1. Phantom Preparation
2. MRI Examination
3. Image Analysis and Post-processing
With the above-mentioned protocol, we evaluated the artifact volume of 2 different dental implants made of Titanium (T; see the Table of Materials) supporting different crowns [porcelain-fused-to-metal non-precious alloy (CCT-T) and monolithic zirconia (Z-T); Figure 1b and 1c]. The CCT-T sample represents a highly paramagnetic material composition predicting large artifacts (Cobalt 61%, Chrome 21%, and Tungsten 11%; CCT). The crown material of the Z-T sample represents a low paramagnetic material (Zirconia 92%; Z). Furthermore, four different, non-fat-saturated, T2-weighted sequences were evaluated to compare their vulnerability to metal artifacts. Specifically, the techniques of multiple slab acquisition with a view-angle-tilting gradient based on a sampling perfection with application-optimized contrasts using different flip angle evolutions (MSVAT-SPACE), slice encoding for metal artifact correction (SEMAC), and their conventional counterparts SPACE and turbo spin echo (TSE) were evaluated (see Table 1 for the detailed sequence parameters). MRI scans were performed on a 3T MRI system (see the Table of Materials) and a 16-channel multipurpose surface coil was used. The variation of the pulse sequence parameters has a strong impact on the artifact size. Thus, pulse sequence parameters were chosen as close as possible to those used in the in vivo dental MRI scans to increase the transferability of the results. The analysis was performed 3x by two independent raters. For multiple comparisons, a two-way analysis of variances and post hoc Tukey tests were used.
The data analysis reveals differences between both samples and the applied sequences. In all sequences, the combined artifact volumes (the sum of the signal loss and pile-up) of the CCT-T sample were larger compared to the Z-T sample (P <0.001; Figure 2 and Figure 3). Within the same sequence, the artifact volume of the CCT-T sample was 19.3x (SEMAC) to 39.6x (MSVAT-SPACE) larger than the artifact volume of the Z-T counterpart.
The choice of pulse sequence had a significant impact on the artifact volume as well (Figure 2 and Figure 3). Regarding the CCT-T sample, the smallest artifact volumes were observed for TSE and SEMAC, and the largest artifacts for SPACE (P <0.001). In addition, MSVAT-SPACE significantly reduced the artifact volume compared to SPACE (P <0.001; 3.8 vs. 7.3 mL). In contrast, no significant differences were observed between MSVAT-SPACE, TSE, and SEMAC for the Z-T sample. The artifact volume for Z-T was largest in SPACE and was significantly reduced by MSVAT-SPACE (P <0.05; 0.26 vs. 0.1 mL).
Figure 1: ROI positioning and implant samples. (a) This panel shows a typical positioning of the regions of interests (ROIs) for measuring the thresholds for pile-up artifacts and signal distribution (ROIB = ROIBackground) and signal loss artifacts (ROIA = ROIAir; ROIC = ROICore). The blue contour resembles the result of the semiautomatic segmentation for signal loss artifacts within that slice. The small red areas correspond to the result of pile-up artifacts. (b and c) These panels show images of used dental implants supporting different single crowns. Cobalt-Chromium-Tungsten-Titanium (CCT-T); Zirconia-Titanium (Z-T). Please click here to view a larger version of this figure.
Figure 2: Artifact volume measurements. (a and b) These are bar graphs showing the mean values with the standard deviations of the three-dimensional artifact volume of the entire implant samples for all 4 evaluated sequences after subtracting the physical implant volume. If applicable, separate standard deviation error bars are given for signal loss and pile-up artifacts. * P ≤ 0.05; ** P≤ 0.001 Please click here to view a larger version of this figure.
Figure 3: Appearance of artifacts. These panels render the artifact volumes of the entire implants (upper row). The blue colored areas represent signal loss artifacts and the red colored areas represent pile-up artifacts. The panels show the colored source images (lower row) for all evaluated T2-weighted sequences. Panel (a) is of the Cobalt-Chromium-Tungsten-Titanium (CCT-T) sample and panel (b) is of the Zirconia-Titanium (Z-T) sample. Please click here to view a larger version of this figure.
Sequence | TR/TE [ms] |
Voxel size [mm3] |
FOV [mm2] |
Matrix | Readout Bandwidth [Hz/Px] |
Slices | Slice encoding steps or oversampling [%] |
VAT | Time [min:sec] |
SPACE | 2,500/131 | 0.55 x 0.55 x 0.55 | 140 x 124 | 256 x 256 | 501 | 72 | 55.6 | No | 14:02 |
MSVAT-SPACE | 2,500/199 | 0.55 x 0.55 x 0.55 | 140 x 84 | 256 x 256 | 528 | 72 | 55.6 | Yes | 6:04 |
TSE | 5,100/44 | 0.59 x 0.59 x 1.5 | 150 x 150 | 256 x 256 | 592 | 25 | No | No | 3:36 |
SEMAC | 5,100/45 | 0.59 x 0.59 x 1.5 | 150 x 150 | 256 x 256 | 592 | 25 | 4 | Yes | 6:19 |
Table 1: Parameters of all used sequences.
Aqua B. Braun Ecotainer | B. Braun Melsungen AG, Melsungen, Germany | ||
Semisynthetic fat: Witepsol W25 | Caelo Caesar & Loretz GmbH, Hilder, Germany | 4051 | |
Macrogol-8-stearate | Caelo Caesar & Loretz GmbH, Hilder, Germany | 3023 | |
Plastic box: not specified | |||
Implants: Nobel Replace | Nobel Biocare, Zürich, Switzerland | ||
Water bath Haake S5P | Thermo Scientific, Waltham, MA, USA | ||
Measuring cylinder Blaubrand Eterna, Class A, Boro 3.3 | BRAND GmbH + Co Kg, Wertheim, Germany | 32708 | |
Coil: Variety | Noras MRI products GmbH, Höchberg, Germany | ||
MRI: Magnetom Trio | Siemens Healthcare GmbH, Erlangen, Germany | ||
Postprocesing software: Amira 6.4 | Thermo Scientific, Waltham, MA, USA |
As the number of magnetic resonance imaging (MRI) scanners and patients with medical implants is constantly growing, radiologists increasingly encounter metallic implant-related artifacts in MRI, resulting in reduced image quality. Therefore, the MRI suitability of implants in terms of artifact volume, as well as the development of pulse sequences to reduce image artifacts, are becoming more and more important. Here, we present a comprehensive protocol which allows for a standardized evaluation of the artifact volume of implants on MRI. Furthermore, this protocol can be used to analyze the vulnerability of different pulse sequences to artifacts. The proposed protocol can be applied to T1- and T2-weighted images with or without fat-suppression and all passive implants. Furthermore, the procedure enables the separate and three-dimensional identification of signal loss and pile-up artifacts. As previous investigations differed greatly in evaluation methods, the comparability of their results was limited. Thus, standardized measurements of MRI artifact volumes are necessary to provide better comparability. This may improve the development of the MRI suitability of implants and better pulse sequences to finally improve patient care.
As the number of magnetic resonance imaging (MRI) scanners and patients with medical implants is constantly growing, radiologists increasingly encounter metallic implant-related artifacts in MRI, resulting in reduced image quality. Therefore, the MRI suitability of implants in terms of artifact volume, as well as the development of pulse sequences to reduce image artifacts, are becoming more and more important. Here, we present a comprehensive protocol which allows for a standardized evaluation of the artifact volume of implants on MRI. Furthermore, this protocol can be used to analyze the vulnerability of different pulse sequences to artifacts. The proposed protocol can be applied to T1- and T2-weighted images with or without fat-suppression and all passive implants. Furthermore, the procedure enables the separate and three-dimensional identification of signal loss and pile-up artifacts. As previous investigations differed greatly in evaluation methods, the comparability of their results was limited. Thus, standardized measurements of MRI artifact volumes are necessary to provide better comparability. This may improve the development of the MRI suitability of implants and better pulse sequences to finally improve patient care.
As the number of magnetic resonance imaging (MRI) scanners and patients with medical implants is constantly growing, radiologists increasingly encounter metallic implant-related artifacts in MRI, resulting in reduced image quality. Therefore, the MRI suitability of implants in terms of artifact volume, as well as the development of pulse sequences to reduce image artifacts, are becoming more and more important. Here, we present a comprehensive protocol which allows for a standardized evaluation of the artifact volume of implants on MRI. Furthermore, this protocol can be used to analyze the vulnerability of different pulse sequences to artifacts. The proposed protocol can be applied to T1- and T2-weighted images with or without fat-suppression and all passive implants. Furthermore, the procedure enables the separate and three-dimensional identification of signal loss and pile-up artifacts. As previous investigations differed greatly in evaluation methods, the comparability of their results was limited. Thus, standardized measurements of MRI artifact volumes are necessary to provide better comparability. This may improve the development of the MRI suitability of implants and better pulse sequences to finally improve patient care.