An approach of hierarchically implementing the spatial prior information in trans-rectal optical
tomography is introduced. Trans-rectal optical imaging of the prostate deals with photon propagation
through the rectum wall, the peri-prostate tissue and the prostate. Reconstructing a lesion in the prostate
is challenging due to the structural complexity as well as the optical heterogeneity. Incorporating spatial
"hard" a priori information available from complementary imaging modalities such as trans-rectal
ultrasound could in principle improve the accuracy of trans-rectal optical tomography reconstruction.
However, the reconstruction is potentially subject to the local-minimum sensitivity if the values of all
regional optical properties are to be initialized simultaneously. We propose a hierarchical spatial prior
approach for trans-rectal optical tomography reconstruction. Instead of assigning the initial values to all
sub-regions at once, a region is initially assumed homogenous, and the reconstructed optical properties
are used as the initial guess for the region as a background when a sub-region is included in the next step.
This approach translates to a 3-step iteration routine whereby the first step reconstructs the entire imaging
volume as a single region, the second step uses these results as the initial guess of peri-prostate tissue to
reconstruct the prostate and the rectum wall, and the third step assigns the updated results as the initial
values of 3 existing regions to reconstruct a lesion inside the prostate. This approach, validated by
simulation and applied to experimental measurements, is more reliable in global convergence, robust in
imaging of single or multiple targets, and accurate for the recovering of optical properties.