Current MRI technologies require subjects to remain largely motionless for achieving high quality magnetic 
resonance (MR) scans, typically for 5-10 minutes at a time. However, 
lying absolutely still inside the tight MR imager (MRI) tunnel is a difficult task, especially for children, very sick patients, or the 
mentally ill. Even motion 
ranging less than 1 mm or 1 degree can corrupt a scan. This invention involves a 
system that adaptively compensates for subject motion in real-time. An 
object orientation marker, preferably a retro-grate reflector (RGR), is placed on a patients' head or other body organ of interest during MRI. The RGR makes it possible to measure the 
six degrees of freedom (x, y, and z-translations, and 
pitch, 
yaw, and roll), or “
pose”, required to track the organ of interest. A camera-based 
tracking system observes the marker and continuously extracts its 
pose. The 
pose from the 
tracking system is sent to the MR 
scanner via an interface, allowing for continuous correction of scan planes and position in real-time. The RGR-based 
motion correction system has significant advantages over other approaches, including faster tracking speed, better stability, automatic calibration, lack of interference with the MR measurement process, improved ease of use, and long-term stability. RGR-based motion tracking can also be used to correct for motion from awake animals, or in conjunction with other 
in vivo imaging techniques, such as computer 
tomography, 
positron emission 
tomography (PET), etc.