Ultrasound (introduction)

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, compared with other modalities such as conventional radiography or CT.

Ultrasound relies on properties of acoustic physics (compression/rarefaction, reflection, impedance, etc.) to localize and characterize different tissue types. 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).

An ultrasound transducer 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.

Why use ultrasound

  • ultrasound uses non-ionizing sound waves and has not been associated with carcinogenesis. This is particularly important for the evaluation of fetal and gonadal tissue.
  • in most centers, ultrasound is more readily available than more advanced cross-sectional modalities such as CT or MRI.
  • ultrasound examination is less expensive to conduct than CT or MRI.
  • there are few (if any) contraindications to the use of ultrasound, compared with MRI or contrast-enhanced CT.
  • the real-time nature of ultrasound imaging is useful for the evaluation of physiology as well as anatomy (e.g. fetal heart rate).
  • Doppler evaluation of organs and vessels adds a dimension of physiologic data, not available on other modalities (with the exception of some MRI sequences).
  • ultrasound images may not be as adversely affected by metallic objects, as opposed to CT or MRI.
  • an ultrasound exam can easily be extended to cover another organ system or evaluate the contralateral extremity.
  • Disadvantages
  • training is required to accurately and efficiently conduct an ultrasound exam and there is non-uniformity in the quality of examinations ("operator dependence").
  • 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).
  • 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.
  • ultrasound has its own set of unique artifacts (US artifacts), which can potentially degrade image quality or lead to misinterpretation. 
  • some ultrasound exams may be limited by abnormally large body habitus.