6.13.1 Ultrasound

• Ultrasound is defined as:

Sound waves with a frequency above the human hearing range of 20 000 Hz

• Ultrasound is, therefore, a longitudinal wave

Ultrasound is above 20 kHz, although in medical applications the frequencies can be up to the MHz range

• There are two types of ultrasound techniques that are used to obtain diagnostic information:
  • A-Scan, or amplitude scan
  • B-Scan, or brightness scan

A-Scan

• An A-scan, or amplitude scan, uses a single transducer to emit a signal and then later receive the reflected signal back
• It is defined as:

A one-dimensional ultrasound scan used to determine the distance or depth of an internal structure

• This is achieved by:
  • Measuring the time delay between generating and receiving the signal
  • Using the speed of sound in the media to calculate the distance travelled by the signal
• This type of scan is used for:
  • Determining distances from the ultrasound device to the point of reflection (usually the boundary between two media)
  • For example, the length of an eye needs to be determined in planning surgeries or assessing the presence of abnormalities, such as tumours
• This type of scan gives measurements only and does not produce an image

B-Scan

• A B-scan, or brightness scan, is a more complex scan that produces a 2D or 3D image of internal structures in the body
• It is defined as:

An ultrasound scan used to build up a two or three-dimensional image of an internal structure using a number of sensors or one sensor in different positions

• This is achieved by:
  • Using pulsed ultrasound waves in different positions to produce several measurements of time intervals between generating and receiving pulses
  • Moving the transducer over the patient’s skin, or using several transducers, to produce a series of A-scans that are combined to form an image

• This type of scan is used for:
  • Creating images of internal structures for diagnostic purposes
  • For example, bones, muscles and organs or checking on the progress of an unborn child
• To achieve the clearest images:
  • Pulsed ultrasound waves are used to allow time for the reflected waves to be received and not interfere with transmitted waves
  • Smaller wavelengths are used to give more detailed images as they will allow the sound waves to diffract around finer points of detail on the internal structure being studied