Biomedical Engineering Reference
In-Depth Information
There also has to be sufficient commonality in the parts of the body or
organ covered by the field of view of both modalities. Sometimes image sets
have to be reformatted to obtain the most appropriate range of both image
sets. This might, for instance, be the case when registering a multiple bed
PET study covering the whole thorax with an MR image set which covers
only a certain fraction of the PET volume. In brain imaging there might be
excess slices acquired with MRI containing part of the neck, which is not
covered with PET.
Artifacts due to unintentional patient movement cause one of the most
severe problems in registration of PET and MR images. The signatures of
motion artifacts are different for PET and MRI. In PET, patient movement
between different steps of an acquisition protocol might, in the worst case,
corrupt the images completely. Also, certain corrective steps like the correc-
tion for photon absorption based on a separate measurement, but assuming
identical patient positioning, might no longer be feasible (see also Chapter 5).
Patient movement of a lesser degree will cause a loss in contrast and reso-
lution of smaller areas with enhanced intensity amplitudes of the activity
distribution. A solution to this problem often applied is the splitting of the
acquisition into smaller time frames, with subsequent registration and
summation of individual frames and exclusion of frames where motion is
evident.
The situation with motion artifacts in MR imaging is different, as it
depends on the pulse sequence as to which artifacts are visible and what
effect they might have. If slices are acquired sequentially, as in certain rapid
imaging sequences, and the subject moves during imaging, the spatial rela-
tionship between the individual slices will be lost and the data cannot be
regarded as a complete regular volume anymore. With fast, true 3D gradi-
ent echo sequences that sample a whole volume, any motion during the
acquisition will tend to produce artifacts throughout the image volume,
which may make the image unusable for registration. One of the pertinent
motion artifacts, occurring frequently during thorax scans, is blurring and
motion-induced ''ghosting'' caused by respiratory motion of the chest.
Many MR acquisition and postprocessing techniques have been proposed
to deal with the motion problem, but motion artifact remains an important
source of error in the many clinical images. Therefore, one of the main
efforts in designing new sequences is to achieve fast and short acquisitions
covering the whole volume to be imaged.
In a clinical setting one often has to deal with a small number of slices
arising from the fact that the scanning time cannot be easily prolonged
without the risk of motion artifacts. This results in a tradeoff between a
lower number of slices, (i.e., coarser sampling of the volume) and less
motion artifacts, or very high sampling density with many slices but the
risk of motion artifacts. Under clinical conditions the intention is to
assure detection and precise localization of pathologies, rather than pro-
viding high-resolution morphological imaging with a large number of
thin slices.
