Technical note: fiducial markers for correlation of whole-specimen histopathology with MR imaging at 7 tesla.

PURPOSE There is increasing interest in the registration of 3-D histopathology with 3-D in vivo imaging, for example, to validate tumor boundary delineation for targeted radiation cancer therapy. However, accurate correlation is compromised by tissue distortion induced by histopathological processing. Reference landmarks that are visible in both data sets are required. In this study, two iridescent acrylic paints, "Bronze" (containing iron oxide coated mica particles) and "Stainless Steel" (containing iron, chromium, and nickel), were evaluated for creating MRI-visible and histology-visible fiducial markers at 7 T, where resolution is more similar to histology, but artifacts are accentuated. Furthermore, a straight-line paint-track fiducial method was developed to assist in registration and 3-D histopathology reconstruction. METHODS First, the paints were injected into ex vivo porcine tissue samples, which were MR imaged prefixation and postfixation, and subsequently prepared for hematoxylin and eosin staining to verify stability through histopathological processing. Second, the severity of marker susceptibility artifacts produced was compared while using spin-echo and gradient-echo MRI pulse sequences. Finally, multiple paint tracks were injected prefixation through an ex vivo canine prostate sample to validate the potential for line-based registration between MR images of prefixation and postfixation tissue and whole mount histology slides. RESULTS The Stainless Steel paint produced excessive susceptibility artifacts and image distortion, while the Bronze paint created stable and appropriate markers in MRI and histology. The Bronze paint produced artifacts approximately three times larger in gradient-echo than in spin-echo MR images. Finally, the paint-track fiducials were visible in the prefixation and postfixation MRI and on whole mount histology. CONCLUSIONS The Bronze iridescent acrylic paint is appropriate for fiducial marker creation in MRI at 7 T. The straight-line paint-track fiducials may assist 3-D histopathology reconstruction and can provide important information on the deformation effects of fixation, and hence may improve registration accuracy.