What is Medical Ultrasound?

Medical Ultrasound > An Introduction to Medical Ultrasound > Medical Imaging

Dr Jack Jellins AM PhD (Hon)MD

Ultrasound Imaging and Education

Medical Ultrasound

Medical ultrasound falls into two distinct categories: diagnostic and therapeutic.

Diagnostic ultrasound 

Diagnostic ultrasound is a non-invasive diagnostic technique used to image inside the body.

Medical ultrasound probes, called transducers, produce sound waves that have frequencies above the threshold of human hearing (above 20KHz). Transducers in current use for medical imaging operate at much higher frequencies, and are in the megahertz (MHz) range..

For most diagnostic ultrasound examinations the operator places the probe on the skin. However, to optimize image quality, physicians may use specially designed probes placed inside the body – for example via the gastrointestinal tract, vagina, or blood vessels.

Surgeons often use ultrasound imaging during surgery. They can place a sterile probe into the patient’s operation area.  

A sub-division of diagnostic ultrasound is anatomical and functional ultrasound. 

Anatomical Ultrasound

Anatomical ultrasound produces images of internal organs or other structures. 

Therapeutic ultrasound

Therapeutic ultrasound uses sound waves above the range of human hearing but does not produce images and has sufficient power to either modify or destroy body tissue.

Among the modifications possible are:

• moving or pushing tissue;
• heating tissue;
• dissolving blood clots; or
• delivering drugs to specific locations in the body.

The very high-intensity beam destroys or ablates diseased or abnormal tissues (such as tumours). The advantage of using medical ultrasound therapies is that, in most cases, they are non-invasive. The procedure requires no incisions or cuts and there are no wounds or scars.

Functional Ultrasound

Functional ultrasound combines anatomical images with information such as:

• the movement and velocity of tissue or blood,;
• softness or hardness of tissue; and
• other physical characteristics,

to create “information maps.”

These maps help doctors visualize changes or differences in function within a structure or organ.

How does it work?

A transducer produces ultrasound waves which can emit the ultrasound waves, as well as detect the ultrasound echoes reflected back.

In most cases, the active elements in ultrasound transducers are made of special ceramic crystal materials called piezoelectrics. These materials are able to produce sound waves when an electric field is applied to them.

But the transducer can also work in reverse, producing an electric field when a sound wave hits it.

When used in an ultrasound scanner, the transducer sends out a beam of sound waves into the body. The sound waves are reflected back to the transducer by boundaries between tissues in the path of the beam (e.g. the boundary between fluid and soft tissue or tissue and bone).

When these echoes hit the transducer, they generate electrical signals that are sent to the ultrasound scanner.

Using the speed of sound and the time of each echo’s return, the scanner calculates the distance from the transducer to the tissue boundary. 

These distances are then used to generate two-dimensional images of tissues and organs.

During an ultrasound examination, the ultrasound technician will apply a gel to the skin. This stops air pockets from forming between the transducer and the skin, which can ‘block’ ultrasound waves from passing into the body.

What is ultrasound used for?

Diagnostic ultrasound.

Diagnostic ultrasound is able to non-invasively image internal organs within the body.

However, it is not good for imaging bones or any tissues that contain air, like the lungs.

Ultrasound provides images of bones in cases such as:

• a foetus
• small baby; or
• lungs and lining around the lungs filled or partially filled with fluid.

One of the most common uses of medical ultrasound is during pregnancy, to monitor the growth and development of the foetus.

But there are many other uses, including imaging the heart, blood vessels, eyes, thyroid, brain, breast, abdominal organs, skin, and muscles.

Medical ultrasound equipment display images as either 2D, 3D, or 4D (which is a 3D image in motion).

Functional ultrasound.

Functional medical ultrasound applications include Doppler and color Doppler ultrasound.

These techniques measure and visualize blood flow in vessels in the body or heart.

The technique can also measure the speed of the blood flow and direction of movement.

This shows the operator colour-coded maps (known as colour Doppler imaging).

A common procedure is to use doppler ultrasound imaging to detect whether plaque has built up inside the carotid arteries and is blocking blood flow to the brain.

Another functional form of ultrasound is elastography.

This is a method that measures and displays the relative stiffness of tissues.

Elastography differentiates tumours from healthy tissue and displayed as either:

• colour-coded maps of relative stiffness;
• black-and white maps that display high-contrast images of tumours compared with anatomical images; or
• color-coded maps overlayed on the anatomical image. 

 Inflammation of the liver can cause fibrosis from excessive scar tissue build-up. If this is suspected, an ultrasound examination (called elastography) can be used to image the patient’s liver.

Medical ultrasound imaging is frequently used to detect interventions in the body. Some examples are when the physician uses:

• image-guided ultrasound procedures to insert a biopsy needle into the patient’s breast cancer; or
• real-time ultrasound imaging to locate the tip of a catheter inserted in a blood vessel and guided along the length of the vessel.

Overall, medical ultrasound imaging helps to:

• minimise invasive surgery in operating procedures; and
• plays a valuable role in clinical medicine and emergency services.

Therapeutic or interventional ultrasound.

For therapeutic procedures, the medical ultrasound equipment produces high levels of acoustic output.

By using a high-powered, focused beam, ultrasound can heat, ablate or break up human tissue.

An ultrasound method, called High Intensity Focused Ultrasound (HIFU), is specially designed to accurately target the treatment site in the patient.

Exposure to the high-intensity HIFU ultrasound, modifies or destroys the patient’s abnormal tissues.

The patient receives HIFU treatment without having to open or tear the skin or cause damage to the surrounding tissue.

The physician:

• initially uses ultrasound and or MRI to identify the treatment target;
• guides and controls the treatment using ultrasound in real time; and
• confirms the effectiveness of the treatment at the end of the operation.

The FDA have approved HIFU for the treatment of:

• uterine fibroids,
• alleviate pain from bone metastases, and
• (most recently) ablation of prostate tissue.

HIFU is also under investigation as a method for

• closing wounds and stopping bleeding,
• break up clots in blood vessels, and
• temporarily open the blood brain barrier to enable medications to pass through.

Are there risks?

Diagnostic ultrasound is generally regarded as safe. It does not produce ionizing radiation like that produced by X-rays.

Still, ultrasound is capable of producing some biological effects in the body under specific settings and conditions.

For this reason, the FDA requires that diagnostic ultrasound devices operate within acceptable limits.

The FDA (as well as many professional societies) discourage using ultrasound for casual purposes (e.g. for keepsake videos).

Medical ultrasound equipment must not be used for non- medical purposes.

J. Jellins AM PhD (Hon) MD, 8 June 2019

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Acknowledgement

This article, What is Medical Ultrasound?, is an extract from the website of the National Institute of Biomedical Imaging and Bioengineering:

https://www.nibib.nih.gov/science-education/science-topics/ultrasound

It is reproduced here to use for educational and patient information purposes.

Copyright Conditions

This material is subject to the conditions laid down by the National Institute of Biomedical Imaging and Bioengineering: https://www.nibib.nih.gov/policies#copyright.

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Medical Ultrasound > An Introduction to Medical Ultrasound > Medical Imaging