Annular arrays are transducer assemblies with circular or ringlike elements, used to focus the beam. Annular arrays must be steered mechanically since they can only be fired in an outward-inward progression due to the rings. Annular arrays reduce section thickness artifacts.

From ESAOTE S.p.A.;
'The AU5 EPI is an ultrasound system for multi-disciplinary use that incorporates all the transducer technologies (linear, convex, phased array, and annular array) adapted to today's most diverse diagnostic requirements. Furthermore it offers the most modern and advanced techniques available today on the market.'

Arrays are spatial arrangements of transducers or transducer elements.
Array types used in ultrasound imaging:
See also Composite Array.

An array transducer is composed of multiple piezoelectric crystal elements arranged in an array. Arrays are transducer assemblies with a row of elements, used to focus the beam.

Types of array transducers:

See also Amplitude Shading, Transducer Types, and Transducer Assembly.

Composite arrays are combinations of piezoelectric ceramics and polymers that form a new material with different properties. Piezocomposites improve the performance of usual arrays such as the mechanically scanned annular array and the linear phased array.
Piezocomposites reduce the acoustic impedance with a better impedance match with tissue. The result is a reduction of the reverberation level in the near field. Unwanted surface waves propagating laterally over the transducer are suppressed. The composite materials allow to vary the electromechanical coupling constant, and to give better control over the trade-off between sensitivity and bandwidth.

See also Narrow Bandwidth, Dead Zone, Ultrasound Phantom.