Ultrasound is a diagnostic imaging technique that uses high-frequency sound waves to create images of internal body structures. Ultrasound is widely used because of how it provides real-time imaging, does not require ionizing radiation, and is non-invasive. Commonly associated with prenatal scans, ultrasound is also used extensively in cardiology, emergency medicine, and anesthesiology 1,2.
How ultrasound operates is by transmitting sound waves at frequencies typically between approximately 2 and 15 megahertz (MHz) through the body using a transducer. These sound waves travel through tissues and are reflected back to the transducer when they encounter boundaries between different tissue types, such as fluid and soft tissue, or soft tissue and bone. The transducer then detects these returning echoes, which are processed by the ultrasound machine to construct an image 3,4.
When ultrasound waves encounter a tissue boundary, how many are reflected depends on the difference in acoustic impedance between the two tissues. Acoustic impedance is the product of tissue density and the speed of sound within that tissue; larger differences in impedance result in stronger echoes. These echoes are timed based on how long they take to return to the transducer, allowing the machine to calculate depth and generate a two-dimensional cross-sectional image 5,6.
Several modes of ultrasound imaging are used depending on the clinical application. B-mode (brightness mode) is the most common, providing grayscale images that represent the intensity of returning echoes. M-mode (motion mode) is used for assessing moving structures such as heart valves. Doppler ultrasound evaluates the movement of blood within vessels by detecting frequency shifts in the reflected sound waves, which occur due to the Doppler effect. Color Doppler overlays directional blood flow on B-mode images, enhancing vascular assessments 7,8.
How sharp the resulting image is and how deep into tissues the scan can penetrate depends on the frequency of the ultrasound waves. Higher frequencies provide better resolution but have less tissue penetration, making them suitable for superficial structures, while lower frequencies penetrate deeper tissues but with reduced image clarity. Various methods can be leveraged to optimize image quality 9,10.
While ultrasound is versatile and safe, it has limitations. It cannot image structures behind air-filled organs or dense bones effectively, as these cause significant reflection or absorption of the sound waves. Ultrasound artifacts, in addition, can mislead interpretation but also offer clues about tissue characteristics when properly understood 11,12.
Ultrasound is a cornerstone of modern diagnostic imaging, valued for its safety, accessibility, and dynamic capabilities. By harnessing high-frequency sound waves and analyzing their reflections, ultrasound provides critical insights into soft tissue structures and vascular flow. Ongoing advancements in technology and techniques continue to expand its diagnostic and therapeutic potential across a broad range of medical specialties.
References
- Ultrasound: MedlinePlus Medical Test. https://medlineplus.gov/lab-tests/sonogram/.
- Ultrasound – Mayo Clinic. https://www.mayoclinic.org/tests-procedures/ultrasound/about/pac-20395177.
- Thapaliya, A., Sithole, A., Welsh, M. & Dana, G. Basic Principles of Ultrasound. (2024).
- Grogan, S. P. & Mount, C. A. Ultrasound Physics and Instrumentation. in StatPearls (StatPearls Publishing, Treasure Island (FL), 2025).
- Wachinger, C., Shams, R. & Navab, N. Estimation of acoustic impedance from multiple ultrasound images with application to spatial compounding. in 2008 IEEE Computer Society Conference on Computer Vision and Pattern Recognition Workshops 1–8 (IEEE, Anchorage, AK, USA, 2008). DOI:10.1109/CVPRW.2008.4563028.
- Bakhru, R. N. & Schweickert, W. D. Intensive Care Ultrasound: I. Physics, Equipment, and Image Quality. Ann Am Thorac Soc 10, 540–548 (2013). DOI: 10.1513/AnnalsATS.201306-191OT
- Doppler Ultrasound: What Is It, Purpose and Procedure Details. Cleveland Clinic https://my.clevelandclinic.org/health/diagnostics/22715-doppler-ultrasound.
- Neumann, D. & Kollorz, E. Ultrasound. in Medical Imaging Systems: An Introductory Guide (eds. Maier, A., Steidl, S., Christlein, V. & Hornegger, J.) (Springer, Cham (CH), 2018).
- Ng, A. & Swanevelder, J. Resolution in ultrasound imaging. Continuing Education in Anaesthesia Critical Care & Pain 11, 186–192 (2011). DOI: 10.1093/bjaceaccp/mkr030
- Sassaroli, E., Crake, C., Scorza, A., Kim, D. & Park, M. Image quality evaluation of ultrasound imaging systems: advanced B‐modes. J Appl Clin Med Phys 20, 115–124 (2019). DOI: 10.1002/acm2.12544
- Ultrasound-pros and cons. European Society for Paediatric Anaesthesiology https://www.euroespa.com/science-education/specialized-sections/espa-pain-committee/us-regional-anaesthesia/ultrasound-pros-and-cons/ (2025).
- Kremkau, F. W. & Taylor, K. J. Artifacts in ultrasound imaging. J Ultrasound Med 5, 227–237 (1986). DOI: 10.7863/jum.1986.5.4.227