The invention relates to a process for preparing the compounds of general formula I by enantioselective hydrogenation of the compounds of formula II in the presence of special hydrogenation catalysts. A filter includes a first filter function having a first response time for filtering received luminance representative values of a particular individual pixel varying in response to a first motion disturbance in the multiple sequential medical images for use in identifying a substantially minimum luminance value of the particular individual pixel in the multiple sequential medical images. The spatial mode of the multimode laser diode is converted to single-mode by enclosing the Nd3-doped host laser crystal within a laser cavity that has a fundamental mode size large enough to encircle the spatial extent of the beam emanating from the laser diode.