Systems, devices, and associated methods including: emitting ultrasound to the subject and receiving, in response, a current ultrasound view; matching the received image to a pre-existing image, such as a three-dimensional reference image; and, for user assistance, generating, based on the matching, feedback for the guidance. The reference image may be a statistical atlas or it may be derived from patient-specific CT or MR scans. The pre-existing image may instead be a database image corresponding to a state in a state space. The feedback can be an image derived from the reference image; a graphic indication of a plane of the target view; the received view fused to an image derived from the reference image; or the received view and an image derived from the reference image, the derived image appearing concurrently and enhanced to spatially indicated where the received view registers to the reference image.