Fat grafting is an efficient method to correct large volumetric defects after mastectomy. There is an ongoing debate regarding the best method of processing the harvested fat before fat grafting. This study aimed to introduce a new MRI model and to compare two fat processing techniques measuring the gain in soft tissue thickness after fat grafting to the chest wall. Fifty-one mastectomy patients (one double sided), who required delayed implant reconstruction, with poor skin conditions were proposed fat grafting prior to implant reconstruction. At the time of fat grafting, patients were randomly assigned to centrifugation or sedimentation of the aspirated fat. The trial was undertaken in a single-center private practice setting. The gain in soft tissue thickness of the chest wall was measured using an MRI model, with 12 predefined points for measurement. Two MRIs were performed, one prior to fat grafting and one 8 weeks thereafter. The radiologist was blinded to the fat graft processing method used. Seven cases were excluded because they did not complete their second MRI. The analyses were thus based on 44 patients (one double sided). Centrifugation was performed in 21 cases and sedimentation in 24 cases. The mean gain in soft tissue thickness was +7.0mm in the centrifugation group and +8.8mm in the sedimentation group (p=.268). The mean operative time was 88min in the centrifugation group and 78min in the sedimentation group (p=.11). There were no adverse events for any of the patients. We developed a simple and reproducible MRI model to objectively measure and evaluate different fat processing techniques prior to fat grafting. At a median time of 8 weeks after one session of fat grafting, there was no benefit of centrifugation over sedimentation.