What Is the Principle of Ultrasonography?

Ultrasound devices send ultrasonic soundwaves through tissue. The device measures how long the waves take to “echo” or return to the device, much like SONAR used by submarines. The programming in the device uses that data to assemble an image nearly in real time; different densities of tissue show up as different concentrations of lighted pixels on the screen, allowing doctors to map the internal structure of the organ or tissue.

Ultrasonography (USG) is a popular diagnostic modality used in medical practice. The test has been around for decades, and the US Food and Drug Administration (FDA) considers it reasonably safe for use in most individuals, including pregnant women.

The USG technique can be used to diagnose medical conditions (diagnostic) or treat medical conditions (therapeutic).

The diagnostic ultrasound, also known as the sonography test, uses the principle of “Doppler effect” or echoes to convert the reflected sound energy into images.

Sound energy is a vibratory disturbance that moves forward in a wave through a substrate, whether that’s air when you’re talking to a friend or human tissue absorbing ultrasound energy. Sound can travel well in air, solid, and liquid mediums. 

The human ear can detect the sound frequency between 20 Hz and 20,000 Hz. The waves having a frequency higher than 20,000 Hz (20 KHz) are called ultrasound waves. 

Ultrasound is not different from “normal” (audible) sound in its physical properties, except that humans cannot hear it.

• During sonography, the doctor will place a transducer (USG probe) directly on the skin or inside a body opening (vagina or rectum). 
• A thin layer of gel is applied to the skin directly above the organ to be scanned. Ultrasound waves are transmitted from the transducer through the gel into the body. 
• These short bursts of sound energy hit the desired organs and return to the probe as an echo. The probe diverts them to a biometer present in the system. 
• The biometer converts the sound wave data into organ images. 
• The image formed depends upon the echoes received and time taken for the sound to be reflected. In a diseased organ, the echoes reflected are different compared with those in a healthy organ. Therefore, the image formed is different. 

These images can provide valuable information for diagnosing and treating various diseases and conditions. A special three-dimensional (3D) ultrasound used in pregnancy can show us some of the facial features of the child inside the uterus. Fingers and toes of the fetus may be visible on this kind of scan. The four-dimensional ultrasound is a 3D ultrasound in motion.

Therapeutic ultrasound uses sound waves with very high vibrations. These interact with the body tissues, heat them up, and destroy them. Therapeutic ultrasound is currently FDA-approved for the treatment of uterine fibroids, to alleviate pain from spread of cancer into the bones (bone metastases), and for prostate surgery.

• The sonography is a noninvasive technique yet a highly accurate scan to view insides of the body.
• Ultrasound images are captured in real time. They can show the movement of the body’s internal organs, fetus, and blood flow through the blood vessels.
• Unlike X-ray imaging, there is no ionizing radiation exposure associated with ultrasound imaging. Hence, it can be used safely in pregnancy.
• Ultrasound can be used in cases where remote diagnosis or teleconsultations are the only option.
• It can be performed as a bedside investigation.

Ultrasound waves can heat up the tissues slightly. In some cases, it can also produce cavitation (small pockets of gas in the body fluids or tissues). The long-term consequences of these effects are still unknown.

A USG cannot be used in organs with large amount of air, for example, for the lungs or a gassy intestine.

• View the uterus and ovaries during pregnancy and monitor the developing baby’s health
• Diagnose gallbladder diseases such as gallstones or cancer of the gallbladder
• Evaluate the blood flow to organs such as the heart, placenta, and testis (Doppler ultrasound)
• Guide a needle for biopsy or tumor treatment
• Examine a breast lump or perform a routine mammography in a young female
• Check the thyroid gland for swelling or nodule
• Detect prostate enlargement and urinary bladder health
• Ultrasound of the eye to visualize the structures inside the eyeball for cataract surgery or diagnosing eye diseases