What is apodization?

Many higher end machines now boast apodization on their list of ultrasound specifications. This technique involves varying the amplitude across the aperture of the transducer, such that the elements at the centre of the probe head are electrically excited with a voltage of greater amplitude to those at the edges. The result of this is a significant reduction in the strength of sidelobes, which (unlike grating lobes, which are unique to linear arrays) are present in all transducer types.

Sidelobes are lobes at the edges of the main beam. Although they are weaker than the main part of the beam, if they insonate a strong reflector, a high enough amplitude echo will return to the transducer and the ultrasound machine will ‘assume’ that this echo originated from the centre of the beam. Thus, it will place this reflection as such in the ultrasound image, resulting in an artefact. Whilst lobe artefacts are more common with linear array transducers in the form of a specific type called grating lobes (due to the regular spacing of the elements with these transducers, at more than one wavelength apart), side lobes are also a particular problem at higher frequencies, which are commonly used when scanning small animals in veterinary ultrasound.

Apodization thus seeks to reduce the amplitude of these sidelobes even further, to minimise their effect on the overall image. There is a price to be paid, however, in that the main beam broadens with increasing suppression of side lobes, resulting in a decrease in lateral resolution. Recent advances include the development of nonlinear sidelobe suppression (i.e. apodization across the transmit and receive apertures is not weighted linearly), which achieves the high signal-to-noise ratio of linear apodization but with little or no sacrifice in lateral resolution (Seo & Yen, 2010).