The applicants proposed to develop intra-procedural magnetic resonance imaging (iMRI) methods that enable radio frequency (RF) ablation of breast cancer. The specific aims of this proposal are: 1) to develop critical MR-targeting and MR- thermometry methods, and 2) to validate them in a preliminary cohort of patients with breast cancer. MR-targeting will facilitate accurate deployment of the RF probe by providing simultaneous visualization of the RF probe and enhancing breast cancers. MR-thermometry will be the key to ensuring that MR-guided RF ablation is successful by revealing whether the entire three-dimensional extent of the tumor and an appropriate margin of surrounding breast tissue have been treated completely. In Aim 1, the applicants proposed to develop a 3-point Dixon rapid acquisition and refocused echo (RARE) pulse sequence with optional view angle tilting (VAT) for MR targeting. They proposed to investigate the ability of this pulse sequence to provide fat-nulled images with artifacts from the RF probe, as well as the sensitivity with which the technique detect enhancing breast lesions. They would also develop and optimize an MR-thermometry method for non-invasively mapping temperatures in breast tissues that generates separate water-specific and fat-specific temperature-sensitive images from sequential Dixon RARE images obtained during heating. In Aim 2, the applicants proposed to validate the MR-targeting and MR-thermometry methods in vivo, performing MR-guided RF ablation in 15 women using a treat an resect protocol; all patients will undergo surgical excision 1 to 4 days after RF ablation of their breast cancer. Temperature measurements from interstitial fiber-optic probes at multiple points in the breast will be used to test the accuracy of MR-thermometry.