Physics of Ultrasound
Generation of ultrasonic sound waves
Ultrasound uses ultrasonic (above the range of human hearing) sound waves that are produced and detected within an ultrasound transducer. The physical concept underlying the transducer is the piezoelectric effect. This is the property of some materials that causes them to change in shape when an electric current is applied to them. Alternatively, a change in shape can provoke an electric current to form. A ultrasound transducer contains a piece of piezoelectric material between two electrodes. An osciliating current is applied, causing the piezoelectric material to vibrate rapidly and generate ultrasound waves.
Properties of ultrasonic sound waves
Ultrasound waves interact with tissue in various ways.
- They are attenuated through absorption and scattering.
- Absorption occurs due to loss of energy as heat, particularly in tissues that are unable to oscillate (eg. bone)
- Scattering occurs with small inhomogeneities (eg. small vessel, soft tissue septa). Some of this scatter returns to the probe, giving the characteristic 'grainy' apperance to ultrasound images
- Waves may be reflected or refracted, similar to light 'rays' passing through different media
- Reflection of the wave returns a signal to the transducer, and occurs at a boundary between two media, perpendicular to the wave direction
- Refraction of the wave occurs at oblique angles between two different mediums, and may lead to errors in depth estimation
Detection of ultrasonic waves
Ultrasonic waves which return to the detector, either by reflection or scattering, cause the piezoelectric material to vibrate, generating an electric signal that is converted into an image.
Use of Ultrasound
Ultrasound is a relatively safe imaging procedure, but needs some experience in use before images can be easily read. It is most useful for interstitial or intracavitary brachytherapy, but can also be used in some single-field applications (eg. breast boost).
Advantages
- No ionising radiation
- Not invasive (with some exceptions, eg. prostate ultrasound may require a rectal probe)
- 3D or 4D ultrasound is available
- Useful in brachytherapy to verify anatomy or location of applicators
- Inexpensive
Disadvantages
- Gives no information on attenuation coefficients of tissue
- Unable to produce DRR
- Operator dependent
- Limited to certain body sites (unable to scan through bone or gas)
- Ultrasound waves are affected by refraction the depth seen on the scan may not be accurate
- Poor image resolution due to scattering artefact
