Biomedical Engineering Reference
In-Depth Information
−
t
/
τ
(8.32)
xy
M
=
e
2
τ
2
relaxa-
tion. Relaxation behavior is strongly dependent on the physicochemical environ-
ment of the nucleus and hence on the tissue type in which the nuclei are located.
This is an important source of image contrast in biomedical MRI. During the in-
terval between excitation and acquisition of an echo, magnetization in different
parts of the sample will be undergoing
Thus the NMR signal detected by the RF coil rapidly decays due to
τ
2
decays at different rates depending on the
environment (i.e., the tissue in which it is located). Thus by varying this interval,
known as the echo time,
τ
E
, it is possible to vary the degree of
τ
2
weighting (i.e., the
extent to which differences in
τ
2
affect the appearance of the image). In addition, as
noted above, it is necessary to repeat acquisition a number of times to allow phase
encoding. During the interval between repetitions, known as the repetition time,
τ
R
, magnetization undergoes differential
τ
1
recovery. By varying this interval, the
user can alter the extent of
τ
1
weighting.
τ
1
and
τ
2
weighting result in very differ-
ent image appearances.
τ
1
-weighted images provide excellent soft tissue contrast,
and material with long
τ
1
, such as cerebrospinal fluid (CSF) in the ventricles of the
brain, appears dark. This type of imaging is often used to depict anatomical struc-
tures. On
τ
2
-weighted images, tissues with increased water content appear bright.
This includes CSF, but also pathological processes such as neoplasm, inflammation,
ischemia, and degenerative changes. Thus
τ
2
-weighted images are often used to
highlight areas of disease.
The advantage of MRI is balanced by the fact that MRI images have less reso-
lution than those from CT. MRI complements X-ray CT in providing different
information. X-ray CT offers details that depend on the density of body structures:
the denser an object, the more X-rays it blocks and the whiter its appearance in an
X-ray or a CT image. MRI responds to the prevalence of particular types of atoms
in the body. Fatty tissues, which have little water, appear bright, while blood vessels
or other fluid-filled areas are dark. MRI is particularly useful for seeing details in
the brain. Gray matter has more fluid than white matter, making it easy to distin-
guish between the two. However, the resolution of a conventional clinical MRI is
in the order or 1-2 mm and it is not ideal for cellular-level imaging. In 1992, the
functional MRI (fMRI) was developed to allow the mapping of the various regions
of the human brain. fMRI detects changes in blood flow to particular areas of the
brain. It provides both an anatomical and a functional view of the brain. Three
types of dynamic brain functions are quantifiable by noninvasive imaging: blood
flow changes in response to sensory or mental activity, neurochemical activity, and
metabolic activity of energy consumption (e.g., glucose and oxygen consumption).
Radiotracer techniques have shown that blood flow in the capillary bed of small
regions of the brain increases 2% to 30% when that region is called on to in-
crease nerve activity by some external or internal stimulus. The more recent advent
of MRI methods for detecting a signal change associated with this local activity
makes it possible to measure mental functioning noninvasively without the use of
radiation.
