Ultrasonography of the Thyroid study notes

Ultrasonography of the Thyroid - Manfred Blum February 28, 2012

http://www.thyroidmanager.org/chapter/ultrasonography-of-the-thyroid/

http://www.thyroidmanager.org/wp-content/uploads/chapters/ultrasonography-of-the-thyroid.pdf

Authors

Manfred Blum, M.D.Professor of Medicine and Radiology, Director Thyroid Unit, New York University School of Medicine

INTRODUCTION

Ultrasonography (US) is the most common and most useful way to image the thyroid gland and its pathology, as recognized in guidelines for managing thyroid disorders published by the American thyroid Association (1) and other authoritative bodies.

In addition to facilitating the diagnosis of clinically apparent nodules, the widespread use of US has resulted in uncovering a multitude of clinically unapparent thyroid nodules, the overwhelming majority of which are benign. The high sensitivity for nodules but poor specificity for cancer has posed a management and economic problem. This chapter will address the method and utility of clinically-effective thyroid US to assess the likelihood of cancer, to enhance fine needle aspiration biopsy and cytology (FNA), to facilitate other thyroid diagnoses, and to teach thyroidology.

Previously, imaging of the thyroid required scintiscanning to provide a map of those areas of the thyroid that accumulate and process radioactive iodine. The major premise of thyroid scanning was that thyroid cancers concentrate less radioactive iodine than healthy tissue. Although, scintiscanning remains of primary importance in patients who are hyperthyroid or for detection of iodine-avid tissue after thyroidectomy for thyroid cancer, US has largely replaced it for the majority of patients who require a graphic representation of the regional anatomy because of its higher resolution, superior correlation of true thyroid dimensions with the image, smaller expense, greater simplicity, and lack of need for radioisotope administration. The other imaging methods, computerized tomography (CT) and magnetic resonance imaging (MRI) are more costly than US, are not as efficient in detecting small lesions, and are best used selectively when US is inadequate to elucidate a clinical problem (2-3).

As with any test, US should be used to refine a differential diagnosis only when it is needed to answer a specific diagnostic question that has been raised by the clinical history and physical examination (4). The image must then be integrated into patient management and correlated precisely with the other data. A technique has been reported that helps the clinician to interpret thyroid scintigrams of goiters and functioning nodules by assembling scintiscans and US side-by-side as one composite image (2).

Although sonography can supply very important and clinically useful clues about the nature of a thyroid lesion, it does not reliably differentiate benign lesions and cancer. However, it can help significantly. US can:

Depict accurately the anatomy of the neck in thyroid region,

Help the student and clinician to learn thyroid palpation,

Elucidate cryptic findings on physical examination,

Assess the comparative size of nodules, lymph nodes, or goiters in patients who are under observation or therapy,

Detect a non-palpable thyroid lesion in a patient who was exposed to therapeutic irradiation,

Give very important and clinically useful clues about the likelihood of malignancy,

Identify the solid component of a complex nodule,

Facilitate fine needle aspiration biopsy of a nodule,

Evaluate for recurrence of a thyroid mass after surgery,

Monitor thyroid cancer patients for early evidence of reappearance of malignancy in the thyroid bed or lymphadenopathy,

Identify patients who have ultrasonic thyroid patterns that suggest diagnoses such as thyroiditis.

Refine the management of patients on therapy such as antithyroid drugs,

Facilitate delivery of medication or physical high-energy therapy precisely into a lesion and spare the surrounding tissue,

Monitor in-utero the fetal thyroid for size, ultrasonic texture, and vascularity,

Scrutinize the neonatal thyroid for size and location,

Screen the thyroid during epidemiologic investigation in the countryside.


TECHNICAL ASPECTS

Sonography depicts the internal structure of the thyroid gland and the regional anatomy and pathology without using ionizing radiation or iodine containing contrast medium (5-6). Rather, high frequency sound waves in the megahertz range (ultrasound), are used to produce an image.

The procedure is safe, does not cause damage to tissue and is less costly than any other imaging procedure. The patient remains comfortable during the test, which takes only a few minutes, does not require discontinuation of any medication, or preparation of the patient.

The procedure is usually done with the patient reclining with the neck hyperextended but it can be done in the seated position.

A probe that contains a piezoelectric crystal called a transducer is applied to the neck but since air does not transmit ultrasound, it must be coupled to the skin with a liquid medium such a gel. This instrument rapidly alternates as the generator of the ultrasound and the receiver of the signal that has been reflected by internal tissues. The signal is organized electronically into numerous shades of gray and is processed electronically to produce an image instantaneously (real-time). Although each image is a static picture, rapid sequential frames are processed electronically to depict motion. Two-dimensional images have been standard and 3-dimentional images are an improvement in certain circumstances (7). There is considerable potential for improving ultrasound images of the thyroid by using ultrasound contrast agents. These experimental materials include gas-filled micro-bubbles with a mean diameter less than that of a red blood corpuscle and Levovist, an agent consisting of granules that are composed of 99.9% galactose and 0.1% palmitic acid. They are injected intravenously, enhance the echogenicity of the blood, and increase the signal to noise ratio (8-9).

Dynamic information such as blood flow can be added to the signal by employing a physics principle called the Doppler effect, which is that the frequency of a sound wave increases when it approaches a listener (the ear or, in the case of ultrasonography, a transducer) and decreases as it departs. The Doppler signals, which are superimposed on real time gray scale images, are extremely bright in black and white images and may be color coded to reveal the velocity (frequency shift) and direction of blood flow (phase shift) as well as the degree of vascularity of an organ (10-11). Flow in one direction is made red and in the opposite direction, blue. The shade and intensity of color can correlate with the velocity of flow. Thus, in general terms, venous and arterial flow can be depicted by assuming that flow in these two kinds of blood vessels is parallel, but in opposite directions. Since portions of blood vessels may be tortuous, modifying orientation to the probe, different colors are displayed within the same blood vessel even if the true direction of blood flow has not changed. Thus, an analysis of flow characteristics requires careful observations and cautious interpretations. The absence of flow in a fluid-filled structure can differentiate a cystic structure and a blood vessel.

Blood flow within anatomic structures can also be depicted by non-Doppler technology that is called B-flow ultrasonic imaging (BFI). This is accomplished by transmitting precisely separated adjacent ultrasound beams and computer-analyzing the reflected echo pairs (12).

The ultrasound is treated differently by the various anatomic features and different kinds of tissues (2, 5). The air-filled trachea does not transmit the ultrasound. Calcified tissues such as bone and sometimes cartilage and calcific deposits in other anatomic structures block the passage of ultrasound resulting in a very bright signal and a linear echo-free shadow distally. Most tissues transmit the ultrasound to varying degrees and interfaces between tissues reflect portions of the sound waves. Fluid-filled structures have a uniform echo-free appearance whereas fleshy structures and organs have a ground glass appearance that may be uniform or heterogeneous depending on the characteristics of the structure.

The depth penetration and resolving power of ultrasound depends greatly on frequency (6). Depth penetration is inversely related and spatial resolution is directly related to the frequency of the ultrasound. For thyroid, a frequency of 7.5 to 10 or 14 megahertz is generally optimal for all but the largest goiters. Using these frequencies, nodules as small as two to three millimeters can be identified.

Routine protocols for sonography are not adequate. Although some technologists become extremely proficient after specific training and experience, supervision and participation by a knowledgeable and interested physician-sonographer is usually required to obtain a precise and pertinent answer to a specific problem that has been posed by the clinician. Standard sonographic reports may provide considerable information about the anatomy, but are suboptimal unless the specific clinical concern is explored and answered. Indeed, because some radiologists cannot address the clinical issue adequately, and for convenience, numerous thyroidologists and a few surgeons perform their own ultrasound examinations, in which case it is essential that they have state-of-the-art equipment (that might not be cost-effective) and that they are willing to expend a considerable amount of time for a complete study. Technical ingenuity, electronic enhancements such as Doppler capability, and even artistry are frequently required. Special maneuvers, various degrees of hyperextension of the neck, swallowing to the facilitate elevation of the lower portions of the thyroid gland above the clavicles, swallowing water to identify the esophagus, and a Valsalva maneuver to distend the jugular veins may enhance the value of the images. Nevertheless, sonography is rather difficult to interpret in the upper portion in of the jugular region and in the areas adjacent to the trachea. Sonography is generally not useful below the clavicles.

It is informative for orientation to survey the entire thyroid gland with a low-energy transducer before proceeding to 10-14 megahertz equipment to delineate the fine anatomy. Protocols have been devised to assemble a montage of images to encompass an unusually large lobe or goiter. For an overview, panoramic ultrasound, which is a variation of conventional ultrasound has been reported to produce images with a large anatomic field of view, displaying both lobes of the thyroid gland on a single image (13).

There may be considerable differences between sonologists in estimating the size of large goiters or nodules. One investigation has reported that curved-array transducers may avoid significant inter-observer variation that may occur when linear-array equipment is employed, especially when the gland is very large(14). The inter-observer variation may be almost 50% among experienced ultrasonographers for the determination of the volume of thyroid nodules, because it is difficult to reproduce a two-dimensional image plane for multiple studies (15). Accuracy in volume estimation becomes most important when one uses ultrasound measurements to calculate an isotope dose or to compare changes over time in the size of a nodule or a goiter. Using planimetry from three-dimensional images reportedly has lower intra-observer variability (3.4%) and higher repeatability (96.5%) than the standard ellipsoid model for nodules and lobes, with 14.4% variability and 84.8% repeatability (p < 0.001) (16).

There may be imperfect concordance between the ultrasonic dimensions of large thyroid nodules compared with surgical excision (17).

SONOGRAPHY OF THE NORMAL THYROID AND ITS REGION

The anterior neck is depicted rather well with standard gray scale sonography. (FIGURE 1) The thyroid gland is slightly more echo-dense than the adjacent structures because of its iodine content. It has a homogenous ground glass appearance. Each lobe has a smooth globular-shaped contour and is no more than 3 – 4 centimeters in height, 1 – 1.5 cm in width, and 1 centimeter in depth. The isthmus is identified, anterior to the trachea as a uniform structure that is approximately 0.5 cm in height and 2 – 3 mm in depth. The pyramidal lobe is not seen unless it is significantly enlarged. In the female, the upper pole of each thyroid lobe may be seen at the level of the thyroid cartilage, lower in the male. The surrounding muscles are of lower echogenicity than the thyroid and tissue planes between muscles are usually identifiable. The air-filled trachea does not transmit the ultrasound and only the anterior portion of the cartilaginous rings is represented by dense, bright echoes. The carotid artery and other blood vessels are echo-free unless they are calcified. The jugular vein is usually in a collapsed condition and it distends with a Valsalva maneuver. There are frequently 1-2 mm echo-free zones on the surface and within the thyroid gland that represent blood vessels. The vascular nature of all of these echo-less areas can be demonstrated by color Doppler imaging to differentiate them from cystic structures (10-11). Lymph nodes may be observed and nerves are generally not seen. The parathyroid glands are observed only when they are enlarged and are less dense ultrasonically than thyroid tissue because of the absence of iodine. The esophagus may be demonstrated behind the medial part of the left thyroid lobe, especially if a sip of water distends it. (FIGURE 2)

Figure 1. Sonogram of the neck in the transverse plane showing a normal right thyroid lobe and isthmus. L=small thyroid lobe in a patient who is taking suppressive amounts of L-thyroxine, I=isthmus, T=tracheal ring ( dense white arc is calcification, distal to it is artefact), C=carotid artery ( note the enhanced echoes deep to the fluid-filled blood vessel), J=jugular vein, S=Sternocleidomastoid muscle, m=strap muscle.

Figure 2. Sonogram of the left lobe of the thyroid gland in the transverse plane showing a rounded lobe of a goiter. L=enlarged lobe, I= widened isthmus, T=trachea, C=carotid artery ( note the enhanced echoes deep to the fluid-filled blood vessel), J=jugular vein, S=Sternocleidomastoid muscle, m=strap muscles, E=esophagus.

GENERAL THOUGHTS ABOUT SONOGRAPHY

Thyroid US may play a useful role in the management of patients even when the thyroid examination is normal but it is debatable if the procedure is cost-effective as a screening test (1). Many thyroidologists/endocrinologists advocate routine use of US at the time of physical examination to discover subclinical, nonpalpable thyroid abnormalities, which will be discussed presently, and to enhance the sensitivity and accuracy of palpation. This practice is called “point of service” US.

Whether US is performed at the point of service or in an US laboratory by ultrasonographers/radiologists, it is important to employ thyroid sonography selectively to supplement or confirm a physical examination especially when clinical perception is confused by obesity, great muscularity, distortion by abnormal adjacent structures, tortuous regional blood vessels, a prominent thyroid cartilage, metastatic tumor, lymphadenopathy, or prior surgery.

In practice, US may be used to supplement an examination when there is uncertainty about the palpation. However, US is time-consuming and the accumulated data of its utility, which are discussed below, were obtained with state of the art equipment by experts. It is important to comprehend that the optimal clinical value of US depends on the quality of the examination, including the maturity of the examiner and the characteristics of the equipment. Grossly misleading results may occur with quick, incomplete studies and unsophisticated machines or substandard readouts. Therefore, routine sonography in a medical office or clinic or in a laboratory by an incompletely trained general radiologist will require proper professional preparation. Without study, training, and practice, there are likely to be unacceptable results, adverse outcomes, and negative publicity. Furthermore, the cost-effectiveness of US as screening or in sub-optimal conditions has yet to be critically examined.

In the academic situation, sonography is useful to teach palpation of the thyroid gland.

There are claims that US can offer insights into thyroid function. For instance, among 4649 randomly selected adult subjects one investigation found that there was correlation between thyroid hypoechogenicity and higher than average levels of serum TSH, even in subjects without overt thyroid disease (18). One group reported TSH elevation in 26 patients with autoimmune thyroiditis when there was a well-defined area of low echogenicity, about 10   mm in diameter, between the lateral margin of one or both thyroid lobes, the medial wall of the carotid artery, and, posteriorly, the pre-vertebral muscles. Euthyroid patients (71) with thyroiditis and controls (154) did not demonstrate a hypoechoic triangle (18A). In contrast, how accurately does a normal thyroid sonogram predict normal thyroid function? In one study of normal-appearing US, TSH was normal 41/48 (85.4%) but was elevated in 7 subjects (14.6%) (p<0.001) and anti-thyroid antibodies were detected in 5 patients (10.4%)(19). Therefore, a normal sonogram does not preclude hypothyroidism or Hashimoto ’ s thyroiditis.

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