Fig. 11

Typical diffraction pattern of an ultrasound probe and presence of side lobes associated with the beam forming reconstruction method, and finite dimension of the transducer aperture (typically determined by the number of transducer elements used at emission and reception). Notice that diffraction and side lobe characteristics are defining the point spread function property schematized in Fig. 4. This example illustrates the acoustic pressure distribution of a typical ultrasound beam produced by an array transducer. Specific beam forming strategies are used to reduce side lobes and multiple focuses improve the uniformity of the acoustic pressure distribution. Nevertheless, wave diffraction occurs and should be compensated to achieve reliable QUS measurements. Because the magnitude of compression waves is not uniform in the lateral (x-axis) and axial (y-axis or depth) directions, calibration with a reference phantom is required to compensate for spatial changes