1.  Field of the Invention
The present invention concerns a focus detector arrangement of an x-ray apparatus for generation of projection or tomographic phase contrast exposures of an examination subject: of the type having a radiation source arranged on a first side of the examination subject, that generates a beam of coherent rays with grid-like origin, a phase grid arranged in the beam path on the opposite second side of the examination subject that generates an interference pattern of the x-ray radiation in a predetermined energy range of the x-ray radiation, and an analysis detector system that locally detects at least one interference pattern generated by the phase grid relative to its phase shift.  The invention also concerns a method for generation of projection or tomographical x-ray phase contrast exposures with such a focus detector arrangement. 
2.  Description of the Prior Art and Related Subject Matter
Focus detector arrangements for generation of projection or tomographic phase contrast exposures of an examination subject or of the type and such methods are generally known.  EP 1 447 046 A1 and German patent applications 10 2006 017 290.6, 10 2006 015 358.8, 10 2006 017 219.4, 10 2006 015 356.1 and 10 2006 015 355.3 are examples. 
In principle two effects that occur upon passage of the radiation through matter, namely an absorption and a phase shift of the radiation passing through an examination subject, can be considered for imaging with ionizing radiation (in particular with x-rays).  It is also known that the effect of the phase shifts upon passage of a beam through an examination subject reacts significantly more strongly than the absorption effects to smaller differences in the composition of the penetrated matter. 
The phase shift caused by the subject must be evaluated for such a phase contrast radiography or phase contrast tomography.  Both projection images of the phase shift or even a number of projection images of tomographical depictions of the phase shift that is caused by a volume element can be calculated, analogous to x-ray radiography and x-ray tomography. 
Such phase shifts for generation of projection or tomographic exposures can be measured by the use of interferometric grids.  The previously cited documents are likewise referenced with regard to these interferometric methods.  In these methods an examination subject is irradiated by a coherent x-ray beam that is subsequently directed through a grid with a period adapted to the wavelengths of the radiation, so an interference pattern arises that is dependent on the occurred radiation shift.  This interference pattern is measured by a subsequent analysis-detector arrangement so that the phase shift can be determined. 
The method described above requires a sufficient degree of spatial coherence in the employed radiation.  This can be achieved by an extremely small focus, for example, but the achievable dose rating is barely usable for medical applications due to the long required exposure time.  Another possibility is the use of synchrotron radiation.  Such applications are much too complicated in practice.  Finally, in the cited prior art it is also proposed to use a focus with a conventional large focal spot as is known in the field of computed tomography and to arrange what is known as a source grid between the focus and the examination subject.  The slits of this source grid generate a field of individual coherent rays of a specific energy having dose rating that is sufficient to generate the known interference pattern, with the use of a phase grid arranged after the subject in the beam direction. 
In this manner it is possible to use radiation sources that have dimensions that correspond to normal x-ray tubes in CT systems and transmission x-ray systems, such that easily differentiated soft tissue exposures can now also be made with the use of x-ray apparatuses in the field of general medical diagnostics, for example. 
One problem given this type of focus detector combination is that, given the use of such source grids, a relatively high dose proportion nevertheless occurs that acts as quasi-coherent radiation, and therefore produces a high background noise and also leads to unnecessary radiation exposure of the examined patient.