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Ultrasound Imaging of the Neck


Key Points

  • High-resolution ultrasound imaging is the “gold standard” modality for nodular thyroid disease and should include assessment of cervical lymph nodes.

  • Thyroid nodules are very common and should be risk stratified by sonographic appearance according to the guidelines of the American Thyroid Association or American College of Radiology to avoid overtreatment.

  • Sonographic features suggestive of thyroid malignancy include microcalcifications, irregular borders, extrathyroidal extension, hypoechogenicity, and increased internal vascularity.

  • Ultrasound-guided fine-needle biopsy is the gold standard technique for thyroid biopsy that reduces sample error and nondiagnostic rates when compared with palpation-guided biopsy

  • Sonographic features suggestive of cancerous lymphadenopathy include loss of echogenic hilum, round shape, microcalcifications, cystic component, irregular borders, and extranodal extension.

  • Ultrasonography is a sensitive tool for localization of enlarged parathyroid glands, and it provides greater anatomic detail than traditional planar radionuclide imaging.

  • Ultrasonography is useful in the assessment of submandibular and parotid gland inflammatory disease, neoplasms, and salivary stones.

Basics of Ultrasonography

Over the past 30 years, sonographic imaging technology has undergone tremendous advancement. As a result of increasing resolution, portability, and affordability, it has gained popularity as an office-based procedure, adding a significant dimension to the physical examination. Although an understanding of anatomy and disease pathophysiology remains the key to interpreting cervical ultrasound (US), the clinician must also understand the general physics of US technology to maximize the information provided.

Sonographic technology is based on the properties of the acoustic wave. The energy generated by a US transducer is transferred to molecules of a medium. The molecules vibrate in a series of rhythmic, mechanical compressions that generate a number of longitudinal waves like ripples on the surface of water. Each wave has a particular number of cycles per second that determines its frequency. A frequency of 1 cycle/s is equal to 1 hertz (Hz); 1,000,000 cycles/s is equal to 1 megahertz (MHz). Audible sound has a frequency between 20 and 20,000 Hz. Frequencies greater than the range of audible hearing are referred to as ultrasonic.

The sonographic signal is generated at the level of the transducer, which contains crystals that demonstrate the piezoelectric effect. That is, their properties permit conversion of electrical energy into mechanical energy (and vice versa) creating a mechanical wave—in this case an ultrasonic wave. These are linearly arranged crystals. As US waves propagate through tissue, a small percentage of the ultrasonic energy (echo) is reflected back to the transducer. The US image is formed by the returning wave, and the strength of the image is proportional to the strength of the returning wave. Substances with a greater density produce stronger “echoes” and appear hyperechoic on imaging compared with reference structures. This reflectance also occurs most readily at the junction between materials with different acoustic materials. Therefore structures of different soft tissue densities can be easily distinguished from one another.

Multiple tissue interfaces emit various sonographic echoes and permit generation of readable images. High-frequency waves provide better resolution, because smaller wavelengths help detect more minute anatomy, but are also subject to greater energy loss. Therefore high-frequency waves are restricted to the evaluation of the superficial structures. In contrast, lower-frequency waves penetrate more deeply with less attenuation but result in images with less resolution. Because of the superficial location of most head and neck structures, clinical US uses fluctuating frequencies between 7 and 15 MHz. This range combines the penetration of lower frequencies and the greater resolution of higher frequencies.

Several terms used in US imaging are unique. The term B-mode sonography refers to a standard gray-scale mode, whereas Doppler sonography is used for the assessment of blood flow and is color coded (blue or red), depending on flow pattern. As the whistle on a train moves toward an observer, the pitch will increase. Echogenicity defines the appearance of tissues on the US image relative to a reference material. Anechoic refers to a complete absence of return signal; it represents complete penetration of the energy through a structure without echoic return. This appears uniformly black on imaging. An isoechoic object has similar echogenicity to surrounding tissue and is typically a midtone; normal thyroid and salivary glands are the referent isoechoic neck structures. Hypoechoic tissue has lower echogenicity than reference tissue and appears darker. Hyperechoic tissue is lighter compared with the reference structure because of higher echogenicity, and it may appear white. Put simply, hyperechoic refers to whiteness and hypoechoic to blackness on imaging.

Utility and Limitations

The majority of normal structures in the face and neck are located within 5 cm of the skin surface, which allows for easy evaluation with high-resolution (high-frequency) sonographic technology. Therefore most extracranial head and neck neoplasms can be accurately assessed using US. Currently, the most common use of US includes assessment of the thyroid, parathyroid, and salivary glands, as well as the central and lateral cervical lymph node basins. Although magnetic resonance imaging (MRI) and computed tomography (CT) can be used to visualize these structures, they generally offer no clear-cut advantage in the assessment of size, margin, or malignant potential of identified pathology. However, as discussed later, these modalities are superior when evaluating mucosal disease.

Although traditional radiology-performed US is a valuable tool, one major advantage of US is that office-based clinician-performed ultrasonography is quite feasible. US lends itself to office examination because it is relatively inexpensive, portable (the size of a laptop computer), quick and easy to perform, and harmless to the patient. It allows both real-time diagnostic imaging and image-guided fine-needle biopsy to be performed at the same visit. US does not involve ionizing radiation nor does it require intravenous contrast.

Limitations of US include an inability to penetrate through bone and cartilage, and a difficulty assessing deep visceral and bony invasion. US cannot reliably evaluate perineural spread, involvement of the skull base, or the presence of pathologic lymph nodes in the parapharyngeal, retropharyngeal, or lower mediastinal region. A large patient body habitus may also limit resolution because neck thickness increases signal attenuation.

In addition, operator dependence is one of the major limitations of office-based US and requires mastery of skills that may initially seem foreign to nonradiologists. Comfort with in-office US requires a steady volume of applicable patients to both maintain competence and justify the cost of an office unit. Yet, once comfort with US is achieved, an office-based exam affords several unique advantages. The mastery of US technique by a clinician who possesses an in-depth appreciation for both cervical anatomy and pathology (1) eliminates the reliance on a separate reporting physician, (2) provides better real-time preoperative localization, (3) provides better assesses site-specific lymphadenopathy, (4) permits single-operator follow-ups with image-based documentation, (5) assists in more accurate fine-needle specimen acquisition, and (6) adds a valuable dimension to the physical examination.

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