Magnetic Tagging Research Articles

Right ventricular midwall surface motion and deformation using magnetic resonance tagging.

We describe a method for reconstructing the three-dimensional motion and deformation of the midwall surface of the right ventricular free wall (RVFW) using magnetic resonance tissue tagging. Tag points were defined where the tag stripes intersected the midwall contour and were tracked through systole in both short- and long-axis images. A finite-element model of the midwall surface of the RVFW was constructed to fit the midwall shape at end diastole. The model was then deformed to each subsequent frame by fitting the tag displacements and midwall contour locations. The method was applied to two human studies, a normal subject and a patient with right ventricular hypertrophy. The root mean squared error between model tag planes and tracked tag points was 0.70 mm for the normal heart (180 points) and 0.67 mm for the hypertrophic heart (52 points), both less than the image pixel size of approximately 1.0 mm. The differences in contraction patterns were visualized between the two studies. We conclude that this method allows accurate, noninvasive measurement of in vivo RVFW deformation.

Effects of afterload on regional left ventricular torsion

To determine if left ventricular torsion, as measured by magnetic resonance tissue tagging, is afterload dependent in a canine isolated heart model in which neurohumoral responses are absent, and preload is constant. In ten isolated, blood perfused, ejecting, canine hearts, three afterloads were studied, while keeping preload constant: low afterload, high afterload (stroke volume reduced by approx. 50% of low afterload), and isovolumic loading (infinite afterload). There were significant effects of afterload on both torsion (P < 0.05) and circumferential shortening (P < 0.0005). Between low and high afterloads, at the anterior region of the endocardium only, where torsion was maximal, there was a significant reduction in torsion (15.1 +/- 2.2 degrees to 7.8 +/- 1.8 degrees, P < 0.05). Between high afterload and isovolumic loading there was no significant change in torsion (7.8 +/- 1.8 degrees to 6.2 +/- 1.5 degrees, P = NS). Circumferential shortening at the anterior endocardium was significantly reduced both between low and high afterload (-0.19 +/- 0.02 to -0.11 +/- 0.02, P < 0.0005), and also between high afterload and isovolumic loading (-0.11 +/- 0.02 to 0.00 +/- 0.02, P < 0.05). Plots of strains with respect to end-systolic volume demonstrated a reduction in both torsion and shortening with afterload-induced increases in end-systolic volume. Torsion, but not circumferential shortening, persisted at isovolumic loading. Maximal regional torsion of the left ventricle is afterload dependent. The afterload response of torsion appears related to the effects of afterload on end-systolic volume.

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.

Angiotensin-converting enzyme inhibition limits dysfunction in adjacent noninfarcted regions during left ventricular remodeling

We hypothesized that angiotensin-converting enzyme inhibitors would limit dysfunction in the first 8 weeks after transmural infarction in adjacent noninfarcted regions, as well as attenuate left ventricular remodeling. Angiotensin-converting enzyme inhibition limits ventricular dilation and hypertrophy and improves survival after anterior infarction, but its effect on regional function during remodeling is not well characterized. Thirteen sheep underwent coronary ligation to create an anteroapical infarction. At postinfarction day 2, eight sheep were randomized to therapy with the angiotensin-converting enzyme inhibitor ramipril, and five sheep received no therapy. Animals were studied with magnetic resonance myocardial tagging before and 8 weeks after infarction. Left ventricular volume, mass and ejection fraction were measured, as were changes in percent circumferential shortening within the subendocardium and subepicardium of infarcted and noninfarcted myocardium, both adjacent to and remote from the infarction. Angiotensin-converting enzyme inhibition limited the increase in end-diastolic volume from a mean (+/- SD) of +1.5 +/- 0.7 ml/kg in control animals to +0.5 +/- 0.8 ml/kg in the treated group (p < 0.04). Segmental function within infarcted and remote noninfarcted tissue did not differ between groups. However, angiotensin-converting enzyme inhibition limited the decline in function in the adjacent noninfarcted region 8 weeks after infarction. Percent circumferential shortening in the subendocardium decreased by -13 +/- 5% in the control group compared with -5 +/- 5% in the treated group (p < 0.03). In concert with a reduction in left ventricular remodeling after anterior infarction, angiotensin-converting enzyme inhibition limits the decline in function in the adjacent noninfarcted region. Dysfunction in adjacent noninfarcted regions may be an important determinant of left ventricular remodeling after infarction.

Assessment of right ventricular motion with magnetic resonance tissue tagging and slice following

Assessment of right ventricular motion with magnetic resonance tissue tagging and slice following

969-97 Regional Strain and Wall Motion of the Transplanted Left Ventricle in Pediatric Patients Using Magnetic Resonance Tagging

The biomechanical properties of the transplanted left ventricle (LV) may not be the same as the normal LV which is innervated, has not undergone ischemic arrest, and is not at risk for cellular rejection. Abnormal LV systolic torsion has been noted during histologic rejection in adult transplant patients. This has not been assessed in pediatric patients. We studied 8 transplant patients (ages 5.1 ± 3.7 years, 2.6 ± 1.2 years post-transplant) who underwent 9 magnetic resonance imaging studies using spatial modulation of magnetization, a tissue tagging technique, to quantify regional wall motion and deformation. Imaging was done within 6 days of endomyocardial biopsy in 7 of 9 studies. Seven normal adults were imaged as controls. Magnetic tagging was performed at 2 LV short axis levels, atrioventricular valve (AW) and apex, through 12 phases in systole. Intersection points were tracked, and regional wall motion and strain (using the 2-dimensional strain tensor) was determined. The myocardium was divided into 4 anatomic regions (ventricular septum. IW = inferior wall, PW = posterior wall. and SW = superior wall) and subdivived further into endocardial and epicardial regions. Review of 7 biopsy specimens revealed 2 with rejection (grade 1A and grade 2). Cardiac catheterization demonslrated normal hemodynamics in all except the patient with grade 2 rejection. Seven of 8 patients had clockwise twist of the SW, IW, and/or PW. and all had regions of no twist. Four patients had akinetic regions and 2 demonstrated paradoxical motion of the ventricular septum. Controls had counterclockwise twist in all regions. In transplant patients, strain and strain heterogeneity were similar between the 4 regions at the AW and the apex. The ralio of endocardial to epicardial strain was similar between all regions at the AW, but not at the apex. Strain was not different between transplant patients and controls for each region. Strain heterogeneity was different between the 2 groups at the SW of the AW, and at the ventricular septum of the AW and apex. When comparing AW to apex, there was no difference in strain, or the endocardial-to-epicardial strain ratio. between transplant patients and controls for each region. Pediatric transplant patients demonstrale regional wall motion abnormalities in the absence of rejection. Compared to controls, the transplanted LV maintains normal strain in the presence of abnormal twist. This may be a compensatory mechanism and may have clinical implications.

972-105 Afterload Reduction Selectively Improves Myocardial Deformation After Infarction in Adjacent Noninfarcted Regions

left ventricular remodeling post-infarction (post-MI) is accompanied by persistent dysfunction in the non-infarcted adjacent region. Chronic afterload reduction (ALR) therapy has a salutary effect on post-MI remodeling. The relative contribution of mechanical unloading to this effect remains unclear. We hypothesized that improved adjacent (ADJ) region dysfunction contributes to the beneficial effect of ALR. We therefore studied myocardial deformation in ADJ and remote (REM) non-infarcted regions in 5 sheep two months after anteroapical infarction during control (C) and after nitroprusside infusion (NP). Using magnetic resonance tissue tagging and finite element analysis techniques, we measured deformation using the orthogonal principal strains, λl (1 + greatest systolic elongation), λ2 (1 - greatest systolic shortening), and β or angular deviation of λl from the radial direction. With NP. LV systolic pressure fell from 94 ± 9 to 69 ± 7 mmHg (p &lt; 0.05) while end-diastolic pressure was unchanged, as measured by a high-fidelity catheter. Results (mean ± S.E.): REM deformation and its orientation were similar in C and NP In contrast: ADJλI ADJ λ2 ADJβ(°) C 1.06 ± 0.02 † 0.88 ± 0.01 † 45.6 ± 7.6 † NP 1.09 ± 0.03 † 0.83 ± 0.01 * † 25.8 ± 2.4 * † ANOVA P NS &lt; 0.05 &lt; 0.05 * p &lt; 0.05 vs. Control † p &lt; 0.05 REM vs. ADJ Thus in the ADJ regions, λl and λ2 were reduced compared to REM, and λ2 decreased after NP. denoting greater systolic shortening. The most important change with NP was the normalization of ADJ β, resulting in a greater contribution to pump function. In conclusion, improved mechanical function in the ADJ non-infarcted regions, particularly the orientation of principal strains, is an important determinant of the beneficial effect of ALR on post-MILV function.

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.

Regional heterogeneity of function in hypertrophic cardiomyopathy.

In patients with hypertrophic cardiomyopathy (HCM), left ventricular ejection performance may be normal while segmental myocardial function is distinctly abnormal. The advent of magnetic resonance tissue tagging has allowed the noninvasive evaluation of intramyocardial segmental shortening in vivo in a topographic and temporal manner. Ten patients with HCM documented by echocardiography and 10 healthy volunteers were studied with magnetic resonance tissue tagging by spatial modulation of magnetization. Percent circumferential myocardial shortening (%S) was compared at endocardium, midwall, and epicardial levels at four regions around the left ventricular short axis and from four short axis slices from apex to base at four or five time intervals during systole. In 8 patients and 8 control subjects, longitudinal shortening was evaluated within the septum and the lateral free wall at three levels from apex to base. Circumferential %S was less in HCM patients than in control subjects in the septal (13 +/- 5% versus 24 +/- 6%, P = .0002), inferior (13 +/- 5% versus 21 +/- 4%, P = .001), and anterior (17 +/- 5% versus 21 +/- 3%, P < .03) regions but not in the lateral region. Circumferential end-systolic %S was reduced in patients with HCM compared with control subjects at all levels from apex to base. The normal transmural gradient in circumferential end-systolic shortening was preserved with greatest %S at the endocardium. Most of the total cumulative circumferential shortening occurred earlier in systole in patients compared with control subjects, especially within the septum. Longitudinal end-systolic %S was depressed throughout the septum in patients compared with control subjects, most markedly at the base, but was normal in the lateral free wall. Circumferential myocardial segment shortening is depressed in HCM in the septum, inferior, and anterior regions and at all levels from apex to base, and much of the total cumulative shortening occurs early in systole. Longitudinal shortening is reduced in the basal septum in HCM. The heterogeneity of regional function in these patients may reflect the regional variation in the myocardial disarray and fibrosis that is characteristic of this disorder.

Evaluation of left ventricular segmental wall motion in hypertrophic cardiomyopathy with myocardial tagging.

Segmental wall motion was assessed noninvasively in eight patients with hypertrophic cardiomyopathy and six healthy volunteers by magnetic resonance myocardial tagging. Localization scans were performed for determination of the true short-axis views of the left ventricle (double-angulated view). Spatial modulation of magnetization was used to produce a rectangular grid of landmarks. Distortion of the grid was assessed at end diastole, mid systole, and end systole with multiphase gradient echoes. Image sets were acquired at three different planes, namely, the base, the equator, and the apex. Quantitative evaluation was carried out by computer-assisted image analysis. Each individual grid crossing point was identified automatically and the displacement calculated. A polar coordinate system with the center of gravity as motion reference point was chosen to assess fractional rotation and radial displacement at the endocardial, midwall, and epicardial layers of the septal, anterior, posterior, and inferior regions. A wringing motion of the left ventricle with a clockwise rotation of 5.0 +/- 2.4 degrees at the base and a counterclockwise rotation of -9.6 +/- 2.9 degrees at the apex was observed in control subjects. An equal rotation of 5.0 +/- 2.5 degrees at the base and a slightly reduced rotation of -7.3 +/- 5.2 degrees at the apex was found in patients with hypertrophic cardiomyopathy. A transmural gradient in fractional rotation and radial displacement was observed, with the highest values in the endocardial layer. Rotation in patients with hypertrophic cardiomyopathy was significantly less than in normal volunteers in the posterior region of the equatorial and apical planes. Furthermore, radial displacement was significantly reduced in the septum and inferior wall. In the anterior and posterior wall segments, a reduction of the radial displacement was observed only in the epicardium and midwall layers. Magnetic resonance myocardial tagging allows the noninvasive assessment of regional wall motion. Both in normal volunteers and in patients with hypertrophic cardiomyopathies, cardiac motion occurs in a complex mode, with the base and the apex rotating in opposite directions and the equatorial plane as a transitional zone (wringing motion). A reduced cardiac rotation can be observed in patients with hypertrophic cardiomyopathy mainly in the posterior region, whereas a reduced radial displacement is found in the inferior septal zone.

Regional heart wall motion: two-dimensional analysis and functional imaging with MR imaging.

Analysis of the transmural distribution or nonradial components of myocardial motion has previously been possible only with use of invasive techniques such as implantation of radiopaque markers. Magnetic tagging of the heart wall in conjunction with magnetic resonance imaging allows noninvasive regional analysis of within-wall motion, including its separation into components of rigid body motion and deformation. The results of this analysis can be displayed as functional images. This provides a new tool for the study of heart wall motion that should be of use for both basic physiologic and clinical research applications.

Acquisition and evaluation of tagged magnetic resonance images of the human left ventricle

Magnetic resonance myocardial tagging was used to noninvasively analyze the complicated contraction pattern of the human cardiac left ventricle. The tagging and imaging sequence was optimized to obtain three to four double-angulated short-axis views during systole. The image contrast between labeled and unlabeled tissue was sufficient to apply a semiautomatic image evaluation procedure. In accordance with the invasively achieved findings of other groups, the measurements indicate a wringing motion of the left ventricle, with a clockwise twist at the heartbase and a contrary rotation at the apical level.

Simulation of magnetic flocculation behaviour of fine minerals

The feasibility of predicting the behaviour of aqueous dispersions of fine particles of magnetic minerals by direct computation of the Brownian, van der Waals, Born, electrical double layer and magnetic forces acting on each particle employing an advanced supercomputer is described. In this manner it is possible to estimate the extent of magnetic flocculation and the size and geometry of the flocs formed as a function of key process variables (particle size, solids concentration, magnetic field strength, ionic composition of electrolyte, temperature, etc.). Such data are of significance to processes involving magnetic mineral colloids, ferromagnetic dense media, and mineral separation methods based on magnetic tagging. The methodology used represents an example of the type of calculations that have been facilitated by the recent developments in high speed computing technology. The notion of using these tools to enable realistic modelling of complex particulate and mineral processes is analogous to the use of molecular modelling in the field of drug design. Future applications and developments of these techniques are discussed and it is envisaged that in due course they will radically transform the modelling and design procedures used for mineral and metallurgical processes.

Quantification of and correction for left ventricular systolic long-axis shortening by magnetic resonance tissue tagging and slice isolation.

Measurement of regional left ventricular (LV) function is predicted on the ability to compare equivalent LV segments at different time points during the cardiac cycle. Standard techniques of short-axis acquisition in two-dimensional echocardiography, cine computed tomography, and standard magnetic resonance imaging (MRI) acquire images from a fixed plane and fail to compensate for through-plane motion. The shortening of the left ventricle along its long axis during systole results in planar images of two different levels of the ventricle, leading to error in any derived functional measurements. LV systolic long-axis motion was measured in 19 normal volunteers using MRI. With a selective radio frequency (RF) tissue-tagging technique, three short-axis planes were labeled at end diastole and standard spin-echo images were acquired at end systole in the two- and four-chamber orientations. Persistence of the tags through systole allowed visualization of the intersecting short-axis tags in the long-axis images and allowed precise quantification of long-axis motion of the septum, lateral, anterior, and inferior walls at the base, mid, and apical LV levels. The total change in position along the long axis between end diastole and end systole was greatest at the base, which moved toward the apex 12.8 +/- 3.8 mm. The mid left ventricle moved 6.9 +/- 2.6 mm, and the apex was nearly stationary, moving only 1.6 +/- 2.2 mm (p less than 0.001). Having quantified the normal range of long-axis shortening, we developed a technique that isolates a slice of tissue between selective RF saturation planes at end diastole. Combining this with a wide end-systolic image slice, end-systolic images were acquired without contamination of signal from adjacent tissue moving into the imaging plane. This technique was validated in a moving phantom and in normal volunteers. Significant LV systolic long-axis shortening exists, and this effect is seen the most at the base and the least at the apex. At a given ventricular level, shortening varied significantly according to location. A method using selective saturation pulses and gated spin-echo MRI automatically corrects for this motion and thus eliminates misregistration artifact from regional function analysis.

Noninvasive quantification of left ventricular rotational deformation in normal humans using magnetic resonance imaging myocardial tagging.

It has been postulated that rotation of the left ventricular apex with respect to the base is a component of normal systolic function in humans, but it has been difficult to measure it noninvasively. Tagging is a new magnetic resonance imaging technique that labels specific areas of myocardium by selective radio-frequency excitation of narrow planes orthogonal to the imaging plane before acquiring an image. Tags appear as black lines and persist in myocardium for 400-500 msec and, if applied at end diastole, will move with the myocardium through systole. Tagging was used to noninvasively quantify left ventricular torsion and circumferential-longitudinal shear (shearCL) in humans. Eight normal volunteers, aged 24-38 years, were imaged in a 0.38-T iron-core resistive magnet. Five short-axis left ventricular images, positioned to encompass the entire left ventricle (LV), were obtained separately at end systole. Four equiangular radial tags had been applied at end diastole, intersecting the myocardium at eight locations. We calculated the difference in angular displacement of each epicardial and endocardial tag point (a tag point being where the tag crossed the epicardium or endocardium) at end systole from the systolic position of the corresponding tag point on the basal plane. This value was called the torsion angle. From this, shearCL, the angle inscribed on the epicardial or endocardial surface between the systolic tag position, the corresponding basal tag position, and its projection onto the slice of interest could be calculated at 32 points in the left ventricular wall.(ABSTRACT TRUNCATED AT 250 WORDS)

High gradient magnetic cell separation with MACS

A flexible, fast and simple magnetic cell sorting system for separation of large numbers of cells according to specific cell surface markers was developed and tested. Cells stained sequentially with biotinylated antibodies, fluorochrome-conjugated avidin, and superparamagnetic biotinylated-microparticles (about 100 nm diameter) are separated on high gradient magnetic (HGM) columns. Unlabelled cells pass through the column, while labelled cells are retained. The retained cells can be easily eluted. More than 10(9) cells can be processed in about 15 min. Enrichment rates of more than 100-fold and depletion rates of several 1,000-fold can be achieved. The simultaneous tagging of cells with fluorochromes and very small, invisible magnetic beads makes this system an ideal complement to flow cytometry. Light scatter and fluorescent parameters of the cells are not changed by the bound particles. Magnetically separated cells can be analysed by fluorescence microscopy or flow cytometry or sorted by fluorescence-activated cell sorting without further treatment. Magnetic tagging and separation does not affect cell viability and proliferation.

High-temperature EPR in superionic fluorites

The EPR of the ${\mathrm{Mn}}^{2+}$-doped superionic conducting fluorites Ca${\mathrm{F}}_{2}$, Sr${\mathrm{F}}_{2}$, and Ba${\mathrm{F}}_{2}$ has been studied at high temperatures at 9.3 GHz, and that of Pb${\mathrm{F}}_{2}$ and Sr${\mathrm{Cl}}_{2}$ at 22 GHz as well. In the fluoride compounds, the $^{19}\mathrm{F}$ transferred hyperfine structure (hfs) that is superimposed upon each $^{55}\mathrm{Mn}$ hfs component is observed to collapse when the ${\mathrm{F}}^{\ensuremath{-}}$ hopping rate ${w}^{\ensuremath{-}}$ exceeds the $^{19}\mathrm{F}$ hfs coupling $\frac{{A}^{(19)}}{\ensuremath{\hbar}}$. The integrated intensity, ${m}^{(55)}$, and frequency dependence of the line profiles have been used to identify (in addition to static dipolar broadening) three distinct line-broadening mechanisms: (1) a quasistatic, nonlocal distortion of the cubic crystal field that results from the growth of ion disorder with increasing $T$, (2) a local, dynamic broadening related to enhanced anharmonic phonon effects, and (3) a combined ${\mathrm{Mn}}^{2+}$-concentration--- and $T$---dependent "collision" broadening which proceeds via dipolar encounters between diffusing ${\mathrm{Mn}}^{2+}$ ions and is proportional to their hopping rate, ${w}^{+}$. The latter mechanism is closely related to the effects of "magnetic tagging" on the $^{19}\mathrm{F}$ NMR relaxation in Mn-doped Pb${\mathrm{F}}_{2}$.

Magnetic tagging of theF19NMR relaxation in aliovalently doped PbF2

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Rotational hysteresis in recording materials measured with a gas turbine

Magnetic properties in recording particles are assessed in terms of their rotational hysteresis WR measured using a low power gas turbine. Values for the peak rotational hysteresis WRp divided by π/M2s and the field at the peak Hp divided by 2πMs are consistent with a model for magnetic reversal by curling. Rotational hystereis measurements within different planes of a sample give the magnetic orientation and the magnitude of the shape demagnetizing fields. From the profile for the curve of WR versus field, the distribution of mixtures which contain components with distinctly different coercive forces is determined, giving this method potential as a technique for magnetic tagging.

Effects of early experience on emotional and social reactivity in CBA mice

During infancy some mice were handled and others were not handled in conjunction with their being reared in groups or isolation until adulthood. Emotional reactivity was assessed at 120 days of age by their activity in the open field, followed by measurement of plasma corticosterone levels, and social interaction was observed during their 15 weeks in a Reimer-Petras population cage, using a magnetic tagging technique. Handling and socialization produced a significant increase in open field activity, with the handled-socialization animals being the most active. Isolated animals had significantly higher plasma corticosterone levels than socialized animals, but the difference between handled and nonhandled males was not significant. The handled-socialized animals developed the most stable social hierarchy in the population cage, successfully differentiated roles, and had the lowest increase in systolic blood pressure. Isolated males failed to develop normal social behavior and severe fighting was observed throughout the 15 weeks; moreover, these animals had a higher elevation of systolic blood pressure than the socialized animals.