A reflection of the beam is called an echo and the production and detection of echoes form the basis of ultrasound. A reflection occurs at the boundary between two materials provided that a certain property of the materials is different. This property is known as the acoustic impedance and is the product of the density and propagation speed. If two materials have the same acoustic impedance, their boundary will not produce an echo. If the difference in acoustic impedance is small, a weak echo will be produced, and most of the ultrasound will carry on through the second medium. If the difference in acoustic impedance is large, a strong echo will be produced. If the difference in acoustic impedance is very large, all the ultrasound will be totally reflected. Typically in soft tissues, the amplitude of an echo produced at a boundary is only a small percentage of the incident amplitudes, whereas areas containing bone or air can produce such large echoes that not enough ultrasound remains to image beyond the tissue interface [1].

The direction from where the sound returns tells it in which direction the structure is. The time taken for the sound waves to reach the structure and return back to the transducer tells how far away a structure is. The longer the sound waves take to return, the further away the structure is. If the difference in tissue density is very different, then sound is completely reflected, resulting in total acoustic shadowing like bones (Fig. 25.2), calculi and air (lung).

Homogenous fluids like urine, simple cysts, ascites, and pleural effusion are seen as echo-free structures due to the missing difference of the density within the tissue or between tissues (Fig. 25.3).

The wavelength is the distance traveled by sound in one cycle or the distance between two identical points in the wave cycle, i.e., the distance from a point of peak compression to the next point of peak compression. It is inversely proportional to the frequency. Wavelength is one of the main factors affecting axial resolution of an ultrasound image. The smaller the wavelength (and therefore the higher the frequency), the higher the resolution, but the lesser the penetration. Therefore, higher-frequency probes (5–10 MHz) provide better resolution but can be applied only for superficial structures and in children. Lower frequency probes (2–5 MHz) provide better penetration albeit lower resolution and can be used to image deeper structures.

For more than 50 years, many kinds of transducers have evolved for medical ultrasound imaging. Transducers operate at different center frequencies, have different physical dimensions, footprints, and shapes, and provide different image formats. Systematic selection criteria that allow matching of transducers to specific clinical needs are available. The criteria include access to and coverage of the region of interest, maximum scan depth and image extent, and coverage of essential diagnostic modes required to optimize a patient’s diagnosis. For completeness, single-element transducers, primarily used in intraluminal or catheter applications, are also included in the considerations. In the field of endobronchial ultrasound, two different techniques are used. Depending on the indication, the examiner needs information from an area close to the transducer (nearfield) or whats to see a larger image [2, 3].

Frequency refers to the number of cycles of compressions and rarefactions in a sound wave per second, with one cycle per second being hertz. Medical ultrasound frequencies range from 2 to approximately 20 MHz, depending on the indications and the situations. The depth of penetration is related to the frequency of the ultrasound wave. Higher frequencies have a shorter depth of penetration. Lower frequencies have a longer depth of penetration. On the other hand, probes with a high frequency are not able to provide information for distant areas. Therefore EBUS-TBNA scopes normally use frequencies around 7.5–12.5 MHz to provide information up to 5–6 cm away from the airways. The radial ultrasound, used to analyze the internal structure of the bronchial wall and solid lesions, uses 20 MHz (Figs. 25.4 and 25.5) [4].