On-line correction and visualization of motion during MRI-controlled hyperthermia.

Displacement of tissue during MRI-controlled hyperthermia therapy can cause significant problems. Errors in calculated temperature may result from motion-related image artifacts and inter-image object displacement, leading to incorrect spatial temperature reference. Here, cyclic navigator echoes were incorporated in rapid gradient-echo MRI sequences, used for temperature mapping based on the proton resonance frequency. On-line evaluation of navigator information was used in three ways. First, motion artifacts were minimized in echo-shifted (TE > TR) gradient-echo images using the phase information of the navigator echo. Second, navigator profiles were matched for a quantitative evaluation of displacement. Together with a novel processing method, this information was employed to correct the reference temperature maps, thereby avoiding persistence of motion-related temperature errors throughout the hyperthermic period. Third, on-line visualization of displacement, together with temperature maps and thermal dose images, was developed, allowing physician intervention at all times. Examples are given of on-line corrections during hyperthermia procedures with focused ultrasound and radiofrequency heat sources. Magn Reson Med 45:128-137, 2001.