BEAM SHAPE

The area through which the sound energy emitted from the ultrasound transducer travels is known as the ultrasound beam. The beam is three-dimensional and is symmetrical around its central axis. It can be subdivided into two regions: a near field (or Fresnel zone) which is cylindrical in shape, and a far field (or Fraunhofer zone) where it diverges and becomes cone-shaped (see Fig. 5.1).

Fig. 5.1 Beam shape

The actual shape of the beam depends on a number of factors, including the diameter of the crystal, the frequency and wavelength, the design of the transducer, and the amount of focusing applied to the beam. Increasing the frequency will result in a longer near field and less far field divergence. A narrow crystal diameter will result in a narrower beam in the near field, but the disadvantage is that the near field is shorter and there is more divergence in the far field.

Where there is continuous sound output from a transducer (primarily Doppler applications), there will be sound energy present throughout the beam for the duration of the scanning period. Most diagnostic applications, however, use pulsed sound, where the output is a series of short pulses of sound. In this case sound energy is not present throughout the beam, but only in small areas or pockets. It is the movement of these pockets of sound which we refer to as the ultrasound beam (see Fig. 5.2).

Fig. 5.2 Pulsed and continuous wave ultrasound beam

INTENSITY OF THE BEAM

The intensity of the beam is the power (measured in watts) flowing through a unit area (see Chapter 12 on Ultrasound safety). The intensity is not uniform throughout the beam’s length nor across its width. This is caused by a number of variable factors such as:

Related posts:

Multiple choice questions and answers The piezoelectric effect Ultrasound interactions and attenuation Transducers Resolution Quality assurance and performance testing

Stay updated, free articles. Join our Telegram channel

Join