Choice of modality
In head and neck cancer the principle role of any imaging modality is to identify the deep infiltration of tumour, and the presence of metastases, particularly lymphadenopathy. Mucosal changes can often be evaluated by direct inspection.
Whilst Magnetic Resonance Imaging (MRI) is recognized as having greater tissue discrimination than other imaging modalities, there is still a significant problem with image degradation due to motion artefact; despite the availability of fast imaging sequences.
In the pharynx the nasopharynx is subject to the least motion, and in assessing tumours of the nasopharynx MRI is accepted as the imaging modality of choice; the better tissue contrast and multiplanar facility combining to improve the accuracy of staging compared with computed tomography (CT).
In assessing tumours of the oral cavity and oropharynx, motion is a problem, and CT and MRI are considered complementary investigations. However, with appropriate patient selection the staging of oral cavity and oropharyngeal tumours can be achieved by MRI alone, and in patients with tumours at certain sites, for example, the posterior third of the tongue, it is more accurate than CT, enabling more precise radiotherapy and surgical treatment planning.
These statements also apply to the patient with recurrent tumour, who is potentially suitable for salvage surgery.
Patients with large oral cavity tumours, and excessive salivation are usually best imaged by CT. Chronic obstructive pulmonary disease often co-exists in these patients.
Parotid gland. Whilst at present CT is the mainstay for evaluation of glandular and peri-glandular masses, MRI can readily distinguish between lesions in the deep lobe of the parotid gland and those in the parapharyngeal space, but more significantly, the facial nerve is often seen distal to the stylomastoid foramen, thereby allowing a more accurate assessment of its relationship to a mass lesion. A disadvantage of MRI is the relatively poor detection of intra-glandular calcification or calculi within the salivary ducts. CT is, therefore, preferred to MRI when a mass is thought to be inflammatory.
Submandibular/sublingual glands. Mass lesions can be assessed by MRI with the multiplanar facility providing information about the relationship with the mass to the floor of the mouth and the parapharyngeal and masticator spaces.
Submental and submandibular lymphadenopathy is readily identified.
CT and MRI are complementary investigations. The choice of imaging modality will often depend on the patient’s status and ability to co-operate.
The advantage of CT is that it is quicker and more readily available than MRI. It is also more sensitive to cortical bone invasion.
Currently, CT is used for radiotherapy treatment planning.