What Is Diagnostic Sonography?

This article was initially published on The Radiology Blog as "What is Ultrasound Imaging?" It provides a basic explanation of the physic behind sonography. 

There are several major modalities in radiology: plain x-ray, computed tomography (CT), magnetic resonance imaging (MRI). Today, we cover another major tool in the radiologist's toolbox, ultrasound.

What is ultrasound imaging or medical sonography?

Ultrasound imaging, also known as medical sonography, utilizes the principles of sound waves and physics to generate images without using radiation. Ultraound is a type of sound wave. All sound waves can be characterized by their frequency and amplitude, which denote the amount of energy carried by the wave. Ultrasound refers to spectrum of acoustic waves with a frequency above the range of human hearing, approximately 20,000 hertz.

The basic question regarding ultrasound is how are sound waves converted into a diagnostic image? The basic process involves three steps: the creation of appropriate sound waves, the reception of the echoes, and the processing of those echoes into an image. A transducer on the ultrasound machine produces sound waves in the 2 to 18 megahertz range. These waves are then transmitted into the body via an interface, typically a rubber-coated probe with a gel interface with the patient's skin in order to produce efficient transfer of waves into body tissues. Once the wave is in the body, it propagates until it hits tissue of sufficient density to create a reflection, or echo. The echo travels in reverse, eventually striking the transducer, which contains a receiver. The receiver's vibration is translated into an electronic signal. The ultrasound machine contains a processor which calculates the time it took the echo to return and how strong the echo was in order to determine which pixel on the screen to light up, and at what intensity. Repeating this process for each wave and each pixel yields the image seen on the ultrasound machine's screen. This covers the basics of ultrasound image production, but more technical discussions are easily found.

What are the benefits of ultrasound?

The main benefit of ultrasound is that it does not use radiation, which makes it safer to use in general. Ultrasound is particularly useful in pregnant women to image the fetus, as well as in newborns who are still undergoing tissue development which may be susceptible to radation. Another major benefit of ultrasound is portability. Although the machines are expensive, they can be wheeled around on a cart, which makes it easy to do bedside imaging or imaging on critical patients who cannot be moved easily. Furthermore, this reduces expense as a dedicated ultrasound facility is not necessarily needed.

What are the applications of ultrasound?

Ultrasound has applications in nearly every field of medicine. Some of the most prominent are cardiology, obstetrics, gastroenterology, and gynecology/urology. In cardiology, ultrasound in combination with Doppler sonography is used to produce echocardiograms, which assess both the structure of the heart as well as flow within the heart. Obstetric ultrasounds are very common, and are used to track healthy pregnancies as well as diagnose problems in difficult pregnancies. In gastroenterology, ultrasound is better at producing images of certain pathologies, such as biliary disease like gallstones and liver imaging. For urology / gynecology, ultrasound is used to image the kidneys, bladder, ovaries, and other pelvic structures for a variety of pathologies.

For more information on radiology topics, including other modalities such as CT or MRI, check out The Radiology Blog.