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.

Damping is a process, material, design, and mounting technique used to reduce the pulse duration or ringing of the transducer. Special material is applied to the back of the transducer in order to reduce the amplitude and pulse length of the sound wave.
Damping improves axial resolution by reducing pulse length. Thereby the lateral resolution increases.

The focal zone is the region within the transmitted sound beam in which the beam narrows to its minimum size. The lateral resolution is best within the focal zone of the beam.

The perfect image quality is dependent on some assumptions of the propagation of ultrasound waves in tissues after generating in an imaging system. These assumptions are important for the developing of optimal ultrasound imaging systems.
A lot of steps can be taken to prevent artifacts and to improve image quality, for example beamforming is used to focus the ultrasound beam, and contrast agents decrease the reflectivity of the undesired interfaces or increase the backscattered echoes from the desired regions.

See also Coded Excitation, Validation and Refraction Artifact, Q-Value, Ultrasound Phantom, Dead Zone, Narrow Bandwidth.

Linear array transducer elements are rectangular and arranged in a line. Linear array probes are described by the radius of width in mm. A linear array transducer can have up to 512 elements spaced over 75-120 mm. The beam produced by such a narrow element will diverge rapidly after the wave travels only a few millimeters. The smaller the face of the transducer, the more divergent is the beam. This would result in poor lateral resolution due to beam divergence and low sensitivity due to the small element size.
In order to overcome this, adjacent elements are pulsed simultaneously (typically 8 to 16; or more in wide-aperture designs). In a subgroup of x elements, the inner elements pulse delayed with respect to the outer elements. The interference of the x small divergent wavelets produces a focused beam. The delay time determines the depth of focus for the transmitted beam and can be changed during scanning.
Linear arrays are usually cheaper than sector scanners but have greater skin contact and therefore make it difficult to reach organs between ribs such as the heart. One-dimensional linear array transducers may have dynamic, electronic focusing providing a narrow ultrasound beam in the image plane. In the z-plane (elevation plane - perpendicular to the image plane) focusing may be provided by an acoustic lens with a fixed focal zone.
Rectangular or matrix transducers with unequal rows of transducer elements are two-dimensional (2D), but they are termed 1.5D, because the number of rows is much less than the number of columns. These transducers provide dynamic, electronic focusing even in the z-plane.

See also Rectangular Array Transducer.