Cengizhan Ozturk

Four-dimensional B-spline based motion analysis of tagged MR images: introduction and in vivo validation.

In MRI tagging, magnetic tags-spatially encoded magnetic saturation planes-are created within tissues acting as temporary markers. Their deformation pattern provides useful qualitative and quantitative information about the functional properties of underlying tissue and allows non-invasive analysis of mechanical function. The measured displacement at a given tag point contains only unidirectional information; in order to track the full 3D motion, these data have to be combined with information from other orthogonal tag sets over all time frames. Here, we provide a method to describe the motion of the heart using a four-dimensional tensor product of B-splines. In vivo validation of this tracking algorithm is performed using different tagging sets on the same heart. Using the validation results, the appropriate control point density was determined for normal cardiac motion tracking. Since our motion fields are parametric and based on an image plane based Cartesian coordinate system, trajectories or other derived values (velocity, acceleration, strains ...) can be calculated for any desired point within the volume spanned by the control points. This method does not rely on specific chamber geometry, so the motion of any tagged structure can be tracked. Examples of displacement and strain analysis for both ventricles are also presented.

Left ventricular motion reconstruction from planar tagged MR images: a comparison

Through recent development of MR techniques, it is now possible to assess regional myocardial wall function in a non-invasive way. Using MR tagging, space is marked with planes which deform with the tissue, providing markers for tracking the local motion of the myocardium. Numerous methods to reconstruct the three-dimensional displacement field have been developed. The aim of this article is to provide a framework to quantitatively compare the performance of four methods the authors have developed. Five sets of experiments are described, and their results are reported. Instructions are also provided to perform similar tests on any method using the same data. The experiments show that some characteristic properties of the methods, such as sensitivity to noise or spatial resolution, can be quantitatively classified. Cross-comparison of performances show what range values for these properties can be considered acceptable.

Four-dimensional B-spline-based motion analysis of tagged cardiac MR images

In recent years, with development of new MRI techniques, noninvasive evaluation of global and regional cardiac function is becoming a reality. One of the methods used for this purpose is MRI tagging. In tagging, spatially encoded magnetic saturation planes, tags, are created within tissues. These act as temporary markers and move with the tissue. In cardiac tagging, tag deformation pattern provides useful qualitative and quantitative information about the functional properties of underlying myocardium. The measured deformation of a single tag plane contains only unidirectional information of the past motion. In order to track the motion of a cardiac material point, this sparse, single dimensional data has to be combined with similar information gathered from other tag sets and all time frames. Previously, several methods have been developed which rely on the specific geometry of the chambers. Here, we employ an image plane based, simple cartesian coordinate system and provide a stepwise method to describe the heart motion using a four-dimensional tensor product of B-splines. The proposed displacement and forward motion fields exhibited sub-pixel accuracy. Since our motion fields are parametric and based on an image plane based coordinate system, trajectories or other derived values (velocity, acceleration, strains...) can be calculated for any desired point on the MRI images. This method is sufficiently general so that the motion of any tagged structure can be tracked.

Interactive four-dimensional segmentation of multiple image sets

We have developed a software tool for interactive visualization and 4D segmentation of multiple sets of images. The segmentation process uses a predefined anatomical template of the structure of interest represented as a polygonal mesh in 3D. This can be obtained from a library of normal or diseased anatomies, or if available, a surface generated from the patients previous studies can be used. The user then deforms the template so that it correctly delineates the region of interest in the underlying images. These deformations can be constrained to maintain spatial and temporal smoothness as is expected in the underlying anatomy. A unique feature of this analysis package is that multiple non-coplanar image sets can be used concurrently to generate accurate contours. This feature is particularly useful in contouring long axis and short axis images of the heart simultaneously. By generating a reliable segmentation from a substrate of images in space and time, we can automatically contour the structure in the remaining images through appropriate interpolation, and thereby significantly reduce the total segmentation time.

Free-breathing respiratory motion of the heart measured from x-ray coronary angiograms (Second Place Student Paper Award)

Respiratory motion compensation for cardiac imaging requires knowledge of the hearts motion and deformation during breathing. We propose a method for measuring the natural tidal respiratory motion of the heart using free breathing coronary angiograms. A 3D deformation field describing the cardiac and respiratory motion of the coronary arteries is recovered from a biplane acquisition. Cardiac and respiratory phase are assigned to the images from an ECG signal synchronized to the image acquisition, and from the diaphragmatic displacement as observed in the images. The resulting motion field is decomposed into cardiac and respiratory components by fitting the field with periodic 2D parametric functions, where one dimension spans one cardiac cycle, and the second dimension spans one respiratory cycle. The method is applied to patient datasets, and an analysis of respiratory motion of the heart is presented.

Interactive visualization of 4D parametric fields

Parametric images are generated from the analysis of image data to help characterize the functional information present in the original images. They address the need for enhancing the spatial and temporal resolutions for analysis. Parametric fields provide an underlying model that can be evaluated at any image location using its analytical formulation. To facilitate interactive display and analysis of such fields, we developed a visualization scheme that can help directly render the parametric field using graphics interpolation methods. This eliminates the need for high resolution storage of such data for visualization purposes. The major advantage of such an approach is that graphics hardware can be used to accelerate the interpolation, thus achieving substantial improvement in the performance. Successive derivative parametric images, such as velocity or acceleration maps from the motion fields, can be displayed in real-time. The example presented here is the 4D B-spline based motion field representation of the cardiac-tagged MR images. A motion field with 7 by 7 by 7 by 15 control points shown to adequately describe the full motion of the heart during cardiac cycle. Using this field, material points can be tracked over time and local mechanical properties can be computed. The visualization method presented here utilizes the similarity between the B-spline representation of the motion fields and the graphics hardware support for NURBS display with texture mapping to achieve high performance visualization of these parametric fields.