Freehand 3D ultrasound systems acquire sets of B-Mode ultrasound images tagged with position information obtained by a tracking device. For both further processing and clinical use of these ultrasound slice images scattered in space, it is often required to reconstruct them into 3D-rectilinear grid arrays. We propose new efficient methods for this so-called ultrasound spatial compounding using a backward-warping paradigm. They allow to establish 3D-volumes from any scattered freehand ultrasound data with superior quality / speed properties with respect to existing methods. In addition, arbitrary MPR slices can be reconstructed directly from the freehand ultrasound slice set, without the need of an extra volumetric reconstruction step. We qualitatively assess the reconstruction quality and quantitatively compare our compounding method to other algorithms using ultrasound data of the neck and liver. The usefulness of direct MPR reconstruction for multimodal image registration is demonstrated as well.