Assessment of fiducial markers to enable the co-registration of photographs and MRI data.

PURPOSE To investigate the visualisation of novel external fiducial skin markers in photography and MRI. To co-register photographs and MR images, and additionally assess the spatial accuracy of these co-registrations with the view of future application in the investigation of forensically relevant soft tissue lesions. METHODS AND MATERIALS Strand-shaped fiducial markers were secured externally over hematomas on the thigh of 10 volunteers. The region of interest was photographed and examined using MRI at 3T in oblique and transversal orientations and the visibility of the markers assessed. Markers provided 'control points' in both sets of images, enabling the computation of an affine transform to register oblique MR images to photographs. The fiducial registration error was evaluated by calculating the root-mean-square error of nine corresponding evaluation points visible in both modalities. RESULTS Fiducial markers were clearly visualised in both photography and MRI. The co-registration of photographs and oblique MR images was achieved for all participants. The overall root-mean-square error for registrations was 1.18mm (TIRM) and 1.46mm (TSE2D with SPAIR fat-suppression). CONCLUSIONS The proposed approach led to the successful visualisation of non-invasive fiducial markers using photography and MRI (TIRM and TSE2D (SPAIR) sequences). This visualisation, combined with an affine transformation process provided a simple, cost-effective way to accurately co-register photographs and MR images of subcutaneous hematomas located on the thigh. Further investigation of the novel markers and the proposed co-visualisation approach holds potential to improve not only the forensic documentation of soft tissue lesions, but to also improve certain clinical applications, including the area of dermatology.