Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

sound beam path is thus largely confined to the dimensions of the active portion ofthe transducer surface, with the beam diameter converging to approximately halfthe transducer diameter at the end of the near field. The near field length is dependenton the transducer frequency and diameter:d 2 r 2Near field length = 4A = Twhere d is the transducer diameter, r is the transducer radius, and A is the wavelengthof ultrasound in the propagation medium. In soft tissue, A = ~~~~~ , andthe near field length can be expressed as a functionof frequency:. d 2 (mm 2 ) {(MHz)Near field length, soft tissue (mm) = -~-~~-~4 X 1.54 mmA higher transducer frequency (shorter wavelength) will result in a longer near field,as will a larger diameter element (Fig. 16-16). For a lOcmm-diameter transducer,the near field extends 5.7 cm at 3.5 MHz and 16.2 cm at 10 MHz in soft tissue.For a 15-mm-diameter transducer, the corresponding near field lengths are 12.8FIGURE 16-16. For an unfocused transducer, the near field length is a function ofthe transducer frequency and diameter; as either increases, the near field lengthincreases. Bottom: A focused single element transducer uses either a curved element(shown) or an acoustic lens. The focal distance (near field length) is broughtcloser to the transducer surface than the corresponding unfocused transducer, witha decrease in the beam diameter at the focal distance and an increase in the beamdivergence angle. A "focal zone" describes the region of best lateral resolution.