Timo Mäkelä

A review of cardiac image registration methods

In this paper, the current status of cardiac image registration methods is reviewed. The combination of information from multiple cardiac image modalities, such as magnetic resonance imaging, computed tomography, positron emission tomography, single-photon emission computed tomography, and ultrasound, is of increasing interest in the medical community for physiologic understanding and diagnostic purposes. Registration of cardiac images is a more complex problem than brain image registration because the heart is a nonrigid moving organ inside a moving body. Moreover, as compared to the registration of brain images, the heart exhibits much fewer accurate anatomical landmarks. In a clinical context, physicians often mentally integrate image information from different modalities. Automatic registration, based on computer programs, might, however, offer better accuracy and repeatability and save time.

A 3-D model-based registration approach for the PET, MR and MCG cardiac data fusion

In this paper, a new approach is presented for the assessment of a 3-D anatomical and functional model of the heart including structural information from magnetic resonance imaging (MRI) and functional information from positron emission tomography (PET) and magnetocardiography (MCG). The method uses model-based co-registration of MR and PET images and marker-based registration for MRI and MCG. Model-based segmentation of MR anatomical images results in an individualized 3-D biventricular model of the heart including functional parameters from PET and MCG in an easily interpretable 3-D form.

Elastic Matching Using a Deformation Sphere

A novel method is proposed for elastic matching of two data volumes. A combination of mutual information, gradient information and smoothness of transformation is used to guide the deformation of another of the volumes. The deformation is accomplished in a multiresolution process by spheres containing a vector field. Position and radius of the spheres are varied. The feasibility of the method is demonstrated in two cases: matching inter-patient MR images of the head and intra-patient cardiac MR and PET images.

A New Method for the Registration of Cardiac PET and MR Images Using Deformable Model Based Segmentation of the Main Thorax Structures

Integration of magnetic resonance (MR) and positron emission tomography (PET) images of the heart has proved its usefulness for the estimation of the myocardial viability. In this paper, a method for the rigid registration of cardiac MR and PET images is presented. It is based on the matching of the surfaces of thorax structures extracted by a deformable model from PET transmission and MR transaxial images. MR short axis registration with PET emission image is easily derived and allows the study viability in the proper anatomic conditions. The method has been evaluated on ten patients suffering from three vessel coronary artery disease. Qualitative results were good with 9 over the 10 available cases. A quantitative estimation of the registration quality confirmed the nice abilities of this approach.

A strategy to quantitatively evaluate MRI/PET cardiac rigid registration methods using a Monte Carlo simulator

The goal of this work is to present a strategy to validate cardiac MRI/PET registration methods. The strategy relies on a MRI/PET image reference data set including a computer generated PET data set of the thorax and its structures. This data set was produced using a Monte Carlo simulator from segmented T1-weighted MRI thorax data. From the reference data set as a gold standard, test transformations are randomly generated and used to quantify registration accuracy. The validation approach has been applied to our own rigid registration method with three different similarity measures: Correlation Ratio, Correlation Coefficient and Mutual Information. In this study, we observed that the Correlation Ratio gave better results both for thorax and heart image registration.

Evaluation of cardiac PET-MRI registration methods using a numerical breathing phantom

The thorax structure movements (breathing, heart motion and patient motion) during the positron emission tomography (PET) scanning cause deformations and blurring to PET scans. These artifacts are important reasons, for the misregistration of PET volumes. In this paper, a breathing simulated PET phantom was constructed by deforming simulated (static) PET volume according to breathing model. The constructed volume was used as a ground truth in the assessment of a nonrigid registration method for cardiac MR and PET volumes. Results showed that the assessed nonrigid registration method was more accurate than a rigid registration method which was used as a reference.

Comparison of current density viability imaging at rest with FDG-PET in patients after myocardial infarction

The assessment of myocardial viability is a major diagnostic challenge in patients with coronary artery disease (CAD) after myocardial infarction. Novel threedimensional current density (CD) imaging algorithms use high-resolution magnetic field mapping to determine the electrical activity of myocardial segments at rest. We, for the first time, compared CD activity obtained with several algorithms to 18-F-fluoro-deoxyglucose positron emission tomography (FDG-PET) in evaluation of myocardial viability. Magnetic field maps were obtained in nine adult patients (pt) with CAD and a history of infarction. The criterion for non-viable myocardium was an FDG-PET uptake with less than 45% of the maximum in the respective segments. CD imaging was applied to the left ventricle by using six different methods to solve the inverse problem. Mean CD activity was calculated for a close meshed grid of 90 locations of the left ventricle. A cardiologist compared bulls eye plots of CD and FDG-PET activity by eye. Spearmans correlation coefficients and specificity at a given level of sensitivity (70%) were calculated. Bulls eye plots revealed a significant correlation of CD/PET in 5 pt and no correlation in 3 pt. One pt had a negative correlation. The six different CD reconstruction methods performed similar. While CD reconstruction has the principal potential to image viable myocardium, we found that the reconstructed CD magnitude was low in scar segments but also reduced in some segments with preserved metabolic activity under resting conditions. New vector measurement techniques, the use of additional stress testing and advances in mathematical methodology are expected to improve CD imaging in future.

Evaluation and comparison of surface and intensity based rigid registration methods for thorax and cardiac MR and PET images

In this paper an evaluation and a comparison of surface and image intensity based (mutual information, normalized mutual information and correlation ratio) rigid registration methods for cardiac magnetic resonance and positron emission tomography images are presented. In both types of rigid image registration methods, PET transmission image was used as a linking mediator to register corresponding PET emission image to MR image coordinates. Also direct rigid registration of PET emission image to MR image coordinates was tested. Methods were evaluated with simulated and ten patient MR-PET images and with three optimization methods. Results indicated that NMI and CR methods with simplex optimization provided the most robust and accurate results.

A 3-D Model-Based Approach for the PET-Functional and MR-Anatomical Cardiac Imaging Data Fusion

In this paper, an approach for the assessment of 3-D functional maps of the heart is proposed. It relies on the model-based coregistration of MR anatomical and PET metabolic images and the extraction of an individualized anatomical heart model from MR images. This results in a 3-D geometrical model of the heart for which functional parameters such as FDG uptake can be attributed and visualized.