Introduction

Ultrasound uses high-frequency sound waves in order to create an image; this can be used to monitor a foetus, diagnose a problem or help guide a surgeon, they are also referred to as sonograms. 

Whilst they are used for creating images, there is also therapeutic uses for ultrasound, where high frequency sound waves can be used to move or break down tissues, such as  kidney stones or even tumours,

There are many uses for ultrasound in modern medicine and there is research suggesting that ultrasound may have even broader uses; such as drug delivery. 

With initial knowledge of ultrasound beginning in the late 1800’s the modern use of ultrasound is very interesting and widespread and there is scope for further development in using sound in medicine.

In this article, we will discuss the usage of ultrasound in details.

Basics of Ultrasound

Ultrasound is operating by using sound waves; beyond the ability of human hearing, these can be used for diagnostic needs or therapeutic purposes.

Ultrasound refers to extremely high frequency sound waves; that are beyond the human range of hearing, this is seen as anything above 20,000 hertz (Hz) or 20 kilohertz (kHz). The frequency of ultrasound; used for diagnosis, is generally between 3 and 10 megahertz (MHz). Abdominal examination requires around 3 MHz higher frequencies are used for examining organs like, the breast and thyroid. Higher frequency can give greater detail in the image produced.¹ 

Ultrasound waves are produced by a transducer. This transducer also detects them. This is important as this is how an image is produced in an ultrasound scan. The transducer emits the ultrasound waves which then travel through tissue, the sound waves are echoed back to the transducer when they hit a boundary between tissues (muscle and bone, muscle and blood vessels etc.). 

These echoes are detected by the transducer, which can create an image based on the time it takes the sound to return to the transducer.²

These images can then be used to understand if there are any abnormalities and their size/positioning, the size and development of a foetus, or to guide a surgeon inside the body to avoid more invasive procedures. 

Application of Ultrasound

Diagnostic Ultrasound

This form of ultrasound is the most common and that which most people will be familiar with, the most common use of this is to view and monitor the growth of a foetus during pregnancy, this can also be used to view heart, blood vessels, muscles, breast or several other tissues. Diagnostic ultrasound images can be viewed as 2D, 3D or 4D (3D but moving).

Functional Ultrasound

This can be used to measure and visualise blood flow through major blood vessels, this can be known as doppler ultrasound. It can measure the speed and direction in which blood travels, creating colour coded maps called doppler imaging. This is used to view whether plaques are blocking blood flow, particularly in the carotid artery that would block blood flow to the brain. 

Elastography is another form of functional ultrasound, this can be used to determine the elasticity of different tissues in the body, particularly differentiating healthy tissue from tumours and in liver fibrosis, showing scar tissue build up in the liver. The images created are maps that can show regions differing in elasticity.

Functional ultrasound imaging can also aid in performing less invasive surgical intervention, by showing a real-time image of the inside of the body, it can also be helpful when completing needle biopsies; guiding the needle, as well as when inserting a catheter into a blood vessel allowing for knowledge of its exact location.⁴ 

Therapeutic Ultrasound

At relatively lower frequencies ultrasound can be used to dilate blood vessels, or increase cellular metabolism through oxygen and nutrient delivery, this is through thermal energy created by the high frequency sound waves. Physical therapies can also be used to treat some shoulder injuries.

High intensity focused ultrasound (HIFU) this form of treatment creates an accurate focal point for the ultrasound waves and the thermal energy it creates, this can be used to break down tumours and treat skin abnormalities.

Cavitation is another therapeutic method for using ultrasound; this involves pressure variations occurring in liquids that are caused by the ultrasound waves. Bubbles then form; bursting on impact with solids, creating a shockwave which can break the solid down the most prominent use of this is breaking down kidney stones so they are easier to pass.⁵ 

Risks and Limitations

There is no real risk to ultrasound imaging, it is safe and unlike other imaging techniques uses no forms of radiation, just sound.²

During therapeutic uses of ultrasound there can be a slightly higher risk of complications, with prolonged exposure possibly causing superficial burns, but using magnetic resonance imaging (MRI) along with HIFU can help avoid any burns or tissue damage. Cavitation can cause more issues such as internal bleeding or scarring.⁵ 

Ultrasound can be limited in what can be viewed, as imaging can not be done accurately deep in the body; as well as ultrasound waves have issues with travelling through bone or gas, so imaging can be difficult in the head or of the lungs. More advanced imaging techniques may be used if ultrasound is not appropriate.³ 

Future Potential Uses of Ultrasound

In the future research and technology may allow for ultrasound to aid in the delivery of drugs; by opening membranes and crossing the blood-brain barrier, allowing drugs entry to areas they may have trouble entering.⁶ 

There is also research showing that ultrasound may potentially be used to 3D print through the skin; by injecting a special ink which focused ultrasound can solidify it, repairing bones or muscles and avoiding invasive surgeries.⁴ 

Summary

Ultrasound is the use of high frequency sound waves; beyond the range of human hearing, this can be used to create images of the inside of the body or for therapeutic needs and breakdown of abnormal tissues.

The images are created by a transducer sending out sound waves; these waves then echo back to the receiver when they hit a boundary between tissues, the image is created by working out where the boundaries are by measuring how long it has taken for the echo to reach the receiver.

This is most prominently used in monitoring a foetus during pregnancy, but can also help in measuring blood flow and speed, helping to determine if there are plaques in blood vessels.

Therapeutic ultrasound can break down abnormal tissues such as tumours or kidney stones.

Ultrasound is very safe and there are no major risks, thanks to only using sound, it also reduces the need for more invasive procedures, helping with imaging during surgery, biopsies or inserting catheters.