Fischer Imaging, a maker of digital mammography and medical imaging systems, announced that Gerald D. Knudson has stepped down as CEO, for family reasons. Harris Ravine, the lead director of Fischer's board, has been appointed CEO. Knudson will remain on the board.   . . .    Medical Coaches Inc., of Oneonta, N.Y., and CTI Molecular Imaging Inc. announced they have completed the design, manufacture and sale of a mobile PET/CT unit to PET Scans of America, of Fort Lee, N.J.   . . .    A panel of researchers from Sweden and the US will meet Wednesday at the Swedish Consulate General in Santa Monica, Calif., to discuss nanotechnology and potential commercial applications. The seminar, "Less is More: Simulating Biosystems through Nanotechnology," is being organized by Consul General Andreas Ekman in cooperation with the State of California as part of the series, "Sweden and California: Partners in Biotechnology." Panelists will include Lars Montelius, of University of Lund, Sweden; Michael Natan, PhD, CEO of Nanoplex Technologies Inc., of Mountain View, Calif.; and Carlo Montemagno, PhD, of UCLA. Alexander Suh, managing director of California Technology Ventures in Pasadena, will be moderator.