1. A method of registering a needle in a patient target volume in an ultrasound imaging system, comprising: capturing a first ultrasound image of the patient target volume using an ultrasound probe prior to insertion of the needle into said patient target volume; capturing a second ultrasound image of a sub-sector of said patient target volume using the ultrasound probe after insertion of the needle into said patient target volume, said sub-sector corresponding generally to a predicted trajectory of the needle within said patient target volume; computing the actual trajectory of said needle in said patient target volume using a computing device, based on differences detected between said first and second ultrasound images; and with the actual needle trajectory computed, computing a needle tip location and an entry location of the needle into said patient target volume, wherein said actual needle trajectory computing comprises: generating a difference map from said first and second examining said difference map from said first and second ultrasound images; examining said difference map from said first and second ultrasound images; fitting a line to the voxels representing the needle; and using the equation of the line to represent said actual needle trajectory.