The earliest ultrasound imaging consisted of a sound transmitted into the body, with the reflected sound waves displayed on an oscilloscope. Referred to as A-mode ultrasound, these images in the 1960s and 1970s were capable of providing measurements of internal structures such as thyroid lobes, nodules, and cysts. Figure 2.7a shows an A-mode ultrasound image of a solid thyroid nodule. Scattered echoes are present from throughout the nodule. Figure 2.7b shows an image from a cystic nodule. The initial reflection is from the proximal wall of the cyst, with no significant signal reflected by the cyst fluid. The second reflection originates from the posterior wall. Figure 2.7c shows the A-mode image from a complex nodule with solid and cystic components. A-mode ultrasound was capable of providing size measurements in one dimension but did not provide a visual image of the structure [1].

In order to provide a visual two-dimensional image, a series of one-dimensional A-mode images are aligned as a transducer is swept across the structure being imaged. Early thyroid ultrasound images were created by slowly moving a transducer across the neck. By scanning over a structure and aligning the A-mode images, a two-dimensional image is formed. The two-dimensional image formed in this manner is referred to as a B-Mode scan (brightness mode) (Fig. 2.8). Most ultrasound transducers used for thyroid imaging use a series of piezo-electric crystals in a linear array to electronically simulate a sweep of the transducer. Firing sequentially, each crystal sends a pulse of sound wave into the tissue and receives subsequent reflections.

The final ultrasound image shows a cross sectional image through the tissue defined by the thin flat beam of sound emitted from the transducer. Resolution is the ability to distinguish between two separate, adjacent objects. For example, with a resolution of 0.2 mm, two adjacent objects measuring <0.2 mm would be shown as a single object. Objects smaller than the resolution will not be realistically imaged. Lateral resolution refers to the ability to discriminate in a transverse, or side to side, direction. Azimuthal resolution refers to the image perpendicular to the axis of the ultrasound beam; this is inherent to the transducer and cannot be adjusted. Axial resolution is the ability to discriminate objects along the path of the ultrasound beam. Axial resolution is determined by the spatial pulse length and therefore frequency. Lateral and azimuthal resolutions are dependent on the quality and focusing of the ultrasound beam.

A number of artifacts commonly occur in ultrasound images. Unlike most other imaging techniques, such as CT scanning, artifacts are very helpful in interpreting ultrasound images. Artifacts such as shadows behind objects or unexpected areas of brightness can provide additional understanding of the properties of the materials being imaged.

When sound waves impact on an area of extreme mismatch of acoustic impedance, such as a tissue-air interface or a calcification within soft tissue, the vast majority of the sound waves are reflected, providing a very bright signal from the object’s surface and an absence of imaging beyond the structure. Figure 2.9 demonstrates acoustic shadowing behind a calcified nodule. Figure 2.10 illustrates a coarse calcification within the thyroid parenchyma with acoustic shadowing behind the calcification. Figure 2.11 shows the typical appearance of the trachea on an ultrasound image. Because there is virtually no transmission of sound through the air-tissue interface of the anterior wall of the trachea, no imaging of structures posterior to the trachea occurs. As explained below the reflections seen behind the trachea represent reverberation artifact.

Conversely, a cystic structure transmits sound with very little attenuation, resulting in a greater intensity of sound waves behind it, compared to adjacent structures. This “through transmission” results in acoustic enhancement with a brighter signal behind a cystic or anechoic structure. This enhancement can be used to distinguish between a cystic and solid nodule within the thyroid. Figure 2.12 illustrates enhancement behind a cystic nodule. Enhancement is not limited to cystic nodules, however. Any structure that causes minimal attenuation of the ultrasound signal will have enhancement posterior to it. Figure 2.13 illustrates enhancement behind a solid parathyroid adenoma . Figure 2.14 illustrates enhancement behind a benign colloid nodule . Due to the high content of fluid and colloid within the nodule, and resultant decrease in cellularity, there is less attenuation of signal within the nodule than within the surrounding thyroid tissue. To establish that a structure is cystic, use Doppler (as described in Chap. 3) to confirm there is no internal flow as would be seen in a vascular solid nodule with which posterior enhancement can also be seen.