In 3D ultrasound imaging, a volume is created by either assembling many 2D planes into a 3D dataset or by using a transducer and beamforming approach that directly acquires and forms a volume in real time. This means you either sweep through the area with multiple 2D slices and then reconstruct those slices into a full volume, or you use a matrix array that steers and focuses sound in three dimensions to produce a true volumetric image. Real-time 4D imaging is simply this volume data updated quickly over time to show motion. A single wide beam that covers the entire volume isn’t how this works, because achieving sufficient resolution and accurate focus throughout a full volume with one beam is not feasible in practice. Relying on a stack of 2D slices or 3D beamforming gives you genuine volumetric information rather than just separate images. While synthetic approaches or Doppler data can contribute to certain advanced techniques, the general principle of 3D ultrasound is forming a volume from multiple 2D planes or 3D beamformed data, with optional real-time updates for 4D imaging, rather than just side-by-side 2D slices.