eRadiograph Volume 7: Imaging of Oral Cavity | Page 15

MRI is the modality of choice in view of its excellent soft tissue contrast. CT scan
does not have the contrast resolution to match MRI. Further, MRI is not so affected by
dental amalgam as CT scan is. The ideal protocol would be to use a dedicated neck
coil with a small FOV of 14-18 cm and thin sections 3-4 cm. A larger FOV of 20 cm may
be used to scan the entire neck to look for lymph nodes. The ideal sequences to be
used would be either Frequency Selected Fat Saturated T2 (FATSAT) or Short Time
Inversion Recovery (STIR) through the oral cavity. FATSAT images are preferred as they
increase the conspicuity of tumour, especially post contrast images. STIR is actually
preferred as not that affected by magnetic field inhomogeneities produced by metallic
hardware. FATSAT images may actually magnify effects of dental amalgam, also
FATSAT sequences increase duration of acquisition, thereby increasing possibilities of
motion artefact. In these situations a non FATSAT sequence may be useful.
On T2 weighted images there may be overestimation of the size of the tumor
due to associated oedema inflammation. Post contrast T1 FATSAT images
provide a more accurate assessment of size of tumor. Tumors are seen as iso to
hypo intense to muscle on T1 weighted images. Tissues like tongue which have a
high intrinsic fat content, tumors will be well appreciated on T1 weighted images as
the hypointense tumor is contrasted against the hyper intense fat background. On T2
weighted images tumors are hyperintense, they are more conspicuous on FATSAT
images as they standout against the fat suppressed background. On post contrast
images, tumor enhances, this is demonstrated well on fat saturated images as the
hyperintense background is supressed. The tongue should be imaged primarily in the
axial plane but also the coronal plane to see for floor of mouth extension and

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Imaging of Oral Cavity

carcinoma in the Indian subcontinent is tobacco chewing. Other risk factors are
smoking, especially reverse smoking, alcohol and HPV infection. Carcinoma tongue
may present as an ulcerated lesion or an infiltrative lesion. Ulcerated lesions are
detected on clinical examination and may present with difficulty in swallowing and
pain. Infiltrative lesions extending within the substance of the tongue present only
when the lesion is large enough to cause disturbance in mobility of the tongue. The
role of imaging is to assess depth and extent of invasion as this is difficult to
detect with physical examination. An accurate assessment of extent aids
treatment planning as well is a useful prognostic indicator. A wide surgical margin of 1
cm is usually adequate in squamous cell carcinomas but in the tongue a wider margin
of 1.5 - 2 cm is ideal. However, a wider excision will result in impairment of speech and
deglutition, therefore an accurate assessment is very important to provide a wide
adequate margin as well as preserve function. The thickness of tumor is directly related
to presence of nodal metastases and prognosis. Thickness of the tumor is measured in
a mediolateral direction and not superioinferiorly. When tumors cross a threshold
of 2 cm, survival decreases from 90 to 63%. Thickness of tumor greater than 4 cm is
associated with a greater incidence of nodal metastases.