Tim Hilgenfeld1, Alexander Heil1, Sebastian Schwindling2, David Grodzki3, Mathias Nittka3, Daniel Gareis4, Peter Rammelsberg2, Martin Bendszus1, Sabine Heiland1, and Marcel Prager1
1Division of Neuroradiology, University Heidelberg, Heidelberg, Germany, 2Division of Prosthodontics, University Heidelberg, Heidelberg, Germany, 3Siemens Healthcare GmbH, Erlangen, Germany, 4NORAS MRI products GmbH, Höchberg, Germany
Synopsis
Dental MRI is a new technique which is often impaired by artifacts due
to metallic dental implants. Several MRI sequences were developed to reduce susceptibility
artifacts (e.g. for orthopaedic implants). Here, we for the first time
systematically evaluated MR sequences for artifact reduction in dental
implants. Smallest artifact volume was measured for 2D-TSE sequences. Since imaging of dental structures
benefit from high resolution and possibility of 3D reconstructions 3D sequences
are advantageous. Significant artifact reduction was noted for SPC-WARP
measuring only 2.1 times artifact volume of TSE sequence instead of 4.8 times
when using standard SPC sequence.Introduction:
Magnetic resonance imaging (MRI) has become a standard diagnostic tool
for head and neck imaging. In addition new potential dental MRI applications
are in developmental stage for example in orthodontics [1, 2], endodontics [3], prosthodontics [4] and periodontology [5]. In patients undergoing
dental MRI, image quality may be impaired due to susceptibility artifacts due
to metallic dental implants [6]. Several MRI techniques to reduce susceptibility artifacts were developed
like view angle tilting (VAT), slice-encoding metal artifact correction
(SEMAC), pointwise encoding time reduction with radial acquisition (PETRA),
multiple slab acquisition with VAT gradient based on a SPACE sequence
(SPC-WARP), and techniques employing combinations of those like TSE-WARP (VAT
and SEMAC). Up to now, these sequences were predominantly evaluated in orthopaedic
or neurosurgical metallic implants [7-10]. The aim of this study was to determine the value of these techniques
in terms of artifact volume reduction at different dental implants while
maintaining reasonable acquisition times compared to standard sequences.
Methods:
Two dental implants with different
composition of crowns (CCT-T implant: cobalt, chromium and tungsten; Z-T
implant: zirconium dioxide), prosthesis screws and abutments made of titanium
were used. For measurement of artefact volume implants were embedded in a
mixture of water and fat. Afterwards image quality was assessed in porcine head
with implants placed in anterior mandible.
All MR measurements were performed on a 3T
MRI system (Tim-Trio; Siemens Healthcare GmbH). A 16-channel multipurpose coil
(Variety; NORAS MRI products; measurement of artefact volume and imaging of
porcine head) and 12-channel head coil (Siemens Healthcare GmbH; signal-to-noise
ratio (SNR) measurement) were used.
TSE-WARP, SPC-WARP and PETRA sequences were
optimized with regards to artifact size (for example matrix size, readout
bandwidth, and slice thickness). In a second step standard TSE and SPACE (SPC) sequences with imaging
parameters identical to WARP sequences as much as possible were implemented for
comparison.
Determination of artifact volume was performed
by subtraction of the true implant volume determined by water displacement from
the volume measured in MRI. Semi- automatic quantification and rendering of
signal loss- and pile up artifact volume were done with Amira 3D (FEI).
Image quality was assessed quantitatively by
calculating SNR and qualitatively by blinded read of porcine heads by two
radiologists. The SNR was determined by measuring the dynamic noise [11]. 25 repetitions of each sequence were performed. Using Matlab R2015a (MathWorks
Inc.) Regions of interest were automatically placed exactly at the same
position in all sequences and a voxel based SNR map was calculated. Radiologists
evaluated image quality of eight anatomical structures for the last molar on a
scale from 1 (best visibility) to 5 (poor visibility) as published before [8].
Results:
Comparing different implants,
the larger artifact volume was always noted for the CCT-T implant which was
between 9.3 ± 2.4 times (TSE-WARP) and 31.6
± 4 times (PETRA) larger than the artifact volume of the Z-T implant. For
both implants no significant reduction of artifact volume was observed using
the TSE-WARP sequence compared to a standard TSE sequence. In contrast,
SPC-WARP reduced the artifact volume of the CCT-T implant by 56.2 ± 3 % and 33.1 ± 6.8 %
for Z-T implant, respectively compared to standard SPC sequence. A significant
increase in artifact volume of CCT-T implant compared to standard SPC sequence
was observed for PETRA (+11.8 ± 4.2%)
whereas artifact volume of Z-T implant decreased by 40.8 ± 6.3%.
Significant but minor differences in SNR were noted between SPC versus SPC-WARP
and TSE versus TSE-WARP, respectively. SNR of PETRA was about 69.5 ± 8.5 % smaller than for standard SPC which resulted in poor performance in
qualitative image review as well (significant worse in six out of eight
anatomical structures compared to standard SPC). No significant differences were
observed between SPC versus SPC-WARP and TSE versus TSE-WARP, respectively.
Better results (but not significant) were obtained for small structures like
periodontal space or apical foramen with SPC and SPC-WARP. Cohen’s κ for interrater agreement was excellent (0.81).
Conclusions:
There was no significant artifact
reduction using TSE-WARP compared to standard TSE sequences. Artifact volume
was smaller in all TSE sequences compared to SPC, SPC-WARP and PETRA sequences.
However, in dental MRI acquisition of 3D sequences is advantageous for various reasons. Artifact
reduction of dental implants can be achieved with SPC-WARP and PETRA technique
compared to standard SPC but the effect depends on composition of dental
implants. Poor image quality was obtained with PETRA. SPC and SPC-WARP
performed slightly better in terms of image quality than TSE and TSE-WARP as a
result of higher resolution.
Acknowledgements
We thank SIEMENS Healthcare GmbH for providing two WIP packages and
Straumann Germany for providing the zirconia prosthesis screw.
Furthermore the authors would like to thank Stefanie Sauer, Ph.D.,
pharmacist in the Department of Pharmacy Heidelberg University Hospital for her
work on this project.References
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