A localization mechanism, or fixture, is used in conjunction with a breast coil for breast compression and for guiding a core biopsy instrument during prone stereotactic biopsy procedures in both open and closed Magnetic Resonance Imaging (MRI) machines. The localization fixture includes a fiducial marker and three-dimensional Cartesian positionable guide for supporting and orienting an MRI-compatible biopsy instrument with detachable probe/thumb wheel probe to the biopsy site of suspicious tissues or lesions.