A-Scans and B-Scans | Medical Physics

Ultrasound Results

Two types of ultrasound scans are A-scans and B-scans:

A-scans (in one dimension) are more simple that B-scans. Standing for "amplitude scan", they use the voltage peaks on a cathode-oscilloscope. The peaks occur when reflected ultrasounds return through the piezoelectric crystal and create a potential difference due to the piezoelectric effect.

These can be used to determine the dimensions of organs, for example in the eye the returned voltage peaks might look like Fig. 1 below.

B-scans are many A-scans put together and viewed as a 2D cross-sectional "brightness" image [1]. This can be seen in the well known example of ultrasound scans of babies in the uterus, and also 2D cross-sectional images of the eye, as shown in Fig. 2 and Fig. 3 below.

References:

[1] https://www.ophthalmologyweb.com (accessed 26/02/19)

[2] https://www.cyberphysics.co.uk/topics/medical/Ultrasound/B_Scan.html (accessed 26/02/19)

The first peak is the pulse from the transducer, the second from the front of the eye cap, the third from the front of the lens, the fourth from the back of the lens, and the fifth from the retina. Then, because we know how fast ultrasound travels in each section of the eye, we can determine the length of each section from the distance between voltage peaks.

Fig. 1: A diagram of the eye with the ultrasound peaks that would result from an a-scan

Fig.2 : A 2D B-Scan of the eye (source: Medicine UIOWA)

The varying brightness throughout the image allows you to see the different parts of the eye in 2D. In this particular scan you can see a tumor on the back wall of the eye.

One important use of this scan is to find the position of the placenta in relation to the entrance of the cervix, because if the entrance is covered a c-section wil be required.

Another use is during amniocentesis (inserting a tube into the uterus to remove cells for genetic tests). The position of the foetus and placenta have to be located accurately, which is done by the B-scan [2].

Fig.3 : A 2D B-Scan of the a baby in the womb

(source: Cyber Physics)