Ultrasonic Two-Dimensional Visualization for Medical Diagnosis

In ultrasonic two-dimensional visualization, the spot size of the display unit is the limit of resolution and determines the design of the echoscope. The shortest echo response is obtained from a nonreflectivity backed transducer, but the usual backed or quarter-wave matched transducer gives adequate axial resolution. The beam-width of the optimum weakly focused transducer is equivalent to four times the spot size, and some form of signal comparison must be used to make the azimuthal resolution compatible with the axial resolution. The finite beamwidth also degrades the axial resolution of inclined structures and distorts the display by elongating structures along the beamwidth. Off-axis receiving transducers may be used to receive larger echoes from inclined structures. An improvement by a factor of 3 may be obtained before the employment of these transducers degrades the resolution. The technique is prone to artifacts that are more troublesome in contact than in water delay scanning. These features are illustrated for an echoscope designed for the visualization of the pregnant uterus.