Leon Axel

Three-dimensional left ventricular deformation in hypertrophic cardiomyopathy.

BackgroundIn hypertrophic cardiomyopathy, ejection fraction is normal or increased, and force-length relations are reduced. However, three-dimensional (3D) motion and deformation in vivo have not been assessed in this condition. We have reconstructed the 3D motion of the left ventricle (LV) during systole in 7 patients with hypertrophic cardiomyopathy (HCM) and 12 normal volunteers by use of magnetic resonance tagging. Methods and ResultsTransmural tagging stripes were automatically tracked to subpixel resolution with an active contour model. A 3D finite-element model was used to interpolate displacement information between short- and long-axis slices and register data on a regional basis. Displacement and strain data were averaged into septal, posterior, lateral, and anterior regions at basal, midventricular, and apical levels. Radial motion (toward the central long axis) decreased slightly in patients with HCM, whereas longitudinal displacement (parallel to the long axis) of the base toward the apex was markedly reduced: 7.5±2.5mm (SD) versus 12.5±2.0 mm, P<.001. Circumferential and longitudinal shortening were both reduced in the septum (P<.01 at all levels). The principal strain associated with 3D maximal contraction was slightly depressed in many regions, significantly in the basal septum (−0.18±0.05 versus −0.22±0.02, P<.05) and anterior (−0.20±0.05 versus −0.23±0.02, P<.05) walls. In contrast, LV torsion (twist of the apex about the long axis relative to the base) was greater in HCM patients (19.9±2.4° versus 14.6±2.7°, P<.01). ConclusionsHCM patients had reduced 3D myocardial shortening on a regional basis; however, LV torsion was increased.

Regional differences in function within noninfarcted myocardium during left ventricular remodeling.

BACKGROUND The mechanisms of ventricular enlargement and dysfunction during postinfarct remodeling remain largely unknown. Although global left ventricular architectural changes after myocardial infarction are well documented, differences in function between adjacent and remote noninfarcted myocardium during left ventricular remodeling have not been investigated. These functional differences may relate to regional differences in wall stress during contraction and may contribute to chamber enlargement and global dysfunction after infarction. METHODS AND RESULTS Anteroapical infarcts were produced in seven sheep by ligation of the mid left anterior descending coronary artery and second diagonal branch at thoracotomy. Magnetic resonance short-axis and long-axis images tagged by spatial modulation of magnetization were obtained before and 1 week, 8 weeks, and 6 months after infarction. Left ventricular volumes, mass, ejection fraction, and lengths of infarcted and noninfarcted segments were measured. Circumferential and longitudinal shortening in the subendocardium and subepicardium, wall thickness, and histopathology were assessed in infarcted segments and regions adjacent to and remote from the infarct border. We found that a difference in circumferential and longitudinal segmental shortening between adjacent and remote noninfarcted myocardium present at 1 week persisted up to 6 months after myocardial infarction. However, partial improvement of function in adjacent regions occurred during infarct healing between 1 and 8 weeks after infarction. Left ventricular volume increased up to 6 months after infarction, out of proportion to the concomitant eccentric hypertrophy, whereas the ejection fraction fell. Left ventricular dilatation late in the remodeling process was secondary to lengthening of noninfarcted segments, which were free of significant fibrosis. CONCLUSIONS Left ventricular dilatation and eccentric hypertrophy during remodeling are associated with persistent differences in segmental function between adjacent and remote noninfarcted regions. These functional differences may reflect increased wall stress in adjacent noninfarcted regions and contribute to the global dilatation and dysfunction characteristic of left ventricular remodeling after infarction.

Tracking and finite element analysis of stripe deformation in magnetic resonance tagging

Magnetic resonance tissue tagging allows noninvasive in vivo measurement of soft tissue deformation. Planes of magnetic saturation are created, orthogonal to the imaging plane, which form dark lines (stripes) in the image. The authors describe a method for tracking stripe motion in the image plane, and show how this information can be incorporated into a finite element model of the underlying deformation. Human heart data were acquired from several imaging planes in different orientations and were combined using a deformable model of the left ventricle wall. Each tracked stripe point provided information on displacement orthogonal to the original tagging plane, i.e., a one-dimensional (1-D) constraint on the motion. Three-dimensional (3-D) motion and deformation was then reconstructed by fitting the model to the data constraints by linear least squares. The average root mean squared (rms) error between tracked stripe points and predicted model locations was 0.47 mm (n=3,100 points). In order to validate this method and quantify the errors involved, the authors applied it to images of a silicone gel phantom subjected to a known, well-controlled, 3-D deformation. The finite element strains obtained were compared to an analytic model of the deformation known to be accurate in the central axial plane of the phantom. The average rms errors were 6% in both the reconstructed shear strains and 16% in the reconstructed radial normal strain.

Quality assurance methods and phantoms for magnetic resonance imaging: report of AAPM nuclear magnetic resonance Task Group No. 1.

DISCLAIMER: This publication is based on sources and information believed to be reliable, but the AAPM and the editors disclaim any warranty or liability based on or relating to the contents of this publication. The AAPM does not endorse any products, manufacturers, or suppliers. Nothing in this publication should be interpreted as implying such endorsement.

Intramural myocardial shortening in hypertensive left ventricular hypertrophy with normal pump function.

BACKGROUND In hypertensive left ventricular hypertrophy (LVH), intrinsic myocardial systolic function may be normal or depressed. Magnetic resonance tagging can depict intramural myocardial shortening in vivo. METHODS AND RESULTS Tagged left ventricular magnetic resonance images were obtained in 30 hypertensive subjects with LVH (mean LV mass index, 142 +/- 41 g/m) and normal ejection fraction (mean, 64 +/- 9%) using spatial modulation of magnetization. In 26 subjects, circumferential myocardial shortening (%S) was compared with results obtained in 10 normal subjects at endocardium, midwall, and epicardium on up to 4 short-axis slices each. Similarly, in 10 subjects, midwall long-axis shortening at basal, midventricular, and apical sites was compared with results obtained in 12 normal volunteers. Circumferential %S was reduced in hypertensive subjects. Mean shortening was 29 +/- 6% at the endocardium in hypertensive subjects versus 44 +/- 6% in normal subjects (P = .0001); 20 +/- 6% at the midwall versus 30 +/- 6% (P = .0001); and 13 +/- 5% at the epicardium versus 21 +/- 5% (P = .0002). However, the transmural gradient in percent shortening from endocardium to epicardium in hypertensive subjects paralleled that in normal subjects. The normal base-to-apex gradient in circumferential %S was absent in LVH. In contrast to normal subjects, circumferential %S showed regional heterogeneity in hypertensive subjects, being maximal in the lateral wall and least in the inferior wall. Longitudinal shortening was also uniformly depressed in hypertensive subjects: 10 +/- 9% at the base versus 21 +/- 6% in normal subjects (P = .0001); 14 +/- 8% at the midventricle versus 18 +/- 3% (P = .03); and 14 +/- 8% at the apex versus 18 +/- 4% (P = .04). CONCLUSIONS In hypertensive LVH with normal pump function, intramural circumferential and longitudinal myocardial shortening are depressed.

Analysis of left ventricular wall motion based on volumetric deformable models and MRI-SPAMM.

We present a new approach for the analysis of the left ventricular shape and motion based on the development of a new class of volumetric deformable models. We estimate the deformation and complex motion of the left ventricle (LV) in terms of a few parameters that are functions and whose values vary locally across the LV. These parameters capture the radial and longitudinal contraction, the axial twisting, and the long-axis deformation. Using Lagrangian dynamics and finite-element theory, we convert these volumetric primitives into dynamic models that deform due to forces exerted by the datapoints. We present experiments where we used magnetic tagging (MRI-SPAMM) to acquire datapoints from the LV during systole. By applying our method to MRI-SPAMM datapoints, we were able to characterize the 3-D shape and motion of the LV both locally and globally, in a clinically useful way. In addition, based on the model parameters we were able to extract quantitative differences between normal and abnormal hearts and visualize them in a way that is useful to physicians.

Two-dimensional left ventricular deformation during systole using magnetic resonance imaging with spatial modulation of magnetization.

BackgroundMyocardial tissue tagging with the use of magnetic resonance imaging allows noninvasive regional analysis of heart wall motion and deformation. However, any evaluation of the effect of disease or treatment requires a baseline reference of normal values and variation. We studied the two-dimensional motion of material points imaged within the left ventricular wall using spatial modulation of magnetization (SPAMM) in 12 normal human volunteers. Methods and ResultsFive parallel short-axis and five parallel long-axis slices were acquired at five times during systole. SPAMM tags were generated at end diastole using a 7-mm grid. Intersection point data were analyzed for displacement, rotation, and torsion, and triangles of points were analyzed for local rotation and principal strains. Short-axis displacement was the least in the septum for all longitudinal levels (P < .001). Torsion about the long axis was uniform circumferentially because of the motion of the centroids used to reference the rotation. In the long-axis images, the base displaced longitudinally toward the apex, with the posterior wall moving farther than the anterior wall (13.4±2.2 versus 9.7±1.8 mm, P < .001) in this direction. The largest principal strain (maximum lengthening) was approximately radially oriented in both views. In the short-axis images, the minimum principal strain (maximum shortening) increased in magnitude toward the apex (P < .001) with little circumferential variation, except at midventricle, where the anterior wall showed greater contraction than the posterior wall (−0.21±0.03 versus −0.19±0.02, P < .02). ConclusionsConsistent regional variations in deformation are seen in the normal human heart. Displacement and maximum shortening strains are well characterized with twodimensional magnetic resonance tagging; however, higherresolution images will be required to study transmural variations.

MR imaging of arrhythmogenic right ventricular cardiomyopathy: Morphologic findings and interobserver reliability

Background: Magnetic resonance (MR) imaging is frequently used to diagnose arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D). However, the reliability of various MR imaging features for diagnosing ARVC/D is unknown. The purpose of this study was to determine which morphologic MR imaging features have the greatest interobserver reliability for diagnosing ARVC/D. Methods: Forty-five sets of films of cardiac MR images were sent to 8 radiologists and 5 cardiologists with experience in this field. There were 7 cases of definite ARVC/D as defined by the Task Force criteria. Six cases were controls. The remaining 32 cases had MR imaging because of clinical suspicion of ARVC/D. Readers evaluated the images for the presence of (a) right ventricle (RV) enlargement, (b) RV abnormal morphology, (c) left ventricle enlargement, (d) presence of high T1 signal (fat) in the myocardium, and (e) location of high T1 signal (fat) on a Likert scale with formatted responses. Results: Readers indicated that the Task Force ARVC/D cases had significantly more (χ2 = 119.93, d.f. = 10, p < 0.0001) RV chamber size enlargement (58%) than either the suspected ARVC/D (12%) or no ARVC/D (14%) cases. When readers reported the RV chamber size as enlarged they were significantly more likely to report the case as ARVC/D present (χ2= 33.98, d.f. = 1, p < 0.0001). When readers reported the morphology as abnormal they were more likely to diagnose the case as ARVC/D present (χ2 = 78.4, d.f. = 1, p < 0.0001), and the Task Force ARVC/D (47%) cases received significantly more abnormal reports than either suspected ARVC/D (20%) or non-ARVC/D (15%) cases. There was no significant difference between patient groups in the reported presence of high signal intensity (fat) in the RV (χ2 = 0.9, d.f. = 2, p > 0.05). Conclusions: Reviewers found that the size and shape of abnormalities in the RV are key MR imaging discriminates of ARVD. Subsequent protocol development and multicenter trials need to address these parameters. Essential steps in improving accuracy and reducing variability include a standardized acquisition protocol and standardized analysis with dynamic cine review of regional RV function and quantification of RV and left ventricle volumes.

Circumferential myocardial shortening in the normal human left ventricle. Assessment by magnetic resonance imaging using spatial modulation of magnetization.

BackgroundConventional cardiac imaging methods do not depict true segmental myocardial shortening, since they cannot determine segment length between fixed points in the myocardium. Methods and ResultsWe used electrocardiographically gated magnetic resonance imaging with spatial modulation of magnetization to noninvasively “tag” the myocardium with dark stripes at uniform 7-mm intervals center to center at end diastole. We then determined end-systolic stripe separation and thereby calculated circumferential shortening. When end systole was not reached in the first image series, a second temporally overlapped series starting in late systole was used to determine late-systolic shortening. Septal, anterior, lateral, and inferior segments were assessed at endocardium, midwall, and epicardium on five midventricular short-axis sections each in 10 normal volunteers. A transmural gradient in circumferential shortening was observed, with the percentage of endocardial segment shortening consistently greater than epicardial segment shortening (epicardial, 22 ± 5%; midwall, 30 ± 6%; and endocardial, 44 ± 6%; p < 0.0001 by analysis of variance). Circumferential shortening varied from apex to base with slices closer to the base of the left ventricle showing less shortening at the midwall (28 ± 9%o) and endocardium (39 ± 6%) than more apical slices at the midwall (34 ± 13%) and endocardium (49 ± 9%o) (p < 0.05 and p < 0.01, respectively, by analysis of variance). ConclusionsTransmural and longitudinal heterogeneity of circumferential shortening is present in the normal human left ventricle. Magnetic resonance imaging with spatial modulation of magnetization is a powerful new tool for assessment of circumferential shortening and provides information unobtainable with conventional imaging methods. (Circulation 1991;84:67–74)

Deformable models with parameter functions for cardiac motion analysis from tagged MRI data

The authors present a new method for analyzing the motion of the hearts left ventricle (LV) from tagged magnetic resonance imaging (MRI) data. Their technique is based on the development of a new class of physics-based deformable models whose parameters are functions. They allow the definition of new parameterized primitives and parameterized deformations which can capture the local shape variation of a complex object. Furthermore, these parameters are intuitive and require no complex post-processing in order to be used by a physician. Using a physics-based approach, the authors convert the geometric models into dynamic models that deform due to forces exerted from the datapoints and conform to the given dataset. The authors present experiments involving the extraction of the shape and motion of the LVs mid-wall during systole from tagged MRI data based on a few parameter functions. Furthermore, by plotting the variations over time of the extracted LV model parameters from normal and abnormal heart data along the long axis, the authors are able to quantitatively characterize their differences.