Showing papers by "Septimiu E. Salcudean published in 2011"

Travelling Wave Expansion: A Model Fitting Approach to the Inverse Problem of Elasticity Reconstruction

Abstract: In this paper, a novel approach to the problem of elasticity reconstruction is introduced. In this approach, the solution of the wave equation is expanded as a sum of waves travelling in different directions sharing a common wave number. In particular, the solutions for the scalar and vector potentials which are related to the dilatational and shear components of the displacement respectively are expanded as sums of travelling waves. This solution is then used as a model and fitted to the measured displacements. The value of the shear wave number which yields the best fit is then used to find the elasticity at each spatial point. The main advantage of this method over direct inversion methods is that, instead of taking the derivatives of noisy measurement data, the derivatives are taken on the analytical model. This improves the results of the inversion. The dilatational and shear components of the displacement can also be computed as a byproduct of the method, without taking any derivatives. Experimental results show the effectiveness of this technique in magnetic resonance elastography. Comparisons are made with other state-of-the-art techniques.

Intra-operative pick-up ultrasound for robot assisted surgery with vessel extraction and registration: a feasibility study

Abstract: We propose the use of a "pick-up" ultrasound transducer for robot-assisted minimally invasive surgeries. Unlike prior approaches, the ultrasound transducer is inserted before the procedure and remains in the abdominal cavity throughout. We present a new design for such an intra-abdominal ultrasound transducer with a handle that can be grasped in a repeatable manner using a da Vinci Pro-Grasp tool. The main application is mapping the vasculature, which is segmented from Doppler and B-mode images using a Kalman-filtering approach. Our goal is employ the vasculature to register pre-operative CT to intraoperative camera images. To demonstrate the feasibility of the approach, we use an ultrasound flow phantom to register a CT surface model to extracted ultrasound vessel center points using an iterative closest point method. The transducer was tracked with electromagnetic sensors and a target registration error of 3.2 mm was calculated. The initial application will be nephrectomy where vessel localization is paramount.

Image-Based Variational Meshing

Abstract: In medical simulations involving tissue deformation, the finite element method (FEM) is a widely used technique, where the size, shape, and placement of the elements in a model are important factors that affect the interpolation and numerical errors of a solution. Conventional model generation schemes for FEM consist of a segmentation step delineating the anatomy followed by a meshing step generating elements conforming to this segmentation. In this paper, a single-step model generation technique is proposed based on optimization. Starting from an initial mesh covering the domain of interest, the mesh nodes are adjusted to minimize an objective function which penalizes intra-element intensity variations and poor element geometry for the entire mesh. Trade-offs between mesh geometry quality and intra-element variance are achieved by adjusting the relative weights of the geometric and intensity variation components of the cost function. This meshing approach enables a more accurate rendering of shapes with fewer elements and provides more accurate models for deformation simulation, especially when the image intensities represent a mechanical feature of the tissue such as the elastic modulus. The use of the proposed mesh optimization is demonstrated in 2-D and 3-D on synthetic phantoms, MR images of the brain, and CT images of the kidney. A comparison with previous meshing techniques that do not account for image intensity is also provided demonstrating the benefits of our approach.

Haptic Simulator for Prostate Brachytherapy with Simulated Needle and Probe Interaction

Abstract: This paper presents a haptic simulator for prostate brachytherapy. Both needle insertion and the manipulation of the transrectal ultrasound (TRUS) probe are controlled via haptic devices. Tissue interaction forces that are computed by a deformable tissue model based on the finite element method (FEM) are rendered to the user by these devices. The needle insertion simulation employs 3D models of needle flexibility and asymmetric tip bevel. The needle-tissue simulation allows a trainee to practice needle insertion and targeting. The TRUS-tissue interaction simulation allows a trainee to practice the 3D intraoperative TRUS placement for registration with the preoperative volume study and to practice TRUS axial translation and rotation for imaging needles during insertions. Approaches to computational acceleration for realtime haptic performance are presented. Trade-offs between accuracy and speed are discussed. A graphics-card implementation of the numerically intensive mesh-adaptation operation is also presented. The simulator can be used for training, rehearsal, and treatment planning.

A robotic system for intra-operative trans-rectal ultrasound and ultrasound elastography in radical prostatectomy

Abstract: A new robotic system for trans-rectal ultrasound (TRUS) imaging during robot-assisted laparoscopic radical prostatectomy is described. The system consists of three main parts: a robotic probe manipulator (robot), an ultrasound machine with a biplane TRUS probe, and control and image processing software. A review of prior use of TRUS during prostatectomy is provided in order to demonstrate the potential benefits of such intra-operative imaging. The ability of the system to capture two-dimensional and three-dimensional B-mode and elastography data is demonstrated using a prostate phantom. A registration method that can be used for instrument tracking in RALRP is described and tested. Initial patient images captured using the system are presented.

Real-time solution of the finite element inverse problem of viscoelasticity

Abstract: The linear dynamic finite element model can be formulated such that the elasticity and viscosity of the elements appear as the parameters in a linear system of equations. The resulting system of equations can be solved directly using singular value decomposition or a similar technique or through defining a quadratic functional. A priori knowledge and regularity measures can be added as equality or inequality constraints. The sensitivity of the inverse problem solution to the displacement noise and model imperfections are tested in simulations, where the parameters were successfully reconstructed with a displacement signal-to-noise ratio as low as 20 dB. Also, the viscoelastic parameters have been successfully estimated for a breast phantom with an embedded hard inclusion. The study of the computation speed demonstrates the potential of the new method for real-time implementations.

Bandpass sampling of high-frequency tissue motion

Abstract: The characterization of tissue viscoelastic properties requires the measurement of tissue motion over a region of interest at frequencies that significantly exceed the frame rates of conventional ultrasound systems. In this paper, we propose that the bandpass sampling technique be applied to tissue motion sampling. With this approach, high-frequency signals limited to a frequency band can be sampled and reconstructed without aliasing at a sampling frequency that is lower than the Nyquist rate. We first review this approach and discuss the selection of the tissue excitation frequency band and of the feasible sampling frequencies that allow signal reconstruction without aliasing. We then demonstrate the approach using simulations based on the finite element method and ultrasound simulations. Finally, we perform experiments on tissue-mimicking materials and demonstrate accurate motion estimation using a lower sampling rate than that required by the conventional sampling theorem. The estimated displacements were used to measure the elasticity and viscosity in a phantom in which an inclusion has been correctly delineated. Thus, with bandpass sampling, it is feasible to use conventional beamforming on diagnostic ultrasound systems to perform high-frequency dynamic elastography. The method is simple to implement because it does not require beam interleaving, additional hardware, or synchronization.

Real-Time Image-Based B-Mode Ultrasound Image Simulation of Needles Using Tensor-Product Interpolation

Abstract: In this paper, we propose an interpolation-based method for simulating rigid needles in B-mode ultrasound images in real time. We parameterize the needle B-mode image as a function of needle position and orientation. We collect needle images under various spatial configurations in a water-tank using a needle guidance robot. Then we use multidimensional tensor-product interpolation to simulate images of needles with arbitrary poses and positions using collected images. After further processing, the interpolated needle and seed images are superimposed on top of phantom or tissue image backgrounds. The similarity between the simulated and the real images is measured using a correlation metric. A comparison is also performed with in vivo images obtained during prostate brachytherapy. Our results, carried out for both the convex (transverse plane) and linear (sagittal/para-sagittal plane) arrays of a trans-rectal transducer indicate that our interpolation method produces good results while requiring modest computing resources. The needle simulation method we present can be extended to the simulation of ultrasound images of other wire-like objects. In particular, we have shown that the proposed approach can be used to simulate brachytherapy seeds.

Bandpass sampling for elastography

Abstract: The characterization of tissue viscoelastic properties requires the measurement of tissue displacements over a region of interest at frequencies that exceed significantly the frame rates of conventional medical imaging devices. The present invention involves using bandpass sampling to track high-frequency tissue displacements. With this approach, high frequency signals limited to a frequency bandwidth can be sampled and reconstructed without aliasing at a sampling frequency that is lower than the Nyquist rate. With bandpass sampling, it is feasible to use conventional beam-forming on diagnostic ultrasound machines to perform high frequency dynamic elastography. The method is simple to implement as it does not require beam interleaving, additional hardware or synchronization and can be applied to magnetic resonance elastography.

Signature-based algorithm for improved needle localization in ultrasound images: A feasibility study

Abstract: Ultrasound imaging is commonly used to guide needle insertion procedures, such as biopsies and anesthesia. In such procedures, the needle can be poorly visualized, making it difficult to determine the needle trajectory. A novel needle detection algorithm is introduced based on the unique needle reflection pattern of circular ultrasound waves. The proposed algorithm was evaluated by locating a needle placed in an agar phantom. Circular waves were created using a 5 MHz, curvilinear transducer array. The coordinates of the needle axis were estimated when the angle between the needle and the transducer central axis was varied between 50° and 90°. The differences between the radial and azimuth coordinates of the needle estimated using the proposed algorithm and those calculated using computed tomography scan of the phantom were within 0.1 to 0.5 mm and 0.0° to 3.1°, respectively.

Comparison between 2-D cross correlation with 2-D sub-sampling and 2-D tracking using beam steering

Abstract: We have previously presented multi-dimensional sub-sample motion estimation techniques that use multi-dimensional polynomial fitting to the discrete cross-correlation function to jointly estimate the sub-sample motion in all three spatial directions. Previous simulation and experimental results showed that these estimators significantly improve the performance of the motion estimation in 2-D and 3-D. In this short communication, we present additional simulation results and compare these techniques to 2-D tracking using beam steering. The results show that beam steering technique performs better in estimating the motion vector especially the lateral component.

Trans-perineal prostate MR elastography

Abstract: The present invention relates to a method and apparatus for imaging the mechanical properties of the prostate of a patient non-invasively. The apparatus generally comprises a magnetic resonance scanner, a vibration assembly coupled to the perineal region of the patient, and a driver that drives the mechanical exciter. The method generally comprises positioning the vibration assembly against the perineal region of the patient, vibrating the mechanical exciter to cause deformational excitation of a tissue region contacted in the perineum, capturing a series of images in time (snapshots) of the tissue region using the MR scanner, and finally processing the displacement images to generate maps of mechanical properties of images tissue.

Prostate implant reconstruction from C-arm images with motion-compensated tomosynthesis

Abstract: Purpose: Accurate localization of prostate implants from several C-arm images is necessary for ultrasound-fluoroscopy fusion and intraoperative dosimetry. The authors propose a computational motion compensation method for tomosynthesis-based reconstruction that enables 3D localization of prostate implants from C-arm images despite C-arm oscillation and sagging. Methods: Five C-arm images are captured by rotating the C-arm around its primary axis, while measuring its rotation angle using a protractor or the C-arm joint encoder. The C-arm images are processed to obtain binary seed-only images from which a volume of interest is reconstructed. The motion compensation algorithm, iteratively, compensates for 2D translational motion of the C-arm by maximizing the number of voxels that project on a seed projection in all of the images. This obviates the need for C-arm full pose tracking traditionally implemented using radio-opaque fiducials or external trackers. The proposed reconstruction method is tested in simulations, in a phantom study and on ten patient data sets. Results: In a phantom implanted with 136 dummy seeds, the seed detection rate was 100% with a localization error of 0.86 {+-} 0.44 mm (Mean {+-} STD) compared to CT. For patient data sets, a detection rate of 99.5% was achieved in approximately 1 min permore » patient. The reconstruction results for patient data sets were compared against an available matching-based reconstruction method and showed relative localization difference of 0.5 {+-} 0.4 mm. Conclusions: The motion compensation method can successfully compensate for large C-arm motion without using radio-opaque fiducial or external trackers. Considering the efficacy of the algorithm, its successful reconstruction rate and low computational burden, the algorithm is feasible for clinical use.« less

Haptic simulation of needle and probe interaction with tissue for prostate brachytherapy training

Abstract: This paper presents a haptic simulator for prostate brachytherapy. Both needle insertion and the manipulation of the transrectal ultrasound (TRUS) probe are controlled via haptic devices. These are used to render tissue interaction forces computed using a deformable tissue model based on the finite element method (FEM). Needle flexibility and lateral needle bevel forces are also simulated. The TRUS-tissue simulation allows a trainee to practice the 3D intra-operative placement of the TRUS probe for registration with the pre-operative volume study. The needle-tissue simulation allows a trainee to practice needle insertion and targeting. The TRUS probe and the needle can be maneuvered simultaneously. Approaches to computational acceleration for real-time haptic performance are presented. Trade-offs between accuracy and speed are discussed. A graphics-card implementation of the numerically intensive mesh-adaptation operation is also presented.

Quantifying stranded implant displacement following prostate brachytherapy

Abstract: We aim to compute radioactive stranded-implant displacement during and after prostate brachytherapy. We present the methods used to identify corresponding seeds in planned, intra-operative and postimplant patient data that enable us to compute seed displacements. A minimum cost network flow algorithm is used, on 8 patients, for needle track detection to group seeds into needles that can be matched between datasets. An iterative best line detection algorithm is used both to help with needle detection and to register the different datasets. Our results show that there was an average seed misplacement of 5.08±2.35 mm during the procedure, which then moved another 3.10±1.91 mm by the time the quality assurance CT was taken. Several directional trends in different regions of the prostate were noted and commented on.

Towards intra-operative prostate brachytherapy dosimetry based on partial seed localization in ultrasound and registration to C-arm fluoroscopy

Abstract: Intraoperative dosimetry during prostate brachytherapy is a long standing clinical problem. We propose a novel framework to address this problem by reliable detection of a subset of seeds from 3D transrectal ultrasound and registration to fluoroscopy. Seed detection in ultrasound is achieved through template matching in the RF ultrasound domain followed by thresholding and spatial filtering based on the fixed distance between stranded seeds. This subset of seeds is registered to the complete reconstruction of the implant in C-arm fluoroscopy. We report results, validated with a leave-one-needle-out approach, both in a phantom (average post-registration seed distance of 2.5 mm) and in three clinical patient datasets (average error: 3.9 mm over 113 seeds).

Comparing student assessed competencies in pbl and traditional ece programs

Abstract: Since 1999, both traditional and project based learning (PBL) programs have been offered to second year ECE students at the University of British Columbia. This paper describes the structure of the PBL program and a student evaluation of their competencies. The evaluation was carried out by a survey of third and fourth year students who passed through either the PBL or the traditional program. Results of the evaluation indicate that the former PBL students felt significantly more improved than the traditional students in their design-related abilities and some of their teamwork abilities evaluated by the survey.

Frequency-locked pulse sequencer for high-frame-rate monochromatic tissue motion imaging

Abstract: To overcome the inherent low frame rate of conventional ultrasound, we have previously presented a system that can be implemented on conventional ultrasound scanners for high-frame-rate imaging of monochromatic tissue motion. The system employs a sector subdivision technique in the sequencer to increase the acquisition rate. To eliminate the delays introduced during data acquisition, a motion phase correction algorithm has also been introduced to create in-phase displacement images. Previous experimental results from tissue- mimicking phantoms showed that the system can achieve effective frame rates of up to a few kilohertz on conventional ultrasound systems. In this short communication, we present a new pulse sequencing strategy that facilitates high-frame-rate imaging of monochromatic motion such that the acquired echo signals are inherently in-phase. The sequencer uses the knowledge of the excitation frequency to synchronize the acquisition of the entire imaging plane to that of an external exciter. This sequencing approach eliminates any need for synchronization or phase correction and has applications in tissue elastography, which we demonstrate with tissue-mimicking phantoms.