Real-time MR (magnetic resonance) images in two perpendicular planes, both of which included the path of the needle, were utilized for MR-guided microwave ablation of liver tumors. The two image planes were automatically and alternately switched by new MR scanner control software installed on an external PC. This technique is possible only with MRI (magnetic resonance imaging) units with multiplanar and multisection capabilities. Reformatted images in the corresponding two planes were also constructed from preoperative three-dimensional volume data. These four images (two real-time and two reformatted) were continuously visible to the surgeons. These images enabled the needle position in the three-dimensional space to be accurately and clearly recognized, in contrast to the difficulty encountered with two-dimensional MR images in a single image plane. This technique was also applied to MR temperature mapping during microwave ablation, as it allowed monitoring of the spread of the heat in a three-dimensional space. This type of computer-integrated image navigation was demonstrated to be feasible for MR-guided microwave ablation of liver tumors.