Basics of Ultrasound

Over the past 30 years sonographic imaging technology has undergone tremendous change. As a result of excellent resolution and portability, it has gained popularity as an office-performed procedure adding another dimension to physical examination and going beyond physicians’ proprioception. Understanding of anatomy and disease pathophysiology remains the key in any radiologic study, yet one is required to understand the principles and physics of ultrasound (US) technology in order to maximize the information provided.

Sonographic technology is based on the properties of the acoustical wave. The energy by a US transducer is transferred to molecules of a medium. The molecules vibrate in a series of rhythmic, mechanical compressions generating a number of longitudinal waves like a ripple in the water. Each wave has a particular number of cycles per second that determines its frequency. One cycle per second is equal to 1 Hz; 10⁶ cycles/second is equal to 1 megahertz (MHz). Audible sound has a frequency between 20 and 20,000 Hz, whereas US has a frequency greater than 20,000 Hz.

The sonographic signal is generated at the level of the transducer that contains crystals with piezoelectric properties. These crystals are linearly arranged and their properties permit conversion of electrical energy into US energy and vice versa. As US waves propagate through tissue, a small percentage of the 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.” Structures of different densities are easier to distinguish from one another.

Multiple tissue interfaces emit various sonographic echoes and permit generation of readable images. High-frequency waves provide better images because of higher reflection, but they are restricted to the evaluation of the superficial structures caused by rapid energy loss. Because of the superficial location of most head and neck structures, clinical US uses fluctuating frequencies between 5 and 10 MHz or 7 and 12, 13, or 16 MHz. This range combines the penetration of lower frequencies and higher resolution of higher frequencies. The frequency of the sound changes when it is reflected from a moving object. The change in frequency is known as the Doppler shift and can be used to determine the flow of red blood cells as they course through vessels.

One should be familiar with terminology used in US imaging. B-mode sonography refers to a gray-scale mode, where Doppler sonography is used for assessment of blood flow and is color coded. Echogenicity defines the appearance of tissues on the US image relative to its ability to reflect the US wave. Anechoic means no return signal; it represents complete penetration of the energy through a structure, appearing black. An isoechoic object has similar echogenicity to surrounding tissue. Hypoechoic tissue has lower echogenicity than reference tissue, appearing darker. Hyperechoic tissue is brighter compared with the reference structure due to higher echogenicity.¹

Brief Description of Technique

The patient is placed in a semi-reclined or fully reclined position with mild neck extension. The monitor is placed near and at the same level as the patient. The examiner should be able to view the monitor and perform the scan without undue stretching or twisting. Convention dictates the orientation of the transducer relative to the patient. Transducers have an embedded indicator that must be directed cephalad during scanning in the sagittal plane and directed toward the patient’s right during scanning in the transverse plane. The examiner’s hand is stabilized on the patient’s skin, with the transducer held between the thumb and first two fingers. The transducer is advanced with constant skin contact with mild consistent pressure. For better acoustic coupling, the skin is covered with aqueous gel. Patient position should not be considered to be static; changes in position are often necessary to improve the quality of the image, to assess a particular structure, or to discriminate mobile from fixed reflectors. The dynamic examination is one of the advantages of real-time US imaging.

It is important to develop a readily replicated, systematic approach for the examination of neck structures. We prefer to use B-mode (gray-scale) scanning in the transverse/axial plane primarily. Starting inferiorly in the lateral neck, the probe is tilted inferiorly to visualize the subclavian vessels and then steadily advanced in superior direction with the carotid along the medial aspect of the field of view. The examination is then repeated on the contralateral side.^6,7

For central compartment evaluation, one half of the trachea should be visible on the medial aspect of the screen and the great vessels visible laterally, proceeding inferiorly to superiorly. A light touch will minimize compression of the jugular vein. Longitudinal/sagittal imaging is less intuitive; thus it is mostly reserved for confirmation and sizing in the cranial-caudal dimension of selected pathology once it is identified. For subtle changes in position, the probe can be slightly tilted back and forth, which allows more precise movement than sliding the probe along the skin surface. Color Doppler imaging can be used to assess vascularity, though most vessels are readily distinguished in gray scale. Once an image is localized in real time, the image can be frozen, labeled, sized, saved, and printed for the patient’s file (Fig. 117-1).⁸

Figure 117-1. Transverse/axial image of a predominantly cystic thyroid nodule with a solid component along the medial aspect. SCM, sternocleidomastoid muscle.

Thyroid Ultrasound

Neck US is widely accepted as the optimal modality for evaluation of thyroid nodularity. The thyroid gland, one of the easiest neck structures to image with US, should be homogeneous and of intermediate gray scale. It is considered the reference echogenicity for comparison with other structures. Solid hyperfunctioning thyroid nodules tend to be mildly hyperechoic with respect to the surrounding gland. Fluid-filled or cystic nodules tend to be hypoechoic, similar to vascular structures. Cystic nodules can be differentiated from vessels as one scans from inferior to superior because nodules will appear and then disappear, whereas vessels will continue in one direction or another. Doppler imaging can be used to assess vascularity of the gland overall or a discrete nodule. Untreated Graves’ patients demonstrate increased vascularity throughout a relatively homogeneous thyroid gland. In contrast, Hashimoto’s thyroiditis usually appears heterogeneous with splotchy ill-defined hypoechoic areas; the micronodular pattern represents lymphocytic infiltrations. As the disease progresses, it results in generalized gland atrophy and fibrosis-induced hypoechoic lobulation that may mimic nodularity. The diseased gland is often associated with benign-appearing reactive lymph nodes frequently located inferior to the gland.

US provides the most accurate sizing of nodular disease for surveillance, and its sensitivity for detection of thyroid malignancy approaches 80% to 90% in experienced hands.^9,10

In the setting of a multinodular thyroid, it is important to carefully evaluate each nodule individually and target which sonographic features increase the predictive value of fine-needle aspiration (FNA) for carcinoma. Although there is no single pathognomonic sonographic feature, the constellation of findings may suggest malignancy: microcalcifications, intranodular intrinsic vascularity, absent “halo sign,” extraglandular extension, and irregular or microlobulated margins (Fig. 117-2, Box 117-1). Unless purely cystic, the presence of cystic component within a lesion does not diminish risk of carcinoma, and is common in papillary carcinomas. One should be aware of inspissated colloid as a common misinterpretation for microcalcifications. The presence of a comet-tail or ring-down artifact posterior to the echogenic foci is seen with colloid (Fig. 117-3). We still perform a biopsy of the largest nodule in spite of benign sonographic features, but will select other nodules for biopsy that appear suspicious independent of size alone.^11,12

Figure 117-2. A and B, Papillary thyroid carcinoma with microcalcifications and nodal metastasis.

Figure 117-3. Transverse/axial image of a colloid nodule with comet-tail artifact.

In addition to evaluation of the thyroid gland, cervical lymph nodes must be carefully examined, especially in patients suspected of having malignancy.¹³ Sonographic identification of round, enlarged, hypoechoic, solid or cystic lymph nodes with microcalcifications and absent echogenic hilum should raise suspicion of papillary or medullary thyroid carcinoma (Fig. 117-4

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