Biomedical Engineering Reference
In-Depth Information
image has its resolution degraded. Software for viewing the registered
images needs to take this into account by providing tools for viewing the
combined images in a variety of ways. For example, giving the user a choice
of overlaying MR on CT (or vice versa) or allowing the images to be com-
bined to form a new image that has higher resolution than either modality.
10.2.1.3
Image Distortion
A detailed discussion of scanner distortion is provided in Chapter 5. It is
worthwhile here, however, to consider the main causes of distortion in MR-CT
registration.
MR images can have distortion resulting either from errors in the gradient
systems or as a result of field inhomogeneity introduced at the boundary
between tissues with different magnetic susceptibility properties, such as
between soft tissue and air and, to a lesser extent, between soft tissue and bone.
This second type of distortion is common in the head, for example in the fron-
tal lobe of the brain near the frontal sinus or the temporal lobes of the brain
near the maxilliary and sphenoid sinuses. While methods to correct for this
distortion are available,
4,5
they are seldom used clinically. Object-dependent
distortion in common diagnostic MR sequences (excluding echo planar
imaging) is greatest in the readout gradient direction and can be reduced, at
the expense of signal-to-noise ratio, by increasing the readout gradient
strength (which is under user control for scanners from some manufacturers).
Distortion in CT is quite different. While it is true that x-rays invariably
travel in straight lines, the same cannot be said for the patient couch. The
patient couch can bend (especially with heavy patients) as it is extended,
which can lead to a variable skew distortion with slice position. There can be
additional skew errors due to poorly calibrated gantry tilt. These skew errors
can be substantial, leading to errors of several millimeters in some parts of
the images. Another cause of distortion in spiral CT can be errors in bed
speed that lead to errors in slice spacing. Errors caused by these distortions
can be quite obvious in registered images, even if they are not at all apparent
when viewing the images on radiographic film on a light box. Figure 10.2
shows sagittally reformatted images through a CT volume before and after
correction of a 22° gantry skew. If the skew had not been corrected (for exam-
ple if the file transfer process had lost this information), or if the skew were
incorrectly corrected due to inaccuracies in the scanner's measurement of
gantry tilt, substantial registration errors would result.
10.2.1.4
Patient Motion
Patient motion during either MR or CT acquisition makes image registration
harder. Subject motion during the acquisition of a CT slice results in streak-
ing which degrades the diagnostic quality. Patient motion between CT slice
acquisitions may not reduce the diagnostic quality of the images, but motion
either during or between slices can make accurate registration more difficult.
