Modern high-resolution ultrasound has excellent spatial and contrast resolution for the near field, and the development of 3D technology, extended field-of-view or panoramic imaging, and color flow and power Doppler applications has led to great improvements in its diagnostic utility and accuracy.
Ultrasound is the second most common method of imaging carried out in hospitals worldwide after plain-film radiography. Modern high-resolution ultrasound has excellent spatial and contrast resolution for the near field, and the development of 3D technology, extended field-of-view or panoramic imaging, and color flow and power Doppler applications has led to great improvements in its diagnostic utility and accuracy. The technology involves no ionizing radiation, is readily available in most centers, and is relatively inexpensive compared with CT, MRI, and PET.
The superficial nature of the neck structures lends itself readily to ultrasound assessment, and ultrasound plays an increasingly important role in head and neck imaging. Ultrasound can provide reliable real-time guidance for fine-needle aspiration cytology (FNAC) or core biopsy, and recognition of its versatility and diagnostic accuracy has led to its routine incorporation in head and neck clinics.
A patient presenting with a neck mass is a common clinical scenario. A meticulous clinical history with physical examination usually provides a reasonable clinical diagnosis. Imaging is necessary for accurate diagnosis and assessment of the extent of a lesion's involvement prior to treatment.
High-resolution ultrasound is an ideal initial imaging investigation for most neck lumps.1 Cross-sectional modalities serve a supplementary role, offering accurate presurgical anatomic localization, particularly for more deep-seated and locally extensive lesions. The differential diagnosis of a neck mass depends on a patient's age, the anatomic location of the lesion, and its appearance on ultrasound. Lesions in the head and neck are site-specific (see table), and practitioners should familiarize themselves with the locations of common lesions and their imaging characteristics.
The basic level of equipment required for head and neck ultrasound is a modern system with a high-frequency transducer ( > 7.5 MHz). High-frequency transducers allow excellent near-field resolution, though they are not as good at visualizing deeper structures. Lower frequency transducers (5 MHz) may occasionally be required for assessing deep-seated lesions, such as those in the deep lobe of the parotid gland.
Color flow imaging is now a routine part of the ultrasound examination. Systems should ideally offer high-sensitivity color-flow imaging and power Doppler functionality. Equipment used for functional imaging should be calibrated to depict slow-flowing vessels in the head and neck without artifacts or background noise caused by oversensitivity. The lingual artery in the floor of mouth is a useful and practical vascular landmark for calibration.
We recommend the following for power Doppler examinations: high sensitivity, low wall filter, pulse repetition frequency of ~700 Hz, and medium persistence.
The patient should be positioned with the neck slightly extended. Placing a pillow behind the shoulders and lower neck allows the patient to adopt a comfortable position that can be maintained throughout the examination.
An easily replicated systematic scanning protocol is essential. We routinely evaluate the following regions consecutively: submental, submandibular, parotid, upper cervical, midcervical, lower cervical, supraclavicular fossa, posterior triangle, and midline of the neck (including the thyroid gland).2
Scanning in a transverse plane is adequate in most circumstances. Supplementary examination in longitudinal and oblique planes helps to delineate the lesion and its relationship with adjacent anatomic structures.
Because most lesions in the neck are site-specific, once a lesion has been located, specific ultrasound features can be used to establish the diagnosis. This can be confirmed from ultrasound-guided FNAC, allowing appropriate clinical management and treatment.
Size alone is a poor criterion. Among the various measurement parameters for nodal size, minimum axial diameter is the most specific dimension for predicting malignancy. Van den Brekel et al recommend a minimum axial diameter of 7 mm for submental/submandibular nodes and 8 mm for other cervical nodes.4 They cite an overall accuracy of 70% when minimum axial diameter is used as a sole diagnostic criterion. If ultrasound examination of a patient with head and neck cancer reveals a lymph node that is increasing in size or new nodes, then these findings should be viewed with a high degree of suspicion.
Malignant lymph nodes are commonly round, while benign nodes tend to have an elliptical shape.5-7 Axis ratios (either short/long or long/short) can be measured, though a visual qualitative assessment is usually sufficient.
The presence of an echogenic hilus within a cervical lymph node reflects preserved sinusoidal architecture and is a good sign for predicting benignity.8,9 The presence of a round node with an absent hilus is highly indicative of malignancy.
A diffusely hypoechoic, or pseudocystic, enlarged lymph node has previously been described as a sign of lymphoma. Newer generation transducers, however, more commonly show a reticulated intranodal pattern.10,11
In patients with primary squamous cell carcinoma (SCC), the presence of necrosis (coagulative and cystic) within an enlarged lymph node is a very strong indicator of malignancy.12 Tuberculous nodes and metastases from papillary thyroid carcinoma also tend to undergo cystic degeneration,13 and these may mimic metastatic nodes from SCC. An ill-defined border or frank invasion of neighboring structures can be detected easily with ultrasound and indicates poor prognosis.
On color Doppler imaging, the distribution of vessels within a node is more reliable than various vascular and resistive indices.14 Benign nodes have a central hilar flow pattern, whereas malignant nodes have a disorganized peripheral pattern. Areas of relative avascularity reflect the presence of necrosis and peripheral subcapsular vessels.14-17
The typical small punctate calcification seen in a papillary thyroid carcinoma is also observed in lymph node metastases from papillary carcinoma.18 The sign is specific and should prompt a diligent search for the thyroid primary.
Appearance on ultrasound depends on whether there is any previous infective or hemorrhagic component.19 Most uncomplicated branchial cleft cysts appear as well-circumscribed round/ovoid anechoic masses with thin walls and posterior acoustic enhancement. Some cysts may exhibit a pseudosolid appearance due to the presence of cellular material and cholesterol. Previous episodes of infection or hemorrhage may cause the lesions to appear ill defined and thick walled, with septae and heterogeneous internal echoes (Figure 2). Lesions with this appearance should be differentiated from metastatic lymph nodes. Further evaluation will require ultrasound-guided FNAC.
Thyroglossal duct cysts typically appear on ultrasound as well-defined thin-walled anechoic cysts with posterior acoustic enhancement (Figure 3). They may have a uniformly pseudosolid echo pattern similar to branchial cleft cysts, owing to proteinaceous content. Previous infection or hemorrhage can result in a heterogeneous appearance.20
The role of ultrasound in patients diagnosed clinically with a thyroglossal duct cyst is to confirm diagnosis and the cyst's relation to the hyoid bone, to detect internal solid components suspicious for malignancy, and to detect the presence of normal thyroid tissue in the neck. If the only functioning thyroid tissue is contained within the thyroglossal duct cyst, then surgery may result in hypothyroidism.20
Macrocystic lymphangioma appears on ultrasound as a multiseptated compressible thin-walled cystic lesion (Figure 7). Intervening infection or hemorrhage may produce thick-walled lesions with poorly defined contours and internal echogenic debris. Although ultrasound is a useful diagnostic tool that guides sclerotherapy treatment, either CT or MRI will be required to map the entire anatomic extent of large lymphangiomas.
High-resolution ultrasound is an ideal initial imaging investigation for patients with a neck lump to characterize the lesion and delineate the anatomic extent of involvement. It can provide real-time guidance for safe FNAC or biopsy to enhance diagnostic accuracy. This information will generally be sufficient for guiding appropriate clinical management.
Dr. Wong and Dr. Lee are radiologists, Prof. King is professor, and Prof. Ahuja is department head, all in the department of diagnostic radiology and organ imaging at the Chinese University of Hong Kong. Dr. Ying is an associate professor in the department of health technology and informatics at the Hong Kong Polytechnic University.