Robust estimation of electron density from anatomical MR imaging of the brain using a unifying multi-atlas approach

Publication date: Available online 14 December 2016
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): Shangjie Ren, Wendy Hara, Lei Wang, Mark Buyyounouski, Quynh-Thu Le, Lei Xing, Ruijiang Li
Purpose/Objective(s)MRI has important advantages over CT as a primary image modality for radiation treatment planning. However, MRI lacks electron density information required for accurate radiation dose calculation. The purpose of this work is to develop a reliable method to estimate electron density based on anatomical MR imaging of the brain.Materials/MethodsWe proposed a unifying multi-atlas approach for electron density estimation based on standard T1 and T2-weighted MRI. First, a composite atlas was constructed through a voxel-wise matching process using multiple atlases, with the goal of mitigating effects of inherent anatomical variations between patients. Next, we computed for each voxel two kinds of conditional probabilities: (1) electron density given its image intensity on T1 and T2-weighted MR images, and (2) electron density given its spatial location in a reference anatomy, obtained by deformable image registration. These were combined into a unifying posterior probability density function using the Bayesian formalism, which provided the optimal estimates for electron density. We evaluated the method on 10 patients using leave-one-patient-out cross validation. ROC analyses for detecting different tissue types were performed.ResultsThe proposed method significantly reduced the errors in electron density estimation, with a mean absolute Hounsfield unit (HU) error of 119, compared with 140 and 144 (p < 0.0001) using conventional T1-weighted intensity and geometry-based approaches. For detection of bony anatomy, the proposed method achieved an 89% AUC, 86% sensitivity, 88% specificity, and 90% accuracy, which improved upon intensity and geometry-based approaches (AUC: 79% and 80%, respectively).ConclusionThe proposed multi-atlas approach provides robust electron density estimation and bone detection based on anatomical MRI. If validated on a larger population, our work could enable the use of MRI as a primary modality for radiation treatment planning.

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Summary: We developed a unifying multi-atlas approach for electron density mapping based on standard-of-care T1 and T2-weighted MRI. The proposed method achieved robust electron density estimation and bone detection in 10 patients. Our work could provide the enabling tools for using MRI as a primary modality for radiation treatment planning.


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