Biomedical Engineering Reference
In-Depth Information
mobile molecules (water and fat), with the contrast between structures in the
images determined by the visible proton density and the relaxation times of
excited spins in the tissues and, in some cases, also by flow, diffusion, and
other parameters. X-ray CT provides a map of x-ray attenuation within the
body.
MR and CT scanners both produce images of anatomical structure, but
the images they generate look quite different. One of the most striking dif-
ferences is that cortical bone has high x-ray attenuation, so is bright in CT,
whereas cortical bone contains virtually no MR visible protons, so is black
in MR. MR images also tend to have better soft tissue contrast than CT.
When imaging the brain, for example, MR images tend to have much higher
contrast between gray matter, white matter, and cerebrospinal fluid (CSF)
than do CT images. This contrast in the MR image can be manipulated in
many ways by changing the timing and strength of the various radiofre-
quency and gradient pulses.
Although there have always been arguments for the inherent superiority of
one or the other of these modalities, it has been the case for many years that
both are considered essential in a well equipped modern hospital, because
the information they provide is complementary. As a result, patients are fre-
quently imaged with both for the purpose of diagnosis, or during the workup
for treatment. The complementary nature of MR and CT images and the reg-
ular use of both modalities to acquire images from the same patient resulted
in the combination of MR and CT images as one of the first applications of
medical image registration.
1-3
All the early applications of MR-CT registra-
tion were within the head, and this remains the predominant application.
Early registration methods required considerable user interaction, but MR and
CT images of the head can now be registered automatically in a matter of sec-
onds or minutes.
In this chapter, the technical issues associated with acquiring MR and CT
images for registration are described, and clinical applications in which reg-
istration has been shown to have benefit are discussed. The majority of this
chapter relates to registration of MR and CT images of the head, with other
applications described in Section 10.3.4.
10.2
Technical Issues
10.2.1
Image Acquisition
10.2.1.1
Field of View
MR and CT images of a region in a patient frequently have quite different
fields of view in the direction perpendicular to the slice plane (the “through-
slice direction”). Due to the risks associated with the ionizing radiation
used in CT acquisitions, the number of slices collected is kept to a minimum.
