Biomedical Engineering Reference
In-Depth Information
inaccurate to use for the accurate geometrical design of radiation
beams and, even if the MRI images are clinically superior, a so-called
“planning CT” taken with the patient immobilized and positioned as
for treatment is generally considered necessary, in addition, for
planning radiation therapy.
The visualization of bone in MRI
Compact bone, with its high concentration of calcium, contains very
few free protons and, hence, yields very
little, if any, MR signal. This lack of
signal from compact bone has given rise
to the mistaken impression that MRI does
not “image” bone. In fact, there is a very
high contrast between bone and neigh-
boring tissue; it is just that, while bone
appears white against a darker back-
ground in CT images, it appears black
against a lighter background in MRI
images. One has but to reverse the MRI
Figure 3.13. MR image
with black and white
reversed, showing the
bone detail inherent in
image as in Figure 3.12 to see how strong
the bone contrast is. To the extent that
bone is eroded by tumor, the signal from
the tumor cells will be visible and the
MRI. Original MRI cour-
tesy of M. Kessler.
bone erosion will be evident.
Clinical information
First and most obviously, as already discussed, CT and MRI scanners
measure different properties of matter. Therefore, they provide the
clinician with different information concerning the biological
properties of the patient's tissues. Whether one or the other provides
the more valuable information from the point of view of clinical
interpretation depends on the site and the tissues being evaluated and
many other factors. However, the two imaging modalities often com-
plement each other, building up a picture of the patient's disease
that neither alone can provide. Here, courtesy of N. Liebsch of the
Massachusetts General Hospital, are two examples of complementary
studies.
Case 1
Figure 3.14 shows two scans of a patient with a low-grade
chondrosarcoma. Figure 3.14a is an axial CT section of the patient,
injected with intravenous contrast medium. Figure 3.14b is an
