An apparatus and method is described for determining the location of a object inside a patient. Three dimensional image data of the patient is obtained by an imaging modality such as a C-arm X-ray device. The same or another imaging modality may be used to obtain projection radiographic images, having a coordinate system that can be registered with respect to that of the three dimensional image. The location of one or more features of a medical treatment device such as a needle or a catheter introduced into the imaging filed of view is determined either by an operator or by image analysis, in at least two projection radiographic images, and the position of the feature located in the three-dimensional volume. The projection on a radiographic image of a vector joining a first and a second position is used to assist in guiding the medical treatment device inside of the patient.