2D is the black and white scan used in the hospital. The ultrasound used to make the 4D/HD-Live images is the same as that used in the 2D black and white you see in the hospital. We use a special probe, and the machines software processes the images differently to make a 4D image. 4D is simply a moving 3D image, during a 4D Scan we use both 3 & 4D, the live scan that you see on the screen is called 4D, and the still images we print out are 3D. The 4D may be purchased on a DVD ( please notify us prior to your scan if you would like one of these) HD (high definition) is another version of 4D providing clearer pictures. Again it is simply a moving 3D image. HD (high definition) is an upgraded version of 3/4D that shows you your baby in a clearer picture