Abstract: Ultrasonic probe apparatus for real time B-mode sector scan examination of a cross section of a structure such as the human heart. A transducer is pivotally mounted near an end of an elongated housing, and linkage means are provided in the housing for oscillating the transducer about an axis whose plane is approximately perpendicular to the longest dimension of the housing. Means are provided for causing the transducer to produce ultrasonic pulses which are reflected by interfaces of the object under examination, and the echoes from these interfaces are returned through the transducer to a B-mode display system. A B-mode sector sweep circuit is provided to produce a B-mode display of the echo pattern received from the structure being observed.