Continuous wave ultrasonic tomography

As an object rotates with respect to a stationary ultrasonic beam, the scattering centers within the object return echoes that are Doppler-shifted in frequency by amounts depending on the velocities of the individual scatterers. The scattering centers that lie on a line of constant cross-range all have the same effective velocity in the direction pointing toward the transducer; therefore, the backscattered echo amplitude at any particular frequency is the line integral of the scattered radiation at the cross-range corresponding to that frequency. The amplitudes of the returned signals at other frequencies give the line integrals for the scatterers at the corresponding cross-ranges. The amplitude as a function of frequency can be interpreted as a tomographic projection. A continuum of the projections at different positions is generated while the object is rotating. A tomographic reconstruction algorithm can produce an image of the distribution of scattering centers in the insonified object from these projections. A microscanner was developed to investigate the approach of using continuous wave (CW) ultrasound for cross-sectional imaging. The resolution is limited by the target size and the ultrasonic wavelength.