A friend of ours sent us a so-called “4D Image” of their developing baby. While the image is 3D (if you let your eyes adjust because at first we couldn’t make out anything), but what the heck is this 4D business? - Jinx
What they sent you was likely a freeze-frame image from a video. A regular ultrasound study shows you a truncated pie-wedge-shaped ‘slice’. Call this a 2-D image. Think of the tech holding the transducer in front of your face, producing a cross-section view of your nose, sinuses, brain, and ears. Now imagine moving the transducer rapidly up and down over your face and using computer programs to integrate the information and produce a 3-D contour map of your facial skin (or, with additional work, a contour map of deeper structures).
Now, if you have advanced enough technology, you can shift the sensor plane electromechanically and perform the reconstruction calculations fast enough that you can show the 3-D image (alright, the 2-D television screen projection of the 3-D image) in real time, as the sensor is passed over your belly to capture the best ‘shot’ of the developing baby’s face. The image changing in real time gives you the fourth dimension - it is a 3-D video rather than a 3-D snapshot. Perhaps this strains the metaphor a bit, but it’s a marketing technique for the ultrasound manufacturer.
When you’re getting it done, you can see the baby move almost in real time, the “fourth dimension”. Early 3D ultrasounds took up to 20 minutes to capture one image, so you couldn’t see movement. Here’s a timeline of 3D/4D ultrasound technology from GE. “4D” is meant to indicate near-real time capture.
Curses! Go for the link, miss the post! Ah well. Good answer, brossa!