An ultrasonic diagnostic imaging system is provided which scans an image region to receive a plurality of spatially arranged lines of ultrasonic image information signals. These ultrasonic line information signals are used in an interpolater to interpolate one or more lines which are spatially interlineated between each pair of spatially adjoining received lines. These interpolated lines are produced using either the received RF or demodulated IF ultrasonic image information signals. The interpolated lines are produced prior to scan conversion, and preferably prior to nonlinear processing such as detection or log compression to reduce spatial aliasing artifacts. In one preferred embodiment the interpolater comprises a transversal filter of four taps which is responsive to received line information signals from a common range or depth. In a second embodiment the interpolater is adaptively responsive to motion to select samples for interpolation which would be spatially at the same range in the absence of motional effects.