An ultrasonic diagnostic imaging system produces spatially compounded trapezoidal sector images by combining component frames acquired from different look directions. A virtual apex scan format is used such that each scanline of a component frame emanates from a different point on the face of an array transducer and is steered at a different scanning angle. For different component frames the scanlines are steered at respectively different angles. In an illustrated example, the scanlines of each component frame are incremented by five degrees relative to the corresponding scanlines in a reference component frame. When the component frames are combined for spatial compounding, the maximum number of component frames are combined over virtually the entire image field.