Imaging Distinct Points in Left Ventricular Myocardium to Study Regional Wall Deformation

Quantification of left ventricular strain and torsion by joint analysis of 3D tagging and cine MR images

Cardiovascular magnetic resonance (CMR) imaging is the gold standard for the non-invasive assessment of left-ventricular (LV) function. Prognostic value of deformation metrics extracted directly from regular SSFP CMR images has been shown by numerous studies in the clinical setting, but with some limitations to detect torsion of the myocardium. Tagged CMR introduces trackable features in the myocardium that allow for the assessment of local myocardial deformation, including torsion; it is, however, limited in the quantification of radial strain, which is a decisive metric for assessing the contractility of the heart. In order to improve SSFP-only and tagged-only approaches, we propose to combine the advantages of both image types by fusing global shape motion obtained from SSFP images with the local deformation obtained from tagged images. To this end, tracking is first performed on SSFP images, and subsequently, the resulting motion is utilized to mask and track tagged data. Our implementation is based on a recent finite element-based motion tracking tool with mechanical regularization. Joint SSFP and tagged images registration performance is assessed based on deformation metrics including LV strain and twist using human and in-house porcine datasets. Results show that joint analysis of SSFP and 3DTAG images provides better quantification of LV strain and twist as either data source alone.

Imaging three-dimensional cardiac function

The three-dimensional (3-D) nature of myocardial deformations is dependent on ventricular geometry, muscle fiber architecture, wall stresses, and myocardial-material properties. The imaging modalities of X-ray angiography, echocardiography, computed tomography, and magnetic resonance (MR) imaging (MRI) are described in the context of visualizing and quantifying cardiac mechanical function. The quantification of ventricular anatomy and cavity volumes is then reviewed, and surface reconstructions in three dimensions are demonstrated. The imaging of myocardial wall motion is discussed, with an emphasis on current MRI and tissue Doppler imaging techniques and their potential clinical applications. Calculation of 3-D regional strains from motion maps is reviewed and illustrated with clinical MRI tagging results. We conclude by presenting a promising technique to assess myocardial-fiber architecture, and we outline its potential applications, in conjunction with quantification of anatomy and regional strains, for the determination of myocardial stress and work distributions. The quantification of multiple components of 3-D cardiac function has potential for both fundamental-science and clinical applications.

Tag and contour detection in tagged MR images of the left ventricle

Tracking magnetic resonance tags in myocardial tissue promises to be an effective tool for the assessment of myocardial motion. The authors describe a hierarchy of image processing steps which rapidly detects both the contours of the myocardial boundaries of the left ventricle and the tags within the myocardium. The method works on both short axis and long axis images containing radial and parallel tag patterns, respectively. Left ventricular boundaries are detected by first removing the tags using morphological closing and then selecting candidate edge points. The best inner and outer boundaries are found using a dynamic program that minimizes a nonlinear combination of several local cost functions. Tags are tracked by matching a template of their expected profile using a least squares estimate. Since blood pooling, contiguous and adjacent tissue, and motion artifacts sometimes cause detection errors, a graphical user interface was developed to allow user correction of anomalous points. The authors present results on several tagged images of a human. A fully automated run generally finds the endocardial boundary and the tag lines extremely well, requiring very little manual correction. The epicardial boundary sometimes requires more intervention to obtain an acceptable result. These methods are currently being used in the analysis of cardiac strain and as a basis for the analysis of alternate tag geometries.

Macroscopic three-dimensional motion patterns of the left ventricle

The pattern of displacements in the left ventricle (LV) can be described by 13 modes of motion and deformation. Three functional modes of deformation are essential for ejection: a decrease in cavity volume, torsion, and ellipticalization. Four additional modes are used to describe asymmetric deformation. Six modes of rigid body motion describe rotation and translation. In the LV 14-20 radiopaque markers were inserted in the wall of the LV. They were distributed more or less evenly from base to apex and around the circumference. Torsion and volume changes require the definition of a cardiac coordinate system. The point at which ejection focuses is used as the origin, and the torsion axis is used as the z-axis. In the present study the coordinate system was positioned objectively by a least squares fit of the kinematic model to the measured motion of markers. In five dogs in the control state the kinematic parameters were determined as a function of time for all 13 modes. The torsion axis was displaced 4 +/- 2 mm (mean +/- sd) from the center of the cross-section of the LV towards the lateral free wall. The direction of the torsion axis closely coincided with anatomical landmarks at the apex and base. During systole, a unique relation was found between the ratio of cavity volume to wall volume and torsion. This relation was universal to all LVs, the cylinder-symmetric mathematical model of cardiac mechanics inclusive. In diastole the patterns of deformation seem less universal and reproducible.

Motion estimation from tagged MR image sequences

A method for reconstructing motion from sequences of tagged magnetic resonance (MR) images is presented. MR tagging is used to create a spatial pattern of varying magnetization so that objects which may otherwise have constant intensity are textured, which reduces the motion ambiguity associated with the aperture problem in computer vision. To compensate for the decay of the tag pattern, a new optical flow algorithm is developed and implemented. The resulting estimated velocity field is then used to recursively update the implied motion reference map over time, thereby tracking the motion of individual particles. If a segmentation of the object is known at the time the tag pattern is created, then an object may be selectively tracked, using the estimated reference map to update the object's position as time progresses. Results are shown for both simulated and actual MR phantom data.

Calculation of three-dimensional left ventricular strains from biplanar tagged MR images

The noninvasive measurement of time-resolved three-dimensional (3D) strains throughout the myocardium could greatly improve the clinical evaluation of cardiac disease and the ability to mathematically model the heart. On the basis of orthogonal arrays of tagged magnetic resonance (MR) images taken at several times during systole, such strains can be determined, but only after heart motion through the image planes is taken into account. An iterative material point-tracking algorithm is presented to solve this problem. It is tested by means of mathematical models of the heart with cylindric and spherical geometries that undergo deformations and bulk motions. Errors introduced by point-tracking interpolation were found to be negligible compared with those due to marker identification on the images. In a human heart studied with this technique, the corrected radial strains at the left ventricular base were approximately 2.5 times the two-dimensional estimates derived from the fixed image planes. The authors conclude that material point tracking allows accurate, time-resolved 3D strains to be calculated from tagged MR images, and that prior correction for motion of the heart through image planes is necessary.

Echo planar imaging of isolated perfused rat hearts at 4.7 T: a comparison of Langendorff and working heart preparations

The implementation of echo planar imaging at 4.7 T is demonstrated using a homebuilt gradient and radiofrequency assembly. The application of such a technique to the study of the isolated perfused rat heart is demonstrated. Langendorff and working heart perfusion preparations are compared and changes in the left ventricular volume shown are to be much larger in the working heart preparation. Such a methodology is expected to provide a useful model for the study of cardiac function and dynamics in the normal and diseased states under controlled perfusion conditions.