1. A method of registering and merging medical image data to guide an interventional instrument through a body channel of a patient, comprising: recording an initial image data set of an examination area of the patient by an imaging system, wherein the initial image data set is a three-dimensional image data set; using at least one external sensor in a location detecting system to determine a first spatial position of an external body part of the patient related to the examination area of the patient, wherein the first spatial position corresponds to the position of the examination area when the initial image data set is recorded by the imaging system; recording a plurality of serial examination image data sets of the examination area at specified time intervals by the imaging system, wherein the serial examination image data sets are two-dimensional image data sets to monitor in real-time the interventional instrument being guided through the body channel of the patient; determining with the location detecting system a current spatial position of the body part of the patient; relating the current spatial position to a reference coordinate frame where coordinates are assigned by the location detecting system to the imaging system while recording in real-time with the imaging system a current serial examination image data set of the examination area, wherein the frame of reference assigned by the location detecting system to the imaging system is connected to the imaging system; determining in a registration unit a spatial position difference between the first and the current spatial position, wherein said spatial position difference is determined in the frame of reference assigned by the location detecting system to the imaging system; adapting the initial three-dimensional image data set using the determined difference such that the adapted initial three-dimensional image data set corresponds to the current spatial position without use of artificial markers or anatomical markers, wherein adapting the initial image three-dimensional data set using the determined spatial position difference includes a translation relative to the current serial examination two-dimensional image data set, said translation having three degrees of translation freedom, the adapting of the initial image data set further includes a rotation having three degrees of rotation freedom, thereby said adapting providing a total of six degrees of freedom; and merging and jointly displaying in an image display unit the adapted initial three dimensional image data set superimposed with respect to the current serial examination two dimensional image data set of the examination area recorded in real time to obtain a three-dimensional road map conducive to guide in real-time the interventional instrument through the body channel of the patient.