Biomedical Engineering Reference
In-Depth Information
Dichomatic
Mirror
Mirror
Laser
CDD
Scope
Cells
Figure 10.3:
Optical mapping hardware.
10.2.2 Functional Magnetic Resonance Imaging
Magnetic Resonance Imaging (MRI) is a technique widely used in neurology clinics for its ability (un-
like X-rays) to image soft tissue and (unlike ultrasound) penetrate the bone of the scull. At its most
fundamental level, MRI is a three-step process as shown in Fig. 10.4 . First a very large ( 1-3 Tesla)
magnetic field,
B
0
, is applied. The effect is to align all protons (
H
+
)tothe
B
O
field. Since
H
+
ions have
a natural spin, they continue to spin like a top even under the influence of the
B
0
field. Second, a fast
radio frequency pulse,
B
1
, is introduced perpendicular to the
B
0
field. The effect is to knock the aligned
spinning molecules into a wobbly orbit. Lastly, the image is created by detecting how these wobbly orbits
decay back to being aligned with the
B
0
field. The decay rate is called the
T
1
time and can be detected by
the flow of current induced in a coil of wire. Superimposed upon the
T
1
decay is a second decay called
T
2
.
This second decay is a result of interactions between the spin of neighboring molecules.Most importantly
in creating medical images, the
T
1
and
T
2
times are dependent upon the type of material.
B
1
T1 Decay
Z
B
0
B
0
B
0
Spin
Figure 10.4:
Overview of the biophysics of MRI.
There are many subtleties to theMRI technique that have been skipped over, but impressive images
of soft tissue can be made quickly in 3D at resolutions approaching 2
mm
3
, allowing for fast diagnosis of
