Developing and evaluating virtual cardiotomy for preoperative planning in congenital heart disease.

Careful preoperative planning is of outmost importance - in particular when considering complex corrective surgery on congenitally malformed hearts. As an aid to such decision-making we describe a system for virtual reconstruction of patient-specific morphology from 3D-capable imaging modalities such as MRI and CT. We introduce and illustrate the concept of virtual cardiotomy as a new tool to preoperatively evaluate the feasibility of different surgical strategies by investigating the anatomical spatial relations through any number of virtual incisions. We review the technical and clinical implementation of the various components of the system, namely 3D imaging, segmentation and reconstruction, visualization, and simulation of tissue elasticity. Finally we summarize the main findings from a recent evaluation study on 42 infants and children.

A framework for shape matching in deformable image registration.

Many existing image registration methods have difficulties in accurately describing significant rotation and bending of entities (e.g. organs) between two datasets. A common problem in this case is to ensure that the resulting registration is physically plausible, i.e. that the registration describes the actual bending/rotation occurring rather than just introducing expansion in some areas and shrinkage in others. In this work we developed a general framework for deformable image registration of two 3D datasets that alleviates this problem. To ensure that only physically feasible and plausible solutions to the registration problem are found, a soft tissue deformable model is used to constrain the search space for the desired correspondence map while minimizing a similarity metric between the source and reference datasets. Results from a deformable phantom experiment were used to verify and evaluate the framework.

The visible ear surgery simulator.

This paper presents a real-time computer simulation of surgical procedures in the ear, in which a surgeon drills into the temporal bone to gain access to the middle or inner ear. The purpose of this simulator is to support development of anatomical insight and training of drilling skills for both medical students and experienced otologists. The key contributions in this application are the visualization and interaction models in the context of ear surgical simulation. The visualization is based on an existing data set, "The Visible Ear", containing a unique volume depicting the inner ear in natural colours. The applied visualization is based on GPU ray casting, allowing high quality and flexible volume rendering using modern graphics card. In connection with the visualization model, different methods for optimizing the GPU ray casting procedure are presented, along with a method for combining polygon based graphics with volume rendering. In addition, different light models are presented that contribute to a realistic rendering of the different parts of the inner ear. To achieve a physically plausible drilling experience, a Phantom Omni force feedback device is utilized. The applied interaction model facilitates a realistic user experience of the response forces from the drilling tool.

Virtual open heart surgery: obtaining models suitable for surgical simulation.

We present a pre-processing strategy including imaging, segmentation, and model reconstruction that is well suited for previously published GPU-accelerated techniques for surgical simulation. In particular we describe these modeling steps as a prerequisite for our virtual open heart surgery simulator. A short description including relevant references is presented for each of the steps.

Smooth haptic interaction from discontinuous simulation data.

When a physical simulation that relies on haptic interaction is temporarily paused due to e.g. visualization, noticeable discontinuities are introduced in the interaction as well as the haptic-feedback. The source of this problem is a discrepancy between the notion of time in the simulation and in the real world. In this paper we analyze the general problem of executing simulation steps that each represent a constant amount of simulation time but are distributed non-uniformly in real world time. We have devised a solution that realigns the two notions of time, hereby insuring smooth interaction data and haptic feedback.

A Vector Flow Imaging Method for Portable Ultrasound Using Synthetic Aperture Sequential Beamforming.

This paper presents a vector flow imaging method for the integration of quantitative blood flow imaging in portable ultrasound systems. The method combines directional transverse oscillation (TO) and synthetic aperture sequential beamforming to yield continuous velocity estimation in the whole imaging region. Six focused emissions are used to create a high-resolution image (HRI), and a dual-stage beamforming approach is used to lower the data throughput between the probe and the processing unit. The transmit/receive focal points are laterally separated to obtain a TO in the HRI that allows for the velocity estimation along the lateral and axial directions using a phase-shift estimator. The performance of the method was investigated with constant flow measurements in a flow rig system using the SARUS scanner and a 4.1-MHz linear array. A sequence was designed with interleaved B-mode and flow emissions to obtain continuous data acquisition. A parametric study was carried out to evaluate the effect of critical parameters. The vessel was placed at depths from 20 to 40 mm, with beam-to-flow angles of 65°, 75°, and 90°. For the lateral velocities at 20 mm, a bias between -5% and -6.2% was obtained, and the standard deviation (SD) was between 6% and 9.6%. The axial bias was lower than 1% with an SD around 2%. The mean estimated angles were 66.70° ± 2.86°, 72.65° ± 2.48°, and 89.13° ± 0.79° for the three cases. A proof-of-concept demonstration of the real-time processing and wireless transmission was tested in a commercial tablet obtaining a frame rate of 27 frames/s and a data rate of 14 MB/s. An in vivo measurement of a common carotid artery of a healthy volunteer was finally performed to show the potential of the method in a realistic setting. The relative SD averaged over a cardiac cycle was 4.33%.

Haptic feedback for the GPU-based surgical simulator.

The GPU has proven to be a powerful processor to compute spring-mass based surgical simulations. It has not previously been shown however, how to effectively implement haptic interaction with a simulation running entirely on the GPU. This paper describes a method to calculate haptic feedback with limited performance cost. It allows easy balancing of the GPU workload between calculations of simulation, visualisation, and the haptic feedback.

Quantitative Neuroimaging Software for Clinical Assessment of Hippocampal Volumes on MR Imaging.

BACKGROUND: Multiple neurological disorders including Alzheimer's disease (AD), mesial temporal sclerosis, and mild traumatic brain injury manifest with volume loss on brain MRI. Subtle volume loss is particularly seen early in AD. While prior research has demonstrated the value of this additional information from quantitative neuroimaging, very few applications have been approved for clinical use. Here we describe a US FDA cleared software program, NeuroreaderTM, for assessment of clinical hippocampal volume on brain MRI. OBJECTIVE: To present the validation of hippocampal volumetrics on a clinical software program. METHOD: Subjects were drawn (n = 99) from the Alzheimer Disease Neuroimaging Initiative study. Volumetric brain MR imaging was acquired in both 1.5 T (n = 59) and 3.0 T (n = 40) scanners in participants with manual hippocampal segmentation. Fully automated hippocampal segmentation and measurement was done using a multiple atlas approach. The Dice Similarity Coefficient (DSC) measured the level of spatial overlap between NeuroreaderTM and gold standard manual segmentation from 0 to 1 with 0 denoting no overlap and 1 representing complete agreement. DSC comparisons between 1.5 T and 3.0 T scanners were done using standard independent samples T-tests. RESULTS: In the bilateral hippocampus, mean DSC was 0.87 with a range of 0.78-0.91 (right hippocampus) and 0.76-0.91 (left hippocampus). Automated segmentation agreement with manual segmentation was essentially equivalent at 1.5 T (DSC = 0.879) versus 3.0 T (DSC = 0.872). CONCLUSION: This work provides a description and validation of a software program that can be applied in measuring hippocampal volume, a biomarker that is frequently abnormal in AD and other neurological disorders.

A GPU accelerated spring mass system for surgical simulation.

There is a growing demand for surgical simulators to do fast and precise calculations of tissue deformation to simulate increasingly complex morphology in real-time. Unfortunately, even fast spring-mass based systems have slow convergence rates for large models. This paper presents a method to accelerate computation of a spring-mass system in order to simulate a complex organ such as the heart. This acceleration is achieved by taking advantage of modern graphics processing units (GPU).

Parameter optimisation for the behaviour of elastic models over time.

Optimisation of parameters for elastic models is essential for comparison or finding equivalent behaviour of elastic models when parameters cannot simply be transferred or converted. This is the case with a large range of commonly used elastic models. In this paper we present a general method that will optimise parameters based on the behaviour of the elastic models over time.

LR-Spring Mass model for cardiac surgical simulation.

The purpose of the research conducted was to develop a real-time surgical simulator for preoperative planning of surgery in congenital heart disease. The main problem simulating procedures on cardiac morphology is the need for a large degree of detail and simulation speed. In combination with a demand for physically realistic real-time behaviour this gives us tradeoffs not easily balanced. The LR-Spring Mass model handles these constraints by the use of domain specific knowledge.

The visible ear simulator: a public PC application for GPU-accelerated haptic 3D simulation of ear surgery based on the visible ear data.

BACKGROUND: Existing virtual simulators for middle ear surgery are based on 3-dimensional (3D) models from computed tomographic or magnetic resonance imaging data in which image quality is limited by the lack of detail (maximum, approximately 50 voxels/mm3), natural color, and texture of the source material.Virtual training often requires the purchase of a program, a customized computer, and expensive peripherals dedicated exclusively to this purpose. MATERIALS AND METHODS: The Visible Ear freeware library of digital images from a fresh-frozen human temporal bone was segmented, and real-time volume rendered as a 3D model of high-fidelity, true color, and great anatomic detail and realism of the surgically relevant structures. A haptic drilling model was developed for surgical interaction with the 3D model. RESULTS: Realistic visualization in high-fidelity (approximately 125 voxels/mm3) and true color, 2D, or optional anaglyph stereoscopic 3D was achieved on a standard Core 2 Duo personal computer with a GeForce 8,800 GTX graphics card, and surgical interaction was provided through a relatively inexpensive (approximately $2,500) Phantom Omni haptic 3D pointing device. CONCLUSION: This prototype is published for download (approximately 120 MB) as freeware at http://www.alexandra.dk/ves/index.htm.With increasing personal computer performance, future versions may include enhanced resolution (up to 8,000 voxels/mm3) and realistic interaction with deformable soft tissue components such as skin, tympanic membrane, dura, and cholesteatomas-features some of which are not possible with computed tomographic-/magnetic resonance imaging-based systems.

Virtual cardiotomy based on 3-D MRI for preoperative planning in congenital heart disease.

BACKGROUND: Patient-specific preoperative planning in complex congenital heart disease may be greatly facilitated by virtual cardiotomy. Surgeons can perform an unlimited number of surgical incisions on a virtual 3-D reconstruction to evaluate the feasibility of different surgical strategies. OBJECTIVE: To quantitatively evaluate the quality of the underlying imaging data and the accuracy of the corresponding segmentation, and to qualitatively evaluate the feasibility of virtual cardiotomy. MATERIALS AND METHODS: A whole-heart MRI sequence was applied in 42 children with congenital heart disease (age 3 +/- 3 years, weight 13 +/- 9 kg, heart rate 96 +/- 21 bpm). Image quality was graded 1-4 (diagnostic image quality > or =2) by two independent blinded observers. In patients with diagnostic image quality the segmentation quality was also graded 1-4 (4 no discrepancies, 1 misleading error). RESULTS: The average image quality score was 2.7 - sufficient for virtual reconstruction in 35 of 38 patients (92%) older than 1 month. Segmentation time was 59 +/- 10 min (average quality score 3.5). Virtual cardiotomy was performed in 19 patients. CONCLUSION: Accurate virtual reconstructions of patient-specific cardiac anatomy can be produced in less than 1 h from 3-D MRI. The presented work thus introduces a new, clinically feasible noninvasive technique for improved preoperative planning in complex cases of congenital heart disease.

Surgical simulation--a new tool to evaluate surgical incisions in congenital heart disease?

We introduce a new concept for preoperative planning and surgical education in congenital heart disease: surgical simulation. Recent advances in three-dimensional image acquisition have provided a new means to virtually reconstruct accurate morphological models while computer visualisation hardware now allows simulation of elastic tissue deformations interactively. Incision simulation is performed in two patients with complex congenital heart disease to preoperatively evaluate potential corrective surgical strategies. The relevant cardiac morphology was correctly depicted by the virtual models on which arbitrary incisions could be performed. By visualising the morphology in respect to each incision, different surgical strategies could be evaluated pre-operatively. We have taken the first step towards a clinically useful incision simulator for procedures in congenital heart disease and made an initial evaluation. With further developments it is likely that new tools for patient-specific preoperative planning and surgical training will emerge based on the presented ideas.