Further physics - The applications of ultrasound
Kwok Yee-shan and Tong Shiu-sing   
Translation by Janny Leung   

Ultrasonography is often used in pre-parturition checkup
Fig. 1   Ultrasonography is often used in pre-parturition checkups (Image Courtesy: Philip Research).

Our ears can only distinguish between sounds of frequency ranging from twenty to twenty thousand hertz; sound waves of a frequency higher than the human audible limit are called ultrasound. The audible limit of different animals is not the same. Dogs can hear some ultrasound, thus a dog trainer can use an ultrasound whistle to call for his dog. Ultrasound is even more important for bats because they rely on ultrasound to "see" the world!

A bat firstly produces a series of ultrasound scream. The sound waves are reflected by obstacles, like the echo produced when we clap in a valley. Due to the high frequency of the ultrasound, the waves are relatively less affected by diffraction and hence the echo is very clear. After analyzing the direction of the echo and the time taken, the bat can get an accurate picture of its environment. Based on the principles involved in how a bat "sees" things, human beings have developed sonar to detect water depth. An emitter on a boat firstly emits ultrasound into the sea, and then a picture of the whole seabed can be sketched based on the reflected messages received and analyzed by other equipment.

The most important application of ultrasound is medical ultrasonography. Ultrasonography does not involve hazardous radiation and is thus safer than other examination tools such as X-ray. Therefore, it is often used in pre-parturition checkups. The doctor would use a high-frequency ultrasound (frequency: 1-5 mega-hertz) handheld transducer to scan the mother's abdomen. Sound waves are reflected to a different degree when they reach the boundaries between different body tissues (such as the boundary between liquid and soft tissues, and that between soft tissues and bones). The strength of reflection and the distance of the reflection surfaces can be calculated after the receiver has received the reflected waves. Using these data, we can distinguish between different body tissues and get an image of the foetus. The receiver utilizes the principle of piezoelectricity and transforms the pressure produced by ultrasound waves into electronic signals, and then transmits them for further analysis. Ultrasonography helps the doctor to measure the size of the foetus and estimate the expected birth date; it is also used to check the foetus' gender, its growth pace, whether its head is positioned correctly downwards, the position of the placenta, the sufficiency of amniotic fluid, and to monitor the process of drawing amniotic fluid to ensure the safety of the foetus etc. Besides, ultrasonography is also used in gynecological checkup; it helps the doctor to efficiently distinguish malignant cells growing in the breast or the ovary.

Doppler ultrasound
Fig. 2  Doppler ultrasound is used to examine the speed of blood flow in the heart (Image Courtesy: Philip Research).

The two branches of ultrasonography - Doppler ultrasound and 3D ultrasound imaging - have even extended the medical applications of ultrasound.

Doppler ultrasound has been used for a rather long time; this technique makes use of the Doppler's effect of wave motion. When an object reflecting ultrasound is in motion, the frequency of the echo will be changed; the frequency will become higher when the object is moving towards the receiver, and conversely it will become lower when the object is moving away. From the frequency change of the echo, the speed of the moving object can be calculated. The major application of Doppler ultrasound is to examine the speed of blood flow in the heart and the major arteries. The flow of blood is shown by a colour image, where different colours represent different speed of flow. This helps the doctor to find out if the foetus has congenital cardiac diseases.

3D ultrasound imaging is a very new technology. An examiner firstly takes 2D ultrasound images of the foetus from various angles, and then uses computer technology to combine them into a 3D image of the foetus. This technology can clearly show the appearance of the foetus, and it can even record subtle movements such as kicking or turning around, bringing surprise to the parents-to-be. Outlook defects such as cleft lips, polydactyl or even as tiny as moles can be clearly shown. 3D imaging technology would be a focus of ultrasound research in the future.

3D ultrasound images of a foetus
Fig. 3  3D ultrasound images of a foetus (Image Courtesy: Philip Research).

In addition, high frequency ultrasound also has a large vibration energy. If we input ultrasound waves into a vessel containing water and an object that needs cleaning, the vibration of water can remove the dirt without the need of touching the object's surface directly. In optical shops spectacles are cleaned in this way. If high frequency ultrasound is focused, the energy can even crush a stone, and hence it can be used to crush the calculus inside the body so that the patient does not have to undergo a surgery.