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Physics, Techniques and Procedures

Near field

also called Fresnel zone (Augustin Jean Fresnel, 17881827, French physicist), the proximal part of an ultrasound beam characterized by a confined, slightly converging beam shape. The length of the near field is equal to r2/l = d2/4l, where r is the radius and d the diameter of the transducer crystal, and l is the ultrasound wavelength in the medium of propagation.

HJS