1. A method of ultrasound imaging, comprising: placing a handheld 2D ultrasound scanner in contact with skin of a patient; obtaining a plurality of 2D ultrasound scans of a patient's heart with the handheld 2D ultrasound scanner; filling a plurality of voxels of a 3D array with the plurality of 2D ultrasound scans; applying an image processing technique to classify the voxels as representing either a tubular blood-filled tissue structure or a non-tubular blood-filled tissue structure; and rejecting the voxels representing the non-tubular blood-filled structure.