1. A method for detecting the position and orientation of a tip of an elongated needle-like medical device inserted into tissue of a patient using a scanning apparatus, said method comprising the steps of: obtaining a plurality of image slices through the tissue having therein the device during a first scan of the tissue using the scanning apparatus, each image slice having a known orientation with respect to a reference co-ordinate system of the scanning apparatus, the plurality of image slices forming a first image volume; determining a position and angular orientation of the device relative to coordinates of the first image volume in each slice obtained from the first scan; determining an appropriate imaging plane for a subsequent scan by referencing the position and angular orientation of the device in each said slice to the reference coordinate system of the scanning apparatus, wherein a longitudinal axis of the device is aligned in an imaging plane of known orientation with respect to the reference coordinate system; obtaining a plurality of image slices through the tissue having therein the device during a subsequent scan of the tissue with the apparatus with said imaging planes aligned with said longitudinal axis of said device, the plurality of image slices forming a subsequent image volume; determining the orientation and position of the device relative to the coordinates of the subsequent image volume from the image slices of the subsequent scan; identifying the tip of the device in a final slice from pixel intensities sampled along the longitudinal axis of the device; wherein determining a position and angular orientation of the device in a slice comprises: calculating a feature image from a normalized cross correlation of the slice with a plurality of device templates, each template having a different angular orientation; thresholding the feature image to obtain a binary image of the device; identifying connected regions in the binary image as candidate device regions; calculating a score for each candidate device region from an area of each region and a ratio of its largest and smallest eigenvalues, and selecting a candidate device region with a highest score as representing the device; and estimating the position and orientation of the device from a center of mass of the selected candidate device region and from the orientation of a largest eigenvector of the candidate device region.