Artefacts in magnetic resonance imaging caused by dental material

Incidental findings and safety events from magnetic resonance imaging simulation for head and neck radiation treatment planning: A single institution experience

Purpose

Having dedicated MRI scanners within radiation oncology departments may present unexpected challenges since radiation oncologists and radiation therapists are generally not trained in this modality and there are potential patient safety concerns. This study retrospectively reviews the incidental findings and safety events that were observed at a single institution during introduction of MRI sim for head and neck radiotherapy planning.

Methods

Consecutive patients from March 1, 2020, to May 31, 2022, who were scheduled for MRI sim after having completed CT simulation for head and neck radiotherapy were included for analysis. Patients first underwent a CT simulation with a thermoplastic mask and in most cases with an intraoral stent. The same setup was then reproduced in the MRI simulator. Safety events were instances where scheduled MRI sims were not completed due to the MRI technologist identifying MRI-incompatible devices or objects at the time of sim. Incidental findings were identified during weekly quality assurance rounds as a joint enterprise of head and neck radiation oncology and neuroradiology. Categorical variables between completed and not completed MRI sims were compared using the Chi-Square test and continuous variables were compared using the Mann-Whitney U test with a p-value of < 0.05 considered to be statistically significant.

Results

148 of 169 MRI sims (88 %) were completed as scheduled and 21 (12 %) were not completed (Table 1). Among the 21 aborted MRI sims, the most common reason was due to safety events flagged by the MRI technologist (n = 8, 38 %) because of the presence of metal or a medical device that was not noted at the time of initial screening by the administrative coordinator. Patients who did not complete MRI sim were more likely to be treated for non-squamous head and neck primary tumor (p = 0.016) and were being treated post-operatively (p < 0.001). CT and MRI sim scans each had 17 incidental findings. CT simulation detected 3 cases of new metastases in lungs, which were outside the scan parameters of MRI sim. MRI sim detected one case of dural venous thrombosis and one case of cervical spine epidural abscess, which were not detected by CT simulation.

Conclusions

Radiation oncology departments with dedicated MRI simulation scanners would benefit from diagnostic radiology review for incidental findings and having therapists with MRI safety credentialing to catch near-miss events.

Assessment of the intrasinusidal volume before and after maxillary sinus augmentation using mri - a pilot study of eight patients

PURPOSE

The purpose of this study was to evaluate the suitability, accuracy, and reliability of a non-invasive 3-Tesla magnetic resonance imaging technique (3 T-MRI) for the visualization of maxillary sinus grafts in comparison to conventional, X-ray-based, established standard imaging techniques.

METHODS

A total of eight patients with alveolar bone atrophy who required surgical sinus floor augmentation in the course of dental implantation were included in this pilot study. Alongside pre-operative cone-beam computed tomography (CBCT), 3 T-MRI was performed before and 6 months after sinus floor augmentation. Two investigators measured the maxillary sinus volume preoperatively and after bone augmentation.

RESULTS

In all cases, MRI demonstrated accurately the volumes of the maxillary sinus grafts. Following surgery, the bony structures suitable for an implant placement increased at an average of 4.89 cm3, corresponding with the decrease of the intrasinusidal volumes. In general, interexaminer discrepancies were low and without statistical significance.

CONCLUSION

In this preliminary study, we could demonstrate the feasibility of MRI bone volume measurement as a radiation-free alternative with comparable accuracy to CT/CBCT before procedures like sinus floor augmentation. Nevertheless, costs and artifacts, also present in MRI, have to be taken into account. Larger studies will be necessary to justify the practicability of MRI bone volume evaluation.

A novel approach for gingiva thickness measurements around lower anterior teeth by means of dental magnetic resonance imaging

OBJECTIVE

This diagnostic accuracy study aims to present the first measurements of gingiva thickness around lower anterior teeth using dental magnetic resonance imaging (MRI) and to compare these measurements with two established methods: (1) gingival phenotype assessment via periodontal probing, and (2) the superimposition of cone-beam computed tomography (CBCT) scans with intraoral scans of teeth and gums.

MATERIALS AND METHODS

Ten patients with substantial orthodontic treatment need and anterior mandibular crowding were consecutively included in this clinical case series. After periodontal probing, each patient underwent a CBCT scan, an intraoral scan of the mandible, and an MRI investigation using a novel mandibula 15-channel dental coil.

RESULTS

The mean gingiva thickness was 0.72 mm measured on MRI and 0.97 mm measured on CBCT, with a mean difference between the measurement methods of 0.17 ± 0.27 mm (p < 0.001). Measurement agreement between the index tests (MRI and CBCT) and the clinical reference standard (probing) yielded an overall percent agreement of 64.94% and 47.02% for MRI and CBCT, respectively. Teeth with thin phenotypes were associated with lower soft tissue dimensions in both free (MRI: 0.56 mm vs. CBCT: 0.79 mm) and supracrestal gingiva (MRI: 0.75 mm vs. CBCT: 1.03 mm) when compared to those with thick phenotypes. However, only the measurements obtained from MRI scans showed statistically significant differences between the two phenotypes.

CONCLUSION

Dental MRI successfully visualizes delicate structures like the gingiva in the anterior mandible and achieves a high correlation with superimposed CBCT scans, with clinically acceptable deviations.

CLINICAL RELEVANCE

The present study helps to establish dental MRI as a radiation-free alternative to conventional radiographic methods.

Can Velopharyngeal MRI be Used in Individuals with Orthodontic Devices?

OBJECTIVE

To evaluate the influence of common pediatric orthodontic appliances on velopharyngeal (VP) MRI and to compare MR image sequences to determine if sequence parameters impact the visibility of key VP structures commonly assessed in clinical VP MRI.

DESIGN

Participants undergoing orthodontic treatment completed a VP MRI study. Level of distortion caused by orthodontic devices on 8 anatomical sites of interest and using variable MRI sequences was evaluated.

SETTING

Single institution.

PARTICIPANTS

Nineteen participants undergoing orthodontic treatment.

MAIN OUTCOME

Level of distortion caused by metal artifacts and MR sequence used.

RESULTS

The results of this study demonstrate that appliances such as hyrax palatal expanders and braces with stainless steel brackets are acceptable for a VP MRI, while class II corrector springs are not recommended. The HASTE MRI sequence with 2D imaging techniques should be utilized if the child has orthodontic devices, while FSE and 3D imaging techniques are not recommended. The presence of wire spring coils and molar bands are likely to not to interfere with the MRI evaluation.

CONCLUSIONS

Findings from this study suggest that the presence of orthodontic appliances does not hinder visualization of all velopharyngeal structures during an MRI. Therefore, careful consideration must be made prior to disqualifying or recommending patients for VP MRI.

Magnetic resonance imaging artefacts caused by orthodontic appliances and/or implant-supported prosthesis: a systematic review

OBJECTIVES

Dental materials, including orthodontic appliances and implants, are commonly mentioned as a possible source of artefacts in magnetic resonance imaging (MRI). The aim of the present study was to undertake a systematic review of the relevant literature on MR image artefacts due to dental materials, limited to orthodontic appliances and implant-supported dental prosthesis, on both technical and diagnostic levels.

METHODS

The MEDLINE (PubMed) bibliographic database was searched up to September 2020. The search was limited to studies published in English, using the search string: (MRI or magnetic resonance) and (artefact or artifact) and (dental or ortho or implant or restoration or restorative). The studies were assessed independently by three reviewers, focusing on the following parameters: MRI sequences, tested materials, assessed parameters, efficacy level and outcome.

RESULTS

The search strategy yielded 31 studies, which were included in this systematic review. These studies showed that metallic dental materials, commonly present in orthodontic appliances and implant-supported dental prosthesis led to diverse types/severities of artefacts in MR images. Fifteen studies were in vivo, based on human subjects. The studies differed substantially in terms of tested materials, assessed parameters, and outcome measurements.

CONCLUSIONS

Metallic dental materials cause artefacts of diverse types and severities in MR images of the head and neck region. However, the diagnostic relevance of the investigated artefacts for the diverse MRI applications is yet to be studied.

Magnetic resonance imaging investigations in patients with metallic dental prosthesis: "The associated dilemma for medical fraternity and the dentist's role"

Background

Interdisciplinary referrals for dental examination in hospital setups are common before radiotherapy, kidney transplants, or magnetic resonance imaging (MRI). The patients who walk in could be random patients with metallic or porcelain-fused-to-metal prostheses done elsewhere but might require an opinion before the MRI. This leaves quite a responsibility on the consulting dentist to green signal the procedure. There is a lack of evidence in the literature, to confirm the absence of any untoward consequence during such MRI, which might leave the dentist in dilemma. Dental materials' magnetic behavior raises concern regarding whether they are 100% nonferromagnetic; furthermore, the examining dentist might be unaware of the metal used (Co-Cr, Ni-Cr, or trace elements). Clinicians may also come across full-mouth rehabilitated patients with multiple crown-bridge prostheses or metallic superstructure for implant prostheses. Research in the area leaves many unanswered questions because most studies have evaluated artifacts during MRI and are in vitro. Titanium is considered to be safe due to its paramagnetic behavior, whereas the literature does not rule out the probability of dislodgment of other porcelain fused to metal (PFM) prostheses. Due to less reported literature there exists dilemma to ascertain MRI in these patients. An online Google Search, PubMed, and gray literature portray the ambiguity associated with metal and PFM crowns and their magnetic behavior during MRI. Most studies were associated with the artifacts caused during MRI and methods of reducing them under in vitro situations. The concern for dislodgment has also been expressed in a few reports.

Technique

Certain steps of a pre-MRI checkup and an innovative technique have been discussed to assure patient safety during the MRI.

Conclusion

The technique explained is inexpensive and a quick aid that can be executed before the investigation.

Clinical and Research Implications

There is a need to study and understand the magnetic behavior of Co-Cr and Ni-Cr crowns in the presence of various MRI strengths.

Orthodontic Retainers-A Critical Review

The achievement of aesthetic, functional occlusion should not mark the end of the orthodontic intervention. To prevent relapse, retention needs advance planning, and may vary in duration. This review aims to present and comment on the available means of retention. The ever-popular, passive Hawley-like removable appliances are credible in maintaining the desired occlusion. Modifications are the removable appliance Wrap Around, having the labial archwire extending to the premolars; the translucent retainer, Astics, a unique aesthetic Hawley-type device; and the reinforced removable retainer, which features a metallic grid reinforcing the acrylic base. Vacuum-formed retainers are easy to fabricate and are readily prescribed. By contrast, fixed retainers are made of orthodontic wire and composite resin bonded on the lingual or palatal surfaces of the anterior teeth. Patient-related variables need evaluation to select the appropriate retainer, while patients ought to realize the importance of retention and comply with offered guidance. Overall, the orthodontist is responsible for keeping the patient informed on the properties and the duration of retention, even before starting active orthodontic treatment.

Orthodontic Retainers-A Critical Review

The achievement of aesthetic, functional occlusion should not mark the end of the orthodontic intervention. To prevent relapse, retention needs advance planning, and may vary in duration. This review aims to present and comment on the available means of retention. The ever-popular, passive Hawley-like removable appliances are credible in maintaining the desired occlusion. Modifications are the removable appliance Wrap Around, having the labial archwire extending to the premolars; the translucent retainer, Astics, a unique aesthetic Hawley-type device; and the reinforced removable retainer, which features a metallic grid reinforcing the acrylic base. Vacuum-formed retainers are easy to fabricate and are readily prescribed. By contrast, fixed retainers are made of orthodontic wire and composite resin bonded on the lingual or palatal surfaces of the anterior teeth. Patient-related variables need evaluation to select the appropriate retainer, while patients ought to realize the importance of retention and comply with offered guidance. Overall, the orthodontist is responsible for keeping the patient informed on the properties and the duration of retention, even before starting active orthodontic treatment.

ARTIFACTS IN MAGNETIC RESONANCE IMAGING CAUSED BY DENTAL MATERIALS: A SYSTEMATIC REVIEW

ARTICLE TITLE AND BIBLIOGRAPHIC INFORMATION

Bohner L, Hanisch M, Sesma N, Blanck-Lubarsch M, Kleinheinz J. Artifacts in magnetic resonance imaging caused by dental materials: a systematic review. Dentomaxillofac Radiol (2022) 10.1259/dmfr.20210450.

SOURCE OF FUNDING

Non-profit, Foundations: Open access funding enabled by Projekt DEAL, Germany.

TYPE OF STUDY/DESIGN

Systematic review.

The effect of metallic dental restorations and implants in causing patient discomfort and artefacts during magnetic resonance imaging of the head and neck

Context

Patients with metallic dental objects in their oral cavity are often associated with discomfort or artefacts during magnetic resonance imaging (MRI) of the head and neck.

Aim

This study was conducted to evaluate the effects of metallic dental objects in causing patient discomfort and artefacts during MRI of the head and neck region.

Design

This is an observational study including 538 participants from various specialized MRI centres in Kozhikode.

Methods and Material

MRI records of 538 participants who underwent head and neck MRI as a part of their medical treatment were collected. With the help of questionnaire and clinical examination, the type of metallic dental object in the patient's oral cavity was identified. After examining the MRI images, four categories of artefacts were established. In MRI brain, artefacts in three different sequences were also studied.

Statistical Analysis Used

The significance of the difference between proportions was analysed by the Chi-square test.

Results

Artefacts were found in 65 cases with metallic dental objects but none of the participants had experienced any kind of discomfort. The artefact formation significantly depended on the type of metallic dental restoration in the patient's oral cavity. Orthodontic braces and titanium bone plate created severe artefacts in head and neck MRI making interpretation almost impossible. In the MRI brain, diffusion-weighted imaging is most sensitive to artefact followed by fluid-attenuated inversion recovery.

Conclusions

Artefacts were found in 65 cases with metallic dental restorations but none of the participants had experienced any kind of discomfort.

Susceptibility artifacts induced by crowns of different materials with prepared teeth and titanium implants in magnetic resonance imaging

This study aimed to investigate the artifacts induced by crowns composed of different materials with prepared teeth and titanium implants. Resin, metal-ceramic, ceramic and zirconia crowns were fabricated and placed onto the prepared teeth on a human cadaver head or titanium implants with prosthesis abutments on a dry human mandible. The samples were scanned on a 1.5 T MRI apparatus, and artifact areas were defined as the signal intensity and signal loss adjacent to the prosthesis and measured by a threshold tool with ImageJ2x. Data were analyzed using SPSS 22.0. Resin, ceramic, zirconia, and precious metal-ceramic crowns barely produced artifacts on the cadaver skull (p > 0.999). By contrast, pure Ti and nonprecious metal-ceramic crowns created significant artifacts (p < 0.001). The average artifacts reduction of double Au-Pt and Ag-Pd metal-ceramic crowns combined with titanium implants and abutments was 79.49 mm² (p < 0.001) and 74.17 mm² (p < 0.001) respectively, while artifact areas were increased in double Co-Cr and Ni–Cr metal-ceramic crowns by 150.10 mm² (p < 0.001) and 175.50 mm² (p < 0.001) respectively. Zirconia, ceramic and precious metal-ceramic crowns induce less MRI artifacts after tooth preparation while precious metal-ceramic crowns alleviate artifacts in combination with titanium implants.

Modal Analysis of the Ancillary During Femoral Stem Insertion: A Study on Bone Mimicking Phantoms

The femoral stem primary stability achieved by the impaction of an ancillary during its insertion is an important factor of success in cementless surgery. However, surgeons still rely on their proprioception, making the process highly subjective. The use of Experimental Modal Analysis (EMA) without sensor nor probe fixation on the implant or on the bone is a promising non destructive approach to determine the femoral stem stability. The aim of this study is to investigate whether EMA performed directly on the ancillary could be used to monitor the femoral stem insertion into the bone. To do so, a cementless femoral stem was inserted into 10 bone phantoms of human femurs and EMA was carried out on the ancillary using a dedicated impact hammer for each insertion step. Two bending modes could be identified in the frequency range [400-8000] Hz for which the resonance frequency was shown to be sensitive to the insertion step and to the bone-implant interface properties. A significant correlation was obtained between the two modal frequencies and the implant insertion depth (R² = 0.95 ± 0.04 and R² = 0.94 ± 0.06). This study opens new paths towards the development of noninvasive vibration based evaluation methods to monitor cementless implant insertion.

Estimating bone mineral density using MRI in medicine and dentistry: a literature review

OBJECTIVES

Studies performed in the medical area have shown that an indirect diagnosis of bone mineral density (BMD) is feasible by assessing the amount of bone marrow fat with non-ionizing magnetic resonance imaging (MRI). In dentistry, radiographic methods are still the most used for alveolar bone diagnosis. The present literature review aimed at addressing the role of MRI in assessing BMD in medicine and dentistry.

METHODS

MEDLINE and EMBASE databases were searched for articles published up to 2019.

RESULTS

A total of 428 potentially eligible papers were screened. Of these, 397 were excluded after title, abstract and keyword assessment, yielding 31 papers that potentially met the inclusion criteria. Eleven studies were then excluded because their full texts did not discuss the role of MRI in the indirect diagnosis of BMD. As a result, a total of 20 studies were finally identified as eligible for inclusion in this literature review. Most studies found satisfactory accuracy of MRI for indirectly assessing BMD by quantifying bone mineral fat (BMF). However, only one of these studies was on dentistry.

CONCLUSION

Within the limitations of this study, the present findings suggest that MRI is accurate to indirectly estimate bone density by assessing BMF, and could be clinically relevant during dental treatment planning.

Comparison of contrast-enhanced videofluoroscopy to unenhanced dynamic MRI in minor patients following surgical correction of velopharyngeal dysfunction

OBJECTIVES

To compare dynamic magnetic resonance imaging (MRI) with videofluoroscopy (VFS) regarding image quality and assessment of gap size between soft palate (SP) and posterior pharyngeal wall (PPW) in children and adolescents following surgical correction of velopharyngeal dysfunction (VPD).

METHODS

Twenty-one patients undergoing unenhanced 3-T MRI and contrast-enhanced VFS were included in this IRB-approved prospective study. The MRI scan protocol comprised refocused gradient-echo sequences in transverse and sagittal planes during speech, with TE 1.97 ms, TR 3.95 ms, flip angle 8°, matrix size 128 × 128, and 5-mm slice thickness. Radial k-space sampling and sliding window reconstruction were used to achieve an image acquisition rate of 28 frames per second (fps). VFS with 30 fps was similarly performed in both planes. Closure of the velopharyngeal port during phonation was evaluated by two experienced radiologists.

RESULTS

Eleven (52.4%) patients displayed a complete closure, whereas ten (47.6%) patients showed a post-operative gap during speech. VFS and MRI equally identified the cases with persistent or recurrent VPD. Differences in SP-PPW distance between VFS (3.9 ± 1.6 mm) and MRI (4.1 ± 1.5 mm) were not statistically significant (p = 0.5). The subjective overall image quality of MRI was rated inferior (p < 0.001) compared with VFS, with almost perfect inter-rater agreement (κ = 0.90). The presence of susceptibility artifacts did not limit anatomical measurements.

CONCLUSION

Dynamic MRI is equally reliable as VFS to assess persistent or recurrent inadequate velum closure in patients following surgical treatment of VPD.

KEY POINTS

• Unenhanced 3-T dynamic MRI and contrast-enhanced videofluoroscopy are equally useful for the identification of patients with incomplete velopharyngeal closure during speech. • MRI using refocused gradient-echo acquisition with radial k-space sampling and sliding window reconstruction generates diagnostic images with 28 frames per second. • MRI can offer a radiation-free alternative to currently established videofluoroscopy for young patients.

In vivo comparison of MRI- and CBCT-based 3D cephalometric analysis: beginning of a non-ionizing diagnostic era in craniomaxillofacial imaging?

OBJECTIVES

To evaluate whether magnetic resonance imaging (MRI) can serve as an alternative diagnostic tool to the "gold standard" cone-beam computed tomography (CBCT) in 3D cephalometric analysis.

METHODS

In this prospective feasibility study, 12 patients (8 males, 4 females; mean age ± SD, 26.1 years ± 6.6) underwent 3D MRI and CBCT before orthognathic surgery. 3D cephalometric analysis was performed twice by two independent observers on both modalities. For each dataset, 27 cephalometric landmarks were defined from which 35 measurements (17 angles, 18 distances) were calculated. Statistical analyses included the calculation of Euclidean distances, intraclass correlation coefficients (ICCs), Bland-Altman analysis, and equivalence testing (linear mixed effects model) with a predefined equivalence margin of ± 1°/1 mm.

RESULTS

Analysis of reliability for CBCT vs. MRI (intra-rater I/intra-rater II/inter-rater) revealed Euclidean distances of 0.86/0.86/0.98 mm vs. 0.93/0.99/1.10 mm for landmarks, ICCs of 0.990/0.980/0.986 vs. 0.982/0.978/0.980 for angles, and ICCs of 0.992/0.988/0.989 vs. 0.991/0.985/0.988 for distances. Bland-Altman analysis showed high levels of agreement between CBCT and MRI with bias values (95% levels of agreement) of 0.03° (- 1.49; 1.54) for angles and 0.02 mm (- 1.44; 1.47) for distances. In the linear mixed effects model, the mean values of CBCT and MRI measurements were equivalent.

CONCLUSION

This feasibility study indicates that MRI enables reliable 3D cephalometric analysis with excellent agreement to corresponding measurements on CBCT. Thus, MRI could serve as a non-ionizing alternative to CBCT for treatment planning and monitoring in orthodontics as well as oral and maxillofacial surgery.

KEY POINTS

• Clinically established 3D cephalometric measurements performed on MRI are highly reliable and show an excellent agreement with CBCT (gold standard). • The MRI technique applied in this study could be used as a non-ionizing diagnostic tool in orthodontics as well as oral and maxillofacial surgery. • Since most patients benefiting from 3D cephalometry are young in age, the use of MRI could substantially contribute to radiation protection and open up new possibilities for treatment monitoring.

Assessment of signal-to-noise ratio and contrast-to-noise ratio in 3 T magnetic resonance imaging in the presence of zirconium, titanium, and titanium-zirconium alloy implants

OBJECTIVE

We quantitatively compared the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) in T1 weighted imaging (WI) and T2 WI sequences in 3 Tesla (T) magnetic resonance imaging (MRI) using zirconium, titanium (grades 4 and 5), and titanium-zirconium alloy implants to evaluate the effect of implant type and imaging sequence.

STUDY DESIGN

MRI was acquired using a 3 T magnet with a 16-channel head coil. Implants of each type were mounted in gel and scanned in axial, coronal, and sagittal planes using fast spin echo sequences in T1 WI (TR = 600, TE = 12 milliseconds) and T2 WI (TR = 3000, TE = 80 milliseconds) sequences. Data were transferred to Synapse 3-D software, and images were measured twice by an oral and maxillofacial radiologist blinded to the type of implants.

RESULTS

Zirconium implants resulted in the lowest SNR and CNR values (P < .05). No significant differences were identified between titanium (grades 4 and 5) and titanium-zirconium implants. The T2 WI sequence had a significantly higher SNR and CNR than T1 WI. There was no difference in intraobserver agreement between T1 WI and T2 WI.

CONCLUSIONS

CNR and SNR at 3 T MRI are dependent on implant type and imaging sequence. Titanium (grades 4 and 5) and titanium-zirconium implants and the T2 WI sequence produced higher SNR and CNR values.

Intensity of artefacts in cone beam CT examinations caused by titanium and glass fibre-reinforced composite implants

OBJECTIVES:

The aim was to compare titanium and glass fibre-reinforced composite (FRC) orbital floor implants using cone beam CT (CBCT). FRC implants are nonmetallic and these implants have not been analysed in CBCT images before. The purpose of this study is to compare the artefact formation of the titanium and the FRC orbital floor implants in CBCT images.

METHODS:

One commercially pure titanium and one S-glass FRC with bioactive glass particles implant were imaged with CBCT using the same imaging values (80 kV, 1 mA, FOV 60 × 60 mm). CBCT images were analysed in axial slices from three areas to determine the magnitude of the artefacts in the vicinity of the implants. Quantified results based on the gray values of images were analysed using analysis-of-variance.

RESULTS:

Compared to the reference the gray values of the titanium implant are more negative in every region of interest in all slices (p < 0.05) whereas the gray values of the FRC implant differ statistically significantly in less than half of the examined areas.

CONCLUSIONS:

The titanium implant caused artefacts in all of the analysed CBCT slices. Compared to the reference the gray values of the FRC implant changed only slightly and this feature enables to use wider imaging options postoperatively.

Unwanted effects due to interactions between dental materials and magnetic resonance imaging: a review of the literature

Magnetic resonance imaging (MRI) is an advanced diagnostic tool used in both medicine and dentistry. Since it functions based on a strong uniform static magnetic field and radiofrequency pulses, it is advantageous over imaging techniques that rely on ionizing radiation. Unfortunately, the magnetic field and radiofrequency pulses generated within the magnetic resonance imager interact unfavorably with dental materials that have magnetic properties. This leads to unwanted effects such as artifact formation, heat generation, and mechanical displacement. These are a potential source of damage to the oral tissue surrounding the affected dental materials. This review aims to compile, based on the current available evidence, recommendations for dentists and radiologists regarding the safety and appropriate management of dental materials during MRI in patients with orthodontic appliances, maxillofacial prostheses, dental implants, direct and indirect restorative materials, and endodontic materials.

3D cephalometric analysis using Magnetic Resonance Imaging: validation of accuracy and reproducibility

The aim of this study was to validate geometric accuracy and in vivo reproducibility of landmark-based cephalometric measurements using high-resolution 3D Magnetic Resonance Imaging (MRI) at 3 Tesla. For accuracy validation, 96 angular and 96 linear measurements were taken on a phantom in 3 different positions. In vivo MRI scans were performed on 3 volunteers in five head positions. For each in vivo scan, 27 landmarks were determined from which 19 angles and 26 distances were calculated. Statistical analysis was performed using Bland-Altman analysis, the two one-sided tests procedure and repeated measures one-way analysis of variance. In comparison to ground truth, all MRI-based phantom measurements showed statistical equivalence (p < 0.001) and an excellent agreement in Bland-Altman analysis (bias ranges: -0.090-0.044°, -0.220-0.241 mm). In vivo cephalometric analysis was highly reproducible among the five different head positions in all study participants, without statistical differences for all angles and distances (p > 0.05). Ranges between maximum and minimum in vivo values were consistently smaller than 2° and 2 mm, respectively (average ranges: 0.88°/0.87 mm). In conclusion, this study demonstrates that accurate and reproducible 3D cephalometric analysis can be performed without exposure to ionizing radiation using MRI.

Evaluation and reduction of magnetic resonance imaging artefacts induced by distinct plates for osseous fixation: an in vitro study @ 3 T

OBJECTIVES:

To analyze MRI artefacts induced at 3 T by bioresorbable, titanium (TI) and glass fibre reinforced composite (GFRC) plates for osseous reconstruction.

METHODS:

Fixation plates including bioresorbable polymers (Inion CPS, Inion Oy, Tampere, Finland; Rapidsorb, DePuy Synthes, Umkirch, Germany; Resorb X, Gebrueder KLS Martin GmbH, Tuttlingen, Germany), GFRC (Skulle Implants Oy, Turku, Finland) and TI plates of varying thickness and design (DePuy Synthes, Umkirch, Germany) were embedded in agarose gel and a 3 T MRI was performed using a standard protocol for head and neck imaging including T₁W and T₂W sequences. Additionally, different artefact reduction techniques (slice encoding for metal artefact reduction & ultrashort echo time) were used and their impact on the extent of artefacts evaluated for each material.

RESULTS:

All TI plates induced significantly more artefacts than resorbable plates in T₁W and T₂W sequences. GFRCs induced the least artefacts in both sequences. The total extent of artefacts increased with plate thickness and height. Plate thickness had no influence on the percentage of overestimation in all three dimensions. TI-induced artefacts were significantly reduced by both artefact reduction techniques.

CONCLUSIONS:

Polylactide, GFRC and magnesium plates produce less susceptibility artefacts in MRI compared to TI, while the dimensions of TI plates directly influence artefact extension. Slice encoding for metal artefact reduction and ultrashort echo time significantly reduce metal artefacts at the expense of scan time or image resolution.

High-resolution MR imaging for dental impressions: a feasibility study

OBJECTIVES

Magnetic resonance imaging is an emerging technology in dental medicine. While low-resolution MRI has especially provided means to examine the temporomandibular joint due to its anatomic inaccessibility, it was the goal of this study to assess whether high-resolution MRI is capable of delivering a dataset sufficiently precise enough to serve as digital impression of human teeth.

MATERIALS AND METHODS

An informed and consenting patient in need of dental restoration with fixed partial dentures was chosen as subject. Two prepared teeth were measured using MRI and the dataset subjected to mathematical processing before Fourier transformation. After reconstruction, a 3D file was generated which was fed into an existing industry standard CAD/CAM process.

RESULTS

A framework for a fixed dental prosthesis was digitally modeled and manufactured by laser-sintering. The fit in situ was found to be acceptable by current clinical standards, which allowed permanent placement of the fixed prosthesis.

CONCLUSIONS

Using a clinical whole-body MR scanner with the addition of custom add-on hardware, contrast enhancement, and data post-processing, resolution and signal-to-noise ratio were sufficiently achieved to allow fabrication of a dental restoration in an acquisition time comparable to the setting time of common dental impression materials. Furthermore, the measurement was well tolerated.

CLINICAL RELEVANCE

The herein described method can be regarded as proof of principle that MRI is a promising option for digital impressions when fixed partial dentures are required.

[Magnetic resonance imaging artefacts and fixed orthodontic attachments]

OBJECTIVES

Orthodontic appliances are often removed before magnetic resonance imaging (MRI) scans because they are known to produce artefacts. The purpose of this study was to find the exact indications for removal of various fixed attachments when imaging four specific areas of the head and neck.

MATERIALS AND METHODS

Sixty patients requiring an MRI scan of the head for medical reasons volunteered for this investigation. One of four different types of fixed attachments (stainless steel brackets, titanium brackets, ceramic brackets with metal slots, and stainless steel retainers) were assigned to a patient. Each patient had two scans at 1.5 T: with an "empty wax jig" and with a wax jig including the attachment. Archwires were not investigated as they are easily removed before a scan. Two radiologists evaluated the scans of each patient and each of the four areas under investigation: maxillary sinus, oral cavity, temporomandibular joints and posterior cerebral fossa.

RESULTS

Stainless steel brackets always caused non-interpretability of all anatomic areas (100 per cent). Titanium brackets, ceramic brackets with metal slots, and stainless steel retainers caused artefact in the oral cavity only (for 20, 16.65, and 86.65 per cent of the subjects).

CONCLUSIONS

Our results show that ceramic brackets with metal slots and titanium brackets do not always have to be removed before MRI scans of the head and neck, depending on the area under investigation. Metal fixed retainers should only be removed if the oral cavity itself is under investigation. Stainless steel brackets should always be removed before MRI scans of the head and neck.

Effects of orthodontic appliances on the diagnostic quality of magnetic resonance images of the head

INTRODUCTION

The influence of 4 commonly used fixed orthodontic appliances on artifact formation and diagnostic quality of magnetic resonance (MR) images of the head produced by a 3-T MR scanner was studied.

METHODS

Stainless steel brackets, ceramic brackets, combination of ceramic brackets and steel molar tubes, and multistranded steel mandibular lingual retainers were embedded into custom-made trays for each of 10 adult subjects. Head MR scans of 9 regions were acquired for each subject wearing these trays. Sagittal T1-weighted, axial T2-weighted, axial gradient-recalled, axial diffusion-weighted, noncontrast axial MR angiography, and axial fluid-attenuated inversion recovery MR sequences were included. Two neuroradiologists evaluated image distortions and diagnostic qualities of the 1314 acquired images (13860 image slices).

RESULTS

The images were affected by appliance, head region, and MR sequence. Stainless steel brackets and molar tubes affected the images the most, and ceramic brackets caused minimal image distortion.

CONCLUSIONS

Head MR images are differentially affected by orthodontic appliances. The appliance, region imaged, and MR sequence should be considered before imaging patients wearing fixed orthodontic appliances.

Magnetic resonance imaging susceptibility artifacts in the cervical vertebrae and spinal cord related to monocortical screw-polymethylmethacrylate implants in canine cadavers

OBJECTIVE To characterize and compare MRI susceptibility artifacts related to titanium and stainless steel monocortical screws in the cervical vertebrae and spinal cord of canine cadavers. SAMPLE 12 canine cadavers. PROCEDURES Cervical vertebrae (C4 and C5) were surgically stabilized with titanium or stainless steel monocortical screws and polymethylmethacrylate. Routine T1-weighted, T2-weighted, and short tau inversion recovery sequences were performed at 3.0 T. Magnetic susceptibility artifacts in 20 regions of interest (ROIs) across 4 contiguous vertebrae (C3 through C6) were scored by use of an established scoring system. RESULTS Artifact scores for stainless steel screws were significantly greater than scores for titanium screws at 18 of 20 ROIs. Artifact scores for titanium screws were significantly higher for spinal cord ROIs within the implanted vertebrae. Artifact scores for stainless steel screws at C3 were significantly less than at the other 3 cervical vertebrae. CONCLUSIONS AND CLINICAL RELEVANCE Evaluation of routine MRI sequences obtained at 3.0 T revealed that susceptibility artifacts related to titanium monocortical screws were considered mild and should not hinder the overall clinical assessment of the cervical vertebrae and spinal cord. However, mild focal artifacts may obscure small portions of the spinal cord or intervertebral discs immediately adjacent to titanium screws. Severe artifacts related to stainless steel screws were more likely to result in routine MRI sequences being nondiagnostic; however, artifacts may be mitigated by implant positioning.

MRI compatibility of several early transition metal based alloys and its influencing factors

Magnetic resonance imaging (MRI) compatibility of three early transition metal (ETM) based alloys was assessed in vitro with agarose gel as a phantom, including Zr-20Nb, near-equiatomic (TiZrNbTa)₉₀ Mo₁₀ and Nb-60Ta-2Zr, together with pure tantalum and L605 Co-Cr alloy for comparison. The artifact extent in the MR image was quantitatively characterized according to the maximum area of 2D images and the total volume in reconstructed 3D images with a series of slices under acquisition by fast spin echo (FSE) sequence and gradient echo (GRE) sequence. It was indicated that the artifacts extent of L605 Co-Cr alloy with a higher magnetic susceptibility (χ_v ) was approximately 3-fold greater than that of the ETM-based alloys with χ_v in the range of 160-250 ppm. In the ETM group, the MRI compatibility of the materials can be ranked in a sequence of Zr-20Nb, pure tantalum, (TiZrNbTa)₉₀ Mo₁₀ and Nb-60Ta-2Zr. In addition, using a rabbit cadaver with the implanted tube specimens as a model for ex vivo assessment, it was confirmed that the artifact severity of Nb-60Ta-2Zr alloy is significantly reduced in comparison with the L605 alloy. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 377-385, 2018.

Artefacts in multimodal imaging of titanium, zirconium and binary titanium-zirconium alloy dental implants: an in vitro study

OBJECTIVES

To analyze and evaluate imaging artefacts induced by zirconium, titanium and titanium-zirconium alloy dental implants.

METHODS

Zirconium, titanium and titanium-zirconium alloy implants were embedded in gelatin and MRI, CT and CBCT were performed. Standard protocols were used for each modality. For MRI, line-distance profiles were plotted to quantify the accuracy of size determination. For CT and CBCT, six shells surrounding the implant were defined every 0.5 cm from the implant surface and histogram parameters were determined for each shell.

RESULTS

While titanium and titanium-zirconium alloy induced extensive signal voids in MRI owing to strong susceptibility, zirconium implants were clearly definable with only minor distortion artefacts. For titanium and titanium-zirconium alloy, the MR signal was attenuated up to 14.1 mm from the implant. In CT, titanium and titanium-zirconium alloy resulted in less streak artefacts in comparison with zirconium. In CBCT, titanium-zirconium alloy induced more severe artefacts than zirconium and titanium.

CONCLUSIONS

MRI allows for an excellent image contrast and limited artefacts in patients with zirconium implants. CT and CBCT examinations are less affected by artefacts from titanium and titanium-zirconium alloy implants compared with MRI. The knowledge about differences of artefacts through different implant materials and image modalities might help support clinical decisions for the choice of implant material or imaging device in the clinical setting.

High isotropic resolution magnetic resonance imaging of the mandibular canal at 1.5 T: a comparison of gradient and spin echo sequences

OBJECTIVES

The precision of localizing the mandibular canal prior to surgical intervention depends on the achievable resolution, whereas identification of the nerve depends on the image contrast. In our study, we developed new protocols based on gradient and spin echo sequences. The results from both sequences were quantitatively compared for their agreement to identify the most suitable approach.

METHODS

By limiting the field of view to one side of the mandible, three-dimensional acquisitions with T₁ weighted gradient and spin echo sequences were performed with 0.5 × 0.5 × 0.5 mm³ resolution within 6.5 min covering the mandibular canal from the mandibular to the mental foramen. Aliasing artefacts were suppressed by different techniques. A manual segmentation of the mandibular canal from seven healthy volunteers was performed on this section by three different observers. The surface distance of the segmented volumes was computed between both sequences as well as between the different observers as a measure of equality.

RESULTS

The quantitative comparison of the segmentation resulted in an average surface distance of 0.26 ± 0.05 mm between both sequences and an interobserver difference of 0.26 ± 0.08 mm for gradient and 0.29 ± 0.07 mm for spin echo data. By repeated evaluation, a difference of 0.15 ± 0.02 mm for gradient and 0.18 ± 0.03 mm for spin echo data was observed, indicating a slightly higher variability for spin echo images.

CONCLUSIONS

Both sequences can be used to achieve high-resolution images with good contrast and can be used for precise localization of the mandibular canal. Despite a slightly increased difference for the spin echo data, the advantage of an easy and robust setup remains.

A standardized evaluation of artefacts from metallic compounds during fast MR imaging

OBJECTIVES

Metallic compounds present in the oral and maxillofacial regions (OMRs) cause large artefacts during MR scanning. We quantitatively assessed these artefacts embedded within a phantom according to standards set by the American Society for Testing and Materials (ASTM).

METHODS

Seven metallic dental materials (each of which was a 10-mm³ cube embedded within a phantom) were scanned [i.e. aluminium (Al), silver alloy (Ag), type IV gold alloy (Au), gold-palladium-silver alloy (Au-Pd-Ag), titanium (Ti), nickel-chromium alloy (NC) and cobalt-chromium alloy (CC)] and compared with a reference image. Sequences included gradient echo (GRE), fast spin echo (FSE), gradient recalled acquisition in steady state (GRASS), a spoiled GRASS (SPGR), a fast SPGR (FSPGR), fast imaging employing steady state (FIESTA) and echo planar imaging (EPI; axial/sagittal planes). Artefact areas were determined according to the ASTM-F2119 standard, and artefact volumes were assessed using OsiriX MD software (Pixmeo, Geneva, Switzerland).

RESULTS

Tukey-Kramer post hoc tests were used for statistical comparisons. For most materials, scanning sequences eliciting artefact volumes in the following (ascending) order FSE-T₁/FSE-T₂ < FSPGR/SPGR < GRASS/GRE < FIESTA < EPI. For all scanning sequences, artefact volumes containing Au, Al, Ag and Au-Pd-Ag were significantly smaller than other materials (in which artefact volume size increased, respectively, from Ti < NC < CC). The artefact-specific shape (elicited by the cubic sample) depended on the scanning plane (i.e. a circular pattern for the axial plane and a "clover-like" pattern for the sagittal plane).

CONCLUSIONS

The availability of standardized information on artefact size and configuration during MRI will enhance diagnosis when faced with metallic compounds in the OMR.

Should the orthodontic brackets always be removed prior to magnetic resonance imaging (MRI)?

Request for temporary removal of orthodontic appliances due to medical conditions that require magnetic resonance (MR) imaging is not uncommon in daily practice in the field of orthodontics. This may be at the expense of time and cost. Metal Orthodontic appliances cause more signal loss and image distortion as compared to ceramic and titanium ones. Stainless steel and large brackets in addition to the oriented miniscrews in relation to the axis of magnetic field may cause severe signal loss and image distortion. Moreover, gradient echo and frequency-selective fat saturation MR protocols are more susceptible to metal artifacts. The spin echo and fat-suppression protocols, low magnetic field strength (e.g., 1.5 Tesla vs. 3 Tesla), small field of view, high-resolution matrix, thin slice, increased echo train length and increased receiver band width could be applied to lessen the metal artifacts in MR images. The larger the distance between an appliance and desired location to be imaged, the lower the distortion and signal loss. Decision to remove brackets should be made based on its composition and desired anatomic location. In this review, first the principles of MR imaging are introduced (Part-I) and then the interactions of orthodontic appliances and magnetic field are farther discussed (Part-II).

Metal artifact reduction in patients with dental implants using multispectral three-dimensional data acquisition for hybrid PET/MRI

BACKGROUND

Hybrid positron emission tomography/magnetic resonance imaging (PET/MRI) shows high potential for patients with oropharyngeal cancer. Dental implants can cause substantial artifacts in the oral cavity impairing diagnostic accuracy. Therefore, we evaluated new MRI sequences with multi-acquisition variable-resonance image combination (MAVRIC SL) in comparison to conventional high-bandwidth techniques and in a second step showed the effect of artifact size on MRI-based attenuation correction (AC) with a simulation study.

METHODS

Twenty-five patients with dental implants prospectively underwent a trimodality PET/CT/MRI examination after informed consent was obtained under the approval of the local ethics committee. A conventional 3D gradient-echo sequence (LAVA-Flex) commonly used for MRI-based AC of PET (acquisition time of 14 s), a T1w fast spin-echo sequence with high bandwidth (acquisition time of 3.2 min), as well as MAVRIC SL sequence without and with increased phase acceleration (MAVRIC, acquisition time of 6 min; MAVRIC-fast, acquisition time of 3.5 min) were applied. The absolute and relative reduction of the signal void artifact was calculated for each implant and tested for statistical significance using the Wilcoxon signed-rank test. The effect of artifact size on PET AC was simulated in one case with a large tumor in the oral cavity. The relative difference of the maximum standardized uptake value (SUVmax) in the tumor was calculated for increasing artifact sizes centered over the second molar.

RESULTS

The absolute reduction of signal void from LAVA-Flex sequences to the T1-weighted fast spin-echo (FSE) sequences was 416 mm(2) (range 4 to 2,010 mm(2)) to MAVRIC 481 mm(2) (range 12 to 2,288 mm(2)) and to MAVRIC-fast 486 mm(2) (range 39 to 2,209 mm(2)). The relative reduction in signal void was significantly improved for both MAVRIC and MAVRIC-fast compared to T1 FSE (-75%/-78% vs. -62%, p < 0.001 for both). The relative error for SUVmax was negligible for artifacts of 0.5-cm diameter (-0.1%), but substantial for artifacts of 5.2-cm diameter (-33%).

CONCLUSIONS

MAVRIC-fast could become useful for artifact reduction in PET/MR for patients with dental implants. This might improve diagnostic accuracy especially for patients with tumors in the oropharynx and substantially improve accuracy of PET quantification.

Advances in diagnostic imaging for pathologic conditions of the jaws

Advances in dental and maxillofacial imaging are delineated along with the advantages and disadvantages of each imaging modality. The imaging modalities that are included are intraoral radiography, panoramic radiography, cone-beam computed tomography, multidetector computed tomography, magnetic resonance imaging, nuclear medicine, and ultrasound.

Does magnetic resonance imaging affect the microleakage of amalgam restorations?

BACKGROUND

The effect of MRI on microleakage of amalgam restorations is an important health issue that should be considered. If MRI application causes increase of microleakage, amalgam fillings should be reassessed after MRI and replaced if necessary.

OBJECTIVES

The aim of this study is to compare the effect of magnetic resonance imaging (MRI) on microleakage of class II bonded amalgam versus classical amalgam restorations.

MATERIALS AND METHODS

Class II cavities (3 mm width × 1.5 mm depth) with gingival margins ending 1 mm below the cementoenamel junction (CEJ) were prepared in 40 permanent molar teeth. The teeth were randomly divided into four groups. Cavities in the first and second groups were restored with dentin adhesive and amalgam (bonded amalgam), and those in the third and fourth groups with amalgam only. MRI was performed with the teeth specimens from the first and third groups. All specimens were then thermocycled at 5° to 55° C with a 30-second dwell time for 1000 cycles. The samples were then immersed in 0.5% methylene blue dye for 24 hours and sectioned longitudinally. Dye penetration at the occlusal and gingival margins was quantified by 15× stereomicroscopy. IBM SPSS Statistics ver. 21.0 (IBM Corp., Released 2012., IBM SPSS Statistics for Windows, Armonk, NY: IBM Corp.) and MS-Excel 2007 programs were used for statistical analyses and calculations. "nparLD" module was used for F2_LD_F1 design analysis at R program. P<0.05 was considered statistically significant.

RESULTS

In teeth with amalgam filling, there were no significant differences of occlusal and gingival surface microleakage after MRI exposure. Occlusal and gingival surface microleakages were also similar with and without MRI in teeth with bonded amalgam filling.

CONCLUSIONS

The results of this study suggest that MRI does not increase microleakage of amalgam restorations.

Magnetic resonance imaging artefacts and fixed orthodontic attachments

BACKGROUND/OBJECTIVES

Orthodontic appliances are often removed before magnetic resonance imaging (MRI) scans because they are known to produce artefacts. The purpose of this study was to find the exact indications for removal of various fixed attachments when imaging four specific areas of the head and neck.

MATERIALS/METHODS

Sixty patients requiring an MRI scan of the head for medical reasons volunteered for this investigation. One of four different types of fixed attachments (stainless steel brackets, titanium brackets, ceramic brackets with metal slots, and stainless steel retainers) were assigned to a patient. Each patient had two scans at 1.5 T: with an 'empty wax jig' and with a wax jig including the attachment. Archwires were not investigated as they are easily removed before a scan. Two radiologists evaluated the scans of each patient and each of the four areas under investigation: maxillary sinus, oral cavity, temporomandibular joints, and posterior cerebral fossa.

RESULTS

Stainless steel brackets always caused non-interpretability of all anatomic areas (100 per cent). Titanium brackets, ceramic brackets with metal slots, and stainless steel retainers caused artefact in the oral cavity only (for 20, 16.65, and 86.65 per cent of the subjects).

CONCLUSIONS/IMPLICATIONS

Our results show that ceramic brackets with metal slots and titanium brackets do not always have to be removed before MRI scans of the head and neck, depending on the area under investigation. Metal fixed retainers should only be removed if the oral cavity itself is under investigation. Stainless steel brackets should always be removed before MRI scans of the head and neck.

Magnetic resonance imaging (MRI) artefacts in hip prostheses: a comparison of different prosthetic compositions

PURPOSE

The purpose of the study was to compare the artefacts produced by different hip prostheses on magnetic resonance imaging (MRI).

MATERIALS AND METHODS

An identical MRI protocol was used to perform a quali-quantitative in vitro evaluation of artefacts caused by different hip prosthetic materials at different field strengths: prosthesis number 1, composed of cobalt-chrome-molybdenum (head and stem); prosthesis number 2, composed of ceramic (head) and titanium (stem); prosthesis number 3, composed of cobalt-chrome (head) and titanium (stem). All prostheses were imaged with both a clinical 1 Tesla (T) (Signa Horizon, General Electrics) and 1.5 T (Achieva, Philips) MRI system, using spin echo (SE) and gradient echo (GRE) sequences: sagittal T1 SE, coronal T2 fast SE (FSE), axial T1 SE, axial T2 FSE, sagittal T2 GRE, axial T2* GRE, coronal T1 GRE, axial T1 GRE. The artefacts produced by each prosthesis were assessed in each sequence at the different field strengths, by measuring the two longest diameters of the artefact in each section and sequence and comparing them to the actual diameters so as to obtain a ratio expressing the effective degree of artefact.

RESULTS

Cobalt-chrome produced the largest artefacts both in SE (1.73 at 1 T and 2.37 at 1.5 T) and GRE sequences (2.8 at 1 T and 3.06 at 1.5 T) followed by titanium (SE, 1.6 at 1 T, 2.13 at 1.5 T; GRE, 2 at 1 T, 2.94 at 1.5 T) and cobalt-chrome-molybdenum (SE, 1.51 at 1 T, 1.67 at 1.5 T; GRE, 2.13 at 1 T and 2.48 at 1.5 T); ceramic produced the smallest artefacts in all sequences (SE, 1.0 at 1 T and 1.18 at 1.5 T; GRE, 1.3 at 1 T and 1.22 at 1.5T). Increasing the magnetic field strength, titanium showed the greatest variations in artefact size, and ceramic the smallest ones.

CONCLUSIONS

The composition of prosthetic implants is decisive in determining the quality of MR imaging.

MR image based approach for metal artifact reduction in X-ray CT

For decades, computed tomography (CT) images have been widely used to discover valuable anatomical information. Metallic implants such as dental fillings cause severe streaking artifacts which significantly degrade the quality of CT images. In this paper, we propose a new method for metal-artifact reduction using complementary magnetic resonance (MR) images. The method exploits the possibilities which arise from the use of emergent trimodality systems. The proposed algorithm corrects reconstructed CT images. The projected data which is affected by dental fillings is detected and the missing projections are replaced with data obtained from a corresponding MR image. A simulation study was conducted in order to compare the reconstructed images with images reconstructed through linear interpolation, which is a common metal-artifact reduction technique. The results show that the proposed method is successful in reducing severe metal artifacts without introducing significant amount of secondary artifacts.

Effects of low-dose microwave on healing of fractures with titanium alloy internal fixation: an experimental study in a rabbit model

Background

Microwave is a method for improving fracture repair. However, one of the contraindications for microwave treatment listed in the literature is surgically implanted metal plates in the treatment field. The reason is that the reflection of electromagnetic waves and the eddy current stimulated by microwave would increase the temperature of magnetic implants and cause heat damage in tissues. Comparing with traditional medical stainless steel, titanium alloy is a kind of medical implants with low magnetic permeability and electric conductivity. But the effects of microwave treatment on fracture with titanium alloy internal fixation invivo were not reported. The aim of this article was to evaluate the security and effects of microwave on healing of a fracture with titanium alloy internal fixation.

Methods

Titanium alloy internal fixation systems were implanted in New Zealand rabbits with a 3.0 mm bone defect in the middle of femur. We applied a 30-day microwave treatment (2,450MHz, 25W, 10 min per day) to the fracture 3 days after operation. Temperature changes of muscle tissues around implants were measured during the irradiation. Normalized radiographic density of the fracture gap was measured on the 10th day and 30th day of the microwave treatment. All of the animals were killed after 10 and 30 days microwave treatment with histologic and histomorphometric examinations performed on the harvested tissues.

Findings

The temperatures did not increase significantly in animals with titanium alloy implants. The security of microwave treatment was also supported by histology of muscles, nerve and bone around the implants. Radiographic assessment, histologic and histomorphometric examinations revealed significant improvement in the healing bone.

Conclusion

Our results suggest that, in the healing of fracture with titanium alloy internal fixation, a low dose of microwave treatment may be a promising method.

Influence of dental materials on dental MRI

OBJECTIVES

To investigate the potential influence of standard dental materials on dental MRI (dMRI) by estimating the magnetic susceptibility with the help of the MRI-based geometric distortion method and to classify the materials from the standpoint of dMRI.

METHODS

A series of standard dental materials was studied on a 1.5 T MRI system using spin echo and gradient echo pulse sequences and their magnetic susceptibility was estimated using the geometric method. Measurements on samples of dental materials were supported by in vivo examples obtained in dedicated dMRI procedures.

RESULTS

The tested materials showed a range of distortion degrees. The following materials were classified as fully compatible materials that can be present even in the tooth of interest: the resin-based sealer AH Plus(®) (Dentsply, Maillefer, Germany), glass ionomer cement, gutta-percha, zirconium dioxide and composites from one of the tested manufacturers. Interestingly, composites provided by the other manufacturer caused relatively strong distortions and were therefore classified as compatible I, along with amalgam, gold alloy, gold-ceramic crowns, titanium alloy and NiTi orthodontic wires. Materials, the magnetic susceptibility of which differed from that of water by more than 200 ppm, were classified as non-compatible materials that should not be present in the patient's mouth for any dMRI applications. They included stainless steel orthodontic appliances and CoCr.

CONCLUSIONS

A classification of the materials that complies with the standard grouping of materials according to their magnetic susceptibility was proposed and adopted for the purposes of dMRI. The proposed classification can serve as a guideline in future dMRI research.

Diagnosis of dental abnormalities in children using 3-dimensional magnetic resonance imaging

PURPOSE

To assess the feasibility of magnetic resonance imaging (MRI) of dental abnormalities in children.

MATERIALS AND METHODS

The study included 16 patients (mean age, 10.8 yr) prospectively selected from 1,500 orthodontic patients. The selected patients included 3 with a mesiodens, 9 with supernumerary teeth other than a mesiodens, 1 with gemination, 1 with dilacerations, 1 with transmigration, and 1 with transposition. Three-dimensional (3D) images were acquired on a 1.5-T MRI scanner using a 3D turbo spin echo pulse sequence with a voxel size of 0.8 × 0.8 × 1 mm. The measurement time was 4 to 5 minutes.

RESULTS

Using natural MRI contrast, the teeth, dental pulp, mandibular canal, and cortical bone could be clearly delineated. The position and shape of malformed teeth could be assessed in all 3 spatial dimensions.

CONCLUSION

MRI was found to be a well-tolerated imaging modality for the diagnosis of dental abnormalities in children and for orthodontic treatment and surgical planning. Compared with conventional radiography, dental MRI provides the advantage of 3-dimensionality and complete elimination of ionizing radiation, which is particularly relevant for repeated examinations in children.

Blood and fibroblast responses to thermoset BisGMA-TEGDMA/glass fiber-reinforced composite implants in vitro

OBJECTIVES

This in vitro study was designed to evaluate both blood and human gingival fibroblast responses on fiber-reinforced composite (FRC) aimed to be used as oral implant abutment material.

MATERIAL AND METHODS

Two different types of substrates were investigated: (a) Plain polymer (BisGMA 50%-TEGDMA 50%) and (b) FRC. The average surface roughness (Ra) was measured using spinning-disk confocal microscope. The phase composition was identified using X-ray diffraction analyzer. The degree of monomer conversion (DC%) was determined using FTIR spectrometry. The blood response, including the blood-clotting ability and platelet adhesion morphology, was evaluated. Fibroblast cell responses were studied in cell culture environment using routine test conditions.

RESULTS

The Ra of the substrates investigated was less than 0.1 μm with no signs of surface crystallization. The DC% was 89.1 ± 0.5%. The FRC substrates had a shorter clotting time and higher platelets activation state than plain polymer substrates. The FRC substrates showed higher (P < 0.01-0.001) amount of adhered cells than plain polymer substrates at all time points investigated. The strength of attachment was evaluated using serial trypsinization, the number of cells detached from FRC substrates was 59 ± 5%, whereas those detached from the plane polymer substrates was 70 ± 5%, indicating a stronger (P < 0.01) cell attachment on the FRC surfaces. Fibroblasts grew more efficiently on FRC than on plain polymer substrates, showing significantly higher (P < 0.01) cell metabolic activities throughout the experiment.

CONCLUSIONS

The presence of E-glass fibers enhances blood and fibroblast responses on composite surfaces in vitro.

Magnetic resonance imaging of solid dental restoration materials using 3D UTE sequences: visualization and relaxometry of various compounds

OBJECT

Due to an increasing scientific interest in MR-imaging of carious lesions and teeth, an accurate signal characterization of dental restoration materials is necessary for optimization of MR sequence protocols and evaluation of material degradation. Therefore, signal yield and relaxation behavior of common dental restoration materials in comparison to those of dentine of extracted human teeth were assessed in vitro by ultrashort echo time (UTE) sequences.

MATERIALS AND METHODS

Eighteen material samples and dentine of two freshly extracted human teeth were investigated on a 3T whole-body clinical MR-scanner. Transverse (T2*) and longitudinal relaxation times (T1) were quantified using a recently published modified Ernst equation that takes relevant in-pulse relaxation effects into account.

RESULTS

All investigated samples could be successfully visualized but maximum signal yield was highly variable between samples. T1-values of the investigated dental restoration materials ranged between 28 and 365 ms, whereas T2*-values ranged between 96 and 917 μs. In contrast, T1-values of dentine (T1=545 ms±299 ms) were higher, while T2*-values (T2*=478 μs±271 μs) showed similar values.

CONCLUSIONS

Dental restoration materials and dentine of extracted human teeth can be visualized by UTE sequences and show a broad range of signal yield and relaxation times.

Spinal fusion-hardware construct: Basic concepts and imaging review

The interpretation of spinal images fixed with metallic hardware forms an increasing bulk of daily practice in a busy imaging department. Radiologists are required to be familiar with the instrumentation and operative options used in spinal fixation and fusion procedures, especially in his or her institute. This is critical in evaluating the position of implants and potential complications associated with the operative approaches and spinal fixation devices used. Thus, the radiologist can play an important role in patient care and outcome. This review outlines the advantages and disadvantages of commonly used imaging methods and reports on the best yield for each modality and how to overcome the problematic issues associated with the presence of metallic hardware during imaging. Baseline radiographs are essential as they are the baseline point for evaluation of future studies should patients develop symptoms suggesting possible complications. They may justify further imaging workup with computed tomography, magnetic resonance and/or nuclear medicine studies as the evaluation of a patient with a spinal implant involves a multi-modality approach. This review describes imaging features of potential complications associated with spinal fusion surgery as well as the instrumentation used. This basic knowledge aims to help radiologists approach everyday practice in clinical imaging.

Artifacts in magnetic resonance imaging and computed tomography caused by dental materials

BACKGROUND

Artifacts caused by dental restorations, such as dental crowns, dental fillings and orthodontic appliances, are a common problem in MRI and CT scans of the head and neck. The aim of this in-vitro study was to identify and evaluate the artifacts produced by different dental restoration materials in CT and MRI images.

METHODS

Test samples of 44 materials (Metal and Non-Metal) commonly used in dental restorations were fabricated and embedded with reference specimens in gelatin moulds. MRI imaging of 1.5T and CT scan were performed on the samples and evaluated in two dimensions. Artifact size and distortions were measured using a digital image analysis software.

RESULTS

In MRI, 13 out of 44 materials produced artifacts, while in CT 41 out of 44 materials showed artifacts. Artifacts produced in both MRI and CT images were categorized according to the size of the artifact.

SIGNIFICANCE

Metal based restoration materials had strong influence on CT and less artifacts in MRI images. Rare earth elements such as Ytterbium trifluoride found in composites caused artifacts in both MRI and CT. Recognizing these findings would help dental materials manufacturers and developers to produce materials which can cause less artifacts in MRI and CT images.

Quantitative assessment of artifacts on cardiac magnetic resonance imaging of patients with pacemakers and implantable cardioverter-defibrillators

BACKGROUND

The safety and clinical utility of MRI at 1.5 T in patients with cardiac implantable devices such as pacemakers (PM) and implantable cardioverter-defibrillators (ICD) have been reported. This study aims to evaluate the extent of artifacts on cardiac magnetic resonance (CMR) in patients with PM and ICD (PM/ICD).

METHODS AND RESULTS

A total of 71 CMR studies were performed with an established safety protocol in patients with prepectoral PM/ICD. The artifact area around the PM/ICD generator was measured in all short-axis (SA), horizontal (HLA), and vertical long-axis (VLA) SSFP cine planes. The location and extent of artifacts were also assessed in all SA (20 sectors per plane), HLA, and VLA (6 sectors per plane) late gadolinium-enhanced CMR (LGE-CMR) planes. The artifact area on cine CMR was significantly larger with ICD versus PM generators in each plane (P<0.001, respectively). In patients with left-sided ICD or biventricular ICD systems, the percentages of sectors with any artifacts on LGE-CMR were 53.7%, 48.0%, and 49.2% in SA, HLA, and VLA planes, respectively. Patients with left-sided PM or right-sided PM/ICD had fewer artifacts. Anterior and apical regions were severely affected by artifact caused by left-sided PM/ICD generators.

CONCLUSIONS

In contrast to patients with right-sided PM/ICD and left-sided PM, the anterior and apical left ventricle can be affected by susceptibility artifacts in patients with left-sided ICD. Artifact reduction methodologies will be necessary to improve the performance of CMR in patients with left sided ICD systems.