When the infarct border zone is stimulated prematurely, a unidirectional block line (UBL) can form and lead to double-loop (figure-of-eight) reentrant ventricular tachycardia (VT) with a central isthmus. The isthmus is composed of an entrance, center, and exit. It was hypothesized that for certain stimulus site locations and coupling intervals, the UBL would coincide with the isthmus entrance boundary, where infarct border zone thickness changes from thin-to-thick in the travel direction of the premature stimulus wavefront.
A quantitative model was developed to describe how thin-to-thick changes in the border zone result in critically convex wavefront curvature leading to conduction block, which is dependent upon coupling interval. The model was tested in 12 retrospectively analyzed postinfarction canine experiments. Electrical activation was mapped for premature stimulation and for the first reentrant VT cycle. The relationship of functional conduction block forming during premature stimulation to functional block during reentrant VT was quantified.
For an appropriately placed stimulus, in accord with model predictions: (1) The UBL and reentrant VT isthmus lateral boundaries overlapped (error: 4.8±5.7 mm). (2) The UBL leading edge coincided with the distal isthmus where the center-entrance boundary would be expected to occur. (3) The mean coupling interval was 164.6±11.0 ms during premature stimulation and 190.7±20.4 ms during the first reentrant VT cycle, in accord with model calculations, which resulted in critically convex wavefront curvature with functional conduction block, respectively, at the location of the isthmus entrance boundary and at the lateral isthmus edges.
Reentrant VT onset following premature stimulation can be explained by the presence of critically convex wavefront curvature and unidirectional block at the isthmus entrance boundary when the premature stimulation interval is sufficiently short. The double-loop reentrant circuit pattern is a consequence of wavefront bifurcation around this UBL followed by coalescence, and then impulse propagation through the isthmus. The wavefront is blocked from propagating laterally away from the isthmus by sharp increases in border zone thickness, which results in critically convex wavefront curvature at VT cycle lengths.
•Ventricular tachycardia is common after myocardial infarction and it is a serious health problem worldwide.•The source of reentrant ventricular tachycardia is often located in the infarct border zone, the layer of thin surviving myocardium adjacent to the infarct.•Although there is electrical activity occurring in the infarct border zone, the conduction velocity of the leading edge of the propagating wavefront is not constant.•Where the infarct border zone dimension changes from thin-to-thick in the travel direction, very slow conduction or even functional block occur at locations where the wavefront becomes critically convex.•In this study, the activation rate and IBZ geometric structure necessary for functional block to form during premature stimulation and during reentrant ventricular tachycardia, are determined.
Activation mapping; Isthmus; Unidirectional block; Ventricular tachycardia; Wavefront curvature
Single-image super resolution is a process of obtaining a high-resolution image from a set of low-resolution observations by signal processing. While super resolution has been demonstrated to improve image quality in scaled down images in the image domain, its effects on the Fourier-based image acquisition technique, such as MRI, remains unknown.We performed high-resolution ex vivo late gadolinium enhancement (LGE) magnetic resonance imaging (0.4 × 0.4 × 0.4 mm3) in postinfarction swine hearts (n = 24). The swine hearts were divided into the training set (n = 14) and the test set (n = 10), and in all hearts, low-resolution images were simulated from the high-resolution images. In the training set, super-resolution dictionaries with pairs of small matching patches of the high- and low-resolution images were created. In the test set, super resolution recovered high-resolution images from low-resolution images using the dictionaries. The same algorithm was also applied to patient LGE (n = 4) to assess its effects. Compared with interpolated images, super resolution significantly improved basic image quality indices (P < 0.001). Super resolution using Fourier-based zero padding achieved the best image quality. However, the magnitude of improvement was small in images with zero padding. Super resolution substantially improved the spatial resolution of the patient LGE images by sharpening the edges of the heart and the scar. In conclusion, single-image super resolution significantly improves image errors. However, the magnitude of improvement was relatively small in images with Fourier-based zero padding. These findings provide evidence to support its potential use in myocardial scar imaging.
Image processing; image quality; magnetic resonance imaging; myocardial scar
Cine MRI is used for assessing cardiac function and flow, and is typically based on a breath-held, segmented data acquisition. Breath-holding is particularly difficult for patients with congestive heart failure or in pediatric cases. Real-time imaging may be used without breath-holding or ECG triggering. However, despite the use of rapid imaging sequences and accelerated parallel imaging, real-time imaging typically has compromised spatial and temporal resolution compared with gated, segmented breath-held studies. A new method is proposed that produces a cardiac cine across the full cycle with both high spatial and temporal resolution from a retrospective reconstruction of data acquired over multiple heart beats during free.
The proposed method was compared with conventional cine images in 10 subjects. The resultant image quality for the proposed method (4.2±0.4) without breath-holding or gating was comparable to the conventional cine (4.4±0.5) on a 5 point scale (p = n.s.). Motion corrected averaging of real-time acquired cardiac images provides a means of attaining high quality cine images with many of the benefits of real-time imaging, such as free-breathing acquisition and tolerance to arrhythmias.
MRI; heart; Real-time; parallel MRI; SENSE; Navigator; Motion correction; Non-rigid; myocardial function
This work improves the performance of interactive real-time imaging with balanced steady-state free precession. The method employs hardware-optimized gradient pulses, together with a novel phase-encoding strategy that simplifies the design and implementation of the optimized gradient waveforms. In particular, the waveforms for intermediate phase-encode steps are obtained by simple linear combination, rather than separate optimized waveform calculations. Gradient waveforms are redesigned in real time as the scan plane is manipulated, and the resulting sequence operates at the specified limits of the MRI gradient subsystem for each new scan-plane orientation. The implementation provides 14–25% improvement in the sequence pulse repetition time over the vendor-supplied interactive real-time imaging sequence for similar scan parameters on our MRI scanner.
MRI; hardware-optimized; gradient; waveform; realtime; imaging; FISP; bSSFP
We examined whether MDCT improves the ability to define peri-infarct zone (PIZ) heterogeneity relative to MRI.
The PIZ as characterized by delayed enhanced (de) MRI identifies patients susceptible to ventricular arrhythmias and predicts outcome after myocardial infarction (MI).
Fifteen mini-pigs underwent coronary artery occlusion followed by reperfusion. MDCT and MRI were performed on the same day approximately 6 months after MI induction followed by animal sacrifice and ex-vivo MRI (n=5). Signal density threshold algorithms were applied to MRI and MDCT data sets reconstructed at various slice thicknesses (1–8mm) to define the PIZ and quantify partial volume effects.
De-MDCT reconstructed at 8mm slice thickness demonstrated excellent correlation of infarct size with post mortem pathology (r2=0.97; p<0.0001) and MRI (r2=0.92; p<0.0001). De-MDCT and de-MRI were able to detect a PIZ in all animals, which correlates to a mixture of viable and non-viable myocytes at the PIZ by histology. The ex-vivo de-MRI PIZ volume decreased with slice thickness from 0.9±0.2cc at 8mm to 0.2±0.1cc at 1mm (p=0.01). PIZ volume/mass by de-MDCT increased with decreasing slice thickness due to declining partial volume averaging in the PIZ, but was susceptible to increased image noise.
De-MDCT provides a more detailed assessment of the PIZ in chronic MI and is less susceptible to partial volume effects than MRI. This increased resolution best reflects the extent of tissue mixture by histopathology and has the potential to further enhance the ability to define the substrate of malignant arrhythmia in ischemic heart disease non-invasively.
MDCT; delayed enhancement; peri-infarct zone; MRI
To develop and test a novel interactive real-time MRI environment that facilitates image-guided cardiovascular interventions.
Materials and Methods
Color highlighting of device-mounted receiver coils, accelerated imaging of multiple slices, adaptive projection modes, live 3D renderings and other interactive features are utilized to enhance navigation of devices and targeting of tissue.
Images are shown from several catheter-based interventional procedures performed in swine that benefit from this custom interventional MRI interface. These include endograft repair of aortic aneurysm, balloon septostomy of the cardiac interatrial septum, angioplasty and stenting, and endomyocardial cell injection, all using active catheters containing MRI receiver coils.
Interactive features not available on standard clinical scanners enhance real-time MRI for guiding cardiovascular interventional procedures.
Interventional MRI; Real-time MRI; Image guided interventions; Catheterization; User Interface
We developed and tested a novel transcatheter circumferential annuloplasty technique to reduce mitral regurgitation in porcine ischemic cardiomyopathy.
Catheter-based annuloplasty for secondary mitral regurgitation exploits the proximity of the coronary sinus to the mitral annulus, but is limited by anatomic variants and coronary artery entrapment.
The procedure, “cerclage annuloplasty,” is guided by MRI roadmaps fused with live X-ray. A coronary sinus guidewire traverses a short segment of basal septal myocardium to reenter the right heart where it is exchanged for a suture. Tension is applied interactively during imaging and secured with a locking device.
We found two feasible suture pathways from the great cardiac vein across the interventricular septum to create cerclage. Right-ventricular septal reentry required shorter fluoroscopy times than right atrial reentry, which entailed a longer intramyocardial traversal but did not cross the tricuspid valve. Graded tension progressively reduced septal-lateral annular diameter but not end-systolic elastance or regional myocardial function. A simple arch-like device protected entrapped coronary arteries from compression even during supra-therapeutic tension.
Cerclage reduced mitral regurgitation fraction (from 22.8 ± 12.7% to 7.2 ± 4.4%, p=0.04) by slice-tracking velocity-encoded MRI. Flexible cerclage reduced annular size but preserved annular motion. Cerclage also displaced the posterior annulus towards the papillary muscles. Cerclage introduced reciprocal constraint to the left ventricular outflow tract and mitral annulus that enhanced leaflet coaptation.
A sample of human coronary venograms and CT angiograms suggested that most have suitable venous anatomy for cerclage.
Transcatheter mitral cerclage annuloplasty acutely reduces mitral regurgitation in porcine ischemic cardiomyopathy. Entrapped coronary arteries can be protected. MRI provided insight into the mechanism of cerclage action.
Image guided intervention; Catheter-based intervention, non-coronary; Magnetic resonance imaging; Multimodality image fusion
Cardiovascular molecular imaging is a new discipline that integrates scientific advances in both functional imaging and molecular probes to improve our understanding of the molecular basis of the cardiovascular system. These advances are driven by in vivo imaging of molecular processes in animals, usually small animals, and are rapidly moving toward clinical applications. Molecular imaging has the potential to revolutionize the diagnosis and treatment of cardiovascular disease. The 2 key components of all molecular imaging systems are the molecular contrast agents and the imaging system providing spatial and temporal localization of these agents within the body. They must deliver images with the appropriate sensitivity and specificity to drive clinical applications. As work in molecular contrast agents matures and highly sensitive and specific probes are developed, these systems will provide the imaging technologies required for translation into clinical tools. This is the promise of molecular medicine.
molecular imaging; cardiovascular; multidisciplinary approach
Earlier studies have yielded conflicting evidence on whether or not cardiac resynchronization therapy (CRT) improves left ventricular (LV) rotation mechanics.
Methods and Results
In dogs with left bundle branch block and pacing-induced heart failure (n=7), we studied the effects of CRT on LV rotation mechanics in vivo by 3-dimensional tagged magnetic resonance imaging with a temporal resolution of 14 ms. CRT significantly improved hemodynamic parameters but did not significantly change the LV rotation or rotation rate. LV torsion, defined as LV rotation of each slice with respect to that of the most basal slice, was not significantly changed by CRT. CRT did not significantly change the LV torsion rate. There was no significant circumferential regional heterogeneity (anterior, lateral, inferior, and septal) in LV rotation mechanics in either left bundle branch block with pacing-induced heart failure or CRT, but there was significant apex-to-base regional heterogeneity.
CRT acutely improves hemodynamic parameters without improving LV rotation mechanics. There is no significant circumferential regional heterogeneity of LV rotation mechanics in the mechanically dyssynchronous heart. These results suggest that LV rotation mechanics is an index of global LV function, which requires coordination of all regions of the left ventricle, and improvement in LV rotation mechanics appears to be a specific but insensitive index of acute hemodynamic response to CRT.
MRI; tagging; ventricular function; mechanics; torsional deformation
Much attention has been focused on the passive mechanical properties of the myocardium, which determines left ventricular (LV) diastolic mechanics, but the significance of the visceral pericardium (VP) has not been extensively studied. A unique en face three-dimensional volumetric view of the porcine VP was obtained using two-photon excitation fluorescence to detect elastin and backscattered second harmonic generation to detect collagen, in addition to standard light microscopy with histological staining. Below a layer of mesothelial cells, collagen and elastin fibers, extending several millimeters, form several distinct layers. The configuration of the collagen and elastin layers as well as the location of the VP at the epicardium providing a geometric advantage led to the hypothesis that VP mechanical properties play a role in the residual stress and passive stiffness of the heart. The removal of the VP by blunt dissection from porcine LV slices changed the opening angle from 53.3 ± 10.3 to 27.3 ± 5.7° (means ± SD, P < 0.05, n = 4). In four porcine hearts where the VP was surgically disrupted, a significant decrease in opening angle was found (35.5 ± 4.0°) as well as a rightward shift in the ex vivo pressure-volume relationship before and after disruption and a decrease in LV passive stiffness at lower LV volumes (P < 0.05). These data demonstrate the significant and previously unreported role that the VP plays in the residual stress and passive stiffness of the heart. Alterations in this layer may occur in various disease states that effect diastolic function.
collagen; diastolic function; myocardial elasticity; residual stress; water permeability; elastin; 2-photon microscopy; porcine heart; optical properties
In catheter ablation of scar-related monomorphic ventricular tachycardia (VT), substrate voltage mapping is used to electrically define the scar during sinus rhythm. However, the electrically defined scar may not accurately reflect the anatomical scar. Magnetic resonance–based visualization of the scar may elucidate the 3D anatomical correlation between the fine structural details of the scar and scar-related VT circuits. We registered VT activation sequence with the 3D scar anatomy derived from high-resolution contrast-enhanced MRI in a swine model of chronic myocardial infarction using epicardial sock electrodes (n=6, epicardial group), which have direct contact with the myocardium where the electrical signal is recorded. In a separate group of animals (n=5, endocardial group), we also assessed the incidence of endocardial reentry in this model using endocardial basket catheters. Ten to 12 weeks after myocardial infarction, sustained monomorphic VT was reproducibly induced in all animals (n=11). In the epicardial group, 21 VT morphologies were induced, of which 4 (19.0%) showed epicardial reentry. The reentry isthmus was characterized by a relatively small volume of viable myocardium bound by the scar tissue at the infarct border zone or over the infarct. In the endocardial group (n=5), 6 VT morphologies were induced, of which 4 (66.7%) showed endocardial reentry. In conclusion, MRI revealed a scar with spatially complex structures, particularly at the isthmus, with substrate for multiple VT morphologies after a single ischemic episode. Magnetic resonance–based visualization of scar morphology would potentially contribute to preprocedural planning for catheter ablation of scar-related, unmappable VT.
ventricular tachycardia; catheter ablation; MRI
We hypothesize that X-ray fused with MRI (XFM) roadmaps might permit direct antegrade crossing and delivery of a VSD closure device and thereby reduce procedure time and radiation exposure.
Percutaneous device closure of membranous ventricular septal defect (VSD) is cumbersome and time-consuming. The procedure requires crossing the defect retrograde, snaring and exteriorizing a guidewire to form an arteriovenous loop, then delivering antegrade a sheath and closure device.
MRI roadmaps of cardiac structures were obtained from miniature swine with spontaneous ventricular septal defect and registered with live X-ray using external fiducial markers. We compared antegrade XFM-guided VSD crossing with conventional retrograde X-ray guided crossing for repair.
Antegrade XFM crossing was successful in all animals. Compared with retrograde X-ray, antegrade XFM was associated with shorter time to crossing (167 ± 103 seconds versus 284 ± 61 seconds; p = 0.025), shorter time to sheath delivery (71 ± 32 seconds versus 366 ± 145 seconds; p = 0.001), shorter fluoroscopy time (158 ± 95 seconds versus 390 ± 137s; p = 0.003), and reduced radiation dose-area product (2394 ± 1522 mG•m2 versus 4865 ± 1759 mG•m2; p = 0.016).
XFM facilitates antegrade access to membranous VSD from the right ventricle in swine. The simplified procedure is faster and reduces radiation exposure compared with the conventional retrograde approach.
Image guided intervention; Interventional magnetic resonance imaging; Congenital heart disease; Multimodality image fusion; Heart Septal Defects, Ventricular
This paper presents two related methods for registering an image of an anatomical object with data from sensors arranged on the object. One method is described with reference to a test case involving a rectangular electrode plaque disposed on a heart surface, which is imaged with MRI. Data from the electrodes is fused with the MRI image at the appropriate locations. The registration scheme involves four parts. First, selected landmarks on a data surface (e.g., electrode plaque) are registered to known locations on a target anatomical surface image. Second, the anatomical surface is represented numerically with a spherical harmonic expansion. Third, given the registration of the select data surface landmarks, the location of the outer four corners of the rectangular electrode plaque are located on the anatomical surface. Fourth, a quasi-evenly spaced grid within these four corners is formed on the anatomical surface. The third and fourth steps involve calculating geodesics on the anatomical surface, preferably by utilizing the spherical harmonic expansion. According to the second registration method, spherical harmonics and geodesics are used to extract a mesh from the anatomical surface. Laplace’s equation is solved on this mesh to generate a mapping from the anatomical surface to the data surface (electrode plaque).
Registration; Geodesics; Spherical harmonics
Catheter visualization and tracking remains a challenge in interventional MR.
Active guidewires can be made conspicuous in "profile" along their whole shaft exploiting metallic core wire and hypotube components that are intrinsic to their mechanical performance. Polymer-based catheters, on the other hand, offer no conductive medium to carry radio frequency waves. We developed a new "active" catheter design for interventional MR with mechanical performance resembling braided X-ray devices. Our 75 cm long hybrid catheter shaft incorporates a wire lattice in a polymer matrix, and contains three distal loop coils in a flexible and torquable 7Fr device. We explored the impact of braid material designs on radiofrequency and mechanical performance.
The incorporation of copper wire into in a superelastic nitinol braided loopless antenna allowed good visualization of the whole shaft (70 cm) in vitro and in vivo in swine during real-time MR with 1.5 T scanner. Additional distal tip coils enhanced tip visibility. Increasing the copper:nitinol ratio in braiding configurations improved flexibility at the expense of torquability. We found a 16-wire braid of 1:1 copper:nitinol to have the optimum balance of mechanical (trackability, flexibility, torquability) and antenna (signal attenuation) properties. With this configuration, the temperature increase remained less than 2°C during real-time MR within 10 cm horizontal from the isocenter. The design was conspicuous in vitro and in vivo.
We have engineered a new loopless antenna configuration that imparts interventional MR catheters with satisfactory mechanical and imaging characteristics. This compact loopless antenna design can be generalized to visualize the whole shaft of any general-purpose polymer catheter to perform safe interventional procedures.
X-ray images acquired on systems with image intensifiers (II) exhibit characteristic distortion which is due to both external and internal factors. The distortion is dependent on the orientation of the II, a fact particularly relevant to II’s mounted on C arms which have several degrees of freedom of motion. Previous descriptions of distortion correction strategies have relied on a dense sampling of the C-arm orientation space, and as such have been limited mostly to a single arc of the primary angle, α. We present a new method which smooths the trajectories of the segmented vertices of the grid phantom as a function of α prior to solving the two-dimensional warping problem. It also shows that the same residual errors of distortion correction could be achieved without fitting the trajectories of the grid vertices, but instead applying the previously described global method of distortion correction, followed by directly smoothing the values of the polynomial coefficients as functions of the C-arm orientation parameters. When this technique was applied to a series of test images at arbitrary α, the root-mean-square (RMS) residual error was 0.22 pixels. The new method was extended to three degrees of freedom of the C-arm motion: the primary angle, α; the secondary angle, β; and the source-to-intensifier distance, λ. Only 75 images were used to characterize the distortion for the following ranges: α, ±45° (Δα =22.5°); β, ±36° (Δβ=18°); λ, 98–118 cm (Δ λ=10 cm). When evaluated on a series of test images acquired at arbitrary (α, β, λ), the RMS residual error was 0.33 pixels. This method is targeted at applications such as guidance of catheter-based interventions and treatment planning for brachytherapy, which require distortion-corrected images over a large range of C-arm orientations.
x-ray image intensifier; x-ray fluoroscopy; angle dependent distortion correction
We have developed and validated a system for real-time X-ray fused with magnetic resonance imaging, MRI (XFM), to guide catheter procedures with high spatial precision. Our implementation overlays roadmaps—MRI-derived soft-tissue features of interest—onto conventional X-ray fluoroscopy. We report our initial clinical experience applying XFM, using external fiducial markers, electrocardiogram (ECG)-gating, and automated real-time correction for gantry and table movement.
This prospective case series for technical development was approved by the NHLBI Institutional Review Board and included 19 subjects. Multimodality external fiducial markers were affixed to patients’ skin before MRI, which included contrast-enhanced, 3D T1-weighted, or breath-held and ECG-gated 2D steady state free precession imaging at 1.5T. MRI-derived roadmaps were manually segmented while patients were transferred to a calibrated X-ray fluoroscopy system. Image spaces were registered using the fiducial markers and thereafter permitted unrestricted gantry rotation, table panning, and magnification changes. Static and ECG-gated MRI data were transformed from 3D to 2D to correspond with gantry and table position and combined with live X-ray images.
Clinical procedures included graft coronary arteriography, right ventricular free-wall biopsy, and iliac and femoral artery recanalization and stenting. MRI roadmaps improved operator confidence, and in the biopsy cases, outperformed the best available alternative imaging modality. Registration errors were increased when external fiducial markers were affixed to more mobile skin positions, such as over the abdomen.
XFM using external fiducial markers is feasible during X-ray guided catheter treatments. Multimodality image fusion may prove a useful adjunct to invasive cardiovascular procedures.
catheterization; magnetic resonance imaging; stereotactic surgery; image guided intervention; myocardial biopsy; chronic total occlusion
Even in experienced hands, X-ray guided needle atrial septal puncture risks non-target perforation and pericardial tamponade. Real-time MRI offers potentially superior target imaging and multiplanar device tracking. We report initial preclinical experience with real-time MRI-guided atrial septal puncture using a MRI-conspicuous blunt laser catheter that perforates only when energized.
Materials and Methods
We customized a clinical excimer laser catheter (0.9mm Clirpath, Spectranetics) with a receiver coil to impart MRI visibility at 1.5T. Seven swine underwent laser transseptal puncture under real-time MRI. MRI signal-to-noise profiles were obtained of the device in vitro. Tissue traversal force was tested with a calibrated meter. Position was corroborated by pressure, oximetry, angiography, and necropsy. Intentional non-target perforation simulated serious complication.
Embedded MRI-antennae accurately reflected the position of the laser catheter tip and profile in vitro and in vivo. Despite increased profile from the microcoil, the 0.9mm laser catheter traversed in vitro targets with similar force (0.22 ± 0.03N) compared with the unmodified laser.
Laser puncture of the atrial septum was successful and accurate in all animals. The laser was activated an average 3.8 ± 0.4 seconds before traversal. There were no sequelae after 6 hour observation. Necropsy revealed 0.9mm holes in the fossa ovalis in all animals.
Intentional perforation of the aorta and of the atrial free wall was evident immediately.
MRI-guided laser puncture of the interatrial septum is feasible in swine, and offers controlled delivery of perforation energy using an otherwise blunt catheter. Instantaneous soft-tissue imaging provides immediate safety feedback.
To develop an imaging and visualization technique for real-time magnetic resonance angiography (rt-MRA) fully integrated with a real-time interactive imaging environment on a clinical MR scanner.
Materials and Methods
Intraarterial injections of contrast agent and imaging processing techniques were employed for rapid catheter-directed assessment of vessel patency and regional tissue perfusion. Operators can image multiple thin slices to maximize anatomic detail or use thick slice or projection imaging to maximize vessel coverage. Techniques in both pulse sequence and image processing were employed to ensure background suppression. Accumulation of maximum pixel values allows persistent display of bolus signal as it passes through the vessels and into tissues. Automatic brightness adjustment was used to ensure visibility at all stages of bolus passage.
Experimental intraarterial rtMRA of coronary, renal, and carotid arteries show that vessel trajectories and perfusion territories are well visualized in swine. Switching between standard real-time imaging and rtMRA imaging after contrast injection was easy to perform during a procedure without stopping the scanner.
The proposed technique facilitates visualization of intraarterial contrast injections using real-time MRI. Although designed for rapid deployment during rtMRI-guided interventional procedures, the technique may also be useful to supplement the study of vessel anatomy, flow, or perfusion.
real-time; angiography; interventional; cardiovascular
Infarct border zone (IBZ) geometry likely affects inducibility and characteristics of postinfarction reentrant ventricular tachycardia, but the connection has not been established.
To determine characteristics of post infarction ventricular tachycardia in the IBZ.
A geometric model describing the relationship between IBZ geometry and wavefront propagation in reentrant circuits was developed. Based on the formulation, slow conduction and block was expected to coincide with areas where IBZ thickness (T) is minimal and the local spatial gradient in thickness (ΔT) is maximal, so that the degree of wavefront curvature ρ ∝ ΔT/T is maximal. Regions of fastest conduction velocity were predicted to coincide with areas of minimum ΔT. In seven arrhythmogenic postinfarction canine heart experiments, tachycardia was induced by programmed stimulation, and activation maps were constructed from multichannel recordings. IBZ thickness was measured in excised hearts from histologic analysis or magnetic resonance imaging. Reentrant circuit properties were predicted from IBZ geometry and compared with ventricular activation maps following tachycardia induction.
Mean IBZ thickness was 231±140µm at the reentry isthmus and 1440±770µm in the outer pathway (p<0.001). Mean curvature ρ was 1.63±0.45mm−1 at functional block line locations, 0.71±0.18mm−1 at isthmus entrance-exit points, and 0.33±0.13mm−1 in the outer reentrant circuit pathway. The mean conduction velocity about the circuit during reentrant tachycardia was 0.32±0.04mm/ms at entrance-exit points, 0.42±0.13mm/ms for the entire outer pathway, and 0.64±0.16mm/ms at outer pathway regions with minimum ΔT. Model sensitivity and specificity to detect isthmus location was 75.0±5.7% and 97.2±0.7%.
Reentrant circuit features as determined by activation mapping can be predicted on the basis of IBZ geometrical relationships.
arrhythmia; border zone; conduction velocity; infarction; mapping; MRI; propagation; ventricular tachycardia
Endoventricular patch plasty (Dor procedure) has gained favor as a surgical treatment for heart failure associated with large anteroapical myocardial infarction. We tested the hypotheses that the Dor procedure increases systolic circumferential shortening and longitudinal shortening in noninfarcted left ventricular regions in sheep.
In 6 male Dorsett sheep, the left anterior descending coronary artery and its second diagonal branch were ligated 40% of the distance from the apex to the base. Sixteen weeks after myocardial infarction, a Dor procedure was performed with a Dacron patch that was 50% of the infarct neck dimension. Two weeks before and 2 and 6 weeks after the Dor procedure, animals underwent magnetic resonance imaging with tissue tagging in multiple short-axis and long-axis slices. Fully three-dimensional strain analyses were performed. All 6 end-systolic strain components were compared in regions 1 cm, 2 cm, 3 cm, and 4 cm below the valves, as well as in the anterior, posterior, and lateral left ventricular walls and the interventricular septum.
Circumferential shortening increased from before the Dor procedure to 6 weeks after repair in nearly every left ventricular region (13/16). The greatest regional change in circumferential shortening was found in the equatorial region or 2 cm below the base and in the posterior wall (from 9.0% to 18.4%; P < .0001). Longitudinal shortening increased 2 weeks after the Dor procedure but then returned near baseline by 6 weeks after the Dor procedure.
The Dor procedure significantly increases systolic circumferential shortening in nearly all noninfarcted left ventricular regions in sheep.
Magnetic resonance imaging (MRI) is an ideal imaging modality to measure blood flow and tissue motion. It provides excellent contrast between soft tissues, and images can be acquired at positions and orientations freely defined by the user. From a temporal sequence of MR images, boundaries and edges of tissues can be tracked by image processing techniques. Additionally, MRI permits the source of the image signal to be manipulated. For example, temporary magnetic tags displaying a pattern of variable brightness may be placed in the object using MR saturation techniques, giving the user a known pattern to detect for motion tracking. The MRI signal is a modulated complex quantity, being derived from a rotating magnetic field in the form of an induced current. Well-defined patterns can also be introduced into the phase of the magnetization, and could be thought of as generalized tags. If the phase of each pixel is preserved during image reconstruction, relative phase shifts can be used to directly encode displacement, velocity and acceleration. New methods for modeling motion fields from MRI have now found application in cardiovascular and other soft tissue imaging. In this review, we shall describe the methods used for encoding, imaging, and modeling motion fields with MRI.
Cardiac magnetic resonance; displacement encoding with stimulated echoes (DENSE); harmonic phase imaging (HARP); magnetic resonance imaging (MRI); magnetic resonance tagging; motion tracking; phase contrast magnetic resonance imaging; review
We investigated whether transmural mechanics could yield insight into the transmural electrical sequence.
Although the concept of transmural dispersion of repolarization has helped explain a variety of arrhythmias, its presence in vivo is still disputable.
We studied the time course of transmural myofiber mechanics in the anterior left ventricle of normal canines in vivo (n = 14) using transmural bead markers under biplane cineradiography. In 4 of these animals, plunge electrodes were placed in the myocardial tissue within the bead set to measure transmural electrical sequence.
The onset of myofiber shortening was earliest at endocardial layers and progressively delayed toward epicardial layers (p < 0.001), resulting in transmural dispersion of myofiber shortening of 39 ms. The onset of myofiber relaxation was earliest at epicardial layers and most delayed at subendocardial layers (p = 0.004), resulting in transmural dispersion of myofiber relaxation of 83 ms. There was no significant transmural gradient in electrical repolarization (p = NS).
Despite lack of evidence of significant transmural gradient in electrical repolarization in vivo, there is transmural dispersion of myofiber relaxation as well as shortening.
A technique is presented for rapidly and noninvasively determining aortic distensibility, by NMR measurement of wave velocity in the aorta. A two-dimensional NMR selective-excitation pulse is used to repeatedly excite a cylinder of magnetization in the aorta, with magnetization read out along the cylinder axis each time. A toggled bipolar flow-encoding pulse is applied prior to readout, to produce a one-dimensional phase-contrast flow image. Cardiac gating and data interleaving are employed to improve the effective time resolution to 2 ms. Wave velocities are determined from the slope of the leading edge of flow measured on the resulting M-mode velocity image. The technique is sensitive over a range of distensibilities from 10−6 to 10−3 m s2/kg. The average value in the descending thoracic aorta in seven normal subjects was found to be 4.8 × 10−5 m s2/kg, with a significant inverse correlation with age.
aortic distensibility; wave speed; compliance; blood velocity