Progress in Respiratory Research

This chapter presents the physics of ultrasound, probe design, and some of the typicel artifacts present in pulmonological applications. Some of the techniques used for optimal imaging of chest anatomy are explained as well as the diagnostic questions that can be answered by a pulmonologist. Copyright © 2009 S. Karger AG, Basel Diagnostic ultrasonography is the only clinical imaging technology currently in use that does not depend on electromagnetic radiation. This modality is based on the properties of sound waves, and hence the mechanical and acoustic properties of tissues. Diagnostic ultrasound is mechanical energy that causes alternating compression and rarefaction of the conducting medium, traveling in the body as a wave usually at frequencies of 2–10 MHz, well beyond audible frequencies. In general it is assumed that the speed of sound in tissue is constant at 1,540 m/s [1]. By knowing the frequency and the speed of sound, one can determine its wavelength (similar to electromagnetic radiation) [2–4]: (wavelength) c(speed)/f(frequency), or using the assumed speed: (wavelength in mm) 1.54/f(frequency in MHz). For example, at a frequency of 2 MHz (which is very close to the necessary frequency usually used to image deeply into the body compared to higher frequencies for more superficial structures) the wavelength is 0.77 mm. When a pulse of ultrasound energy is incident upon the body, it interacts with the tissue in a variety of ways which will be discussed. Some of the incident energy is directed back towards the source and is detected. The time delay between the energy going into the body and returning to the ultrasound probe determines the depth from which the signal arises, with longer times corresponding to greater depths (depth velocity time/2). This information is used in the creation of an image. Other factors that make the tissues distinguishable on a screen are their slightly different acoustical properties; one is known as the acoustic impedance defined as Z density speed of sound [2–4]. At the boundary between two different tissue types the sound waves can be: (1) reflected, like light off a mirror, this being the primary interaction of interest for diagnostic ultrasound, as it allows the major organ outlines to be seen; the diaphragm and pericardium are specular reflectors; (2) refracted, like light rays passing through a lens and hence having their directions altered; (3) scattered, like sunlight in the sky, sending sound waves off in different directions; this occurs when the ultrasound wave encounters a surface that is ‘rough’ [3, 4] or whose shape and density vary on a spatial scale which is small compared to the wavelength of the ultrasound, and (4) and attenuated or absorbed, as they lose energy, which is converted to heat in the tissue. These last 3 effects will, in general, cause the sound waves that are reflected back to the transducer from deeper tissues to be much weaker, causing the image to get increasingly noisy (too many echoes or small visible densities in the background compared with those of the desired image). The amount of attenuation that occurs as the sound wave passes through the tissue increases with higher frequency. However, for pulsed ultrasound the axial resolution (the ability to distinguish between adjacent dots in the direction of the sound wave) is improved at high frequencies. This difference in resolution produced comparing higher and lower frequency transducers is because for a given number of acoustic cycles, the pulse length is less at higher frequenBolliger CT, Herth FJF, Mayo PH, Miyazawa T, Beamis JF (eds): Clinical Chest Ultrasound: From the ICU to the Bronchoscopy Suite. Prog Respir Res. Basel, Karger, 2009, vol 37, pp 2–10 Physics of Diagnostic Ultrasound