What causes grating lobes in ultrasound imaging?

Answer:
"Grating lobes are unique to array transducers and are caused by the regular, periodic spacing of the small array elements. When the energy of these lobes is reflected by off-axis structures and detected by the transducer, the signal produced is artifactual and produce "ghost images" blurring the main image. To overcome this problems each individual element has been subdivided into a half wavelength wide. This effectively eliminated the grating lobes by increasing the angle to greater than 90 degrees. Eliminating grating lobes also improves the signal-to-noise ratio by increasing the size of the main lobe energy relative to the background energy. This further improves image contrast."

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