Improved multi B-value diffusion-weighted MRI of the body by simultaneous model estimation and image reconstruction (SMEIR)

Real-time stereo reconstruction of intra-operative scene and registration to pre-operative 3D models for augmenting surgeons' view during RAMIS

PURPOSE

During Minimally Invasive Surgery (MIS) procedures, there exists an ever-growing/apparent need for providing computer generated visual feedback to the surgeon(s), through a visualization device. While multiple solutions have been proposed in the literature, there is limited evidence of such a system performing reliably in practice, and when it does, it is often tailored to a specific operation type. Another important aspect is regarding the usability of such systems, which typically include complicated and time-consuming steps, and often require the assistance of specialized personnel. In this study, we propose an auxiliary visualization system for surgeons, which includes streamlined process to use pre-operative data of the patient, and apply it to two different MIS cases, namely Robot-assisted Partial Nephrectomy (RAPN) and Robot-assisted Partial Lateral Meniscectomy (RaPLM).

METHODS

The visualization and processing pipeline consists of an intra-operative 3D reconstruction of the surgical area, using an optimized version of the Quasi Dense method, aimed to perform with good accuracy while maintaining real-time speed. A set of pre-processing and post-processing techniques further contribute to the result by providing a smoother and more dense point cloud. DynamicFusion is used for the registration of the pre-operative model to the intra-operative scene. Two silicon kidney phantoms and an ex-vivo porcine meniscus are used for evaluation, representing subjects for the examined surgical cases.

RESULTS

Performance is evaluated qualitatively using the two datasets. The pre-operative model of the subject is projected on top of the actual 2D image and also in 3D space. The model is superimposed on top of the actual physical structure it represents, and remains in the correct position throughout the experiments, even when abrupt camera movements are taking place. Finally, when deformation is introduced, the model is deformed as well, resembling the real subject's structure.

CONCLUSIONS

Results demonstrate and validate the use of the presented algorithms for each separate task of the pipeline. A complete methodology to provide surgeon(s) with visual information during surgery is presented. Its operation is evaluated over two different surgical scenarios, paving the way for a single visualization methodology that can adapt and perform robustly for multiple cases, with minimal effort. This article is protected by copyright. All rights reserved.

Spatially-constrained probability distribution model of incoherent motion (SPIM) for abdominal diffusion-weighted MRI

Quantitative diffusion-weighted MR imaging (DW-MRI) of the body enables characterization of the tissue microenvironment by measuring variations in the mobility of water molecules. The diffusion signal decay model parameters are increasingly used to evaluate various diseases of abdominal organs such as the liver and spleen. However, previous signal decay models (i.e., mono-exponential, bi-exponential intra-voxel incoherent motion (IVIM) and stretched exponential models) only provide insight into the average of the distribution of the signal decay rather than explicitly describe the entire range of diffusion scales. In this work, we propose a probability distribution model of incoherent motion that uses a mixture of Gamma distributions to fully characterize the multi-scale nature of diffusion within a voxel. Further, we improve the robustness of the distribution parameter estimates by integrating spatial homogeneity prior into the probability distribution model of incoherent motion (SPIM) and by using the fusion bootstrap solver (FBM) to estimate the model parameters. We evaluated the improvement in quantitative DW-MRI analysis achieved with the SPIM model in terms of accuracy, precision and reproducibility of parameter estimation in both simulated data and in 68 abdominal in-vivo DW-MRIs. Our results show that the SPIM model not only substantially reduced parameter estimation errors by up to 26%; it also significantly improved the robustness of the parameter estimates (paired Student's t-test, p < 0.0001) by reducing the coefficient of variation (CV) of estimated parameters compared to those produced by previous models. In addition, the SPIM model improves the parameter estimates reproducibility for both intra- (up to 47%) and inter-session (up to 30%) estimates compared to those generated by previous models. Thus, the SPIM model has the potential to improve accuracy, precision and robustness of quantitative abdominal DW-MRI analysis for clinical applications.

Multi-shell diffusion signal recovery from sparse measurements

For accurate estimation of the ensemble average diffusion propagator (EAP), traditional multi-shell diffusion imaging (MSDI) approaches require acquisition of diffusion signals for a range of b-values. However, this makes the acquisition time too long for several types of patients, making it difficult to use in a clinical setting. In this work, we propose a new method for the reconstruction of diffusion signals in the entire q-space from highly undersampled sets of MSDI data, thus reducing the scan time significantly. In particular, to sparsely represent the diffusion signal over multiple q-shells, we propose a novel extension to the framework of spherical ridgelets by accurately modeling the monotonically decreasing radial component of the diffusion signal. Further, we enforce the reconstructed signal to have smooth spatial regularity in the brain, by minimizing the total variation (TV) norm. We combine these requirements into a novel cost function and derive an optimal solution using the Alternating Directions Method of Multipliers (ADMM) algorithm. We use a physical phantom data set with known fiber crossing angle of 45° to determine the optimal number of measurements (gradient directions and b-values) needed for accurate signal recovery. We compare our technique with a state-of-the-art sparse reconstruction method (i.e., the SHORE method of Cheng et al. (2010)) in terms of angular error in estimating the crossing angle, incorrect number of peaks detected, normalized mean squared error in signal recovery as well as error in estimating the return-to-origin probability (RTOP). Finally, we also demonstrate the behavior of the proposed technique on human in vivo data sets. Based on these experiments, we conclude that using the proposed algorithm, at least 60 measurements (spread over three b-value shells) are needed for proper recovery of MSDI data in the entire q-space.