Biomedical Engineering Reference
In-Depth Information
32
In the brain, MR techniques such as
functional studies or perfusion imaging can also provide information about
eloquent areas of the brain which would have significant consequences for
the patient if damaged, or the physiology of an already treated tumor, which
may influence whether further treatment is required. When a tumor is irradi-
ated, the margins of the radiation beams carefully and precisely calculated on
the planning images must correspond exactly spatially to the beams used to
irradiate the patient. The images used for planning purposes must be geo-
metrically accurate or as free from distortion as possible. MR is subject to dis-
tortions to a greater extent than CT as described in Chapter 5. CT has a
further important advantage over MR in that the intensities of image voxels
(measured in Hounsfield units) represent electron density and can be used to
calculate dose distributions directly. These two factors have limited the use
of MR scans so far for radiotherapy planning. Registration of CT and MR
images provides one way of overcoming these problems and utilizing the dif-
ferent information from both modalities to optimize treatment.
adjacent normal tissues and structures.
6,26,33,34
10.3.4
Applications outside the Head
Techniques for registering MR and CT images are almost invariably restricted
to finding a rigid-body or affine transformation. Outside the head, a nonrigid
transformation is normally necessary because of soft tissue deformation result-
ing from change in patient positioning, respiration, etc. When registering MR or
CT images with PET or SPECT images, some parts of the body such as the pelvis
can be treated as rigid bodies because careful patient positioning can make
tissue deformation smaller than the resolution of the PET or SPECT images. For
MR-CT registration, this assumption is not valid. There is nevertheless consid-
erable interest in registration of MR and CT outside the head for staging cancer
and planning radiotherapy; for example, in the spine and prostate.
10.4
Conclusions
The registration of MR and CT images was one of the first applications of
medical image registration. Despite a large number of algorithms devised for
MR and CT registration, these algorithms are not used routinely outside a
small number of highly specialized centers. Furthermore, the applicability of
MR-CT registration is restricted primarily to the head by current algorithms
that can only determine rigid-body or affine transformations. MR-CT regis-
tration is likely to become more widely used in the future. The driving factors
are fully digital x-ray departments that make access to the data more straight-
forward, increasing use of image-guided surgery and stereotactic radiosurgery
systems that include registration software, and greater use of MRI and CT as
interventional modalities. When MR or CT is used as an interventional
modality, it can nevertheless be desirable to have access to accurately registered
