Biomedical Engineering Reference
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tion of certain molecules (MR spectroscopy), functional imaging (fMRI), and
diffusion-tensor tracking [6].
The combination of MR and PET is very attractive. High soft-tissue con-
trast (MRI) combined with specific PET tracers may lead to completely new
and effective work flows and higher diagnostic accuracy. High sensitivity and
specificity are expected. In addition, acquisition times of PET and MRI are in
the same order of magnitude and thus this coupling suggests several synergetic
advantages. But the integration is also technically very challenging.
A simultaneous acquisition of PET and MRI images requires full PET-
hardware compatibility to MR. This means, for example, that the standard
PET detector (i.e., scintillation crystal coupled to a photomultiplier tube
(PMT)) has to be replaced by a combination of the scintillator and a photo
avalanche diode or SiPM, i.e., a detector that is significantly less sensitive to
magnetic fields. Another challenge is the fact that quantitative PET imag-
ing requires that the PET data are corrected for attenuation. While the CT
information can easily be scaled to correct for attenuation and scatter [10],
this is not quite that simple for MRI data sets. Unfortunately, the standard
MRI acquisition shows only very little bone contrast, one of the sources of
PET attenuation. In addition, the soft-tissue information of the MRI image
is not correlated to the attenuation coecient necessary for PET attenuation
correction [2, 3]. Either specific MR pulse sequences have to be implemented
to enable CTsuch as datasets which could directly be used for attenuation
correction or theoretical patient models (e.g., based on an atlas) have to be
implemented. Special care has to be taken when magnetic and paramagnetic
material is used for implants. Some of these implants may exclude the patient
from an MR-PET scan altogether; others will lead to significant artifacts in
the MR data which do require specific software tools to align the MR infor-
mation for PET attenuation correction and to guarantee a perfect match of
the PET and the MR information.
On the other hand, a simultaneous acquisition of PET and MR may allow
for even more sophisticated data corrections. One possibility would be an
online motion correction of the PET information due to the fact that fast
MR sequences are a very sensitive tool to detect and track organ and patient
movement.
First prototypes of such a combined MR-PET system (Figures 10.4 and
10.5) have demonstrated the performance of this type of hybrid imaging de-
vice and are being used to explore the clinical possibilities. While these initial
systems are limited to brain applications, they nevertheless allow the extrap-
olation to whole-body designs [8, 5].
At RSNA 2010 the first fully integrated whole-body MR-PET system has
been introduced combining state of the art 3T MR technology with high
performance PET technology for simultaneous MR-PET whole-body appli-
cations. A prototype system is demonstrating first results at the Technical
University of Munich, University Hospital rechts der Isar, Department of Nu-
clear Medicine (Figure 10.6).
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