Biomedical subjects
Publications and source records attributed to L Axel.
Left main coronary artery compression by the pulmonary artery may be seen in patients with pulmonary hypertension who are undergoing cardiac catheterization. Cardiac magnetic resonance imaging is useful in these patients to document extrinsic compression, which might otherwise be mistaken for intrinsic atherosclerotic disease.
The effect of inotropic stimulation on the pattern and magnitude of regional left ventricular contraction was studied using tagged magnetic resonance imaging to assess whether dobutamine exacerbates variation in regional contraction at rest. Dobutamine stress testing defines a normal response as a homogeneous increase in regional wall motion. In 8 normal subjects, 4 equally spaced left ventricular short-axis levels were imaged through systole using tagged magnetic resonance imaging. The baseline imaging sequence was repeated with 5-, 10-, 15-, and 20-microg/kg/min dobutamine infusion. Regional myocardial displacement, radial thickening, and circumferential shortening were measured. The left ventricle was analyzed by level (base to apex) and wall (septum, inferior, lateral, anterior). Dobutamine did not alter baseline regional functional heterogeneity. Dobutamine infusion resulted in a uniform increase in displacement, radial thickening, and circumferential shortening from baseline to 10-microg/kg/min infusion without additional increases at higher doses.
PURPOSE: To evaluate the interobserver and intermodality variability of conventional angiography and gadolinium-enhanced magnetic resonance (MR) angiography in the assessment of renal artery stenosis. MATERIALS AND METHODS: Fifty-four patients underwent conventional angiography and gadolinium-enhanced three-dimensional gradient-echo MR angiography. Three angiographers blinded to each other's interpretations and the MR angiographic findings assessed the conventional angiograms for renal artery stenosis. Similarly, three blinded MR imagers evaluated the MR angiograms. RESULTS: Interobserver variability for the degree of renal artery stenosis in the 107 kidneys evaluated was not significantly different between the two modalities. The mean SD of the degree of stenosis was 6.9% at MR angiography versus 7.5% at conventional angiography (alpha < or = .05, P > .05). In 70 kidneys (65%), the average degree of stenosis reported by the readers for the two modalities differed by 10% or less. In 22 cases (21%), the degree of stenosis was overestimated with MR angiography by more than 10% relative to the results of conventional angiography. In 15 cases (14%), the degree of stenosis was underestimated with MR angiography by more than 10%. CONCLUSION: Gadolinium-enhanced MR angiography permits evaluation of renal artery stenosis with an interobserver variability comparable with that of conventional angiography.
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Right ventricular (RV) regional function, in both normal and diseased states, is not well characterized. Using 1D MR myocardial tagging, RV and septal intramyocardial segmental shortening was noninvasively measured in ten healthy subjects and in seven patients with chronic pulmonary hypertension. The normal RV free wall regional shortening was not uniform. A pattern of increasing RV free wall short-axis shortening was found from the RV outflow tract to the RV apex, and a more complex pattern of RV free wall long-axis shortening was observed. Both regional short- and long-axis shortening were globally reduced in pulmonary hypertension patients, with the greatest decreases in the RV outflow tract and in the basal septal wall region. Regional RV function can be quantitatively evaluated using MR tagging to determine the impact of chronic pulmonary hypertension on RV performance.
A single integrated examination using regional measurements of perfusion from contrast-enhanced MRI and three-dimensional (3D) strain from tissue-tagged MRI was developed to differentiate infarcted myocardium from adjacent tissue with functional abnormalities. Ten dogs were studied at baseline and 10 days after a 2-hour occlusion of the left anterior descending coronary artery (LAD). Strain was determined using a 3D finite element model. Two-dimensional measurements of hypoenhancing regions were highly correlated with myocardial viability (r = 0.96). Signal intensity versus time curves obtained from contrast-enhanced MRI were used for quantitative perfusion analysis. The remote and adjacent noninfarcted tissue of the dogs with LAD occlusion, as well as the infarcted tissue, exhibited abnormal deformation patterns as compared to normal dogs (positive predictive value (PPV) of strain determination of infarction = 66%). Integration of contrast-enhanced MRI results with 3D strain analysis enabled the delineation of the myocardial infarction (PPV = 100%) from functionally compromised myocardium. This integrated cardiac examination shows promise for noninvasive serial assessment of potentially jeopardized noninfarcted myocardium to study the process of infarct remodeling and expansion.
MR imaging has made rapid progress and promises to be of great utility in the evaluation of the cardiovascular system. Some of the features that make it so promising are its safety (with appropriate guidelines); its ability to produce high-quality tomographic images in arbitrary orientations; and the possibility to obtain unique data, such as on regional myocardial function and metabolism. Ongoing technical developments in such areas as more rapid imaging and newer contrast agents should continue to increase the usefulness of cardiovascular MR imaging.
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Chemical shift differences, field inhomogeneity, and gradient nonlinearity result in artifacts in magnetic resonance imaging. Three artifacts are characterized for linogram imaging and it is shown that, based on computer simulations and theory, linogram MRI behaves similarly to 2DFT. A correction technique similar to a scheme for 2DFT imaging based on the Dixon technique and coordinate transform methods is proposed. The algorithm is applied to correct for field inhomogeneity and gradient nonlinearity-induced artifacts in both simulations and images of a clinical phantom. The results show good correlation with the theory. It is concluded that linogram imaging offers certain attractive features of both 2DFT and PR imaging techniques, and is a potentially viable alternative to PR imaging in the presence of field inhomogeneity.
Calculation of global cardiac function parameters has been validated using fast, segmented k-space, breath-hold, gradient-echo, magnetic resonance images. Images of phantoms, experimental animals, normal volunteers, and patients were acquired with a 1.5 T clinical scanner. Humans were imaged using two phased-array surface coils in multicoil mode. Myocardial contours were extracted using a new interactive, semi-automated method based on the active contour model method. Images were acquired in the short-axis orientation, and, using a new imaging and analysis strategy, in rotating plane long-axis orientations, to provide better definition of the valve planes and the apex, and also to reduce the number of slices (compared with the short-axis method) required to sample the whole heart. Validation was accomplished through calculation of the volumes of phantoms and left and right ventricular masses of animal hearts. Functional parameters from MRI were compared with those from echocardiograms and radionuclide angiograms in normal volunteers and patients, respectively.
An alternate scheme for linogram image reconstruction, which is more logical from the viewpoint of its applicability to MRI data, is presented here. As a result, an intermediate step for this method, the direct Fourier method (DFM) gives the same results as the earlier developed general reconstruction algorithm labeled as the linogram method (LM). However, the two differ in the pathways taken to the solution. As a result, an intermediate step for DFM corresponds directly with the geometry of linogram data collected for MRI, in contrast to the LM reconstruction. The two reconstruction methods are delineated within the context of MRI data reconstruction, and applied to reconstruct images from linogram spin-echo data of a physical phantom, obtained on a clinical 1.5 T scanner.
Early after infarction in the perfusion bed of the left anterior descending coronary artery, cine MRI with spatial modulation of magnetization (SPAMM) tagging (7-mm grid) was used for short- and long-axis cardiac imaging. Two-dimensional strain analysis of triangular finite elements was performed between end-diastole and end-systole. Patients (n = 10) were compared with age-matched healthy subjects (n = 8). The anteroseptal region at midventricular level was considered representative for "infarcted" and the posterolateral region at basal level was considered "remote". The left ventricular end-diastolic volume index was larger in the patients (69 +/- 15 ml/m2 versus 56 +/- 4 ml/m2, P < 0.05). Short-axis images showed in the infarcted region a decrease of first principal strain (greatest systolic lengthening: 1.10 +/- .06 versus 1.27 +/- 0.04, P < 0.0001), and in the remote region an increase (1.48 +/- 0.11 versus 1.36 +/- 0.07, P < 0.025). The lateral and inferior ventricular regions at mid- and basal levels were found to function normally. Long-axis images yielded similar results. Early after infarction, regions with dysfunction, normal function, and hyperfunction can be delineated with MR tagging. The compensatory increased contraction in the remote region is possibly triggered by the Frank-Starling mechanism.
The motion sensitivity of cardiac magnetic resonance imaging (MRI) can be exploited to measure the motion patterns within the heart wall and thus to noninvasively calculate the intramyocardial strain. The resulting large data sets pose a challenge for visualization, but offer the potential of a greatly improved picture of cardiac dynamics. This may have both basic research and clinical applications.
UNLABELLED: Tomographic techniques separate overlying structures, permitting measurements of absolute ventricular volumes. The purpose of this study was to determine absolute right and left ventricular volume and ejection fraction measurements with tomographic gated equilibrium blood-pool scintigraphy (TMUGA) compared to MRI and conventional planar scintigraphy. METHODS: Eighteen patients were studied. Ventricular volumes for TMUGA and MRI were calculated by modified Simpson's rule. TMUGA regions were defined by constraints including phase analysis, intensity threshold and visual inspection. MRI studies were acquired with a fast gradient-echo, ECG-gated, breath-hold technique and boundaries were defined by a semiautomated contour method. Conventional gated first-pass radionuclide angiography (FP) and planar gated equilibrium blood-pool scintigraphy (PMUGA) were performed for RV EF and LV EF, respectively. RESULTS: TMUGA absolute right ventricular volumes showed excellent-correlation with MRI for both right ventricular volumes (r = 0.91, slope = 0.90, s.e.e. = 15.7) and left ventricular volumes (r = 0.96, slope = 0.88, s.e.e. = 18.2). For left ventricular ejection fraction, TMUGA also showed excellent correlation with MRI (r = 0.94, slope = 1.10, s.e.e. = 9.0) and planar MUGA (r = 0.97, slope = 1.23, s.e.e. = 6.2). For right ventricular ejection fraction, TMUGA showed good correlation with both MRI (r = 0.88, slope = 0.79, s.e.e. = 6.0) and first-pass planar scintigraphy (r = 0.86, slope = 1.2, s.e.e. = 7.9). CONCLUSION: Tomographic gated blood-pool scintigraphy absolute right and left ventricular volumes and ejection fractions show good correlation with accepted techniques. Further studies are necessary to define the reproducibility of this method.
MRI studies of first-pass contrast enhancement with polylysine-Gd-DTPA and myocardial tagging using spatial modulation of magnetization (SPAMM) were performed to assess the feasibility of a combined regional myocardial blood flow and 2D deformation exam. Instrumented closed-chest dogs were imaged at a baseline control state (Cntl) followed by two interventions: moderate coronary stenosis (St) achieved by partial occlusion of the left anterior descending (LAD) and moderate coronary stenosis with dobutamine loading (StD). Hypoperfusion of the anterior region (ANT) of the myocardium (LAD distribution) relative to the posterior wall (POS) based on the upslope of the signal intensity time curve from the contrast-enhanced MR images was demonstrated only with dobutamine loading (ANT:POS Cntl = 1.077 +/- 0.15 versus ANT:POS StD = 0.477 +/- 0.11, P < 0.03) and was confirmed with radiolabeled microspheres measurements (ANT:POS Cntl = 1.18 +/- 0.2 ml/min/g versus ANT:POS StD = 0.44 +/- 0.1 ml/min/g; P < 0.002). Significant changes in regional myocardial shortening were only seen in the StD state (P < 0.02); the anterior region showed impaired myocardial shortening with dobutamine loading (P = NS), whereas the nonaffected POS region showed a marked increase in shortening when compared with Cntl (Cntl = 0.964 +/- 0.02 versus StD = 0.884 +/- 0.03; P < 0.001). These results demonstrate that an integrated quantitative assessment of regional myocardial function and semiquantitative assessment of myocardial blood flow can be performed noninvasively with ultrafast MRI.
OBJECTIVES: 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. BACKGROUND: 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. METHODS: 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. RESULTS: 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). CONCLUSIONS: 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.
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.
Motion results in various artifacts such as blurring and streaks in clinical imaging of subjects based on reconstruction from projections. We model subject motion-induced artifacts due to scaling, translational and rotational motion. A correction algorithm based on the Ludwig-Helgason consistency conditions is derived here. These conditions are satisfied whenever the projection data are consistent. We apply the algorithm to simulated data collected on linogram (LR) and projection reconstruction (PR) geometries, and to real PR geometry data, in magnetic resonance imaging (MRI). The results show that motion-induced in-plane, interview artifacts can be reduced with application of the algorithm. The algorithm is general enough to be applied to certain other cases arising in tomographic imaging.