Showing papers by "Derek L. G. Hill published in 2004"

Non-rigid image registration: theory and practice

Abstract: Image registration is an important enabling technology in medical image analysis. The current emphasis is on development and validation of application-specific non-rigid techniques, but there is already a plethora of techniques and terminology in use. In this paper we discuss the current state of the art of non-rigid registration to put on-going research in context and to highlight current and future clinical applications that might benefit from this technology. The philosophy and motivation underlying non-rigid registration is discussed and a guide to common terminology is presented. The core components of registration systems are described and outstanding issues of validity and validation are confronted.

Novel Method of Quantifying Pulmonary Vascular Resistance by Use of Simultaneous Invasive Pressure Monitoring and Phase-Contrast Magnetic Resonance Flow

Abstract: Background— Pulmonary vascular resistance (PVR) quantification is important in the treatment of children with pulmonary hypertension. The Fick principle, used to quantify pulmonary artery flow, may be a flawed technique. We describe a novel method of PVR quantification by the use of magnetic resonance (MR) flow data and invasive pressure measurements. Methods and Results— In 24 patients with either suspected pulmonary hypertension or congenital heart disease requiring preoperative assessment, PVR was calculated by the use of simultaneously acquired MR flow and invasive pressure measurements (condition 1). In 19 of the 24 patients, PVR was also calculated at 20 ppm nitric oxide +30% (condition 2) and at 20 ppm nitric oxide +100% oxygen (condition 3), with the use of the MR method. This method proved safe and feasible in all patients. In 15 of 19 patients, PVR calculated by Fick flow was compared with the MR method. At condition 1, Bland-Altman analysis revealed a bias of 2.3% (MR > Fick) and limits of agre...

Visualization and tracking of an inflatable balloon catheter using SSFP in a flow phantom and in the heart and great vessels of patients.

Abstract: Passive catheter tracking involves direct interaction between the device and its surroundings, creating a local signal loss or enhancement of the image. Using only standard balloon catheters filled with CO2 and imaged with a steady-state free precession sequence, it was possible to visualize and passively track catheters in a flow phantom and in the heart and great vessels of 20 patients without any additional image processing. The phantom work demonstrated that it was advantageous to sacrifice spatial resolution in order to increase temporal resolution. Frame rates greater than 10/sec were necessary for ease of catheter manipulation. Although only the tip of the catheter was visualized, this technique proved to be effective in patients undergoing cardiac catheterization. Magn Reson Med 51: 988-995, 2004. (C) 2004 Wiley-Liss, Inc.

Coil-based artifact reduction.

Abstract: Multiple MRI receiver coils provide extra information and canenable the reconstruction of multiple images using data fromdifferent combinations of coils. Comparison of these imagesshows that artifacts due to motion or flowing blood appear withdifferent intensities due to the differing coil sensitivities. Typically,the artifact appears amplified in regions of low coil sensitivity.An optimization routine was developed to correct for theartifact by comparing reconstructions from various coil combinationsand favoring a self-consistent solution. It is demonstratedthat images artifacted by blood flowing in the aorta, ortranslational motion of the head, can be improved. Multiple MRI receiver coils provide extra information and canenable the reconstruction of multiple images using data fromdifferent combinations of coils. Comparison of these imagesshows that artifacts due to motion or flowing blood appear withdifferent intensities due to the differing coil sensitivities. Typically,the artifact appears amplified in regions of low coil sensitivity.An optimization routine was developed to correct for theartifact by comparing reconstructions from various coil combinationsand favoring a self-consistent solution. It is demonstratedthat images artifacted by blood flowing in the aorta, ortranslational motion of the head, can be improved.

Catheter tracking and visualization using 19F nuclear magnetic resonance.

Abstract: This work presents an investigation into catheter visualization and localization using 19F nuclear magnetic resonance (NMR) in conjunction with proton imaging. For this purpose, the imaging capabilities of a standard system were extended to allow for 19F excitation and signal detection. Two modes of operation were implemented: 1) a real-time tracking mode that provides tip tracking and automatic slice position updates interleaved with real-time, interactive proton imaging; and 2) a non-real-time catheter length visualization mode in which the entire length of a catheter can be assessed. Initial phantom experiments were conducted with the use of an angiographic balloon catheter filled with the blood substitute perfluorooctylbromide (PFOB). Using limited bandwidth excitation centered at the resonances of the CF2 groups of PFOB, we found that sufficient signal could be received to facilitate tip tracking during catheter motion and length visualization for various catheter configurations. The present approach is considered a promising alternative to existing methods, which either are associated with safety concerns (if active markers are employed) or suffer from insufficient, direction-dependent contrast (if passive visualization is used). Furthermore, our approach enables visualization of the entire length of the catheter. The proposed method provides a safe technique that, unlike electrical or optical devices, does not require modification of commercially available catheters.

Using a Maximum Uncertainty LDA-Based Approach to Classify and Analyse MR Brain Images

Abstract: Multivariate statistical learning techniques that analyse all voxels simultaneously have been used to classify and describe MR brain images. Most of these techniques have overcome the difficulty of dealing with the inherent high dimensionality of 3D brain image data by using pre-processed segmented images or a number of specific features. However, an intuitive way of mapping the classification results back into the original image domain for further interpretation remains challenging. In this paper, we introduce the idea of using Principal Components Analysis (PCA) plus the maximum uncertainty Linear Discriminant Analysis (LDA) based approach to classify and analyse magnetic resonance (MR) images of the brain. It avoids the computation costs inherent in commonly used optimisation processes, resulting in a simple and efficient implementation for the maximisation and interpretation of the Fisher’s classification results. In order to demonstrate the effectiveness of the approach, we have used two MR brain data sets. The first contains images of 17 schizophrenic patients and 5 controls, and the second is composed of brain images of 12 preterm infants at term equivalent age and 12 term controls. The results indicate that the two-stage linear classifier not only makes clear the statistical differences between the control and patient samples, but also provides a simple method of analysing the results for further medical research.

Free-breathing radial acquisitions of the heart.

Abstract: There is considerable interest in performing free-breathing acquisitions of the heart in order to obtain high-quality images without the need for multiple, long breathholds. In this article a 3D motion-correction method is described that is based on image registration of in-plane data and through-plane slice tracking. A number of fast radial undersampled images are acquired, each of which is free of motion artifacts. Initially, in-plane translational and rotational motion between each image was corrected before combining the data to give a fully sampled image. At the next stage, correction of in-plane deformation, in addition to translations and rotations, was performed in the image domain. Through-plane translational motion was compensated using a navigator echo to move the acquisition plane. Using this method, information on the motion of the heart was captured at the same time as acquiring the image data. No motion model, assumptions about the motion, or training data are required. The method is demonstrated on phantom data and cardiac images acquired on free-breathing volunteers.

The Automatic Identification of Hibernating Myocardium

Abstract: Delayed enhancement imaging is a recently described technique that enables for the first time, the direct observation of areas of myocardium that have scarred following infarction. When this information is combined with information about myocardial contraction, areas that are neither dead, nor contracting can be identified. Such areas will resume contraction following revascularisation (hibernating myocardium). The identification of such areas is consequently of great interest to clinicians. This paper describes how registration can be used to align the images prior to the identification of areas that will benefit from revascularisation. Patient data is used to demonstrate image alignment and image-derived information combination. This is then mapped onto patient-specific 2D and 3D representations of the heart.

Multi-channel Mutual Information Using Scale Space

Abstract: We propose a new voxel similarity measure which utilises local image structure and intensity information. Gaussian scale space derivatives provide the structural information. Each derivative is assigned an information channel of N-D normalised mutual information. We illustrate the behaviour of the measure for a simulated signal and 2D medical brain images and demonstrate its potential for non-rigid, inter-subject registration of 3D thorax MR images as a proof of concept.

XMR guided cardiac electrophysiology study and radio frequency ablation

Abstract: XMR systems are a new type of interventional facility in which patients can be rapidly transferred between x-ray and MR systems on a floating table. We have previously developed a technique to register MR and x-ray images obtained from such systems. We are carrying out a program of XMR guided cardiac electrophysiology study (EPS) and radio frequency ablation (RFA). The aim of our work was to apply our registration technology to XMR guided EPS/RFA in order to integrate anatomical, electrophysiological and motion information. This would assist in guidance and allow us to validate and refine electromechanical models. Registration of the imaging modalities was achieved by a combination of system calibration and real-time optical tracking. Patients were initially imaged using MR imaging. An SSFP volume scan of the heart was acquired for anatomical information, followed by tagged scans for motion information. The patients were then transferred to the x-ray system. Tracked biplane x-ray images were acquired while electrical measurements were made from catheters placed in the heart. The relationship between the MR and x-ray images was determined. The MR volume scan of the heart was segmented and the tagged scans were analysed using a non-rigid registration algorithm to compute motion. The position of catheters was reconstructed within the MR cardiac anatomy. The anatomical, electrophysiological, and motion information were displayed in the same coordinate system. Simulations of electrical depolarisation and contraction were performed using electromechanical models of the myocardium. We present results for 2 initial cases. For patient 1, a contact mapping system was used for the EPS and for patient 2, a non-contact mapping system was used. Our XMR registration technique allows the integration of anatomical, electrophysiological, and motion information for patients undergoing EPS/RFA. This integrated approach has assisted in interventional guidance and has been used to validate electromechanical models of the myocardium.

Simulation of the Electromechanical Activity of the Heart Using XMR Interventional Imaging

Abstract: Simulating cardiac electromechanical activity is of great interest for a better understanding of pathologies and therapy planning. Design and validation of such models is difficult due to the lack of clinical data. XMR systems are a new type of interventional facility in which patients can be rapidly transferred between x-ray and MR systems. Our goal is to design and validate an electromechanical model of the myocardium, using this XMR system. The proposed model is computationally fast and uses clinically observable parameters. We present the integration of anatomy, electrophysiology, and motion from patients. Pathologies are introduced in the model and the simulations are compared to measured data. Initial qualitative comparison is encouraging. Quantitative local validation is in progress. Once validated, these models will make it possible to simulate different interventional strategies.

Registered computed tomography images as an alternative to postimplantation magnetic resonance imaging in the assessment of subthalamic electrode placement.

Abstract: Magnetic resonance imaging (MRI) after implantation of electrodes in the subthalamic nuclei is currently performed at a number of sites, but a recent adverse incident and changes in MRI technology may heighten safety concerns. In this report, it is demonstrated that given whole-head image data, registration of postimplantation computed tomography to preimplantation MRI can enable verification of the position of electrodes to an accuracy of 2 mm. This registration technique can remove the need for potentially risky postoperative MRI.

Detecting the onset of myocardial contraction for establishing inverse electro-mechanical coupling in XMR guided RF ablation

Abstract: Radio-frequency (RF) ablation uses electrode-catheters to destroy abnormally conducting myocardial areas that lead to potentially lethal tachyarrhythmias. The procedure is normally guided with X-rays (2D), leading to errors in location and excessive radiation exposure. In order to provide preand intraoperative 3D MR guidance, we define a probabilistic measure of regional motion activation. Nonrigid registration of tagged MR sequences is used to track heart motion. Regional motion is also compared between different acquisitions, thus assisting in diagnosing arrhythmia, in follow up of treatment, and in determining whether ablation succeeded. We validate using an electro-mechanical model, synthetic tagged MRI, stress data on healthy volunteers, and one patient with tachyarrhythmia, before and after ablation.

Detecting regional changes in myocardial contraction patterns using MRI

Abstract: Measuring changes in cardiac motion patterns can assist in diagnosing the onset of arrhythmia and ischaemia and in the follow-up of treatment. This work presents a methodology for measuring such motion changes from MR images. Non-rigid registration is used to track cardiac motion in a sequence of 3D tagged MR images. We use a cylindrical coordinate system to subdivide the myocardium into smaller anatomically meaningful regions and to express motion derived measurements such as displacement and strain for each myocardial region during the cardiac cycle. In the first experiment we have evaluated the proposed methods using synthetic image sequences where the ground truth was available. These images were generated using a cardiac motion simulator for tagged MRI. Normal and abnormal motion fields were produced by modifying parameters in a small region of the myocardium. In the second experiment we have acquired two separate tagged MR image sequences from five healthy volunteers. Both acquisitions have been carried out without moving the volunteer inside the scanner, thus avoiding potential misregistration errors due to subject motion between scans. In addition, one of volunteers was subjected to stress during one of the scans. In the final experiment we acquired tagged MR images from a patient with super-ventricular tachyarrhythmia, before and after radio frequency ablation. The image acquisition and catheter intervention were performed with a combined X-ray and MRI system. Detection results were correct on synthetic data and no region was incorrectly classified as having significant changes in the repetition studies. Significant changes in motion pattern were measured in the stress and ablation studies. Furthermore, results seem to corroborate that the ablation regularised cardiac contraction.

Intraoperable Medical Image Registration Grid Service

Abstract: Image Registration is widely used in medical image analysis for determining the geometrical transformation, or mapping, relating two or more images. These images can be of the same subject using different modalities, or the same modality at different time points, or of different subjects. There is a large literature on medical image registration, which has become a core technology in a number of medical imaging applications. Current registration algoritms that can determine non-rigid mappings are extremely computationally demanding, with run times of 10s of minutes or hours on desktop computers. Despite the large amount of research activity in this field, there has been up to now very few comparitive evaluations of these algorithms on reference datasets.

Alignment of dynamic contrast-enhanced MR volumes of the breast for a multicenter trial: an exemplar grid application

Abstract: Image registration is a very important procedure in medical imaging analysis. However, the intensive computations involved in image registration have to some extent made it impractical for interactive use as well as limiting its general availability. This paper presents our current Grid project to facilitate image registration tasks. We have set up an image registration Grid by combining the attractive features of both Globus and Condor distributed computing environments. In order to make it more convenient to use, we have also developed a web interface for potential clients to specify and submit their image registration jobs to the Grid. The initial experiments in 3D breast MR images have shown encouraging results and demonstrated the suitability of Grid technology to this type of computationally intensive applications. The image registration Grid makes it much more straightforward for different institutes to use the identical registration program and protocols to register images consistently, quickly and efficiently. This can greatly improve data sharing and comparative studies in multi-centre trials. The Grid presented here could be an important step for clinical applications of image registration. Future work will focus on refining the Grid with the aim of upgrading it to a Grid Service and testing the system more extensively with medical imaging dataset.

Analysis of serial MR images of joints

Abstract: Medical research and drug discovery rely increasingly on making comparisons between MR images from large numbers of subjects, often with multiple time points for each subject. We show how image registration and visualisation technique can be used for quantitative and qualitative assessment of changes over time in an experimental model of rheumatoid arthritis. We applied the technique to automatically delineate two separate bones (the calcaneus and talus) from the in vivo MR images of an ankle (6 subjects, 6 time points each) by propagating labels from pre-labelled "atlas" MRI scans. As an initial analysis, we calculated their volume and rendered their surfaces in 3D to study the disease progression over time. Finally, we investigated the use of grid computing by coupling it with image analysis and visualisation algorithms to enable remote invocation, parallel execution, and source data input from heterogeneous distributed image repositories.

Use of Data Provenance and the Grid in Medical Image Analysis and Drug Discovery - An IXI Exemplar

Abstract: Medical research and drug discovery rely increasingly on comparisons between magnetic resonance images from large numbers of subjects, often with multiple time points for each subject. We applied image registration and visualisation techniques for quantitative and qualitative assessment of changes over time in an experimental model of rheumatoid arthritis. We used the Grid to enable remote invocation and parallel execution of the algorithms. Data provenance was stored in a database to provide information about the validity, accuracy and timeliness of the data, which establishes a data integrity framework that is very important in the process of drug discovery and development.

Reconstruction of the heart boundary from undersampled cardiac MRI using Fourier shape descriptors and local basis functions

Abstract: A new method for shape reconstruction in cardiac MRI is presented in this work. Using a combined shape representation and local basis functions this method models the shape of the left ventricle. To solve this problem of shape reconstruction a nonlinear approach is required. Fitting the model to the data is done by an iterative nonlinear method, which reconstructs both the intensity and the contour. Results are shown for an undersampled set of ten k-space profiles.

Electromechanical Modelling of the Myocardium using XMR Interventional Imaging

Abstract: Introduction & Aim: Myocardial electrophysiology study (EPS) is carried out prior to procedures such as radio frequency ablation or biventricular pacing for heart failure. Under x-ray fluoroscopic guidance, the procedures are often lengthy and there is considerable delivered x-ray dose. Electrophysiological mapping systems such the EnSite system have made some progress to facilitate EPS procedures. We are undertaking a programme of XMR guided EPS procedures using the EnSite system. Cardiac MR imaging can be used to obtain anatomical and functional information prior to and after the procedure. We have previously reported a technique to register MR and x-ray images obtained in the XMR environment. In the current work we apply our technique to EPS procedures with the aim to validate our electromechanical model of the heart. We present the initial results from our first case.

Multiple Coils for Reduction of Flow Artefacts in MR Images

Abstract: Flowing blood can cause streak or blob artefacts in MR images and these may degrade subsequent image analysis. Multiple MRI receiver coils enable the reconstruction of images using data from different combinations of coils. The artefact intensities differ in these images due to the differing coil sensitivities. The artefact cause is parameterised and an optimisation routine is used to find self-consistent image reconstructions which have reduced artefacts.

Parallel Imaging and MR Motion Correction

Abstract: where is some similarity between images, here . Thus, the energy would take the form for small displacements ,w here is a function of the coil spatial patterns which is zero when the coils are identical; is the image. We plot here the energy in a simulation where the motion is a 1-parameter ( ) nonrigid radial deformation ( : distance