Ultrasound Testing

More on Ultrasound

ultrasound imaging, carotid ultrasound Diagnostic ultrasound (ultrasonography) is an imaging technique used to visualize underlying body structures including tendons, muscles, joints, vessels and internal organs for possible pathology or lesions.

Diagnostic Applications

Typical diagnostic scanners operate in the frequency range of 2 to 18 megahertz, hundreds of times greater than the limit of human hearing. The choice of frequency is a trade-off between spatial resolution of the image and imaging depth: lower frequencies produce less resolution but image deeper into the body.

Ultrasonography is widely used in medicine. It is possible to perform both diagnosis and therapeutic procedures, using ultrasound to guide interventional procedures (for instance biopsies or drainage of fluid collections). Sonographers are medical professionals who perform scans for diagnostic purposes. Sonographers typically use a hand-held probe (called a transducer) that is placed directly on and moved over the patient.

Sonography is effective for imaging soft tissues of the body. Superficial structures such as muscles, tendons, testes, breast and the neonatal brain are imaged at a higher frequency (7-18 MHz), which provides better axial and lateral resolution. Deeper structures such as liver and kidney are imaged at a lower frequency (1-6 MHz) with lower axial and lateral resolution but greater penetration.

Sound in the Body

Ultrasonography uses a probe containing one or more acoustic transducers to send pulses of sound into a material. Whenever a sound wave encounters a material with a different density (acoustical impedance), part of the sound wave is reflected back to the probe and is detected as an echo. The time it takes for the echo to travel back to the probe is measured and used to calculate the depth of the tissue interface causing the echo. The greater the difference between densities, the larger the echo is. If the pulse hits gases or solids, the density difference is so great that most of the acoustic energy is reflected and it becomes impossible to see deeper.

Strengths

  • It images muscle, soft tissue, and bone surfaces very well and is particularly useful for delineating the interfaces between solid and fluid-filled spaces.

  • It renders "live" images, where the operator can dynamically select the most useful section for diagnosing and documenting changes, often enabling rapid diagnoses.

  • It shows the structure of organs.

  • It has no known long-term side effects and rarely causes any discomfort to the patient.

  • Equipment is widely available and comparatively flexible.

  • Small, easily carried scanners are available; examinations can be performed at the bedside.

  • Relatively inexpensive compared to other modes of investigation, such as computed x-ray tomography or magnetic resonance imaging.

  • Spatial resolution is better in high frequency ultrasound transducers than it is in most other imaging modalities.

Weaknesses

  • Sonographic devices have trouble penetrating bone. For example, sonography of the adult brain is very limited though improvements are being made in transcranial ultrasonography.
  • Sonography performs very poorly when there is a gas between the transducer and the organ of interest, due to the extreme differences in density. For example, overlying gas in the gastrointestinal tract often makes ultrasound scanning of the pancreas difficult, and lung imaging is not possible (apart from demarcating pleural effusions).

  • Even in the absence of bone or air, the depth penetration of ultrasound may be limited depending on the frequency of imaging. Consequently, there might be difficulties imaging structures deep in the body, especially in obese patients.

  • The method is operator-dependent. A high level of skill and experience is needed to acquire good-quality images and make accurate diagnoses.

Adapted from Wikipedia, 25 March 2010. Available under the Creative Commons-ShareAlike license.