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Beam widths (−3 dB and −6 dB) as functions of depth for a 400 kHz ultrasound beam emanating from circular piston transducer with a diameter of 25.4 mm and a focal length of 38.1 mm. The −6 dB beam width is completely contained within the calcaneal section (15 mm × 15 mm). The −3 dB and −6 dB beam widths at the focal plane predicted by Fourier Optics are also shown.
