Imaging modalities
Due to the easy access and low radiation dose, conventional radiographs such as panoramic radiographs (orthopantomographies [OPTs] or panoramix X-rays) as well as dental intraoral radiographs traditionally form the backbone in the diagnosis of osseous changes in the mandible [
1,
3]. In addition, the introduction of digital radiographs has lead to further dose reduction (up to 80 %) with the possibility of densitometric and subtraction techniques [
4]. Conventional radiographs of the mandible, typically OPTs, may reveal radiolucent, radiodense or mixed pattern lesions [
1,
3]. In many cases, such as in radicular cysts, the diagnosis is straightforward and no additional imaging is required for diagnosis and treatment. As conventional radiographs are two-dimensional projections of three-dimensional structures, they have a limited value for the assessment of lesion size, lesion margins, as well as extension into important anatomic structures or soft tissues. Computed tomography (CT), cone beam CT (CBCT), magnetic resonance imaging (MRI) and positron emission tomography combined with CT (PET/CT) and more recently positron emission tomography combined with MRI (PET/MRI) complement conventional radiographs overcoming the above-mentioned limitations and providing more specific information in terms of diagnosis and therapeutic options.
Thin-slice (1-mm), high-resolution CT with bone window settings is mainly used pre-operatively to precisely assess lesion size, margins, destruction and expansion patterns, as well as the relationship of the lesion to the mandibular canal. Although coronal slices are sufficient in many situations, dental CT with orthoradial and panoramic reconstructions are superior to standard coronal reconstructions for the evaluation of the relationship of a lesion to the dental structures and to the mandibular canal. Intravenous contrast material is mainly used in cases of suspected jaw infection or in neoplastic diseases to assess the intraosseous and extraosseous involvement. Although CBCT has gained increasing popularity over the past years, it does not allow evaluation of extraosseous structures; use of CBCT may therefore lead to underestimation of disease extent.
High-resolution MRI is mainly used as a complementary tool to CT or CBCT, as it allows precise depiction of intraosseous and extraosseous lesion components, cyst wall architecture (thin versus irregular walls, mural nodules, papillary projections), enhancement patterns after intravenous administration of gadolinium chelates (mild to strong), and type of soft tissue involvement (displacement versus infiltration) [
5]. In inflammatory and infectious lesions, MRI is more sensitive than CT or CBCT for the detection of bone marrow involvement [
6].
Diffusion weighted imaging (DWI) is a functional MRI technique based on the assessment of random (Brownian) motion of water molecules. Biological barriers can impair the free displacement of water molecules, thus resulting in restricted diffusivity. Restricted diffusivity is seen in a variety of conditions, including stroke, tumours with increased cellularity, infection and inflammation, as well as abscesses. Diffusion in biological tissues can be quantified using the apparent diffusion coefficient (ADC). ADC measurements (in mm
2/s) have been shown to be reproducible with excellent intraobserver and interobserver reproducibility in the head and neck [
7]. MRI with DWI and ADC measurements helps in the differential diagnosis of cysts, ameloblastomas and malignant tumours (see below). Although ADC values cannot predict the histological grade in head and neck squamous cell carcinoma (SCC), lower values are observed in poorly differentiated lesions whereas higher values are seen in well-differentiated tumours [
7]. In lymphomas, ADC measurements typically yield very low values (see below).
As a general rule, PET/CT is uncommonly used for the work-up of mandibular lesions. Nevertheless, it may be employed as a complementary examination for the staging of malignant tumours invading the mandible, such as SCC of the oral cavity with secondary mandibular invasion, in primary intraosseous SCC or in mandibular lymphoma. In cases of metastases to the mandible, PET/CT may reveal the location of the primary tumour—if unknown—or may effectively show involvement of multiple organs. The most commonly used radiotracer is
18F-fluorodeoxyglucose (FDG). FDG is a glucose analogue that is taken up by metabolically active tumour cells using facilitated glucose transport. In clinical routine, quantification of tracer uptake is performed using the standardised uptake value (SUV). High SUVs reflect high glucose metabolism mainly seen in aggressive tumours (typically SCC and lymphoma), while lower SUV values are rather seen in slowly growing, less aggressive tumours, in tumours with large areas of necrosis or in inflammatory conditions. The recent implementation of integrated hybrid PET/MRI systems in clinical head and neck oncology [
8] holds promise as it combines morphological, functional and molecular information at the same time, thereby providing additional diagnostic gain. However, research into the potential clinical role of PET/MRI in comparison with PET/CT, MRI with DWI or the combination thereof, is still ongoing.
This review article is based on the retrospective evaluation of 11,725 panoramic radiographs seen during a period of 6 years at our institution. From a practical point of view, radiolucent mandibular lesions can be divided into lesions with well-defined borders and lesions with poorly defined borders.
Conclusions
The vast majority of radiolucent lesions of the mandible seen on conventional radiographs represent benign lesions that require no further work-up. Nevertheless, certain radiological features, such as large lesion size, bone scalloping, relationship to an impacted tooth or the mandibular canal, tooth resorption, as well as ill-defined lesion borders, require further radiological work-up. CT, CBCT, MRI and PET/CT are of additional help if the nature of the lesion is unclear and for the identification of those lesions, where biopsy is indicated for definitive histology.