Biomedical Engineering Reference
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where no MR information is available. The feasibility of such an approach
still needs to be validated. In imaging scenarios with highly specific tracers,
the arms may be dicult to segment automatically in the uncorrected PET
images. Yet another approach could predict the body cross-section, including
parts which were outside the field of view of the MR image, from the atlas
registration. In theory, these approaches could even be combined such that
the registration is performed based on the MR image where the MR image is
available, and elsewhere based on the uncorrected PET image.
11.4.6 MR coils and positioning aids
The fact that the MR coils are located inside the FOV of the PET system
is a challenge for MR-AC. For brain scans, the head coil is rigid and its
attenuation values can be estimated from a baseline CT. Subsequently for
any PET/MR study knowledge of the relative position of the head coil inside
the PET/MR system would be required. Similarly other rigid objects inside
the PET system could be detected and included in the MR-based attenuation
map [22].
For extra-cranial examinations the situation is far more dicult. Surface
coils are required to avoid suboptimal signal generation. Surface coils contain
elastic components and hence cannot be located easily with respect to the
gradient coil or the patient. MR sequences with UTE could possibly help
detect landmarks of surface coils and thus help account for their attenuation.
However, in any case for PET/MR systems the RF coils need to be designed
such that low photon attenuation and scatter are ensured.
Positioning aids are commonly used to reduce patient movement over the
duration of the scan. As they are typically made of foam or other soft materials
that can be adapted to the patient outline, their shape and corresponding
attenuation map changes from one patient scan to the next. This position
seems dicult to predict even if MR-visible markers were placed on them.
Mantlik et al. [21] recently performed a quantitative analysis of the effect of
ignoring the attenuation of positioning aids. For 5 head/neck patients who
were fixed in a vacuum mattress they obtained a mean underestimation of
activity of 9.1% if the mattress was not included in the attenuation map.
11.4.7 User intervention
Ideally, for the application in clinical PET/MR scenarios, MR-AC should
be fully automatic in order to limit user interaction and, subsequently, exami-
nation and processing times. Despite claims of some groups that their method
for MR-AC is \robust," problems remain that require \some manual interven-
tion of the operator" [41]. Thus, automation of MR-AC remains a challenge,
particularly in cases of large deviation from normal anatomy.
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