Figure 1. 3D ultrasound acquisition protocols. Dedicated freehand probes contain motors that sweep a 2D B-scan over the area of interest: rotational and fan shaped geometries are common. Freehand systems rely on the clinician to guide a conventional 2D probe over the area of interest. The relative positions of the B-scans can be tracked using an add-on spatial locator or, alternatively and less accurately, estimated from the B-scan images themselves.