Creation of a VHF-based anatomic atlas of extrinsic tongue musculature

The anatomy of the extrinsic tongue muscles was mapped in three dimensions using images from the Visible Human Project (VHP). The VHP obtained high-resolution computed tomography (CT) and MRI images prior to freezing, sectioning, and photographing two human cadavers (male 1994; female 1995). This produced three-dimensional (3D) datasets of the complete human anatomy, available free to the scientific community. The Visible Human Female (VHF) is the more comprehensive of the datasets. Photographs are in 24-bit colour with a slice resolution of 2048 × 1216 pixels, and CT/MRI images have a pixel size of 0.33 mm × 0.33 mm and slice thickness of 0.33 mm (isometric voxels). The isometric parameters make the data ideal for multiplanar reformatting (MPR) and 3D rendering. The VHF provides unrivalled anatomical imaging of the lingual myostructure without the distortion effects and shrinkage of formalinized tissues, which can be navigated using an image stack similar to viewing sectional images of CT/MRI.

With the assistance of an anatomist, the boundaries of the extrinsic muscles were individually identified on each imaging slice of the VHF. Each muscle was manually segmented and sequential slices combined to produce a 3D computer model of their anatomical position. The model was validated and updated by comparison with existing models in a variety of forms, including point cloud, 3D renders of triangular meshes, and parallelepipeds. The accuracy of the model was confirmed by co-registration with MRI datasets from healthy volunteers.

Assessment of re-staging archived pre-treatment MRI oral tongue carcinomas using the strict anatomical criteria utilizing co-registration with the computer-generated anatomical model

Ninety-seven consecutive patients with oral tongue carcinomas who had undergone pre-treatment MRI prior to primary surgical ablation between 1999 and 2008 were identified from the departmental database and their pre-treatment MRI images reviewed. Of these, nine patients were excluded because the MRI images were of poor quality, and one case, because of image file corruption. As a result, 87 image sets were available for analysis. MRI images were viewed on eFilm (Version 1.83, Merge Healthcare, Chicago, IL, USA) and all available MRI pulse-sequences utilized to evaluate image quality for susceptibility and motion artefact. Patient demographic data were gathered from the departmental database in combination with a retrospective review of the patient records. Preoperative clinical tumour staging, according to the TNM classification, was completed at multidisciplinary review of the patient. This included clinical and radiological examination with a consensus opinion reached by the team.

The VHF-based anatomical computer model created in the first part of the study was co-registered with the pre-treatment MRI images ( Fig. 1 ; see also Supplementary Material, movie clip). The model was limited to the coronal plane for easy identification of the relevant anatomy, necessitating co-registration with T2-weighted coronal images for optimal tumour identification. The tumour-containing images were identified and exported as JPEG files. In order to identify the most appropriate model slice in which to morph the tumour-containing MRI image, a ratio was calculated of the distance from the tumour to the posterior limit of the tongue representing this distance as a proportion of the entire length of the tongue. This ratio was then multiplied by the total number of slices that were contained in the model ( n = 44), with the result rounded to the next whole slice to provide the most appropriate model slice.

The VHF tongue computer atlas. Images taken from the VHF-derived tongue atlas, with extrinsic tongue muscles in blue, intrinsic muscle mass in red.

Movie 1

The 3D model depicted in the video is that of the extrinsic muscles of the tongue: genioglossus (green), styloglossus (cyan), and hyoglossus (purple).

The MRI-derived JPEG and model slice were imported into FantaMorph (v4.1, Abrosoft, http://www.fantamorph.com ), a commercially available fusion software. FantaMorph uses a non-rigid image co-registration algorithm to warp the source image onto the recipient image. In this case the source image was the model slice and the recipient was the tumour-containing MRI slice. A maximum of 20 corresponding control points were manually defined on the source and recipient images to perform the non-rigid deformation ( Fig. 2 ). The majority of these points were placed on the tongue surface. The first four were placed at the cardinal directions, defining the superior, lateral, and inferior limits of the tongue contour. Where the midline of the tongue was visible, control points were added to compensate for deformation of the midline by any tumour mass effect. The resulting composite image was exported as a JPEG file and stored for reference. The presence of tumour invasion of the extrinsic muscle was assessed visually by two of the authors working in collaboration.

MRI/VHF model fusion. A screen shot of FantaMorph graphical user interface during an MRI–atlas fusion. The green dots represent control points assigned to the tongue contour. The final product is shown in the lower image. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

A 3D animation was subsequently created to aid anatomical visualization of the extrinsic muscles of the tongue, excluding palatoglossus (see Supplementary Material, movie clip). This was designed using references available from drawings, computer renderings, and an existing 3D model of a tongue and its extrinsic muscles based on the VHF project. The objects were constructed in a subdivision modeller (Wings3D, Open-source software) that permits derivation of organic models from two-dimensional references. The shape patterns were first reconstructed as rough polyhedra (with fewer than 50 vertices each) and then the polygons subdivided to obtain a smooth appearance.

Ethical considerations

Consent is routinely obtained from patients for the use of their medical records and imaging in audit and research. Ethical approval for this study, which had no direct impact on patient care, was not required in the UK.