Managing patients with hydrocephalus and cerebrospinal fluid disorders requires repeated head imaging. In adults, this is typically done with computed tomography (CT) or less commonly magnetic resonance imaging (MRI). However, CT poses cumulative radiation risks and MRI is costly. Transcranial ultrasound is a radiation-free, relatively inexpensive, and optionally point-of-care alternative. The initial use of this modality has involved measuring gross brain ventricle size by manual annotation. In this work, we explore the use of deep learning to automate the segmentation of brain right ventricle from transcranial ultrasound images. We found that the vanilla U-Net architecture encountered difficulties in accurately identifying the right ventricle, which can be attributed to challenges such as limited resolution, artifacts, and noise inherent in ultrasound images. We further explore the use of coordinate convolution to augment the U-Net model, which allows us to take advantage of the established acquisition protocol. This enhancement yielded a statistically significant improvement in performance, as measured by the Dice similarity coefficient. This study presents, for the first time, the potential capabilities of deep learning in automating hydrocephalus assessment from ultrasound imaging.
KEYWORDS: Medical image reconstruction, Cone beam computed tomography, Education and training, Deep learning, Brain, Computed tomography, Error analysis, Data modeling, 3D modeling, Monte Carlo methods
Deep Learning (DL) image synthesis has gained increasing popularity for the reconstruction of CT and cone-beam CT (CBCT) images, especially in combination with physically-principled reconstruction algorithms. However, DL synthesis is challenged by the generalizability of training data and noise in the trained model. Epistemic uncertainty has proven as an efficient way of quantifying erroneous synthesis in the presence of out-of-domain features, but its estimation with Monte Carlo (MC) dropout requires a large number of inference runs, variable as a function of the particular uncertain feature. We propose a single-pass method–the Moment Propagation Model–which approximates the MC dropout by analytically propagating the statistical moments through the network layers, removing the need for multiple inferences and removing errors in estimations from insufficient dropout realizations. The proposed approach jointly computes the change of the expectation and the variance of the input (first two statistical moments) through each network layer, where each moment undergoes a different numerical transformation. The expectation is initialized as the network input; the variance is solely introduced at dropout layers, modeled as a Bernoulli process. The method was evaluated using a 3D Bayesian conditional generative adversarial network (GAN) for synthesis of high-quality head MDCT from low-quality intraoperative CBCT reconstructions. 20 pairs of measured MDCT volumes (120kV, 400 to 550mAs) depicting normal head anatomy, and simulated CBCT volumes (100 to120kV, 32 to 200mAs) were used for training. Scatter, beam-hardening, detector lag and glare were added to the simulated CBCT and were corrected (assuming unknown) prior to reconstruction. Epistemic uncertainty was estimated for 30 heads (outside of the training set) containing simulated brain lesions using the proposed single-pass propagation model, and results were compared to the standard 200-pass dropout approach. Image quality and quantitative accuracy of the estimated uncertainty of lesions and other anatomical sites were further evaluated. The proposed propagation model captured >2HU increase in epistemic uncertainty caused by various hyper- and hypo-density lesions, with <0.31HU error over the brain compared to the reference MC dropout result at 200 inferences and <0.1HU difference to a converged MC dropout estimate at 100 inference passes. These findings indicate a potential 100-fold increase in computational efficiency of neural network uncertainty estimation. The proposed moment propagation model is able to achieve accurate quantification of epistemic uncertainty in a single network pass and is an efficient alternative to conventional MC dropout.
KEYWORDS: Video, Education and training, Deformation, 3D acquisition, Voxels, 3D image processing, Imaging systems, Endoscopy, 3D image reconstruction, Visualization
Purpose: Navigating deep-brain structures in neurosurgery, especially under deformation from CSF egress, remains challenging due to the limitations of current robotic systems relying on rigid registration. This study presents the initial steps towards vision-based navigation leveraging Neural Radiance Fields (NeRF) to enable 3D neuroendoscopic reconstruction on the Robot-Assisted Ventriculoscopy (RAV) platform. Methods: An end-to-end 3D reconstruction and registration method using posed images was developed and integrated with the RAV platform. The hyperparameters for training the dual-branch network were first identified. Further experiments were conducted to evaluate reconstruction accuracy using projected error (PE) while varying the volume density threshold parameter. Results: A 3D volume was reconstructed using a simple linear trajectory for data acquisition with 300 frames and corresponding camera poses. The density volume threshold was varied to obtain an optimal value of 96.55 percentile, with a corresponding PE of 0.65 mm. Conclusions: Initial methods for end-to-end neuroendoscopic video reconstruction were developed in phantom studies. Experiments identified the optimal parameters, yielding a geometrically accurate reconstruction along with fast network convergence runtime of < 30 s. The method is highly promising for future clinical translation in realistic neuroendoscopic scenes. Future work will also develop a direct surface-to-volume registration method for improving reconstruction accuracy and runtime.
Neurosurgical techniques often require accurate targeting of deep-brain structures even in the presence of deformation due intervention and egress of Cerebrospinal Fluid (CSF) during surgical access. Prior work reported Simultaneous Localization and Mapping (SLAM) methods for endoscopic guidance using 3D reconstruction. In this work, methods for correcting the geometric distortion of a neuroendoscope are reported in a form that have been translated intraoperative use in first clinical studies. Furthermore, SLAM methods are evaluated in first clinical studies for real-time 3D endoscopic navigation with near real-time registration in the presence of deep-brain tissue deformation. A custom calibration jig with swivel mounts was designed and manufactured for neuroendoscope calibration in the operating room. The process is potentially suitable to intraoperative use while maintaining sterility of the endoscope, although the current calibration system was used in the Operating Room (OR) immediately following the case for offline analysis. A six by seven checkerboard pattern was used to obtain corner locations for calibration, and the method was evaluated in terms of Reprojection Error (RPE). Neuroendoscopic video was acquired under an IRB-approved clinical study, demonstrating rich vascular features and other structures on the interior walls of the lateral ventricles for 3D point-cloud reconstruction. Geometric accuracy was evaluated in terms of Projected Error (PE) on a ground truth surface defined from MR or cone-beam CT (CBCT) images. Intraoperative neuroendoscope calibration was achieved with sub-pixel [0.61 ± 0.20 px] error. The calibration yielded a focal length of 816.42 px and 822.71 px in X and Y directions respectively, along with radial distortion coefficients of -0.432 (first order term [𝑘1]) and 0.158 (second order term [𝑘2]). The 3D reconstruction was performed successfully with a PE of 0.23 ± 0.15 mm compared to the ground truth surface. The system for neuroendoscopic guidance based on SLAM 3D point-cloud reconstruction provided a promising platform for the development of 3D neuroendoscopy. The studies reported in this work presented an important means of neuroendoscope calibration in the OR and provided preliminary evidence for accurate 3D video reconstruction in first clinical studies. Future work aims to further extend the clinical evaluation and improve reconstruction accuracy using ventricular shape priors.
Normal Pressure Hydrocephalus (NPH) is a brain disorder associated with ventriculomegaly. Accurate segmentation of the ventricle system into its sub-compartments from magnetic resonance images (MRIs) could help evaluate NPH patients for surgical intervention. In this paper, we modify a 3D U-net utilizing probability maps to perform accurate ventricle parcellation, even with grossly enlarged ventricles and post-surgery shunt artifacts, from MRIs. Our method achieves a mean dice similarity coefficient (DSC) on whole ventricles for healthy controls of 0.864 ± 0.047 and 0.961 ± 0.024 for NPH patients. Furthermore, with the benefit of probability maps, the proposed method provides superior performance on MRI with grossly enlarged ventricles (mean DSC value of 0.965 ± 0.027) or post-surgery shunt artifacts (mean DSC value of 0.964 ± 0.031). Results indicate that our method provides a high robust parcellation tool on the ventricular systems which is comparable to other state-of-the-art methods.
Purpose: Neuro-endoscopic surgery requires accurate targeting of deep-brain structures in the presence of deep-brain deformations (up to 10 mm). We report a deep learning-based method to solve deformable MR-to-CBCT registration using a joint synthesis and registration (JSR) network. Method: The JSR network first encodes the MR and CBCT images into latent variables via MR and CBCT encoders, which are then decoded by two branches: image synthesis branches for MR-CT and CBCT-CT synthesis; and a registration branch for intra-modality registration in an intermediate (synthetic) CT domain. The two branches are jointly optimized, encouraging the encoders to extract features pertinent to both synthesis and registration. The algorithm was trained and tested on a dataset of 420 paired volumes presenting a wide range of simulated deformations. The JSR method was trained in a semi-supervised manner and evaluated in comparison to an alternative, state-of-the-art, inter-modality registration method (VoxelMorph). Results: The JSR method achieved Dice of 0.67 ± 0.11, surface distance error (SD) of 0.47 ± 0.26 mm, and target registration error (TRE) of 2.23 ± 0.80 mm in a simulation study – each superior to the alternative methods considered in this work. Moreover, JSR maintained diffeomorphism and exhibited a fast runtime of 2.55 ± 0.03 s. Conclusion: The JSR algorithm demonstrates accurate, near real-time deformable registration of preoperative MRI to intraoperative CBCT and is potentially suitable to intraoperative guidance of intracranial neurosurgery.
Purpose: Recent neurosurgical techniques require accurate targeting of deep-brain structures even in the presence of deformation due to egress of cerebrospinal fluid (CSF) during surgical access. Prior work reported Structure-from-Motion (SfM) based methods for endoscopic guidance using 3D reconstruction. We are developing feature detection and description methods for a real-time 3D endoscopic navigation system using simultaneous localization and mapping (SLAM) to for accurate and near real-time registration. Methods: Feature detectors and descriptors were evaluated in SLAM reconstruction in anthropomorphic phantom studies emulating neuroendoscopy. The experimental system utilized a mobile UR3e robot (Universal Robots, Denmark) and ventriculoscope (Karl Storz, Tuttlingen, Germany) affixed to the end effector as a repeatable ventriculoscopy platform. Experiments were conducted to quantify optimal feature detection parameters in scale-space. Neuroendoscopic images acquired in traversal of the lateral and third ventricles provided a rich feature space of vessels and other structures on ventricular walls supporting feature detection and 3D point-cloud reconstruction. Performance was evaluated in terms of the mean number of features detected per frame and the algorithm runtime. Results: Parameter search in scale-space for feature detection demonstrated the dependence on the mean number of features per image and the points of diminishing return in parameter selection (e.g., the number of octaves and scale levels) and tradeoffs in runtime. Nominal parameters were identified as 3 octaves and 9 scale levels, with a mean number of features detected as 492 and 806 respectively. Conclusions: The system for neuroendoscopic guidance based on SLAM 3D point-cloud reconstruction provided a promising platform for the development of robot-assisted endoscopic neurosurgery. The studies reported in this work provided an essential basis for rigorous selection of parameters for feature detection. Future work aims to further develop the SLAM framework, assess the geometric accuracy of reconstruction, and translate methods to clinical studies.
Purpose: Emerging deep-brain stimulation (DBS) procedures require a high degree of accuracy in placement of neuroelectrodes, even in the presence of deformation due to cerebrospinal fluid (CSF) egress during surgical access. We are developing ventriculoscope and hand-eye calibration methods for a robot-assisted guidance system to augment accurate electrode placement through transventricular approach. Methods: The ventriculoscope camera was modelled and calibrated for lens distortion using three different checkerboards, followed by evaluation on a separate board. The experimental system employed a benchtop UR3e robot (Universal Robots, Denmark) and ventriculoscope (Karl Storz, Tuttlingen, Germany) affixed to the end effector – referred to as the robotassisted ventriculoscopy (RAV) platform. Performance was evaluated in terms of three error metrics (RPE, FCE and PDE). Experiments were conducted to estimate the camera frame of reference using hand-eye calibration methods, and evaluated using a ChAruco board, using five different solvers and residual calibration error as the metric. Results: Camera calibration demonstrated subpixel (0.81 ± 0.11) px reprojection error and projection distance error (PDE) <0.5 mm. The error was observed to converge for any checkerboard used given a sufficient number of calibration images. The hand-eye calibration exhibited sub-mm residual error (0.26 ± 0.18) mm insensitive to the solver used. Conclusions: The RAV system demonstrates sub-mm ventriculoscope camera calibration error and robot-to-camera handeye residual error, providing a valuable platform for the development of advanced 3D guidance systems for emerging DBS approaches. Future work aims to develop structure-from-motion (SfM) methods to reconstruct a 3D optical scene using endoscopic video frames and further testing using rigid and deformable anatomical phantoms as well as cadaver studies.
Purpose. Deep brain stimulation is a neurosurgical procedure used in treatment of a growing spectrum of movement disorders. Inaccuracies in electrode placement, however, can result in poor symptom control or adverse effects and confound variability in clinical outcomes. A deformable 3D-2D registration method is presented for high-precision 3D guidance of neuroelectrodes. Methods. The approach employs a model-based, deformable algorithm for 3D-2D image registration. Variations in lead design are captured in a parametric 3D model based on a B-spline curve. The registration is solved through iterative optimization of 16 degrees-of-freedom that maximize image similarity between the 2 acquired radiographs and simulated forward projections of the neuroelectrode model. The approach was evaluated in phantom models with respect to pertinent imaging parameters, including view selection and imaging dose. Results. The results demonstrate an accuracy of (0.2 ± 0.2) mm in 3D localization of individual electrodes. The solution was observed to be robust to changes in pertinent imaging parameters, which demonstrate accurate localization with ≥20° view separation and at 1/10th the dose of a standard fluoroscopy frame. Conclusions. The presented approach provides the means for guiding neuroelectrode placement from 2 low-dose radiographic images in a manner that accommodates potential deformations at the target anatomical site. Future work will focus on improving runtime though learning-based initialization, application in reducing reconstruction metal artifacts for 3D verification of placement, and extensive evaluation in clinical data from an IRB study underway.
Purpose: Deep-brain stimulation via neuro-endoscopic surgery is a challenging procedure that requires accurate targeting of deep-brain structures that can undergo deformations (up to 10 mm). Conventional deformable registration methods have the potential to resolve such geometric error between preoperative MR and intraoperative CT but at the expense of long computation time. New advances in deep learning methods offer benefits to inter-modality image registration accuracy and runtime using novel similarity metrics and network architectures. Method: An unsupervised deformable registration network is reported that first generates a synthetic CT from MR using CycleGAN and then registers the synthetic CT to the intraoperative CT using an inverse-consistent registration network. Diffeomorphism of the registration is maintained using deformation exponentiation “squaring and scaling” layers. The method was trained and tested on a dataset of CT and T1-weighted MR images with randomly simulated deformations that mimic deep-brain deformation during surgery. The method was compared to a baseline method using inter-modality deep learning registration, VoxelMorph. Results: The methods were tested on 10 pairs of CT/MR images from 5 subjects. The proposed method achieved a Dice score of 0.84±0.04 for the lateral ventricles, 0.72±0.09 for the 3rd ventricle, and 0.63±0.10 for the 4th ventricle, with target registration error (TRE) of 0.95±0.54 mm. The proposed method showed statistically significant improvement in both Dice score and TRE in comparison to inter-modality VoxelMorph, while maintaining a fast runtime of less than 3 seconds for a typical MR-CT pair of volume images. Conclusion: The proposed unsupervised image synthesis and registration network demonstrates the capability for accurate volumetric deformable MR-CT registration with near real-time performance. The method will be further developed for application in intraoperative CT (or cone-beam CT) guided neurosurgery.
Purpose: Intraoperative cone-beam CT (CBCT) plays an important role in neurosurgical guidance but is conventionally limited to high-contrast bone visualization. This work reports a high-fidelity artifacts correction pipeline to advance image quality beyond conventional limits and achieve soft-tissue contrast resolution even in the presence of multiple metal objects – specifically, a stereotactic head frame. Methods: A new metal artifact reduction (MAR) method was developed based on a convolutional neural network (CNN) that simultaneously estimates metal-induced bias and metal path length in the projection domain. To improve generalizability of the network, a physics-based method was developed to generate highly accurate simulated, metalcontaminated projection training data. The MAR method was integrated with previously proposed artifacts correction methods (lag, glare, scatter, and beam-hardening) to form a high-fidelity artifacts correction pipeline. The proposed methods were tested using an intraoperative CBCT system (O-arm, Medtronic) emulating a realistic setup in stereotactic neurosurgery, including nominal (20 cm) and extended (40 cm) field of view (FOV) protocols. Results: The physics-based data generation method provided accurate simulation of metal in projection data, including scatter, polyenergetic, quantum noise, and electronic noise effects. The artifacts correction pipeline was able to accommodate both 20 cm and 40 cm FOV protocols and demonstrated ~80% improvement in image uniformity and ~20% increase in contrast-to-noise ratio (CNR). Fully corrected images in the smaller FOV mode exhibited ~32% increase in CNR compared to the 40 cm FOV mode, showing the method’s ability to handle truncated metal objects outside the FOV. Conclusion: The image quality of intraoperative CBCT was greatly improved with the proposed artifacts correction pipeline, with clear improvement in soft-tissue contrast resolution (e.g., cerebral ventricles) even in the presence of a complex metal stereotactic frame. Such capability gives clearer visualization of structures of interest for intracranial neurosurgery, and it provides an important basis for future work aiming to deformably register preoperative MRI to intraoperative CBCT. Ongoing work includes clinical studies now underway.
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