Introduction

For years, ultrasound has been the primary imaging tool for the thyroid gland. It is cost effective, rapid in information accrual, and avoids radiation exposure. The resolution capability of ultrasound machines has improved dramatically since the new millennium, such that even small, portable systems are now able to provide meaningful imaging information. Thus, ultrasound has become an indispensable tool of the clinician in the outpatient setting. The machines have become more affordable, and there are numerous formal educational opportunities to learn the technology and its applications. There are many advantages for the patient who has the ultrasound performed in the office. The opportunity to have the diagnostic ultrasound performed and interpreted and specimens for cytology procured all at the same visit maximizes convenience. With thyroid malignancy, ultrasound has become integral to preoperative planning as well as long-term post-treatment follow-up.

Please see the Expert Consult website for a brief discussion of the limitations of ultrasound.

Patients with Hurthle cell carcinoma usually do not demonstrate thyroglobulin elevation. Some patients with papillary carcinoma have thyroglobulin antibodies. The recurrences of these conditions may depend primarily on ultrasound to adequately detect recurrent or metastatic disease. Prior to the mid-1990s, preoperative imaging in hyperparathyroidism was limited. Nuclear scans and ultrasound technology were both relatively low in resolution. The advent of sestamibi enabled surgeons performing parathyroidectomy to have a strong level of confidence that they can predict the precise nature of disease and its location. Intrathyroidal parathyroid adenomas should be identified prior to surgery as a result of systematic ultrasound. The operating surgeon has a strong incentive to identify the offending enlarged parathyroid gland with complete preop US exam looking for atypical locations including intrthyroidal as well as lesions within the upper thymus, carotid sheath, para-esophagus, and undescended in the upper neck.

This chapter discusses the basics of ultrasound and many of its clinical applications to surgery of the thyroid and parathyroid glands. Although it is a marvelous tool that rightfully has entered the province of the clinician, it does have some limitations. In the beginning, it is time consuming. The equipment can be expensive. Some patients have thick neck tissues or abnormal anatomy, either of which render the images difficult to interpret. When a parathyroid gland is posterior to the esophagus or low in the mediastinum, ultrasound will not be able to detect it. Certification and mentorship opportunities may be limited for some surgeons, and radiologists in the hospital or community may exercise an objection to use of this modality by clinicians. In spite of these few and mostly surmountable issues, ultrasound has become an indispensable tool for the thyroid and parathyroid surgeon. Finally, it complements the surgeon’s passion for anatomy and allows a view of the relevant vascular, muscular, endocrine, lymphatic, and aerodigestive structures in real time with minimal effort.

Physics and Principles of Ultrasound

The ultrasound system has three basic components: (1) transducer, which delivers and receives sound energy directly to and from the tissues; (2) console, which among other things contains the sophisticated computer software, conversion algorithms, storage, and Doppler methodology; (3) display, which permits the observer to view and select anatomic areas of interest. An understanding of how sound energy and tissues interact has permitted a marriage of theory and technology. Whereas artifacts with other radiologic modalities (e.g., dental restoration production of scatter and degradation of computed tomography [CT] images) present an unwanted problem, artifacts in ultrasound enhance clinical information, which can be used to advantage.

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As with many technological advances in photography and computer imaging, resolution in ultrasound has reached such a level of sophistication and miniaturization that portability has become a reality. Whereas expensive large systems by necessity resided in radiology departments, now clinicians are able to adapt ultrasound to use in multiple offices and even the operating room. It is easy to forget that these machines are remarkably sophisticated beneath their user-friendly interface to the clinician.

Sound waves travel relatively slowly in comparison to light and require some form of support medium. When traveling through air, the relative lack of density of the transferring molecules prevents rapid transport and permits attenuation. In contrast, sound waves in fluid travel over long distances with much less degradation depending on their frequency. The process of sono localization in many sea creatures such as dolphins and whales serves both to communicate with others of the same species and to receive feedback information from emitted sounds to locate food. Dolphins and other odontocetes such as toothed whales emit single high-frequency clicks for echolocalization.² Many other animals, including bats, owls, and elephants, emit and receive sound waves for localization and communication.

Many of the applied concepts have been extracted and simplified from the most commonly referenced text by Kremkau³ titled “Diagnostic Ultrasound, Principles and Instruments.” Sound is transmitted as linked sine waves with peak and trough elements whose height represents amplitude or loudness. A single full cycle is measured from peak to peak or trough to trough, and the number of these cycles per 1-second unit of time represents the frequency. The human ear can recognize sounds as low as 20 Hz and as high as 20,000 Hz or 20 kilohertz (20 kHz). Ultrasound is so named because its frequency emission is well above these frequency levels, in the range of millions of cycles per second or megahertz (MHz). In fact, ultrasound systems used in medicine vary from 3 to 17 MHz with advantages and disadvantages for selected emissions in low, intermediate, or high ranges. Just as a fluid wave along the surface of a body of water is generated from the physical deformation produced by a dropped pebble, an ultrasound wave is transmitted to human tissues through the transducer by physical deformation. One of the key elements of the transducer is the piezoelectric crystal, which elongates and shortens in response to applied alternating electrical current. The actual physical change in these crystals imparts vibrational contact with the skin and deeper tissues to produce a transmitting wave. Special backing materials surrounding the transducer, except for the surface of the footprint that touches the skin, prevent random scatter of the wave energy. This design produces directional transmission. In a linear array transducer where the crystals are oriented such that emitted waves are perpendicular to its footprint, commonly a composite of 128 individual crystals in parallel array will generate the signal. Each crystal vibrates in synchrony with adjacent crystals such that multiple parallel sound waves of equal frequency enter the tissue.

Piezoelectric crystals exist naturally; quartz is well known for its physical vibratory properties. However, synthetically manufactured ferromagnetic crystals are generally used in diagnostic ultrasound transducers, PZT (lead zirconate titanate) being the most common. The generated sound waves from these crystals are low energy and therefore do not disrupt cell membranes or produce appreciable heat. In contrast, some ultrasound systems that have been employed in physical therapy do produce significant energy to impart deep heat for certain therapeutic benefits.

As the sound waves enter the skin and deeper tissues, they meet elements of varying density, shape, and reflectivity. Most waves continue to penetrate the tissues and pass on in a linear fashion or scatter oblique to the path of the sound wave. In fact, only 1% of the transmitted sound is reflected back to the transducer.

Sound waves are attenuated and progressively lose amplitude as they pass through tissues. This attenuation is dependent on the density of the tissues and depth of a targeted anatomic structure. The frequency of emitted sound has differing attenuation characteristics in tissue. Low frequencies in the range of 3 to 5 MHz are not attenuated readily and thus are more suited to demonstrating deeper structures. In contrast, high-frequency sound waves attenuate rapidly and reach only moderate depths.

Wave emission from the transducer is not linear but has an hourglass configuration. The optimum point of tissue resolution is at the narrowest portion of this configured wave and designated the focal zone. Stated another way, the reflected waves from adjacent point tissue targets are seen as separated entities rather than combined blurred images when the focal point is properly aligned. The focal zone can be manipulated to a shallow or deeper plane within the region of interest. At the focal zone, tissue properties show the best “lateral resolution.” Similarly, the frequency of the transmitted wave is important. High-frequency waves produce better abilities than low-frequency waves to resolve adjacent tissue elements in the direct path of the sound wave. This property is designated “axial resolution.” Just as a newspaper image with relatively few points of dark ink and luminosity produces a relatively coarse image on magnification, while a high megapixel photograph rendered from a digital single-lens reflex camera produces a sharper image with many more of these points per unit area, so lateral and axial resolution characteristics of penetrating and resolving sound waves combine to produce the ideal image. Where depth of penetration is the most important priority, the lower frequency waves must be utilized with some sacrifice in resolution. In ultrasound of the head and neck where structures of interest are only a few centimeters below the skin surface, higher frequency waves with better axial resolution can be utilized. In summary, resolution or clarity on the monitor depends on both frequency and to a lesser degree alignment of the focal zone relative to the target. Selection of emitted transducer frequency will depend on whether deep or superficial anatomic structures are to be studied.

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The reflected sounds and their interpretation are responsible for allowing a processed image with meaningful information to form. The variations in tissues produce an acoustical mismatch, and these interfaces serve as reflectors. When there is a uniform surface from which much of the signal is reflected directly back in adjacent parallel waves, a discrete linear structure is defined. The mechanical waves are generated in packets of 1000 or more emissions per second, but then the transducer spends more time listening than transmitting. In principle, three transmitting cycles are performed and then the transducer is silent to allow the reflected waves to be received. These mechanical pulses are received by the temporarily nonvibrating piezoelectric crystals and converted back into electrical energy that the console can interpret and display.

The image can be further refined by manipulation after the echoes are returned to the signal processor. Brightness can be adjusted by a turn of the gain control knob. As previously described, ultrasound waves attenuate at greater depth from surface entry. The attenuation is especially problematic when higher frequencies are employed as in thyroid and parathyroid imaging. For example, the deeper aspects of a large goiter (with anterior-posterior dimension of 6 to 7cm or more) will be difficult to see, as these areas are affected by attenuation. These deeper attenuated waves can be selectively recovered with time-gain compensation by increasing the gain of these waves while leaving unaltered the more superficial waves. In this way the overall image has a more even distribution of illumination. Other methodologies such as “sono CT”⁴ change the way sound waves are delivered from the transducer. Rather than simple linear delivery of sound packets from the center of the transducer, some ultrasound units and transducer combinations have a selected option that sends sound waves in parallel from the edges of the long axis of the transducer. The waves converge at intervals somewhat similar to CT scan configurations to improve image resolution. Electronic noise is an undesirable but unavoidable element in amplification systems. Bandpass filtering can reduce the noise, which eliminates the frequencies above and below the ideal selected frequency. Harmonic imaging is a refinement that observes both the fundamental and second harmonic frequency echo reception. The fundamental frequency is filtered while permitting passage of the second harmonic, a postprocessing method that improves image quality. These are but a few examples of methods that modern ultrasound units have employed to refine image quality to a remarkable degree without any risk to the patient. In fact, even the small, basic portable machines now produce images that were simply unimaginable less than a decade ago.

Artifacts

In ultrasonography, artifacts are images that appear on the display and do not represent real anatomic structures. Rather, they are shadows or enhanced representation of tissue elements that are consistent and tell a story. Pure cysts have a thin discrete capsule and are entirely fluid-filled without significant solid components. Sound enters from a superficial direction and as yet unattenuated waves easily penetrate the anterior capsule. Because the interior of the cyst is fluid without elements that reflect sound, the parallel sound waves strike the posterior capsule, which through the acoustical mismatch acts as a reflector. A large proportion of these waves penetrate this capsule and return as bright, high amplitude signals from just beyond. This produces a relatively broad reflected area that is hyperechoic to adjacent tissues and the cyst itself. This artifact is designated “posterior enhancement” and is usually diagnostic of a cyst (Figure 13-1). In contrast, coarse calcifications block transmission of the sound waves to deeper tissue planes, producing a dark rectangular void deep to the densely hyperechoic structure. Known as posterior shadowing (Figure 13-2), this particular artifact represents coarse calcifications often seen in portions of a multinodular goiter and to a lesser degree in some cancers. Microcalcifications (Figure 13-3) generally seen in papillary carcinoma of the thyroid gland, unless closely aggregated do not produce posterior shadowing artifact as a result of their small size. These microcalcifications are small points of hyperechoic signal and represent psammoma bodies defined histologically in papillary carcinoma of either the primary thyroid or metastatic adenopathy. When planning fine-needle aspiration cytology, these areas are good sampling targets under ultrasound guidance. Other artifacts may be confused with microcalcifications. One such confusion can occur with “comet tail” artifact (Figure 13-4). These are hyperechoic points with a tapering core of hyperlucency extending from and deep to the circular dot. When examined more closely, the “tail” portion is actually a form of reverberation artifact. One accepted explanation is that areas of colloid crystallize and serve both as finite obstructions to transmission and deeper reverberation of the ultrasound waves. Ahuja has studied comet tail artifacts in a large number of thyroid conditions, and invariably he has found that the underlying processes are benign.⁵ Examples of structures that commonly demonstrate reverberation (Figure 13-5, A-C) are the anterior wall of the trachea, anterior wall of the carotid artery, comet tails from small colloid crystals, and biopsy needles in their long axis.

Figure 13-1 Posterior enhancement deep to a parathyroid cyst.

Figure 13-2 Hyperechoic dense calcification in a thyroid nodule prevents penetration of sound, and a resultant deep shadow is identified.

Figure 13-3 Microcalcifications do not produce posterior shadowing artifact.

Figure 13-4 Comet-tail artifact is similar in appearance to microcalcifications, but the comet tail clearly differentiates it from the representations of psammoma bodies.

Figure 13-5 Reverberation artifact can be seen in the following: A, anterior wall of the carotid artery; B, biopsy needles in the long axis; C, anterior tracheal wall. Arrows demonstrate the reverberating artifacts.

Doppler

Doppler is a relevant and technically different process than grayscale ultrasound in the assessment of vascularity of anatomic and pathologic elements.^6,7 In simplistic terms, the Doppler shift of sound waves occurs when waves imparted at an angle to a blood vessel strike directionally moving red blood cells and are reflected. If the waves are reflected toward the transducer, the velocity of this reflection is augmented. If the sound waves are reflected away from the transducer signifying blood flow away from the transducer, the velocity is reduced. This velocity of red cell movement can be calculated and directional flow given a color designation—that is, flow toward the transducer is red and away from it blue by convention. These Doppler images are superimposed over the corresponding B mode display in a rapidly alternating fashion such that the eye sees a moving color video rendition of this activity. This color Doppler imaging and flow interpolation is highly relevant to the study of carotid and peripheral vascular anatomy and restriction of flow. Power Doppler is a separate technique that ignores these calculations and directional relationships. Power Doppler is more sensitive to and has better resolution for small vessels with low flow such as those found within a lymph node or parathyroid adenoma (Figure 13-6, A and B). In fact, power Doppler can often be used as a differentiating tool between these two structures in the clinical setting. Power Doppler will still reveal large vessels in the head and neck but cannot render actual flow values.

Figure 13-6 Power Doppler demonstrates small vessels and their pattern in a hyperplastic lymph node (A, grayscale; B, power Doppler overlay).

Thyroid Ultrasound

Ultrasound (US) is the first-line recommended imaging modality for thyroid nodules.^8,9 Its use in thyroid disorders is widely acknowledged, and its benefits and indications continue to expand. Surgeons and endocrinologists in the office-based setting have adopted thyroid US for the evaluation and management of patients with thyroid, parathyroid, and many other head and neck disorders. Its versatility, convenience, safety profile, ability to offer dynamic real-time images, and low cost compared to other radiologic modalities, combined with the outstanding quality of the images produced by high frequency, high-resolution ultrasound, have all contributed to its popularity. Table 13-1 lists the principal goals of and indications for thyroid ultrasonography.

Table 13-1 Thyroid Ultrasonography: Principal Goals and Indications

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In past decades, palpable thyroid nodules were surgically excised to establish a pathologic diagnosis. Advances in two techniques, namely US and fine-needle aspiration cytology, and ultimately the combination of these techniques served to revolutionize the treatment of thyroid nodules and to reduce the incidence of unnecessary surgery for nodules that are predictably benign. Continued improvements in ultrasound technology have further improved the ability to identify and distinguish malignant features of thyroid lesions.^10–13 These same improvements have led to an enhanced ability to localize pathologic parathyroid glands, as will be discussed later in this chapter.

The thyroid gland is ideally suited to ultrasound evaluation because of the gland’s superficial position and easy accessibility, its distinctive echotexture, and the fact that different thyroid disorders tend to demonstrate characteristic sonographic findings. Still, the value of the thyroid US examination depends on the skill, the experience, and the motivation of the examiner, as well as on the quality of the equipment used. Thyroid US provides greater anatomic detail than CT, magnetic resonance imaging (MRI), or radionuclide studies.

Role of Ultrasound in the Initial Evaluation of the Thyroid Nodule

Thyroid US is recommended for all patients with suspected thyroid nodules,^8,9 including patients with palpable abnormalities, nodular goiter, and thyroid lesions incidentally found by other imaging studies.

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Routine screening thyroid US is not recommended for the general population because of the high incidence of thyroid nodules that are not clinically significant. Palpable thyroid nodules occur in approximately 7% of the general adult population, but the incidence of nonpalpable thyroid nodules visible by ultrasound is up 70%.^14–16

US can help confirm the presence of a thyroid nodule; objectively characterize the nodule’s size, location, and appearance; evaluate for benign or suspicious features; and evaluate for the presence of other thyroid nodules or cervical lymphadenopathy.¹ Although certain ultrasound characteristics of thyroid nodules are associated with malignancy, fine-needle aspiration (FNA) cytology remains the gold standard for diagnosis. FNA has until recently been recommended for cytologic evaluation of all thyroid nodules greater than 1 cm in diameter or nodules under 1 cm that exhibit suspicious features.^8,9 The 2009 Revised ATA Guidelines for Management of Thyroid Nodules and Differentiated Thyroid Cancer include more selective recommendations for FNA of certain thyroid lesions based on ultrasound criteria. These include FNA for mixed solid and cystic nodules 1.5 to 2 cm or greater in diameter with any suspicious ultrasound characteristics and limiting FNA of predominantly spongiform nodules to 2 cm or greater in diameter.⁸ Despite its utility, the diagnostic role of FNA is limited by an overall 3% false-negative rate and a 10% nondiagnostic rate.¹⁷

The use of ultrasound guidance improves the overall sensitivity, specificity, and accuracy of fine-needle aspiration (FNA) compared to palpation-guided FNA.^18–20 Ultrasound-guided FNA appears to be most valuable in patients with nonpalpable nodules, small palpable nodules, posterior nodules, multiple nodules, partially cystic nodules, or concomitant glandular disease. It is also beneficial for sampling specific areas of a nodule, such as the solid part of a mixed solid-cystic nodule. Cesur et al. found the rates of inadequate FNA samples to be significantly improved in palpable nodules 1 to 1.5 cm using ultrasound- versus palpation-guided FNA (37.6% versus 24.4%, p = 0.009), but not for palpable nodules 1.6 cm or larger.¹⁸ Ultrasound-guided FNA is also specifically recommended when repeating FNA for a nodule with an initial nondiagnostic cytology result.⁸

Ultrasonography Technique and Measurements

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Ultrasonography Technique and Measurements

US of the thyroid is a painless and relatively quick procedure that does not require any preparation such as hormone withdrawal, low-iodine diet, or fasting. The exam is best performed with the patient in a supine position with the neck extended, although it can be performed on the seated patient. Images are obtained in the transverse and sagittal planes, noting first the overall dimensions of each thyroid lobe. A linear-array, high-frequency (7.5 MHz or greater) transducer, with color Doppler capability, is recommended. By convention the left end of the transducer is directed to the patient’s right side for the transverse plane and in a cranial direction for the sagittal plane. The respective images show the patient’s right side toward the left side of the screen on transverse view and the cranial aspect toward the left side on sagittal view. Attention is given to the echogenicity and vascularity of the tissue as well as the presence of any discrete nodules.

A normal thyroid lobe measures approximately 1.5 to 2 cm in the transverse dimension, 4 to 5 cm in the sagittal plane, and 2 cm in the anterior-posterior dimension. If the sagittal length is greater than the width of the ultrasound transducer and its borders extend beyond the image, the length can be approximated by using the dual-screen option when available and “splicing” together the superior and inferior portions of the lobe (Figure 13-7). The thyroid isthmus has an average length of 1.2 to 1.5 cm in the sagittal plane, and the pyramidal lobe is seldom visible. Normal thyroid parenchyma echoes are fine, uniform, and slightly hyperechoic when compared to the surrounding muscles because of the gland’s iodine content.^5,21

Figure 13-7 Sagittal view of thyroid lobe with superior and inferior portions spliced together using the split-screen feature.

Methods exist for estimating thyroid volume by applying a mathematical formula to measurements obtained from US.^22,23 However, prospective studies have shown high interobserver variability in estimating thyroid nodule volume²⁴ and poor correlation between predicted and actual thyroid volumes.²⁵ Nevertheless, some use thyroid volume to facilitate ¹³¹I dosimetry in patients with thyrotoxicosis.²⁶

A complete US examination of the thyroid gland should also include examination of the regional lymphatics, noting any abnormally enlarged or atypical lymph nodes. Lymphatic metastases of thyroid carcinoma most often occur in the pretracheal and paratracheal lymph nodes or along the lateral neck jugulodigastric chain. Absence of a cervical thyroid gland in an untreated patient or presence of a thyroglossal duct cyst (Figure 13-8) should also prompt ultrasound inspection of the midline neck from the base of the tongue to the sternum, to assess for possible lingual or undescended thyroid. Laryngeal function can often be evaluated in the process.

Figure 13-8 Thyroglossal duct cyst within thyrohyoid space, seen as a well-defined hypoechoic cystic structure overhanging the superior aspect of the larynx (midline transverse view.)

Ultrasound Characteristics of Thyroid Nodules

Many distinctive ultrasound characteristics of malignant thyroid nodules have been identified (Table 13-2). Although these ultrasound characteristics offer high sensitivity, no single criterion offers sufficient specificity to differentiate benign from malignant lesions.²⁷ When characteristics are combined, however, specificity improves. One large prospective, observational study comparing US and FNA results with surgical pathology found that performing FNA on nodules with one of three ultrasound criteria—microcalcifications, blurred margins, or hypoechoic pattern—missed only 2% of cancers.²⁸ Kim et al. prospectively analyzed 155 incidentally discovered, nonpalpable, solid thyroid nodules and found a mean number of 2.6 suspicious ultrasound characteristics per malignant nodule and an overall sensitivity and specificity of 94% and 66%, respectively.²⁹

Table 13-2 Ultrasound Features Associated with Malignancy

Particular ultrasound features of thyroid nodules and their ability to suggest benign versus malignant lesions include the following.

Margins and Halo/Rim

Benign lesions are often associated with a hypoechoic circumferential vascular “halo” (Figure 13-9, A and B) thought to represent compressed vascularity within thyroid parenchyma adjacent to a nodule.³³ Neoplasms may display a partial or absent halo, and its presence or absence has been found to be suggestive but not diagnostic.^34,35 For malignant lesions, the halo is typically avascular and represents a true capsule that may by invaded by neoplastic cells. Blurred or ill-defined margins have been associated with an increased risk of malignancy.^28,34,35 The mobility of the nodule with respect to surrounding structures can be assessed by dynamic palpation during US, and fixation suggests malignant invasion of the surrounding tissue.

Figure 13-9 A, Thin hypoechoic circumferential “halo” surrounding a benign thyroid nodule. B, Vascularity within the “halo” on Doppler imaging.

Shape

Nodule shape has been implicated as a prognostic factor. Nodules that are more tall than wide on transverse view (i.e., greater anteroposterior diameter than transverse diameter; Figure 13-10) are more likely to harbor cancer.²⁹ This characteristic has been found to predict malignancy in breast US as well.³⁶ However, one retrospective analysis also found that thyroid nodules with a more spherical shape had a higher incidence of malignancy.³³ Irregular shape and microlobulated borders (Figure 13-11) have also been implicated in malignancy.^29,35

Figure 13-10 Taller than wide (and markedly hypoechoic) nodule that proved to be malignant on surgical pathology (right transverse view of thyroid.)

Figure 13-11 Irregularly shaped nodule with microlobulated borders, confirmed to be a papillary carcinoma.

Echo Structure

Many thyroid nodules are cystic or have cystic components, such as cystic degeneration of a follicular adenoma (Figure 13-12) or in the setting of multinodular goiter. Malignancy has been more closely associated with solid nodules than with cystic or mixed nodules.^28,35 Purely cystic nodules are unlikely to be malignant,³⁷ and those with a spongiform appearance (Figure 13-13), defined as an aggregation of multiple microcystic components in more than 50% of the nodal volume, are more than 99% specific for benign histology.^38,39

Figure 13-12 Cystic degeneration of a benign thyroid nodule.

Figure 13-13 Benign, spongiform thyroid nodule, with multiple microcystic components.

Echogenicity

The echogenicity of a thyroid nodule should be compared with that of surrounding thyroid tissue. Most benign adenomas or adenomatous nodules are slightly hyperechoic to slightly hypoechoic when compared to normal thyroid tissue (Figure 13-14), whereas malignant nodules are frequently markedly hypoechoic (Figure 13-15), or more hypoechoic than adjacent muscle.^29,35

Figure 13-14 Slightly hypoechoic benign adenomatous nodule.

Figure 13-15 Moderately hypoechoic and elongated nodule that proved to be a follicular carcinoma.

Calcifications

The presence of calcifications has variable significance. Peripheral calcification, also referred to as “eggshell calcification,” has traditionally been considered a benign feature representing previous hemorrhage and degenerative change, although debate exists about its correlation with benignity, especially when the calcification is disrupted. Coarse calcifications, however, are more worrisome for potential malignancy when seen in the center of a solid hypoechoic nodule (see Figure 13-2). Microcalcifications are even more strongly associated with an increased risk of malignancy.²⁸ Forty-five percent to 60% of malignant nodules demonstrate microcalcifications, as opposed to 7% to 14% of benign nodules.^29,40 Approximately 60% of patients with microcalcifications have malignant disease.⁴¹ Microcalcifications in malignant nodules are often attributed to psammoma bodies in papillary thyroid carcinoma (Figure 13-16) but are also frequently seen in medullary thyroid carcinoma. Although suggestive of malignancy, the overall specificity of microcalcifications for thyroid carcinoma has been reported to range from 71% to 94% with a sensitivity of 35% to 72%,^28,42,43 and therefore should not be solely relied on to differentiate benign from malignant lesions.

Figure 13-16 Microcalcifications in a papillary thyroid carcinoma.

Vascular Pattern

The vascular pattern around or within a nodule may correlate with the probability of malignancy. Chammas et al. classified thyroid nodules according to the pattern of vascularity seen with power Doppler into five types: absent blood flow, perinodular flow only, perinodular flow as great or greater than central blood flow, mainly central nodular flow, and central flow only.⁴⁴ Nodules with exclusively central blood flow or central blood flow greater than perinodular flow had a higher incidence of malignancy (Figure 13-17). Follicular carcinomas also tend to show a moderate increase in central vascularity by power Doppler compared to follicular adenomas that favor peripheral flow (Figure 13-18).⁴⁵ In general, increased vascularity in the interior of a thyroid nodule suggests malignancy but should not be considered a pathognomonic feature.

Figure 13-17 Increased central blood flow in a papillary carcinoma.

Figure 13-18 Follicular adenoma with peripheral blood flow.

Capsular Contact

The presence of contact between a nodule and the adjacent thyroid capsule is defined when there is no intervening thyroid tissue between the thyroid nodule and the capsule. Capsular contact of greater than 25% by US has been found to be a useful marker for predicting extrathyroidal extension of cancer⁴⁶ (Figure 13-19).

Figure 13-19 Thyroid nodule with greater than 25% capsular contact, confirmed to be papillary carcinoma with extrathyroidal extension.

Elastography

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Elastography

Elastography, which uses pressure applied with the ultrasound transducer to measure tissue stiffness, appears to hold promise in differentiating benign from malignant lesions and may help guide surveillance and selection of nodules for FNA. Elastography is discussed in greater detail later in this chapter.

Ultrasound Characteristics of Benign Thyroid Nodules

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Ultrasound Characteristics of Benign Thyroid Nodules

Overall, the most common types of nodules are benign hyperplastic (colloid) nodules and benign follicular adenomas (Figure 13-20). Colloid nodules consist of colloid and benign follicular cells and are associated with small, hyperechoic, internal lucencies on US, known as the “comet tail” sign (see Figure 13-4).⁴⁷ Follicular adenomas are the most common type of thyroid neoplasm and are, with few exceptions, not considered a forerunner of carcinoma.⁴⁸ They are typically round, well-encapsulated lesions with a clear margin or “halo” distinguishing them from the surrounding normal thyroid tissue. Spontaneous or traumatic hemorrhage may occur into the nodule (Figure 13-21).

Figure 13-20 Colloid nodule, right sagittal view.

Figure 13-21 Hemorrhage (right panel) into a follicular adenoma (left transverse view). Note halo surrounding the nodule.

Ultrasound Characteristics of Malignant Lesions

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Ultrasound Characteristics of Malignant Lesions

Although the majority of thyroid nodules are benign, a top priority in the management of thyroid nodules is to identify the approximately 10% of nodules that harbor malignancy.⁴⁹ Most thyroid cancers are of follicular cell origin, including papillary, follicular, and Hurthle cell carcinoma, collectively known as well-differentiated thyroid cancers. Other malignancies including medullary thyroid carcinoma, anaplastic carcinoma, lymphoma, and metastatic disease are less common.

Although ultrasound characteristics of thyroid nodules do not offer sufficient specificity to diagnose malignancy, US represents an invaluable tool for identifying thyroid lesions and determining which lesions should undergo further evaluation through biopsy or other investigations. Clinical features of the various thyroid malignancies are discussed elsewhere throughout this textbook (see Chapters 18, Papillary Thyroid Cancer, 19, Papillary Thyroid Microcarcinoma, 20, Follicular Thyroid Cancer, 21, Dynamic Risk Group Analysis for Differentiated Thyroid Cancer, 22, Hurthle Cell Tumors of the Thyroid, 23, Sporadic Medullary Thyroid Carcinoma, 24, Syndromic Medullary Thyroid Carcinoma: MEN 2A and MEN 2B, 25, Sporadic Medullary Thyroid Microcarcinoma, and 26, Anaplastic Thyroid Cancer and Thyroid Lymphoma). Certain ultrasound characteristics are associated with certain thyroid malignancies, as described in this chapter.