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Abstract:
Ziel/Aim: Combined PET/MR imaging is an evolving field in small animal as
well as human medical imaging. Our group successfully constructed a second
generation small animal PET-insert, which allows for large field of view PET/
MR imaging. Here we present technical details about the new PET-insert as
well as an evaluation of its PET and MR performance.
Methodik/Methods: The new PET-insert uses a total of 16 circularly arranged,
copper shielded, detector cassettes. Each cassette consists of 3 avalanche photodiode
(APD) arrays, each attached to a 15x15 LSO crystal block (single crystal
size 1.5x1.5x10mm). The PET-insert is air cooled via vortex tubes and a
temperature feedback system to an operating temperature of 15°C. The PETinsert
is mounted inside a 7T small animal MR system. The total PET/MR
field of view with installed whole body MR coils is ca. 72x70mm, allowing for
whole body mouse as well as rat imaging. The PET spatial resolution was evaluated
for different reconstruction algorithms, using a glass capillary tube, filled
with 1MBq of F-18-FDG solution. Furthermore the sensitivity of the PET-insert
was measured via a small glass sphere filled with 340kBq of F-18 in an
energy window of 350keV-650keV. On the MR side the signal to noise ratio
(SNR) as well as image homogeneity was measured for different MR sequences
(n=18). In vivo images of a rat heart were also obtained.
Ergebnisse/Results: The PET spatial resolution for an OSEM reconstruction
(16 iterations) was found to be (2.22±0.06)mm without an MR sequence running,
and (2.19±0.05)mm with an EPI MR sequence applied. The sensitivity at
the center FOV of the PET insert was (3.19±0.22)% without and (3.1±0.08)%
with the MR sequence. Neither spatial resolution nor sensitivity of the PET
changed significantly when an MR EPI sequence was applied. The MR SNR
with the PET insert varied, depending on the used MR sequence, between
25 for turbospinecho and 8 for flash sequences. Only for the TSE sequence a
small, but significant (p<0.05) drop of the SNR (without PET: 24.9±0.3 with
PET: 24.4±0.3) on the order of 2% was observed. The other sequences showed
no significant influence of the PET on the MR performance. In vivo images of
a rat heart revealed, that gated acquisitions of PET and MR data were simultaneously
possible with this setup. The high quality images allowed identification
of the heart ventricles.
Schlussfolgerungen/Conclusions: Our data clearly shows that the next generation
of small animal PET/MR systems is able to perform on par with stand
alone solutions. Small fluctuations of the SNR with the PET insert installed are
in the range of the typical experimental variability for small animal studies.
The in vivo imaging data reveals the huge potential of this technique in preclinical
and clinical research.
