Flat images (like TV, computer monitors, and photographs) are 2D. There is an indication of depth from perspective and light. Conventional ultrasound images are maps of how tissues reflect sound pulses when a sonic beam is moved along a line path - revealing a thin plane of tissue from the skin surface down. Each little box of data in the flat image is a 'pixel' or picture element. We get 3D images by stacking up a series of 2D ultrasound views in computer memory as volume elements or 'voxels'. Nearly all of the images shown on AmnioNet are high resolution 'surface' renderings, showing the outside of the fetus bathed in echo free amniotic fluid. We use a digital scalpel to cut away voxels in front of or behind our areas of interest.

There has been some popularization of '4D' imaging, which is a way of making multiple, rapid individual 3D images. There is an apparent advantage of 4D in looking at some ephemeral movements, like facial expressions, however, we do NOT use this technique, because detail resolution of current 4D units is poor, limiting their use to late pregnancy, when the fetus is large. When 4D image resolution and noise suppreession are equivalent to what we now achieve regularly with our special 3D techniques, we will adopt it. Our aim is to use the best imaging technology expertly.