The thyroid is well suited to ultrasound examination due to its size, superficial location, and characteristic echotextures in health and disease. Ultrasound imaging uses the reflection of high frequency sound waves to create a visual image. Unlike electromagnetic energy, sound transmission is dependent on, and greatly influenced by, the conducting medium. Sound is reflected at interfaces of mismatch of acoustic impedance. The resolution of an ultrasound image is dependent on the frequency, the focused beam width, and the quality of the electronic processing. Resolution improves, but the depth of imaging decreases, with higher frequencies. Image artifacts such as shadowing and posterior acoustic enhancement provide useful information, rather than just interfering with creation of a clear image. The current image quality, affordable cost, and ease of performance make real-time ultrasound an integral part of the clinical evaluation of the thyroid patient.
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