DIAGNOSTIC ULTRASOUND: TECHNIQUES AND GENERAL PRINCIPLES

ANTHONY A. MANCUSO

KEY POINTS

Ultrasound is an extremely useful triage tool in head and neck imaging for limited indications and especially in the pediatric population.
The use of ultrasound in known diagnostic conditions amenable to follow-up with this diagnostic technique can eliminate cost and radiation in highly selected cases.
Ultrasound should be integrated with other imaging tools so that its added cost produces incremental useful data and preferably eliminates other diagnostic testing.
Ultrasound finds its greatest usage in the head and neck—aside from carotid occlusive disease—in the thyroid gland, where it has supplanted much of radionuclide imaging.
Ultrasound often cannot evaluate the entire extent of pathology or potential associated findings; this is a major shortcoming that must be considered when planning imaging resource allocation in a particular patient or condition.

Bistable diagnostic ultrasound (US) was introduced in the early 1970s with a very rapid progression in technologic development from its tranducers mounted on mechanical arms and analog display of gray-scale data to digital form with much improved transducer technology in the late 1970s. US development to include real-time and Doppler techniques with color flow enhancements followed very rapidly in the early 1980s and then followed by the more recent three- and four-dimensional versions. Essentially handheld systems will continue to evolve to the point where this technology will become like a stethoscope and will likely be incorporated into the modern version of the physical examination—a practice already present in the emergency room for almost a decade.

US is a noninvasive imaging tool of great value when used appropriately in the head and neck region. It has the potential to eliminate more costly computed tomography (CT) and magnetic resonance imaging (MRI) studies (Fig. 4.1).^1,2 It may also help avoid unnecessary irradiation, a goal especially desirable in the pediatric population. The triage job is to make US the only examination necessary as often as possible. US should be avoided as a first choice if it is known in advance that CT and/or MRI will also be done regardless of any US findings (Fig. 4.2). Supplemental US can always be added if it might contribute some incremental data that will advance medical decision making, limit risk, and/or improve outcome. For instance, US is frequently the only study necessary when it is problem focused (Fig. 10.4), if it is being used to direct a biopsy (Fig. 6.6), or if it is being used to follow a process with a known or near-certain diagnosis that can be completely evaluated with US (Fig. 4.1).

FIGURE 4.1. A five-year-old boy presenting with acute onset of a left parotid region mass. The mass was relatively discreet at palpation and slightly tender. A: Ultrasound of the mass showed it to likely be an abnormal lymph node with multiple sonolucent zones (arrows) replacing most of the node parenchyma (arrowhead). B: Color flow Doppler image shown in gray scale shows areas of abnormality in the node that are relatively sonolucent (arrow), compressing normal nodal parenchyma (arrowhead) and displacing nodal vessels. Based on this ultrasound, this was felt to be a suppurative lymph node possibly due to catscratch disease. The patient was treated and followed with ultrasound. C, D: After about 14 days of antibiotic therapy, the node returned to a relatively normal nodal morphology, including the relatively normal hilar flow pattern seen in (D).

FIGURE 4.2. Three different ultrasound studies on three different patients, all with venolymphatic malformations. A: Predominantly cystic hygroma. B: Predominantly lymphangioma but also had a small vascular component (not shown). C: This was also a predominantly lymphatic space malformation, but the color flow images showed it to have some vascular components. (NOTE: Ultrasound, especially in pediatric patients, very frequently confirms the diagnosis of a venolymphatic malformation. However, in a considerable majority of patients, additional magnetic resonance imaging or computed tomography is required to fully map the lesion. In this way, ultrasound is very frequently a redundant examination, and its cost can be eliminated if the diagnosis is fairly certain from the outset.)

PHYSICAL BASIS OF THE ULTRASOUND IMAGE

US is a technique based on the likelihood of focused sound waves projected into the body to be transmitted, reflected, and refracted by the various tissues and disease processes in its path.³ The differences in tissues result in large part from differences in acoustic impedance and the size of the structures probed relative to the wavelength of the sound emitted by the transducer. The stronger and larger the interfaces in tissues, the more specular reflection will be present and the more sound will be returned to the transducer to create the range of gray tones that make up the image. Lesser specular reflectors, such as those caused by point reflectors or surface roughness, cause sound to diffuse so that less sound will return and cause the diffuse reflectors to be manifest as lower-level gray tones. If essentially no reflectors or very low-level diffuse reflectors are present, much of the sound will be transmitted and little or none returned. The object of interest will then contain few gray tones—low level or otherwise—and be considered mainly or completely sonolucent. The cystic or solid nature of the sonolucent lesion then needs to be judged by other characteristics such as through transmission or vascularity preferably by color flow Doppler imaging (Fig. 4.3).^4,5

FIGURE 4.3. Three patients with abnormal neck masses. A, B: Child with a nontender neck mass. In (A), the mass is seen as well circumscribed with a relatively strong back wall and enhanced through transmission. However, the mass contains many low-level echoes creating some ambiguity about whether it is a cystic or solid mass. In (B), the color flow Doppler image shows no evidence of vascularity within the mass. This was a second brachial cleft cyst. C, D: Images from a study of a parotid lesion mass. In (C), the mass has a well-defined, fairly strong back wall with enhanced through transmission, but it contains many low-level gray tones. This creates ambiguity about whether the mass is cystic or solid. In (D), the vascular pattern throughout the mass identifies it as solid and likely neoplastic. This was a benign mixed tumor of the parotid gland. It likely has cystic characteristics because of being composed of many microcystic spaces. E, F: Young adult with a neck mass. In these images, which are two-dimensional (E) and three-dimensional (F) ultrasound images, respectively, the mass appears sonolucent. It has a strong back wall. The tissues beyond the mass are well seen, and the presence of enhanced through transmission is suggested but perhaps not as strongly as in (A–D). Almost no low-level gray tones are seen within the mass. However, in (G), a vascular pattern within the mass is present on the color flow image. The vascular pattern is not that of a normal lymph node. Biopsy showed this to be lymphoma.

US is generally performed with 7.5- to 10.0-MHz transducers with a narrow internal focus. In thicker patients, a 5-MHz transducer may be necessary to get adequate depth penetration. Some understanding of how near field and far field effects of focused transducers influence US images is useful. It is also useful to understand how gains should be set to produce the best images with the least artifact or spurious echo patterns. In general, the neck vasculature and most lymph node groups, as well as more superficial head and neck structures including the parotid superficial portion and submandibular gland, are optimally imaged and the narrow internal focus zones of these high-frequency transducers. It is relatively easy to set the gains properly for these neck studies.

The normal tissues of interest in the practice of head and neck imaging include skin, fat, muscle, glandular tissue, and blood vessels. These have the following general appearance:

(1) Skin may not be seen because it is in the near field where the data image data might be unreliable, as this layer is somewhat hidden in reverberation artifact.

(2) Subcutaneous and other fat is generally sonolucent with low-level gray tones as its texture except for the reflective interface of the superficial fascia and platysma within the fatty layers of the subcutaneous and deeper fascial planes and compartments of the neck. The more fibrous tissue within fat such as around the carotid sheath, the more there will be higher-level echoes in that fat (Fig. 4.4).

FIGURE 4.4. Comparative computed tomographic and ultrasound images to demonstrate some general appearances of tissues as seen on gray-scale ultrasound. The images are in a patient with thyroid carcinoma in the left lobe. A: Contrast-enhanced computed tomographic (CECT) study showing calcification associated with thyroid cancer in the thyroid left lobe. This image correlates with that in (C). Note the subcutaneous fat (arrow) and infrahyoid strap and sternocleidomastoid muscles (arrowheads) for particular comparison with (A). B: CECT image at a slightly different position showing an abnormal level 4 lymph node containing a cystic (arrow) and solid enhancing (arrowhead) component. C: Transverse ultrasound image at about the same slice position as seen in (A) showing the echogenicity of the subcutaneous fat (arrows) beneath the skin surface. The infrahyoid strap muscle is seen as a relatively sonolucent structure (white arrowheads) correlating with (A). This is also true of the sternocleidomastoid muscle (most lateral white arrowhead). Fat around the carotid sheath region shows higher-level echoes than other fat due in part to transmission of sound through the carotid artery but also because of the more reflective interfaces of the fascia within the fat making up the carotid sheath region (black arrows). The calcification seen in the thyroid lobe in (A) causes acoustic shadowing (black arrowheads) within an otherwise relatively normal-appearing glandular texture. D: Image shows that the lymph node identified in (B) is actually one of a chain of abnormal partially cystic lymph nodes (white arrows) identified as partially cystic due to the enhanced through transmission present (black arrows) and the strong back wall of these partially cystic lymph nodes.