![]() ![]() 2 Being relatively inexpensive, office-based US technology is readily available, portable (size of laptop computer), quick and easy to perform, and harmless to the patient. Magnetic resonance imaging (MRI) or computed tomography (CT) scans generally offer no additional information regarding size, margin, or malignant potential. With high-resolution sonographic technology, virtually all extracranial structures can be accurately assessed, though the primary utility is imaging of thyroid, parathyroid, and salivary glands, as well as lymph nodes. The majority of structures and pathologies in the neck are located within 5 cm distance from the skin surface. Hyperechoic tissue is brighter compared with the reference structure due to higher echogenicity. Hypoechoic tissue has lower echogenicity than reference tissue, appearing darker. An isoechoic object has similar echogenicity to surrounding tissue. Anechoic means no return signal it represents complete penetration of the energy through a structure, appearing black. Echogenicity defines the appearance of tissues on the US image relative to its ability to reflect the US wave. B-mode sonography refers to a gray-scale mode, where Doppler sonography is used for assessment of blood flow and is color coded. One should be familiar with terminology used in US imaging. 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. The frequency of the sound changes when it is reflected from a moving object. ![]() This range combines the penetration of lower frequencies and higher resolution of higher frequencies. 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. 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. Multiple tissue interfaces emit various sonographic echoes and permit generation of readable images. Substances with a greater density produce stronger “echoes.” Structures of different densities are easier to distinguish from one another. The US image is formed by the returning wave, and the strength of the image is proportional to the strength of the returning wave. As US waves propagate through tissue, a small percentage of the energy (echo) is reflected back to the transducer. These crystals are linearly arranged and their properties permit conversion of electrical energy into US energy and vice versa. The sonographic signal is generated at the level of the transducer that contains crystals with piezoelectric properties. Audible sound has a frequency between 20 and 20,000 Hz, whereas US has a frequency greater than 20,000 Hz. One cycle per second is equal to 1 Hz 10 6 cycles/second is equal to 1 megahertz (MHz). Each wave has a particular number of cycles per second that determines its frequency. The molecules vibrate in a series of rhythmic, mechanical compressions generating a number of longitudinal waves like a ripple in the water. The energy by a US transducer is transferred to molecules of a medium. Sonographic technology is based on the properties of the acoustical wave. 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. 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. Over the past 30 years sonographic imaging technology has undergone tremendous change. ![]()
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