Ultrasonography

Definition

Ultrasonography is a noninvasive imaging method that uses high-frequency sound waves to capture real-time images of internal body structures. It is commonly used in diagnostics, monitoring, and interventional procedures.[1]

Physical Principles

Piezoelectric effect ultrasound

Ultrasonography uses high-frequency sound waves (around 2–15 MHz) to look at internal structures for diagnosis. The ultrasound beam is produced by the transducer using the piezoelectric effect, where special crystals convert electrical energy into sound waves and then convert the returning echoes back into electrical signals. These waves travel through the body and get reflected at tissue boundaries. The echoes are used to create the image.

How much gets reflected depends on the difference in acoustic impedance between tissues. If the difference is large, the echos are strong, while similar tissues have only weak reflections. At interfaces like bone or air, almost all sound is reflected, which causes acoustic shadowing. Fluids such as blood or urine don’t reflect much sound, so they appear black (anechoic).[2][3]

Ultrasound lesions echogenicity

Image Formation

  • Hyperechoic (bright) – bone, fibrous tissue, fat
  • Hypoechoic (dark) – soft tissues
  • Anechoic (black) – fluid (e.g., cysts, blood)[4]

Modes of Ultrasonography

B-mode

The most commonly used mode in ultrasonography is the B-mode (brightness mode), which produces standard two-dimensional images and allows clear visualization of anatomical structures.

M-mode

The M-mode (motion mode) can evaluate movement, which is particularly useful in cardiology, where it helps assess the motion of the heart walls and valves over time.

Doppler effect

Most modern ultrasound systems can use the Doppler effect, where the changes in the frequency of ultrasound waves occur when they meet moving objects, to determine the direction and speed of an object in motion. There are different ways to apply the Doppler effect. In general two modes are used to collect Doppler data and two types of visualisation are used to show the Doppler information.

Pulsed Wave Doppler (PW)

  • Emits ultrasound in pulses and measures flow at a specific depth
  • Allows precise localization of blood flow
  • Limited in measuring high velocities

Continuous Wave Doppler (CW)

  • Continuously emits and receives ultrasound waves
  • Accurately measures very high blood flow velocities
  • Cannot determine the exact depth of the signal

Spectral Doppler

  • Displays blood flow as a graph (velocity vs. time)
  • Provides quantitative and directional information
  • Used especially in cardiology (valves, chambers)

Colour Doppler

  • Displays blood flow as colors on the B-mode image
  • Shows direction [5]
modern ultrasound scanner

Equipment

  • Transducer (probe)
  • Processing unit
  • Monitor
  • Ultrasound gel

Types of transducers

  • Linear – superficial structures
  • Convex – abdominal imaging
  • Phased array – cardiac imaging

Clinical Applications

Abdominal imaging

  • Liver, gallbladder, spleen, pancreas
  • Detection of gallstones, tumors, cysts
Ultrasonography of sludge and gallstones, annotated

Obstetrics and gynecology

  • Fetal development monitoring
  • Detection of congenital anomalies

Cardiology (echocardiography)

  • Heart function and valve assessment

Vascular imaging

  • Detection of thrombosis
  • Blood flow assessment

Musculoskeletal system

  • Tendons, ligaments, joints

Interventional procedures

  • Guidance for biopsies and drainage[1]

Advantages

  • Non-invasive
  • No ionizing radiation
  • Real-time imaging
  • Portable and cost-effective [6]

Limitations

  • Operator-dependent
  • Limited in obese patients
  • Poor visualization through bone and air
  • Lower resolution compared to CT and MRI [6]

References

  1. ↑ a b https://www.fda.gov/radiation-emitting-products/medical-imaging/ultrasound-imaging
  2. ↑ https://radiopaedia.org/articles/physical-principles-of-ultrasound-1
  3. ↑ https://vetsuisse.iml.unibe.ch/radiosurfvet/allgemeine-radiologie/ultraschall/grundlagen/piezoelektrischer-effekt-1
  4. ↑ https://www.veterinaryradiology.net/4161/what-do-hyperechoic-and-hypoechoic-mean/
  5. ↑ https://www.imv-imaging.com/en/2023/04/news-the-a-b-ms-ultrasound-modes-explained/
  6. ↑ a b https://www.euroespa.com/science-education/specialized-sections/espa-pain-committee/us-regional-anaesthesia/ultrasound-pros-and-cons/