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Ultrasound

Ultrasonic is the word used to describe the range of sound beyond the limit of human hearing. It has a frequency of at least 30 kHz. The 'sound' produced in this frequency range is called ultrasound.

Ultrasound is used as a diagnostic tool instead of other forms of radiation because it is less damaging to the body. At higher energies it is used for therapy.

Uses and effects of ultrasonics in medicine:

An ultrasound source (or transducer) sends a beam of ultrasonic pulses into the body. These pulses are reflected from different surfaces and organs and the time taken for the pulses to be reflected to the detector gives on formation about their distance from the transducer. To give good contact with the skin and to prevent a large amount of reflection from the skin surface a layer of jelly paste is put between the transducer and the skin.

1. Pregnancy scan. Ultrasonic scans are used to look at unborn babies because the sound waves are safer than X rays.

2. Physiotherapy the high frequency 'sound' heats up the damaged tissue and so aids healing

3. Measurement of blood flow by using the Doppler effect with the ultrasound beam reflecting from the moving red blood corpuscles. Very high frequencies in the range 5-10 MHz are used in this measurement. The same idea can be used to measure the speed of movement of the foetal heart.

4. The heating effect of an ultrasound beam can be used to treat small volumes of the body and so destroy small objects such as bladder stones. The destruction of a kidney stone may need as many as 2000 pulses of ultrasound, each pulse carrying an energy of 20 J or more. Some small tumours can also be removed in this way.

5. It is also possible to use ultrasonics to map out the shape of structures beneath the skin. The image is formed by a kind of echolocation the time delay between reflections from each point below the skin surface and the intensity of these reflections being measured. For these measurements the speed of the ultrasonic beam in various tissue structures needs to be known.

6. Echoencephalography
Pulses of ultra sound are transmitted into the brain through the thin region of the skull just above the ear. Echoes are then received from different structures within the skull.

7. Ophthalmology
Ultrasonics is used in ophthalmology to obtain information for use in the diagnosis of eye disease and also fro the measurement of distances within the eye such as lens thickness and the distance of the retina from the cornea.

To see fine detail in an image formed using ultrasonics we need to use an ultrasound beam with a wavelength no larger than the detail we are trying to resolve. So if two points in some fatty tissue are 0.1 mm apart an ultrasound beam with a wavelength of 0.1 mm or less must be used to resolve them. Since the speed of ultrasound in fat is 1450 m/s this means that a frequency of 14.5 MHz is needed.


Just as with light, when the ultrasound passes from one material to another its speed changes and some of the ultrasound is reflected back at the interface of the two materials. The image is formed by this reflected beam from different materials within the body.

Material Speed of sound (ms-1)
Air (at STP) 331
Fat 1450
Water 1495
Soft tissue 1540
Kidney 1561
Muscle 1585
Bone 4080

It is important to remember that even ultrasound can be dangerous. The ultrasound beam carries energy, some of which can be absorbed by the body and so cause damage to the tissue, especially bone. This is only the case if the beam is of a high intensity and the patient is exposed to it for a long period of time. It is also possible for small body parts to resonate in the ultrasound beam and literally shake themselves to pieces.

At low power levels (0.1-20 W/square cm) there is little danger but as the power is increased ultrasound is useful in therapy but as such is producing damage to cells. This is fine as long as it is the right cells that are being damaged as in a tumour. Power levels for deep heating in physiotherapy are around 1 W/square cm but rise to 1000 W/square cm when used to destroy tissue.

The main non-thermal effect of ultrasound is called cavitation. Body tissues and fluids contain dissolved gases. When some of the sound energy is absorbed by the tissue and this could create bubbles from the dissolved gases. The bubbles then vibrate due to more absorption and finally burst. This effect also only occurs at very high dose power levels.

 

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© Keith Gibbs