Joni Taylor

Novel technique for cardiac electromechanical mapping with magnetic resonance imaging tagging and an epicardial electrode sock.

AbstractNear-simultaneous measurements of electrical and mechanical activation over the entire ventricular surface are now possible using magnetic resonance imaging tagging and a multielectrode epicardial sock. This new electromechanical mapping technique is demonstrated in the ventricularly paced canine heart. A 128–electrode epicardial sock and pacing electrodes were placed on the hearts of four anesthetized dogs. In the magnetic resonance scanner, tagged cine images (8–15 ms/frame) and sock electrode recordings (1000 Hz) were acquired under right-ventricular pacing and temporally referenced to the pacing stimulus. Electrical recordings were obtained during intermittent breaks in image acquisition, so that both data sets represented the same physiologic state. Since the electrodes were not visible in the images, electrode recordings and cine images were spatially registered with Gd-DTPA markers attached to the sock. Circumferential strain was calculated at locations corresponding to electrodes. For each electrode location, electrical and mechanical activation times were calculated and relationships between the two activation patterns were demonstrated. This method holds promise for improving understanding of the relationships between the patterns of electrical activation and contraction in the heart.

Multislice first-pass cardiac perfusion MRI: Validation in a model of myocardial infarction†

The purpose of this study was to validate a first‐pass MRI method for imaging myocardial perfusion with multislice coverage and relatively small analyzable regions of interest (ROIs). A fast gradient‐echo (FGRE) sequence with an echo‐train (ET) readout was used to achieve multislice coverage, and a high dose of a contrast agent (CA) was used to achieve a high signal‐to‐noise ratio (SNR). Dogs (N = 6) were studied 1 day after reperfused myocardial infarction, and fluorescent microspheres were used as a standard for perfusion. First‐pass MRI correlated well vs. microsphere flow, achieving mean R values of 0.87 (range = 0.82–0.93), 0.71 (range = 0.46–0.85), and 0.72 (range = 0.49–0.95) for subendocardial ROIs, transmural ROIs, and the endocardial‐epicardial ratio, respectively. Additionally, analysis of myocardial time‐intensity curves (TICs) indicated that 15.8 ± 6.6° sectors, corresponding to 260 μl of endocardium, can be analyzed (R2 > 0.95). Magn Reson Med 47:482–491, 2002. Published 2002 Wiley‐Liss, Inc.

Manganese enhanced magnetic resonance imaging of normal and ischemic canine heart

The ability of MnCl2 to enhance canine myocardium and to delineate ischemic areas is demonstrated. A dose–response curve was measured using T1 weighted images in 11 dogs. MnCl2 (36, 113, 360, and 3600 μmol) was infused over a period of 3 min. Signal intensity increased linearly with MnCl2 dose. At 113 μmol (∼10 μmol/kg) the steady‐state increase in intensity averaged 212 ± 34%. No significant physiologic effects due to the infused MnCl2 were detected except at the highest dose where there was a cardiac depressive effect. Ischemia was induced by occluding the left anterior descending coronary artery in 5 dogs. At an infused dose of 113 μmol, MnCl2 clearly demarcated the ischemic zone during coronary occlusion. Contrast enhancement in the ischemic zone was less than 30% compared with normal tissue (P < 0.03). In conclusion, the intracellular contrast agent MnCl2 enhances the canine heart and shows promise in detecting ischemia at doses that do not cause adverse cardiac effects. Magn Reson Med 54:196–200, 2005. Published 2005 Wiley‐Liss, Inc.

Distinction of salvaged and infarcted myocardium within the ischaemic area-at-risk with T2 mapping

AIM Area-at-risk (AAR) measurements often rely on T2-weighted images, but subtle differences in T2 may be overlooked with this method. To determine the differences in oedema between salvaged and infarcted myocardium, we performed quantitative T2 mapping of the AAR. We also aimed to determine the impact of reperfusion time on T2 in the AAR. METHODS Twenty-two dogs underwent 2 h of coronary occlusion followed by 4 or 48 h of reperfusion before cardiac magnetic resonance imaging at 1.5 T. Late gadolinium enhancement images were used to define the infarcted, salvaged, and remote myocardium. T2 values from T2 maps and signal intensities on T2-weighted images were measured in the corresponding areas. RESULTS At both imaging time points, the T2 of the salvaged myocardium was longer than of remote (66.0 ± 6.9 vs. 51.4 ± 3.5 ms, P < 0.001 at 4 h, and 56.7 ± 7.3 vs. 48.1 ± 3.5 ms, P < 0.001 at 48 h). The T2 was also longer in the infarcted myocardium compared with remote at both 4 and 48 h (71.4 ± 7.6 ms, P < 0.01 vs. salvage and 64.0 ± 6.9 ms, P = 0.03 vs. salvage, both P < 0.001 vs. remote). The increase in T2 in the salvaged myocardium compared with remote was greater after 4 h than after 48 h (14.7 ± 5.6 vs. 8.7 ± 5.1 ms, P = 0.02). CONCLUSIONS T2 relaxation parameters are different in the infarcted and salvaged myocardium, and both are significantly longer than remote. Furthermore, the magnitude of increase in T2 was less in the salvaged myocardium after longer reperfusion, indicating partial resolution of oedema in the first 48 h after reperfusion.

In Vivo Microscopy Reveals Extensive Embedding of Capillaries within the Sarcolemma of Skeletal Muscle Fibers

To provide insight into mitochondrial function in vivo, we evaluated the 3D spatial relationship between capillaries, mitochondria, and muscle fibers in live mice.

Detection of myocardial capillary orientation with intravascular iron‐oxide nanoparticles in spin‐echo MRI

In mammalian hearts the capillaries are closely aligned with the muscle fibers. We report our observation of a main‐field direction‐dependent contrast in MR spin‐echo (SE) images of the heart in the presence of Ferumoxtran‐10, an intravascular iron‐oxide nanoparticle contrast agent (CA). We describe a novel MRI method for mapping the preferential orientation of capillaries in the myocardial wall. The eigenvector corresponding to the minimum eigen value of the R2 relaxation rate tensor is consistent with the expected orientation of the capillary network. Preliminary results also demonstrate the feasibility of this method for in vivo application to rodent imaging. Magn Reson Med, 2006. Published

Three‐dimensional motion tracking for high‐resolution optical microscopy, in vivo

When conducting optical imaging experiments, in vivo, the signal to noise ratio and effective spatial and temporal resolution is fundamentally limited by physiological motion of the tissue. A three‐dimensional (3D) motion tracking scheme, using a multiphoton excitation microscope with a resonant galvanometer, (512 × 512 pixels at 33 frames s−1) is described to overcome physiological motion, in vivo. The use of commercially available graphical processing units permitted the rapid 3D cross‐correlation of sequential volumes to detect displacements and adjust tissue position to track motions in near real‐time. Motion phantom tests maintained micron resolution with displacement velocities of up to 200 μm min−1, well within the drift observed in many biological tissues under physiologically relevant conditions. In vivo experiments on mouse skeletal muscle using the capillary vasculature with luminal dye as a displacement reference revealed an effective and robust method of tracking tissue motion to enable (1) signal averaging over time without compromising resolution, and (2) tracking of cellular regions during a physiological perturbation.

Optical spectroscopy in turbid media using an integrating sphere: mitochondrial chromophore analysis during metabolic transitions.

Recent evidence suggests that the activity of mitochondrial oxidative phosphorylation complexes (MOPCs) is modulated at multiple sites. Here, a method of optically monitoring electron distribution within and between MOPCs is described using a center-mounted sample in an integrating sphere (to minimize scattering effects) with a rapid-scanning spectrometer. The redox-sensitive MOPC absorbances (∼465-630 nm) were modeled using linear least squares analysis with individual chromophore spectra. Classical mitochondrial activity transitions (e.g., ADP-induced increase in oxygen consumption) were used to characterize this approach. Most notable in these studies was the observation that intermediates of the catalytic cycle of cytochrome oxidase are dynamically modulated with metabolic state. The MOPC redox state, along with measurements of oxygen consumption and mitochondrial membrane potential, was used to evaluate the conductances of different sections of the electron transport chain. This analysis then was applied to mitochondria isolated from rabbit hearts subjected to ischemia/reperfusion (I/R). Surprisingly, I/R resulted in an inhibition of all measured MOPC conductances, suggesting a coordinated down-regulation of mitochondrial activity with this well-established cardiac perturbation.

Simultaneous myocardial strain and dark-blood perfusion imaging using a displacement-encoded MRI pulse sequence.

The purpose of this study is to develop and evaluate a displacement‐encoded pulse sequence for simultaneous perfusion and strain imaging. Displacement‐encoded images in two to three myocardial slices were repeatedly acquired using a single‐shot pulse sequence for 3 to 4 min, which covers a bolus infusion of Gadolinium contrast. The magnitudes of the images were T1 weighted and provided quantitative measures of perfusion, while the phase maps yielded strain measurements. In an acute coronary occlusion swine protocol (n = 9), segmental perfusion measurements were validated against microsphere reference standard with a linear regression (slope 0.986, R2 = 0.765, Bland‐Altman standard deviation = 0.15 mL/min/g). In a group of ST‐elevation myocardial infarction patients (n = 11), the scan success rate was 76%. Short‐term contrast washout rate and perfusion are highly correlated (R2 = 0.72), and the pixelwise relationship between circumferential strain and perfusion was better described with a sigmoidal Hill curve than linear functions. This study demonstrates the feasibility of measuring strain and perfusion from a single set of images. Magn Reson Med, 2010.

Quantitative assessment of myocardial fibrosis in an age-related rat model by ex vivo late gadolinium enhancement magnetic resonance imaging with histopathological correlation

Late gadolinium enhanced (LGE) cardiac magnetic resonance (CMR) imaging can detect the presence of myocardial infarction from ischemic cardiomyopathies (ICM). However, it is more challenging to detect diffuse myocardial fibrosis from non-ischemic cardiomyopathy (NICM) with this technique due to more subtle and heterogeneous enhancement of the myocardium. This study investigates whether high-resolution LGE CMR can detect age-related myocardial fibrosis using quantitative texture analysis with histological validation. LGE CMR of twenty-four rat hearts (twelve 6-week-old and twelve 2-year-old) was performed using a 7T MRI scanner. Picrosirius red was used as the histopathology reference for collagen staining. Fibrosis in the myocardium was quantified with standard deviation (SD) threshold methods from the LGE CMR images and 3D contrast texture maps that were computed from gray level co-occurrence matrix of the CMR images. There was a significant increase of collagen fibers in the aged compared to the young rat histology slices (2.60±0.27 %LV vs. 1.24±0.29 %LV, p<0.01). Both LGE CMR and texture images showed a significant increase of myocardial fibrosis in the elderly compared to the young rats. Fibrosis in the LGE CMR images correlated strongly with histology with the 3 SD threshold (r=0.84, y=0.99x+0.00). Similarly, fibrosis in the contrast texture maps correlated with the histology using the 4 SD threshold (r=0.89, y=1.01x+0.00). High resolution ex-vivo LGE CMR can detect the presence of diffuse fibrosis that naturally developed in elderly rat hearts. Our results suggest that texture analysis may improve the assessment of myocardial fibrosis in LGE CMR images.