Imaging is an essential tool in the management of head and neck cancer. Oral, oropharyngeal, laryngeal, and hypopharyngeal lesions are initially imaged with computed tomography (CT) because it allows rapid image acquisition and reduces artifacts related to respiration and swallowing, which can degrade image quality and limit evaluation. Sinonasal, nasopharyngeal, and salivary gland tumors are better approached with MRI because it allows for better delineation of tumor extent. PET/CT is usually reserved for advanced disease to evaluate for distant metastatic disease and posttreatment residual and recurrent disease. Imaging is best used in combination with expert clinical and physical examination.
Key points
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Imaging facilitates staging, treatment planning, posttreatment evaluation, and detection of recurrent/residual disease and provides prognostic information regarding survival outcomes.
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Primary tumor staging can be adequately performed with either computed tomography (CT) or MRI. MRI is more useful for assessing perineural spread of disease.
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Ultrasound is most useful for evaluating cervical lymphadenopathy and for obtaining specimens through fine-needle aspirations and core biopsies.
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PET/CT is most useful for identifying regional and distant metastatic disease.
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Posttreatment imaging may be performed with CT, MRI, or PET/CT; however, an imaging-free period immediately after treatment is needed to present false-positive studies.
Head and neck cancer
Head and neck carcinomas include neoplasms of the nose and nasal cavity, nasopharynx, lips and oral cavity, oropharynx, larynx, hypopharynx, proximal trachea and esophagus, and maxillofacial sinuses. Tumors of the salivary glands, thyroid gland, cervical lymph nodes (lymphoma), and skin cancers can be included in head and neck cancers. Most nonthyroid head and neck neoplasms consist of squamous cell carcinomas of the mucosal lining of the upper respiratory tract and oral cavity.
In order to reduce treatment toxicity, the extent of disease must be assessed efficiently. This assessment includes a combined multidisciplinary effort to examine, image, diagnose, and stage patients properly. Imaging is only one element of the process. Collaboration between specialists in head and neck imaging and treating oncologists is an essential element of multidisciplinary collaboration. Modern imaging methods, including ultrasound (US), computed tomography (CT), MRI, and PET, have dramatically enhanced the process of diagnosis and staging as well as the evaluation for tumor recurrence.
Head and neck cancer
Head and neck carcinomas include neoplasms of the nose and nasal cavity, nasopharynx, lips and oral cavity, oropharynx, larynx, hypopharynx, proximal trachea and esophagus, and maxillofacial sinuses. Tumors of the salivary glands, thyroid gland, cervical lymph nodes (lymphoma), and skin cancers can be included in head and neck cancers. Most nonthyroid head and neck neoplasms consist of squamous cell carcinomas of the mucosal lining of the upper respiratory tract and oral cavity.
In order to reduce treatment toxicity, the extent of disease must be assessed efficiently. This assessment includes a combined multidisciplinary effort to examine, image, diagnose, and stage patients properly. Imaging is only one element of the process. Collaboration between specialists in head and neck imaging and treating oncologists is an essential element of multidisciplinary collaboration. Modern imaging methods, including ultrasound (US), computed tomography (CT), MRI, and PET, have dramatically enhanced the process of diagnosis and staging as well as the evaluation for tumor recurrence.
Rationale for modality selection
How should a clinician determine which imaging modality to use? This decision is complex and multifactorial. Elements include the body part under evaluation, histology of the cancer, stage of disease, and anticipated treatment. Proper modality selection aids the choice of operative approach, potential organ and functional preservation of affected or adjacent tissue, as well as helping to determine the role of surgical, radiation, and medical oncology in treatment. Imaging enhances clinical information beyond examination by assessing the submucosal extent of head and neck tumors and the presence of cervical nodal metastases.
CT and MRI are the primary techniques for evaluating head and neck cancers. CT is superior to MRI because of its speed, easy access, and patient tolerance. The disadvantages of CT include exposure to ionizing radiation and need for iodinated contrast agents. CT is preferable for evaluating metastatic lymph node necrosis. MRI has better soft tissue contrast resolution and multiplanar imaging capability. MRI is superior for evaluating perineural tumor spread along the skull base. However, important disadvantages with MRI include the long time required for imaging acquisition (leading to motion degradation), patient intolerance, metallic implant noncompliance (pacemakers), and gadolinium contrast. In the head and neck, swallowing can cause significant artifact, limiting evaluation of the adjacent soft tissue.
US is especially useful to evaluate cervical nodes and salivary gland tumors. It is particularly helpful for biopsy of small lymph nodes or salivary gland masses ( Figs. 1 and 2 ). It provides a ready way to guide the needle to the lesion in real time. Doppler sonography helps avoid vasculature, thus reducing risk of bleeding.
For head and neck malignancies that are deep rooted and challenging to palpate, fine-needle aspiration (FNA) with CT guidance can be a beneficial alternative option to US-guided biopsies. Many different approaches to deep-seated malignancies using CT guidance have been described. These approaches include needle biopsy of lesions of the parotid gland, Meckel cave, foramen ovale, middle cranial fossa, nasopharynx, sphenoid ridge and maxillary sinus, nasal cavity, infratemporal fossa, and the parapharyngeal space. CT-guided FNA is safe, well tolerated, and accurate for the diagnosis of head and neck lesions.
MRI has strengths for tumor localization and assessing for tumor extension through T1- and T2-weighted imaging. T1-weighted multiplanar postcontrast fat-suppressed images allow for detection of possible perineural spread of malignancy.
Nasopharyngeal malignancy is often best evaluated by PET/CT, MRI, and CT, rather than US, because of its anatomic location and natural history of ready metastatic disease. MRI is the primary imaging tool to image nasopharyngeal malignancy because of its superior anatomic detail in the absence of motion. MRI can detect tumor invasion into the adjacent soft tissues and in particular invasion of the pharyngobasilar fascia, sinus of Morgagni, skull base, intracranial involvement, and perineural invasion. CT is suboptimal compared with MRI for assessing adjacent soft tissue involvement but better for bone. However, PET/CT is better than MRI and CT for evaluating for cervical, supraclavicular, and mediastinal lymph node involvement as well as distant metastases. In patients with nasopharyngeal carcinomas, PET/CT allows for posttreatment functional response assessment. Such a PET/CT should be obtained approximately 3 months following treatment to decrease false-positive results secondary to treatment-related inflammatory changes.
CT, MRI, and PET/CT are of benefit in evaluating sinonasal malignancy. CT gives important information regarding possible osseous invasion and extent of the primary tumor, providing the clinician with a detailed anatomic picture when preparing for open or endoscopic sinus surgery. MRI should be used to define the tumor extent (distinguishing the lesion from edema and retained mucus) and assessing orbital and intracranial extension. US has a limited role in the evaluation of sinonasal malignancy. PET/CT is used for cervical lymph node involvement and staging. PET/CT evaluation is usually obtained at 3 months following treatment because this waiting period allows for minimal false-positive fluorodeoxyglucose (FDG) avidity from post-therapy inflammatory changes.
The choice of imaging modality in carcinomas of the oral cavity and oropharynx is also multifactorial. CT is usually the initial imaging modality for tumor staging. Dental hardware artifact may represent a problem if angled maxilla and mandibular views are not obtained. CT can define osseous invasion of the mandible and maxilla. MRI is more useful for evaluating bone marrow invasion and perineural spread. MRI may be superior to CT to evaluate for primary tongue tumor’s submucosal extent as well as small tumors. The limitations of MRI imaging of the oral cavity and oropharynx are related to artifacts from patient breathing and swallowing. PET/CT is chiefly used for evaluating nodal involvement and systemic metastases in patients with advanced stage. It represents an excellent imaging tool for posttreatment assessment for residual tumor or recurrence in the oral cavity and oropharynx. US again has a limited role in assessment of primary tumors of the oral cavity and oropharynx, though estimation of tumor thickness with US has been studied.
CT is the preferred modality for evaluating the primary laryngeal or hypopharyngeal malignancy, including its involvement of adjacent cartilaginous, bony, and soft tissue structures. MRI is suboptimal in evaluating tumors in this region because the long image acquisition time introduces possible swallowing artifact. CT imaging does not suffer this limitation because a study of the neck can be completed in less than a minute.
Detection of nodal metastases is important. It is present in 50% of patients at diagnosis of head and neck cancers and typically reduces survival by half. CT and MRI are used on a wide scale to assess for cervical lymphadenopathy. Studies that have compared the accuracy of CT and MRI for the assessment of cervical lymphadenopathy have found no significant difference between these two modalities. However, as a rule, between 40% and 60% of all occult metastases are identified by using either CT or MRI. US, less commonly used in North America, has a main advantage in that it can readily be used with FNA biopsy, which results in a specificity of 100% for the combined technique. However, US-guided FNA biopsy identifies clinically occult metastases with a sensitivity of no more than 48% to 73%. Diagnostic accuracy for metastatic nodal detection does not seem to improve considerably by the fusion of FDG-PET with CT when compared with just PET. Problems that limit the accuracy are the suboptimal anatomic resolution of PET even after fusion with low-dose noncontrast CT as well as the incapability to reliably find metastatic nodes less than 5 mm. Thus, the sensitivity of PET/PET-CT detection of metastatic node metastases is similar to CT and MRI. There are new technological advances to improve the ability of MRI to distinguish metastatic from benign cervical lymph nodes with diffusion-weighted imaging (DWI), dynamic contrast-enhanced MRI, or the use of new (lymphotropic) contrast agents as enhancers. DWI MRI may be a useful tool to differentiate tumoral from nontumoral signal with a sensitivity and specificity for the detection of nodal metastasis of 98% and 88%.
Malignant cervical lymphadenopathy will demonstrate a heterogeneous internal architecture, central necrosis, and even extracapsular spread. Metastatic lymphadenopathy usually demonstrates necrosis when the lesion is greater than 3 cm. These areas of necrosis usually show a characteristic pattern on imaging, which helps the radiologist identify malignant lesions quickly. With MRI they have hypointense signal in T1-weighted imaging as well as hyperintense signal on T2-weighted imaging. Postcontrast CT images display necrosis as a central area without enhancement, usually with peripheral areas of enhancing malignant soft tissue. Necrotic tumor on PET/CT imaging does not show FDG avidity and appears hypometabolic or as a cold defect.
Extracapsular spread (ECS) in cervical lymph node metastases from head and neck squamous cell carcinoma is a poor prognostic factor. ECS has been linked with a 50% decrease in survival and about 1.5- to 3.5-fold increase in local relapse. Contrast-enhanced CT is the imaging modality that is usually used to assess cervical lymph node for metastatic disease. CT criteria for abnormal lymph nodes include size, if there is central necrosis, and the presence of a cluster of lymph nodes in the expected drainage path of a tumor. However, this may not be precise if patients have had prior surgery, radiation, or infection. On CT or MRI, in the setting of head and neck carcinomas, cervical lymph nodes larger than 1.5 cm with a rounded contour and loss of the fatty hilum are malignant if they are in the nodal drainage path of the primary malignancy. Additionally central nodal necrosis within any size node is an indication of neoplastic involvement. The most accurate radiological predictor of lymph node metastasis is the presence of central lymph node necrosis. Radiologically, lymph node central necrosis is described as a central area of hypodensity with a peripheral irregular rim of enhancing tissue. Lymph node central necrosis has been reported to be almost 100% accurate in identifying metastatic disease.
Radiological signs that indicate lymph node ECS include nodal capsular enhancement, infiltration of adjacent fat or muscle planes, and capsular contour irregularity. Using CT to evaluate for of ECS has a sensitivity of 81% and a specificity of 72%, compared with 57% to 77% and 57% to 72%, respectively, for MRI. With regard to MRI, precontrast T1- and T2-weighted images are more sensitive than gadolinium-enhanced T1-weighted images. US has demonstrated to be acceptably sensitive, but less specific, for the detection of ECS. On US imaging, abnormal cervical lymph nodes will demonstrate rounded contour with loss of hilar echogenicity, and power Doppler may demonstrate loss of normal hilar flow and increased peripheral vascularity. Nodal matting and soft tissue edema can be seen in pretreatment and posttreatment nodes.
Perineural Spread
A significant predictive element of treatment outcome is perineural spread of tumor in patients with suprahyoid neoplasms. CT imaging can be dependably used to localize perineural spread of malignancy by looking for clues, such as enlargement of the neural foramina at the skull base, degeneration of the muscles that are innervated by the trigeminal nerve (cranial nerve V), or even fatty infiltration of the pterygopalatine fossa. However, perineural spread of malignancy on MRI is usually denoted by irregular thickening and postcontrast abnormal enhancement of the affected nerve. Adenoid cystic carcinomas usually demonstrate more perineural spread than squamous cell carcinomas.
Diagnostic imaging technique
Patients are usually supine for CT and MRI head and neck imaging. Artifacts are common in CT, and they may obscure or mimic pathology. CT artifacts include noise, beam hardening, scatter, pseudoenhancement, and motion, cone beam, helical, ring, and metal artifacts. For head and neck imaging, metal streak artifacts from dental fillings or surgical hardware are the main concern. The artifacts are triggered by many mechanisms, some of which are related to the metal itself and others related to the metal edges. Metal causes beam hardening, scatter effects, and Poisson noise. Beam hardening and scatter cause dark streaks with surrounding bright streaks. The metal edges cause streaks because of undersampling, motion, cone beam, and windmill artifacts. Metal artifacts are usually seen with higher atomic number materials, such as iron or platinum, and less so with lower atomic number metals, such as titanium. In most head and neck cases with dental fillings, patient positioning or gantry tilt may be used to angle the metal outside of the axial slices of interest.
For contrast-enhanced CT and MRI, the patients should have blood tests within approximately 2 weeks of the examination to evaluate renal function. Diabetic patients who are having intravenous contrast for their CT or MRI scans must stop taking the following medications for 48 hours: metformin hydrochloride (Glucophage), glyburide and metformin (Glucovance), metformin hydrochloride, and metformin or its derivatives. Noncontrast imaging does not require prior preparation. During the MRI examination metallic items must be removed. Individuals with metallic implants, pacemakers, metallic clips, and stents must disclose their presence before the study to prevent excessive heating (burns that have been associated with implants and metallic devices). Additionally, ferromagnetic implants may move or even dislodge causing pain and injury to the patients.
Interpretation/assessment of clinical images
A radiologist must have a strong knowledge of anatomy when interpreting images of the head and neck. General features specific to each imaging modality allow a radiologist to discern benign from malignant tumors.
Benign Tumors
Benign lesions on US may appear hypoechoic or hyperechoic but typically demonstrate well-circumscribed, lobulated margins with posterior acoustic enhancement, internal homogeneity, as well as peripheral calcification. Soft tissue lesions that are hypodense (or even cystic) with intact muscle borders and osseous cortex on CT are also usually benign. On MRI, lesions that are homogenous in signal intensity with hypointense signal on T1 and hyperintense signal on T2 are also mostly benign. With regard to PET/CT, low level of FDG activity compared with blood pool activity is usually benign.
Malignant Tumors
Malignant lesions on US usually demonstrate an irregular shape, border, and margin. The lesion may also have increased vascularity. On CT, cancers typically demonstrate soft tissue thickening or a bulky mass with surrounding infiltration and stranding of the adjacent tissues (with or without osseous invasion). Malignant tumors generally may have a suboptimally defined border and may also demonstrate heterogeneous internal signal with possible cystic change and necrosis on MRI. Aggressive lesions on MRI may show heterogeneous enhancement on postcontrast imaging. On PET/CT, malignant lesions often demonstrate increased FDG avidity when compared with blood pool or liver pool uptake images.
Nasopharyngeal Masses
Approximately 80% of neoplastic nasopharyngeal masses are of squamous cell carcinoma origin. The other nasopharyngeal masses are adenocarcinoma, minor salivary gland tumors, and lymphoma. The lateral pharyngeal recess is the most common location for nasopharyngeal carcinomas. As individuals age, the fat content increases and lymphoid and muscular tissue content decreases over time. MRI and CT are used in concert for the initial evaluation of the primary tumor. However, PET/CT is more useful to detect recurrence as well as lymph node and metastatic disease. T1-weighted imaging without contrast is best used for assessing skull base invasion and involvement of the different fat-containing spaces in the skull base. T2-weighted fast spin echo imaging is an additional imaging tool that is helpful when evaluating for parapharyngeal, paranasal, or cervical lymph node involvement. In order to assess intracranial tumor extension or perineural spread, the most useful MRI sequence is T1-weighted contrast-enhanced imaging with fat suppression. Some calculate the choline/creatine ratio using magnetic resonance spectroscopy. The choline/creatine ratio is usually elevated in cases of metastatic lymphadenopathy.
Nasopharyngeal carcinomas may present as infiltration of adjacent soft tissues with little mass effect. Infiltration of the adjacent tissue then leads to effacement of the margins of the muscular tissues. MRI allows for better delineation of the soft tissue infiltration and discerns lymphoid tissue from adjacent soft tissue and muscle. For example, tumor infiltration of the region between the tensor and levator veli palatini muscles (part of the parapharyngeal space) or even fatty infiltration of the region between the nasopharyngeal mucosa and longus capitis muscle (which is known as the retropharyngeal region) will be depicted as an area of fatty infiltration or even obliteration of the usually seen normal hyperintense signal on T1-weighted imaging.
In patients with nasopharyngeal carcinomas, tumors begin in the fossa of Rosenmüller and have local extension with initial metastases to the retropharyngeal lymph nodes of Rouvière. This extension can usually be seen in the region between the skull base to the level of C3 vertebral body. Lymph node involvement usually progresses laterally in nasopharyngeal carcinomas. Nodal spread starts in the retropharyngeal region, and then it moves to the spinal accessory chain (Va/Vb), internal jugular chain (level III-IV), and lastly supraclavicular lymph nodes. Nodal involvement in the region posterior to the jugular vein (IIa/b) is not considered retropharyngeal nodal involvement.
Parapharyngeal Space Masses
The parapharyngeal space contains fat, the V3 segment of the trigeminal nerve, the internal maxillary artery, as well as the ascending pharyngeal artery and veins. This space is divided into a prestyloid compartment (about 80% of lesions occurring there are pleomorphic adenomas) and the poststyloid compartment. The lesions that arise in the poststyloid compartment are of neurogenic origin and paragangliomas. Squamous cell carcinomas may arise from the adjacent pharyngeal mucosa and invade the pharyngeal space.
Parotid Space Masses
The parotid space contains the parotid glands, lymph nodes, retromandibular vein, facial nerve, and external carotid artery. Although surgeons use the facial nerve to divide the gland into the deep and superficial lobes, the retromandibular vein divides the gland into superficial and deep lobes on imaging studies. The most common lesion in this space is a benign tumor, such as a pleomorphic adenoma (about 80% of parotid neoplasms are benign). The second most common lesion in this location is a Warthin tumor. These lesions are usually seen in elderly men who smoke and may be bilateral. Malignant lesions include mucoepidermoid carcinomas, adenoid cystic carcinomas, other primary parotid cancers, metastases, and lymphoma. With respect to imaging by MRI, the benign lesions are usually hyperintense on T2-weighted imaging, whereas malignant neoplasms are hypointense on T2-weighted imaging. DWI will demonstrate restricted diffusion with malignant lesions.
Sublingual and Submandibular Space Masses
Sublingual space is in the superior/medial region to the mylohyoid muscle and houses the tongue, lingual neurovascular plexus, sublingual glands/ducts, and Wharton ducts. The most common malignancy to occur in this region is squamous cell carcinoma. However, the sublingual gland tumors, cellulitis, abscesses, calculi, ranulas, lymphangioma, epidermoid/dermoid cysts, ectopic thyroid, and hemangioma produce masses there. The submandibular space is inferior/lateral to the mylohyoid muscle and superior to the hyoid bone as well as medial to the horizontal ramus of the mandible. This region houses the submandibular gland, nodes, anterior belly of the digastric muscle, as well as the facial vein and artery. The most common malignant masses to occur in this region are metastatic squamous cell carcinoma spread from the mouth. Submandibular gland tumors, lipoma, epidermoid/dermoid cysts, ranulas, infections, second branchial cleft cyst, and lymphangioma are seen.
Salivary Gland Tumors
The benign and malignant features of salivary gland tumors on imaging demonstrate moderate similarities, and it may be difficult to distinguish between them. MRI has been shown to be superior in imaging salivary gland tumors. US uses irregular morphology of the lesion and its margin as well as the lesion’s internal architecture to help differentiate between malignant and begin salivary gland tumors, but characteristic features may not always be present in a lesion. Highly malignant lesions of the salivary glands may demonstrate rapid uptake of contrast on MRI images as well as slow washout. Additional features that are helpful when assessing for salivary gland tumors is perineural spread and (for large lesions) infiltration within the parapharyngeal space and adjacent musculature as well as osseous involvement. Perineural spread associated with salivary gland tumors on MRI will appear as fatty replacement in the neural foramina at the skull base with thickening of the nerve root with enhancement on postcontrast imaging.
CT imaging of salivary gland tumors may underestimate the malignant potential because of a lack of contrast enhancement. Lesions such as adenoid cystic carcinoma do not demonstrate enhancement on CT postcontrast imaging. CT is an excellent adjunct to MRI if there is clinical concern for invasion of adjacent osseous structures as well as to assess for malignant cervical lymphadenopathy. PET/CT is accurate for assessing the extent of high-grade lesions and their nodal metastasis, but differentiating benign and low-grade malignant salivary tumors may be a challenge. Hence, with regard to salivary gland tumors, PET/CT is mainly used to detect nodal involvement or distant metastasis.
Oral Cavity and Oropharyngeal Space Masses
Approximately 90% of oral cavity and oropharyngeal tumors are of squamous cell origin, followed by lymphoma and minor salivary gland tumors.
Most oral cavity as well as oropharyngeal malignancies can be initially evaluated by contrast-enhanced CT imaging. MRI may be used to aid in revealing small tongue lesions and to delineate submucosal extension that cannot be detected by physical examination. Oral cavity lesions may benefit from multi-modality imaging: CT/MRI to assess tumor invasion in the soft tissue and mandible ( Fig. 3 ) and PET/CT and US to search for lymph node metastasis. The maximum accuracy of PET/CT is still relativity low at approximately 76%. PET/CT has demonstrated the highest specificity for staging oral cavity/oropharyngeal carcinomas, and US-guided FNA has shown the highest sensitivity. For oral cavity cancers, imaging is of great benefit for surgical planning regarding the tongue and mandible. Squamous cell carcinomas invading the mandible usually do so through the alveolar crest, and mandibular involvement may need marginal resection or segmental mandibulectomy. Osseous involvement of the mandible or maxilla is usually evaluated through CT or fused CT images and PET images ( Fig. 4 ).
