Continuous wave ultrasonic tomography

As an object rotates with respect to a stationary ultrasonic beam, the scat tering centers within the object return echoes that are Doppler-shifted in frequency by amounts depending on the velocities of the individual scattere rs. The scattering centers that lie on a line of constant cross-range all h ave the same effective velocity in the direction pointing toward the transd ucer; therefore, the backscattered echo amplitude at any particular frequen cy is the line integral of the scattered radiation at the cross-range corre sponding to that frequency. The amplitudes of the returned signals at other frequencies give the line integrals for the scatterers at the correspondin g cross-ranges. The amplitude as a function of frequency can be interpreted as a tomographic projection. A continuum of the projections at different p ositions is generated while the object is rotating. A tomographic reconstru ction algorithm can produce an image of the distribution of scattering cent ers in the insonified object from these projections. A microscanner was developed to investigate the approach of using continuou s wave (CW) ultrasound for cross-sectional imaging. The resolution is limit ed by the target size and the ultrasonic wavelength.