CrCest Study in Primary Mitochondrial Disease

This research study visit will occur on the same day as the participants clinically scheduled MRI. The clinically indicated and study-specific scanning time will not exceed 120 minutes. During the study visits: * A study team member will review inclusion and exclusion criteria with the participant * A study team member will review medical history review with the participant * A safety assessment will occur prior to study procedures * Study MRI scan * During which the participant will use an MRI-safe machine to stimulate exercise, similar to pressing a gas pedal If eligible participants return to CHOP for future clinical imaging, we will also request a study-specific MRI assessment.