The invention provides for a medical apparatus (100) with a magnetic resonance coil assembly (102, 102') comprising a magnetic resonance antenna with a first antenna portion (108, 108') and a second antenna portion (110, 110') for receiving magnetic resonance location data (1246) from a fiducial marker (118, 300, 400, 500). The magnetic resonance coil assembly further comprises a clamp with a first clamping portion (104, 104') and a second clamping portion (106, 106') operable for being moved between an open and a closed configuration. The first clamping portion comprises the first antenna portion. The second clamping portion comprises the second antenna portion. The first and second clamping portions are operable for securing the fiducial marker within a signal reception volume (111) in the closed configuration. When in the open position, the first and second clamping portions enable the fiducial marker being moved into or out of the signal reception volume.