CMOS Circuits for Biological Sensing and Processing by Srinjoy Mitra & David R. S. Cumming

Author:Srinjoy Mitra & David R. S. Cumming
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

(8.3)

where the ratio of the focal depth to the aperture size is called the f-number, f #. Increasing f c and the bandwidth of the transducer will thus improve both the axial and lateral resolutions, resulting in the ability to distinguish smaller features [33].

λ can be calculated from the speed of sound, v, divided by the frequency of the signal, f. The speed of sound in most biological tissues lies between v = 1484 m/s (water) and v = 1590 m/s (liver) [3]. When calculating values, it is common to use v = 1500 m/s as a convenient approximation. Based on this, λ = 1 mm at f = 1.5 MHz and λ = 0.1 mm at f = 15 MHz. This tenfold increase in resolution allows the detection of structures of interest to radiologists, corresponding with the limit of viewing with the naked eye [62]. Further increases in f provide resolutions of 10s of microns, which allow imaging of discrete layers with thicknesses <1 mm and identification of small changes in tissue structure characteristic of early-stage disease progression [13].

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