  Magnetic resonance imaging (MRI) has great potential to improve prostate
cancer diagnosis. It can spare men with a normal exam from undergoing invasive
biopsy while making biopsies more accurate in men with lesions suspicious for
cancer. Yet, the subtle differences between cancer and confounding conditions,
render the interpretation of MRI challenging. The tissue collected from
patients that undergo pre-surgical MRI and radical prostatectomy provides a
unique opportunity to correlate histopathology images of the entire prostate
with MRI in order to accurately map the extent of prostate cancer onto MRI.
Here, we introduce the RAPSODI (framework for the registration of radiology and
pathology images. RAPSODI relies on a three-step procedure that first
reconstructs in 3D the resected tissue using the serial whole-mount
histopathology slices, then registers corresponding histopathology and MRI
slices, and finally maps the cancer outlines from the histopathology slices
onto MRI. We tested RAPSODI in a phantom study where we simulated various
conditions, e.g., tissue specimen rotation upon mounting on glass slides,
tissue shrinkage during fixation, or imperfect slice-to-slice correspondences
between histology and MRI. Our experiments showed that RAPSODI can reliably
correct for rotations within $\pm15^{\circ}$ and shrinkage up to 10%. We also
evaluated RAPSODI in 89 patients from two institutions that underwent radical
prostatectomy, yielding 543 histopathology slices that were registered to
corresponding T2 weighted MRI slices. We found a Dice coefficient of 0.98$ \pm
$0.01 for the prostate, prostate boundary Hausdorff distance of 1.71$ \pm $0.48
mm, a urethra deviation of 2.91$ \pm $1.25 mm, and a landmark deviation of
2.88$ \pm $0.70 mm between registered histopathology images and MRI. Our robust
framework successfully mapped the extent of disease from histopathology slices
onto MRI.
