Ultrasound (introduction)

Last revised by Raymond Chieng on 19 Jul 2023

Ultrasound (US) is an imaging technology that uses high-frequency sound waves to characterize tissue. It is a useful and flexible modality in medical imaging, and often provides an additional or unique characterization of tissues, when compared to other modalities such as conventional radiography or CT.

Ultrasound is the commonest term used for this modality however occasionally ultrasonography (USG), or just sonography are used. When abbreviated, USS, short for ultrasound scanning, may be used as an alternative. Echography is a rare synonym but is seen especially with relation to ultrasound of the eye 3.

For historical reasons, ultrasound of the heart tends to be called echocardiography, or often just echo.

A sound wave is transmitted through liquids as a longitudinal wave, in which the movements of particles in a medium are parallel to the direction of propagation of the sound wave 2. Sound wave transmits their energy mechanically, through pressure variations on the particles. Regions of high pressure and density are called "compressions" while regions of low pressure and density are called "rarefactions" 1.

The frequency of the sound waves used in medical ultrasound is in the range of millions of cycles per second (megahertz, MHz). In contrast, the upper range of audible frequencies for human is around 20 thousand cycles per second (20 kHz) 2.

Ultrasound images are produced by relying on properties of acoustic physics (reflection, refraction, absorption 2, and scattering). These properties cause attenuation of ultrasound that is used to localize and characterize different tissue types 2. The amount of attenuation of ultrasound is described by the attenuation coefficient. Acoustic impedance is a physical property of a tissue in which how much resistance it offers to stop the transmission of an ultrasound beam 2. Differences between the acoustic impedance of the two mediums govern the proportions of reflected and transmitted sound waves 2.

The angle of the transmitted sound waves (refracted waves) is governed by Snell's law 2. The velocity of transmitted wave can be either be higher or slower than the incident wave depending on the type of material it passes through 2. During the change in velocity, the wavelength changes while the frequency remains constant 2. Compressibility (or stiffness) of the material and density of the material affects the velocity of the ultrasound wave. The lower the compressibility (or higher the stiffness), or the lower the density, the higher the velocity of ultrasound because the frequency remains constant while the wavelength increases 2.

The intensity/loudness/amplitude of ultrasound is measured as milliwatts cm-2 or watts cm-2. Diagnostic and low-intensity ultrasound range from 0.1 to 1 watts cm-2, while high-intensity ultrasound is more than 10 watts cm-2 4. Power is energy is generated per unit time, measured in joules per second or watts 5. The intensity or power of the ultrasound is directly proportional to the square of the amplitude 5. Meanwhile, relative sound intensity is measured by decibels, which compares the relative intensity of two sound beams 2. The loss of 3 decibels will reduce the sound intensity by half 2.

An ultrasound transducer, also known as a probe, operates based on the physical principles of ultrasound. It sends an ultrasound pulse into tissue and then receives echoes back. The echoes contain spatial and contrast information. The concept is analogous to sonar used in nautical applications, but the technique in medical ultrasound is more sophisticated, gathering enough data to form a rapidly moving two-dimensional grayscale image.

Some characteristics of returning echoes from tissue can be selected out to provide additional information beyond a grayscale image. Doppler ultrasound, for instance, can detect a frequency shift in echoes, and determine whether the tissue is moving toward or away from the transducer. This is invaluable for evaluation of some structures such as blood vessels or the heart (echocardiography).

Ultrasound continues to evolve additional functions, including 3D ultrasound imaging, elastography, and contrast-enhanced ultrasound using microbubbles.

  1. ultrasound uses non-ionizing sound waves and has not been associated with carcinogenesis - this is particularly important for the evaluation of the fetal and gonads

  2. in most centers, ultrasound is more readily available than more advanced cross-sectional modalities such as CT or MRI

  3. ultrasound examination is less expensive to perform than CT or MRI

  4. ultrasound is straightforward to perform portably, unlike CT/MRI

  5. there are few (if any) contraindications to the use of ultrasound, compared with MRI or contrast-enhanced CT

  6. the real-time nature of ultrasound imaging is useful for the evaluation of physiology as well as anatomy (e.g. fetal heart rate)

  7. Doppler evaluation of organs and vessels adds a dimension of physiologic data, not available on other modalities (with the exception of some MRI sequences)

  8. ultrasound images may not be as adversely affected by metallic objects, as opposed to CT or MRI

  9. an ultrasound exam can easily be extended to cover another organ system or evaluate the contralateral extremity

  1. training is required to accurately and efficiently conduct an ultrasound exam and there is non-uniformity in the quality of examinations ("operator dependence")

  2. ultrasound is not capable of evaluating the internal structure of tissue types with high acoustical impedance (e.g. bone, air). It is also limited in evaluating structures encased in bone (e.g. cerebral parenchyma inside the calvaria)

  3. the high frequencies of ultrasound result in a potential risk of thermal heating or mechanical injury to tissue at a microscopic level, this is of most concern in fetal imaging

  4. ultrasound has its own set of unique artifacts (US artifacts), which can potentially degrade image quality or lead to misinterpretation

  5. some ultrasound exams may be limited by abnormally large body habitus

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