A combined magnetic resonance imager (MRI) and positron emission tomography (PET) imager and a method of performing combined MRI-PET imaging of an object is disclosed herein. The combined MRI-PET imager includes an MRI bore configured to perform MR imaging of the object. The MRI bore is sized so as to provide clearance between the MRI bore and the object within the MRI bore. The dedicated MRI-PET imager further includes a PET detector system is disposed outside the MRI bore to detect PET emissions from the object. The PET detector system includes at least one detector element retractably arranged exterior to the MRI bore. During the PET acquisition, the PET detector elements contract to a size so as to provide optimal clearance between the PET detectors and the object. During MRI acquisition, the PET detectors retract to allow the object to traverse into the MRI field of view
