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

Identifying the Anatomical Variations of the Inferior Alveolar Nerve with Magnetic Resonance Imaging

BACKGROUND

The inferior alveolar nerve (IAN) is located in the mandibular canal (MC). It is critical to evaluate the position of the MC during treatment planning to prevent intra or postoperative complications.

AIMS

This retrospective study aimed to identify the anatomy and anatomical variations of the IAN using soft tissue imaging (pulse sequence magnetic resonance imaging [MRI]).

MATERIALS AND METHODS

This study was designed as a retrospective Consolidated Standards of Reporting Trials (CONSORT) study. In total, 220 MR images were obtained. Nutrient canals (NCs) were classified as intraosseous and dental NCs, while bifid MCs (BMCs) were classified as forward, retromolar, and buccolingual canals. IBM SPSS Statistics 22 was used. Kolmogorov-Smirnov and Shapiro-Wilk tests, descriptive statistical methods (means, standard deviations, and frequencies), and the Chi-square test were used. Statistical significance was set at P < 0.05.

RESULTS

In total, 220 patients (172 females and 48 males) were evaluated. NCs were present in 92.3% of all MCs and were significantly higher in patients aged <25 years. BMCs were observed in 106 patients (24.1%). The most common BMC of MC/IAN was in the forward canal (14.4%), followed by the retromolar canal (7.5%).

CONCLUSION

Although previously, the dental canal was considered as an anatomical variation, this study revisited the classification and suggested that dental canals are anatomical structures.

Dental MRI-only a future vision or standard of care? A literature review on current indications and applications of MRI in dentistry

MRI is increasingly used as a diagnostic tool for visualising the dentoalveolar complex. A comprehensive review of the current indications and applications of MRI in the dental specialities of orthodontics (I), endodontics (II), prosthodontics (III), periodontics (IV), and oral surgery (V), pediatric dentistry (VI), operative dentistry is still missing and is therefore provided by the present work.The current literature on dental MRI shows that it is used for cephalometry in orthodontics and dentofacial orthopaedics, detection of dental pulp inflammation, characterisation of periapical and marginal periodontal pathologies of teeth, caries detection, and identification of the inferior alveolar nerve, impacted teeth and dentofacial anatomy for dental implant planning, respectively. Specific protocols regarding the miniature anatomy of the dentofacial complex, the presence of hard tissues, and foreign body restorations are used along with dedicated coils for the improved image quality of the facial skull.Dental MRI poses a clinically useful radiation-free imaging tool for visualising the dentoalveolar complex across dental specialities when respecting the indications and limitations.

MRI for the display of autologous onlay bone grafts during early healing-an experimental study

OBJECTIVES

Autologous bone grafts are the gold standard to augment deficient alveolar bone. Dimensional graft alterations during healing are not known as they are not accessible to radiography. Therefore, MRI was used to display autologous onlay bone grafts in vivo during early healing.

METHODS AND MATERIALS

Ten patients with alveolar bone atrophy and autologous onlay grafts were included. MRI was performed with a clinical MR system and an intraoral coil preoperatively (t0), 1 week (t1), 6 weeks (t2) and 12 weeks (t3) postoperatively, respectively. The graft volumes were assessed in MRI by manual segmentation by three examiners. Graft volumes for each time point were calculated and dimensional alteration was documented. Cortical and cancellous proportions of bone grafts were assessed. The intraobserver and interobserver variability were calculated. Statistical analysis was performed using a mixed linear regression model.

RESULTS

Autologous onlay bone grafts with cortical and cancellous properties were displayed in vivo in eight patients over 12 weeks. The fixation screws were visible as signal voids with a thin hyperintense fringe. The calculated volumes were between 0.12-0.74 cm³ (t1), 0.15-0.73 cm³ (t2), and 0.17-0.64 cm³ (t3). Median changes of bone graft volumes of -15% were observed. There was no significant difference between the examiners (p = 0.3).

CONCLUSIONS

MRI is eligible for the display and longitudinal observation of autologous onlay bone grafts. Image artifacts caused measurements deviations in some cases and minimized the precise assessment of graft volume. To the knowledge of the authors, this is the first study that used MRI for the longitudinal observation of autologous onlay bone grafts.

Is MRI a viable alternative to CT/CBCT to identify the course of the inferior alveolar nerve in relation to the roots of the third molars?

Objectives

To assess the reliability of judging the spatial relation between the inferior alveolar nerve (IAN) and mandibular third molar (MTM) based on MRI or CT/CBCT images.

Methods

Altogether, CT/CBCT and MRI images of 87 MTMs were examined twice by 3 examiners with different degrees of experience. The course of the IAN in relation to the MTM, the presence/absence of a direct contact between IAN and MTM, and the presence of accessory IAN were determined.

Results

The IAN was in > 40% of the cases judged as inferior, while an interradicular position was diagnosed in < 5% of the cases. The overall agreement was good (κ = 0.72) and any disagreement between the imaging modalities was primarily among the adjacent regions, i.e., buccal/lingual/interradicular vs. inferior. CT/CBCT judgements presented a very good agreement for the inter- and intrarater comparison (κ > 0.80), while MRI judgements showed a slightly lower, but good agreement (κ = 0.74). A direct contact between IAN and MTM was diagnosed in about 65%, but in almost 20% a disagreement between the judgements based on MRI and CT/CBCT was present resulting in a moderate overall agreement (κ = 0.60). The agreement between the judgements based on MRI and CT/CBCT appeared independent of the examiner’s experience and accessory IAN were described in 10 cases in MRI compared to 3 cases in CT/CBCT images.

Conclusions

A good inter- and intrarater agreement has been observed for the assessment of the spatial relation between the IAN and MTM based on MRI images. Further, MRI images might provide advantages in the detection of accessory IAN compared to CT/CBCT.

Clinical relevance

MRI appears as viable alternative to CT/CBCT for preoperative assessment of the IAN in relation to the MTM.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00784-020-03716-4.

Virtual implant planning and fully guided implant surgery using magnetic resonance imaging-Proof of principle

OBJECTIVES

To present a workflow of virtual implant planning and guided implant surgery with magnetic resonance imaging (MRI) and virtual dental models without the use of ionizing radiation.

METHODS

Five patients scheduled for implant placement underwent an MR examination at three Tesla using individualized 2D and 3D turbo spin-echo (TSE) sequences and dedicated head coils. The MRI data and virtual dental models derived from either optical model scans or intraoral scans were imported to a virtual implant planning software (coDiagnostiX, Dental Wings, Montreal, Canada). Virtual prosthetic planning and implant planning were performed regarding the hard and soft tissue anatomy. A drill guide was designed on the virtual dental model using computer-aided design (CAD) and manufactured in-house, using a 3D printer (Eden 260V, Stratasys, Eden Prairie, MN, USA).

RESULTS

The MRI displayed all relevant anatomical structures for dental implant planning such as cortical and cancellous bone, floor of the nasal and maxillary sinus, inferior alveolar nerve and neighboring teeth. The manual alignment of virtual dental models with the MRI was possible using anatomical landmarks. Dental implant planning, CAD/CAM of a drill guide and fully guided implant placement were successfully performed.

CONCLUSIONS

Guided implant surgery is feasible with MRI without ionizing radiation. Further studies will have to be conducted to study the accuracy of the presented protocol and compare it to the current workflow of guided surgery using CBCT.

Magnetic resonance imaging-a diagnostic tool for postoperative evaluation of dental implants: a case report

OBJECTIVE

Compared with cone beam computed tomography (CBCT), magnetic resonance imaging (MRI) might be superior for the diagnosis of nerve lesions associated with implant placement.

STUDY DESIGN

A patient presented with unilateral pain associated with dysesthesia in the region of the right lower lip and chin after implant placement. Conventional orthopantomography could not identify an association between the position of the inferior alveolar nerve and the implant. For 3-dimensional display of the implant in relation to the surrounding anatomy, CBCT was compared with MRI.

RESULTS

MRI enabled the precise depiction of the implant position and its spatial relation to the inferior alveolar nerve, whereas the nerve position and its exact course within the mandible could not be directly displayed in CBCT.

CONCLUSION

MRI may be a valuable, radiation-free diagnostic tool for the visualization of intraoral hard and soft tissues, offering an objective assessment of nerve injuries by a direct visualization of the inferior alveolar neurovascular bundle.

Mandibular canal visibility using a plain volumetric interpolated breath-hold examination sequence in MRI

OBJECTIVES

To evaluate the validity of plain volumetric interpolated breath-hold examinations (VIBEs) for detecting the course of the mandibular canal, and to compare the results with contrast-enhanced (CE) VIBE images.

METHODS

From our imaging archives, we collected 28 cases taken with a VIBE sequence both before and after intravenous administration of gadolinium hydrate, and then two observers evaluated neurovascular bundle (NVB) visibility in the VIBE images. For the invisible NVB cases, we identified the invisible areas and analysed the causes of invisibility. For cases that also had corresponding CT thin slice images, we obtained a fusion image between MRI and CT, and investigated the relationship between the NVB in VIBE and the mandibular canal in CT images.

RESULTS

The visibility of the NVBs in plain VIBE was 89%, the same as on CE VIBE. There were three invisible cases in each plain and CE VIBE images. The invisible areas were premolar in three cases, and molar in one case, and the causes of the invisibility were a metallic artefact in one case and motion artefacts in the other two cases.

CONCLUSIONS

A plain VIBE can depict the NVB at the same rate as CE VIBE, and is suitable for detecting NVBs.