Currently, dentistry imaging is almost exclusively based on micro CT or X-ray, whereas the use of MRI has been rare [1]. This is primarily due to the fact that teeth exhibit little signal, also air tissue interface in the oral cavity presents large magnetic susceptibility. However not only CT is associated with ionizing radiation, very limited contrast for the mandible canal (MC) can be obtained. As illustrated in Fig.1, position of the mandibular canal needs to be precisely known in tooth planting and extraction of wisdom tooth to avoid accidental damage of the inferior alveolar nerve (IAN): in the former case, safe distance needs to be kept in between the root of teeth and the MC (often > 2mm); in the latter case, it needs to be determined if the improperly erupted wisdom tooth has already invaded the MC for extraction planning. In this work, we investigate if zero TE (zTE) imaging [2,3] can be used as an alternative for dentistry imaging, for its similar to CT dense tissue contrast as well as excellent soft tissue contrast. A patient that was diagnosed with horizontally erupted wisdom tooth using dental micro CT further received MRI scan. Zero TE imaging of the jaw was performed on a 3.0T whole body scanner (MR Discovery 750W) equipped with an 8 channel head neck coil. As a comparison, conventional T1 and T2 MR imaging were also performed for similar coverage using 3D SPGR and 3D FSE respectively. In all three cases, 3D isotropic resolution of approximate 1mm was achieved. The resulting MR images were judged by the clarity of visualization of the teeth and the mandible, quantitative assessment was also performed by measuring the relative separation of the root of the teeth to the mandible canal (MC) in both zTE image and CT image. In the MR images, the MC was visually identified based on signal heterogeneity, and then the distance between the mandibular canal was manually measured using vendor supplied MR workstation. In CT image, due to the faint contrast of MC (only the bony out layer is visible, Fig. 2a), operator manually identified several points along the canal using sagittal section views, and the then contour was then obtained by interpolating the identified spots semi-automatically (Fig. 2b). Then the distance from a specific tooth root may be determined. In Fig.3 (a-c), the curled reconstruction of MR images of T1, T2 and zTE are shown, inverse color map was used. The mandibular canal is seen to exhibit different signal levels in different contrast: hypo-intense in T1 image, hypo-intense in T2 image (weak) and hyper-intense in zTE image; whereas the teeth are hypo-intense in all contrasts (note they are inverse color mapped). It can be seen that comparing to the T1 and T2 images, zTE image showed a much clearer depiction of the MC. The MC exhibits poor contrast to the surrounding tissue in T1 and T2 images. On the other hand, CT image offers very clear contrast of the teeth, only the outline of the mandibular canal can be visualized, for it is a layer of bone enclosing the content (nerve and vessel) of the canal. The sagittal and coronal view of zTE and CT images are shown in Fig. 4(a) and (b) respectively. Measurements of the distance between the upper, lower edge of the wisdom tooth and the MC on the sagittal view, between the root of a healthy tooth and the MC on the coronal view, obtained using both image contrasts are shown. It is seen that almost identical measurements were obtained. Hence the excellent contrast of the mandibular canal in zTE image allows the user to easily identify the separation from the root of the teeth; whereas computer aided contouring was needed in CT image to perform such measurement, due to the weak contrast of MC.

Zero TE imaging has the advantage of being able to visualize the teeth that has a very short T2, and also is minimally affected by susceptibility in the oral cavity. As a result, much better contrast among the teeth, mandible, and mandible canal was obtained as compared to conventional T1 and T2 images. The distances measured between the teeth and the mandible canal were seen to be in excellent agreement with those obtained using CT. As a potential alternative to CT, zTE offers radiation free scan as well as much stronger contrast of the mandible, which reduces the error of measurement error. A clinical trial with consenting patients using zTE for dental imaging is underway.