Ultrasound - Standard | Medical Physics

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Purposes

Ultrasound is used in 3 main areas [1]:

Obstetrics and gynaecology:

Measuring the size of the foetus and seeing its position
Checking the position of the placenta
Seeing the number of foetuses in the uterus
Finding the sex of the baby
Determining if there is a good amount of amniotic fluid cushioning the baby
Seeing tumours of the ovary and breast

Cardiology:

Seeing the inside of the heart to identify abnormal structures or functions
Measuring blood flow through the heart and major blood vessels

Urology:

Measuring blood flow through the kidney
Seeing kidney stones
Detecting prostate cancer early

What to expect?

Most ultrasound scans last between 15 and 45 minutes, and they usually take place in a hospital radiology department, performed by either a radiologist or a sonographer

You will have a different experience depending on which type of ultrasound scan is used [2]:

External ultrasound scan:

Often used to examine the heart or an unborn baby in the womb, but also the liver, kidneys and other organs in the tummy and pelvis, or muscles and joints
A handheld probe is placed on the skin and passed over the part of the body that needs to be examined
A gel is put on the skin to help the ultrasound waves go in and out of the body, and it also lubricates the skin to let the probe move smoothly.

This will probably feel cold, but you shouldn’t feel anything other than the gel and sensor on your skin!

If you are having a scan of the womb or pelvic area, you might have to a have a full bladder which may be uncomfortable, but a toilet will be near for when the procedure is finished
If you are having a scan of the digestive system, like the liver and gallbladder, you might have to avoid eating or drinking for a few hours before the scan
After the scan you should be able to drive, eat, drink and go back to normal straightaway!

Internal or transvaginal ultrasound scan:

Often used to examine organs such as the prostate gland, ovaries, or womb, because it allows the doctor to see further inside the body
During this procedure you will be asked to lie in a position to give the doctor easier access
A small probe, which will have a sterile cover on, will be gently passed into the body through the vagina or rectum, and images are transmitted onto a screen
These internal examination might not be very comfortable, but they shouldn’t hurt and won’t last very long
After the scan you should be able to drive, eat, drink and go back to normal straightaway!

Endoscopic ultrasound scan:

Often used to examine areas such as your stomach or throat (oesophagus), by having an endoscope inserted into the body, usually through the mouth
During the procedure you will be asked to lie on your side so that the endoscope can be pushed carefully down towards the stomach

Once inside the body, the ultrasound device creates images just like in an external ultrasound

You will most probably be given a sedative to help you keep calm and a local anaesthetic spray that will mean you can’t feel your throat, because the scan can make you feel a bit sick
You might also have to wear a mouth guard to stop your mouth closing and your teeth biting the endoscope
After the procedure you will have to stay for a few hours until the sedative stops working, and someone will have to pick you up and keep an eye on you for the day

An external ultrasound being carried out

(source: News Medical)

Ultrasound scan of a baby in the womb

(source: Ultrasoundkk)

How does it work?

Ultrasound is a sound wave with a frequency above 20 kHz. In medicine it usually has a range of 2 - 10 MHz [3]

The Piezoelectric Effect

Some crystals can vibrate and create ultrasound waves when electricity flows through them, and when a waves comes back through the crystal it can create an electric flow, this is called the "Piezoelectric Effect"

These crystals are used in “transducers” in ultrasound probes

Echoing waves in the body

The probe sends ultrasound waves (produced by the crystal) into the body, and then “echoes” come back

Different tissues in the body have different “acoustic impedances”, which is very similar to density.

When there is a boundary between two things with different “acoustic impedances” this causes the wave to reflect and go back to the probe, like an echo.

Once the reflected wave moves out of the body and back through the crystal, an electric flow is made, and this can be used to create an image on a screen

What does the gel do?

Air and skin have very different acoustic impedances, which means without something to help, only a tiny percentage of ultrasound waves would enter the body from the probe and you wouldn’t be able to see the organs you are trying to scan!

The gel that is placed on the skin has a very similar acoustic impedance to skin, so the waves from the probe travel through this rather than the air, and most of the waves can enter the body rather than reflecting, which means we get the most useful results!

Measuring the speed of flow

Ultrasound can also be used to find the speed of blood flow using the doppler effect

The ultrasound waves are sent in line with the direction of the blood stream and reflect off the moving fluid, but the echoes return with a changed frequency (pitch).

If the blood is moving towards the transducer the frequency is higher, and vice versa.

Using this change in frequency you can calculate how fast the blood is flowing.

Want to know how ultrasound technology is progressing into the future? Click the button below!

References:

[1] Major Uses Of Ultrasound: https://science.howstuffworks.com/ultrasound4.htm (accessed 19/02/19)

[2] NHS Ultrasound Scan: https://www.nhs.uk/conditions/ultrasound-scan/ (accessed 13/02/19)

[3] National Institute of Biomedical Imaging and Bioengineering: https://www.nibib.nih.gov/science-education/science-topics/ultrasound (accessed 17/02/19)

[4] Go Imaging: https://go-imaging.com/blog/ultrasound/may-need-full-bladder-ultrasound/ (accessed 24/02/19)

[5] American College for Medical Careers: https://www.acmc.edu/fun-facts-about-sonography-and-ultrasound-technology/

[6] Nanomotion: https://www.nanomotion.com/piezo-ceramic-motor-technology/piezoelectric-effect/ (accessed 19/02/19)

Ultrasound In The Future

Why use ultrasound?

Ultrasound scans are inexpensive to run and are also a lot safer than other more invasive methods that use (potentially dangerous) ionising radiation, with little to no side effects!

What is a probe?

While it might sound like something to do with aliens, the ultrasound probe is a just a small device (the size of a bar of soap) that is dragged across the skin to carry out the scan

Why would I need a full bladder?

During scans of the pelvic region, having a full bladder can help to see the pelvic organs in more precise detail, which means you can get clearer results! [4]

Want to know what ultrasound was first developed for?

Surprisingly, the first ultrasound machines were used in industry to measure flaws in metal castings in the 1940s!

[5]

How do the echoing sound waves get turned into images?

We know how fast ultrasound moves through different materials in the body, so by seeing how long it took the waves to travel to certain objects, we can work out how far away the different objects in the body are and turn this into a picture

Piezoelectric effect in more detail...

An electric field going through these unique materials causes the positive and negative charges within to move and so the material either stretches or compresses [6]

What is the doppler effect?

It is when the frequency of a wave changes as the source or observer (of the wave) move towards (or away) from eachother. You can hear it when an ambulance goes past and the pitch of the siren changes from high to low!