Many different methods for registering a body on the basis of different image data sets are already known in the prior art, wherein “registering a body” means that when said body is imaged, the exact spatial position of the body is ascertained. This enables spatial positions to be assigned to image data. In navigation-assisted operations, for example, this assignment enables surgical instruments to be moved precisely into positions which the surgeon has previously defined on the basis of recordings.
In the known registering methods, a distinction is drawn between manual methods and automatic registration methods. In the manual methods, for example, a pointer is used which indicates certain positions on a body which is to be imaged, wherein the position of the pointer is spatially determined. On the other hand, the position at which the pointer is situated is additionally specified manually in an available image data set. In this way, a body which is to be registered can be scanned using the pointer, and step by step, an assignment between spatial positions and image data points is obtained by point-mapping.
In automatic registering methods, a so-called calibrating phantom is for example used. While recording an image data set which is suitable for navigation purposes, a calibrating reference body is simultaneously also recorded, in addition to the body on which an operation is to be performed. The spatial position of this reference body is, however, known and is for example recorded by means of a camera system, wherein so-called marker devices are normally used which are identified by a camera system, wherein such a marker system is situated on the reference body, and another marker system which is simultaneously identified by the camera system is situated in a prior-known position relative to the recording apparatus—in most cases, on the recording apparatus itself. In this way, it is possible to automatically assign image data to spatial coordinates.
Although a number of different registering methods are already known, these registering methods are currently limited to certain types of image data sets. Image data sets on the basis of which registration can be performed currently include for example volumetric image data sets, i.e. three-dimensional data sets such as are for example generated by means of computer tomographs and magnetic resonance tomographs. Automatic registering methods for certain two-dimensional image data sets also exist, in which a conical recording ray is used. This is for example the case with recordings by means of C-arm-type x-ray apparatuses and with ultrasound recordings.
Fluoroscopic x-ray recordings in which the spatial position of the recorded body is known are also currently used to determine the spatial positions of points in three-dimensional CT recordings (so-called CT-fluoroscopic matching).
By contrast, a registration method which uses images generated using fan-shaped imaging rays is not known from the prior art, wherein a two-dimensional image data set is assembled from a number of fan-shaped imaging rays which are spatially offset relative to each other (for example shifted in parallel).
Fan-shaped imaging rays are characterized in that the imaging ray is broadly fanned in a certain preferred direction (the fanning direction), whereas in a direction orthogonal to the fanning direction, the imaging ray has extremely low flaring. The ratio approximately corresponds to 1 m (the detector-source distance) to 1 mm (the approximate thickness of a series of detectors in the Z direction).
A fan-shaped imaging ray is thus a specifically directed ray which is composed of a number of individual rays and lies substantially in one plane. The geometric properties of the fanned ray can thus be characterized in a good approximation by an angle in the plane, which describes the fanning of the fanned ray. Such fan-shaped imaging rays are for example generated prior to an actual CT recording using a computer tomograph. These so-called topograms or scout views are normally used to define the scanning region of the main recording. While in the actual CT recording, the radiation source is rotated around the body to be x-rayed and individual focused rays with no appreciable flaring are emitted, in two-dimensional CT-scan data, the radiation source is spatially fixed relative to the body, and a fan-shaped imaging ray is emitted. The body to be x-rayed is then passed under the radiation source and a number of fan-shaped recordings are taken, from which a two-dimensional image data set is then assembled. It is of course also possible to move the radiation source rather than the body. What is critical is the shifting of the body and the radiation source relative to each other.
It is the object of the present invention to provide an improved method for automatically registering a body on the basis of two-dimensional image data which is suitable for use by medical navigation systems, and a system for automatically registering a body on the basis of two-dimensional image data, for use in medical navigation systems.