, thanks to the horizontal dewar, which
eliminates subject vibrations.
Clinical Applications.
With MEG, neural activity may be imaged with
a time resolution of 1 ms or better and a spatial resolution of a few to se-
veral mm. MEG is basically noninvasive measurement, so it gives profound
benefits not only to the researcher but also to the people to be investigated.
In clinical application, MEG is used to study psychiatric conditions such as
epilepsy, to carry out presurgical localization of brain function, and to eva-
luate the level of recovery of brain function. In scientific research, MEG is
used to study human cognitive functions or so-called higher brain functions.
In this section, the actual clinical applications of MEG are described.
They are neurosurgery, ophthalmology, otorhinolaryngology, neurology, the
examination of epilepsy, and psychiatric conditions.
Presurgical Localization of Brain Function.
A 31-year-old patient with left
frontal lobe astrocytoma was examined before surgical operation in order to
identify the primary somatosensory area (Fig. 3.58). This information helps
a medical doctor to avoid damage to the important parts of the brain. Before
surgery, electrical stimulation to the median nerve in the right-hand joint was
applied to the patient and the position of the primary somatosensory area in
the left brain was identified with MEG.
Postoperative examination was done to confirm that the important part
had survived.
Electrical stimulation was applied to the median nerve in the right hand
of the patient. The signals on the affected side of the brain were more or
less weak, but a somatosensory evoked field was observed 20.5 ms after sti-
mulation, as shown in Fig. 3.59, which proves that the important part has
survived and the sensory has remained.
Objective Examination.
MEG sometimes can examine defects in brain func-
tion that cannot be detected by other modalities. A 56-year-old female was
examined, who complained about the absence of the right half of the vi-
sual field after injury in a trac accident. MR images failed to identify any
