Biomedical Engineering Reference
In-Depth Information
11.4.8 Potential benefits of MR-AC
In PET/CT the PET image is acquired over several minutes, while the
CT scan is a matter of seconds and frequently acquired during a single breath
hold. As a result, patient or organ motion typically causes local misalignment
between the PET and CT images and may lead to serious artifacts for AC,
for example near the diaphragm [4]. Some authors have recommended 4D
PET/CT acquisition and AC [27, 31], however, this involves a substantially
higher patient radiation dose. As MR scans generally take much longer than
CT scans, patients conceivably spend an even longer time in the PET/MR
scanner compared to PET/CT and consequently patient motion is likely to
cause even more severe artifacts. Here, the use of periodic MR navigator sig-
nals in conjunction with a 4D model of the human torso may help to correct for
motion-induced image degeneration in PET/MR data following 4D-MR-AC,
which would be a major advantage over CT-AC.
11.4.9 Additional potential benefits of simultaneous
PET/MR acquisition
As early as 1991, Leahy et al. [19] suggested that PET reconstruction
could be improved by using anatomical MR images from the same patient as
prior information. This remains a field of active research and the potential
of the method can be seen in Figure 11.11. While it is commonplace today
that almost all neurology patients who receive a PET scan also receive a MR
scan, MR-guided PET reconstruction has not yet made the transition from
research into clinical routine. Aside from logistical problems of automatically
retrieving the matching MR image from the PACS, one of the reasons for this
might be that mis-registrations, which are unavoidable in retrospective PET-
MR co-registration, quickly deteriorate the image quality [20]. In combined
PET/MR tomographs, the co-registration accuracy is improved and may help
promote the concept of MR-guided PET image reconstruction.
Even if the PET image is reconstructed independently of the MR image,
it is still possible to use the MR image of the patient as an aid for improved
quantification. In particular MR-guided partial volume correction (PVC) was
suggested as early as 1990 [26, 25]. Again, PET and MR images from com-
bined PET/MR examinations may facilitate improvements in MR-based PET
quantification through the use of MR-based PVC.
11.5 Conclusion
With the onset of a research interest in combined PET/MR imaging sev-
eral studies have appeared on the use of MR for AC of the PET data. MR-AC
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