Discontinuity Preserving Liver MR Registration with 3D Active Contour Motion Segmentation.

OBJECTIVE: The sliding motion of the liver during respiration violates the homogeneous motion smoothness assumption in conventional non-rigid image registration and commonly results in compromised registration accuracy. This paper presents a novel approach, registration with 3D active contour motion segmentation (RAMS), to improve registration accuracy with discontinuity-aware motion regularization. METHODS: A Markov random field-based discrete optimization with dense displacement sampling and self-similarity context metric is used for registration, while a graph cuts-based 3D active contour approach is applied to segment the sliding interface. In the first registration pass, a mask-free L1 regularization on an image-derived minimum spanning tree is performed to allow motion discontinuity. Based on the motion field estimates, a coarse segmentation finds the motion boundaries. Next, based on MR signal intensity, a fine segmentation aligns the motion boundaries with anatomical boundaries. In the second registration pass, smoothness constraints across the segmented sliding interface are removed by masked regularization on a minimum spanning forest and masked interpolation of the motion field. RESULTS: For in vivo breath-hold abdominal MRI data, the motion masks calculated by RAMS are highly consistent with manual segmentations in terms of Dice similarity and bidirectional local distance measure. These automatically obtained masks are shown to substantially improve registration accuracy for both the proposed discrete registration as well as conventional continuous non-rigid algorithms. CONCLUSION/SIGNIFICANCE: The presented results demonstrated the feasibility of automated segmentation of the respiratory sliding motion interface in liver MR images and the effectiveness of using the derived motion masks to preserve motion discontinuity.

Brain Iron Distribution after Multiple Doses of Ultra-small Superparamagnetic Iron Oxide Particles in Rats.

The purpose of this study is to determine the effects of high cumulative doses of ultra-small paramagnetic iron oxide (USPIO) used in neuroimaging studies. We intravenously administered 8 mg/kg of 2 USPIO compounds daily for 4 wk to male Sprague-Dawley rats (Crl:SD). Multiecho gradient-echo MRI, serum iron levels, and histology were performed at the end of dosing and after a 7-d washout period. R2* maps and quantitative susceptibility maps (QSM) were generated from multiecho gradient-echo data. R2* maps and QSM showed iron accumulation in brain ventricles on MR images acquired at the 4- and 5-wk time points. Estimates from QSM data showed ventricular iron concentration was equal to or higher than serum iron concentration. Histologic analysis revealed choroid plexus hemosiderosis and midbrain vacuolation, without iron deposition in brain parenchyma. Serum iron levels increased with administration of both compounds, and a 7-d washout period effectively reduced serum iron levels of one but not both of the compounds. High cumulative doses from multiple, frequent administrations of USPIO can lead to iron deposition in brain ventricles, resulting in persistent signal loss on T2*-weighted images. Techniques such as QSM are helpful in quantifying iron biodistribution in this situation.