In this letter, we present the design and development of a new robotic system for MRI-guided breast biopsy. The robot's structure is nonmagnetic, therefore, it can safely operate from within the scanner's magnetic bore; it has a compact Cartesian mechanism that allows it perform needle insertion tasks from both frontal and lateral directions inside an open bore scanner. A combination of piezoelectric and pneumatic actuators drive the motion of the proposed three degrees-of-freedom robot. To control the needle's insertion, we present an adaptive position regulator that uses the feedback from multiple sensors and that is robust to external disturbances. A detailed experimental study is presented to validate the performance and magnetic properties of the new mechanical prototype.