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Biomedical subjects

J L Prince

Publications and source records attributed to J L Prince.

16 recordsLinked to original sources
PubMed2001

Imaging longitudinal cardiac strain on short-axis images using strain-encoded MRI.

This article presents a new method for measuring longitudinal strain in a short-axis section of the heart using harmonic phase magnetic resonance imaging (HARP-MRI). The heart is tagged using 1-1 SPAMM at end-diastole with tag surfaces parallel to a short-axis imaging plane. Two or more images are acquired such that the images have different phase encodings in a direction orthogonal to the image plane. A dense map of the longitudinal strain can be computed from these images using a simple, fast computation. Simulations are conducted to study the effect of noise and the choice of out-of-plane phase encoding values. Longitudinal strains acquired from a normal human male are shown.

Adult↗
PubMed2000

Fast determination of regional myocardial strain fields from tagged cardiac images using harmonic phase MRI.

BACKGROUND: Tagged MRI of the heart is difficult to implement clinically because of the lack of fast analytical techniques. We investigated the accuracy of harmonic phase (HARP) imaging for rapid quantification of myocardial strains and for detailed analysis of left ventricular (LV) function during dobutamine stimulation. METHODS AND RESULTS: Tagged MRI was performed in 10 volunteers at rest and during 5 to 20 microg(-1). kg(-1). min(-1) dobutamine and in 9 postinfarct patients at rest. We compared 2D myocardial strains (circumferential shortening, Ecc; maximal shortening, E(2); and E(2), direction) as assessed by a conventional technique and by HARP. Full quantitative analysis of the data was 10 times faster with HARP. For pooled data, the regression coefficient was r=0.93 for each strain (P<0.001). In volunteers, Ecc and E(2) were greater in the free wall than in the septum (P<0.01), but recruitable myocardial strain at peak dobutamine was greater in the LV septum (P<0.01). E(2) orientation shifted away from the circumferential direction at peak dobutamine (P<0.01). HARP accurately detected subtle changes in myocardial strain fields under increasing doses of dobutamine. In patients, HARP-determined Ecc and E(2) values were dramatically reduced in the asynergic segments as compared with remote (P<0.001), and E(2) direction shifted away from the circumferential direction (P<0.001). CONCLUSIONS: HARP MRI provides fast, accurate assessment of myocardial strains from tagged MR images in normal subjects and in patients with coronary artery disease with wall motion abnormalities. HARP correctly indexes dobutamine-induced changes in strains and has the potential for on-line quantitative monitoring of LV function during stress testing.

Adult↗
PubMed2000

A k-space analysis of MR tagging.

We present a k-space approximation that directly relates a pulse sequence to its residual pattern of z-directed magnetization M(z), in a manner akin to the k-space approximation for small tip-angle excitation. Our approximation is particularly useful for the analysis and design of tagging sequences, in which M(z) is the important quantity-as opposed to the transverse magnetization components M(x) and M(y) considered in selective excitation. We demonstrate that our approximation provides new insights into tagging, can be used to design novel tag patterns, and, more generally, may be applied to selective presaturation sequences for purposes other than tagging.

Humans↗
PubMed2000

Transmural contractile reserve after reperfused myocardial infarction in dogs.

OBJECTIVES: The goal of this study was to characterize detailed transmural left ventricular (LV) function at rest and during dobutamine stimulation in subendocardial and transmural experimental infarcts. BACKGROUND: The relation between segmental LV function and the transmural extent of myocardial necrosis is complex. However, its detailed understanding is crucial for the diagnosis of myocardial viability as assessed by inotropic stimulation. METHODS: Short-axis tagged magnetic resonance images were acquired at five to seven levels encompassing the LV from base to apex in seven dogs 2 days after a 90-min closed-chest left anterior descending coronary occlusion, followed by reflow. Myocardial strains were measured transmurally in the entire LV by harmonic phase imaging at rest and 5 ig x kg(-1) x min(-1) dobutamine. Risk regions were assessed by radioactive microspheres, and the transmural extent of the infarct was assessed by 2,3,5 triphenyltetrazolium chloride staining. RESULTS: Circumferential shortening (Ecc), radial thickening (Err) and maximal shortening at rest were greater in segments with subendocardial versus transmural infarcts, both in subepicardium (-1.1+/-1.0 vs. 2.5+/-0.6% for Ecc, -0.5+/-1.9 vs. -1.8+/-1.0% for Err, p < 0.05) and subendocardium (-2.0+/-1.4 vs. 2.8+/-0.8%, 2.4+/-1.7 vs. 0.0+/-0.9%, respectively, p < 0.05). Under inotropic stimulation, risk regions retained maximal contractile reserve. Recruitable deformation was found in outer layers of subendocardial infarcts (p < 0.01 for Ecc and Err) but also in inner layers (p < 0.01). Conversely, no contractile reserve was observed in segments with transmural infarcts. CONCLUSIONS: Under dobutamine challenge, recruitment of myofiber shortening and thickening was observed in inner layers of segments with subendocardial infarcts. These results may have important clinical implications for the detection of myocardial viability.

Animals↗
PubMed2000

Visualizing myocardial function using HARP MRI.

Harmonic phase magnetic resonance imaging (HARP) is a new technique for measuring the motion of the left ventricle of the heart. HARP uses magnetic resonance tagging, Fourier filtering and special processing algorithms to calculate key indices of myocardial motion including Eulerian and Lagrangian strain. This paper presents several new methods for visualizing myocardial motion based on HARP. Quantities that are computed and visualized include motion grids, velocity fields, strain rates, pathlines, tracked Eulerian strain, and contraction angle. The computations are fast and fully automated and have the potential for clinical application.

Algorithms↗
PubMed2000

Imaging heart motion using harmonic phase MRI.

This paper describes a new image processing technique for rapid analysis and visualization of tagged cardiac magnetic resonance (MR) images. The method is based on the use of isolated spectral peaks in spatial modulation of magnetization (SPAMM)-tagged magnetic resonance images. We call the calculated angle of the complex image corresponding to one of these peaks a harmonic phase (HARP) image and show that HARP images can be used to synthesize conventional tag lines, reconstruct displacement fields for small motions, and calculate two-dimensional (2-D) strain. The performance of this new approach is demonstrated using both real and simulated tagged MR images. Potential for use of HARP images in fast imaging techniques and three-dimensional (3-D) analyses are discussed.

Algorithms↗
PubMed2000

Bandpass optical flow for tagged MRI.

MR tagging has shown great promise for detailed noninvasive cardiac motion imaging. Our research uses low-frequency tags coupled with gradient-based optical flow estimation to compute cardiac motion. We develop here a novel, fast, fully automated optical flow method for tagged MRI by exploiting the Fourier content of the tagged images. This new method, called bandpass optical flow, works by extracting various subband images from tagged cardiac data, and then formulating multiple optical flow constraints for each subband. The resulting system is solved by least squares pseudo-inversion. The proposed method is validated on simulated and real tagged data.

Algorithms↗
PubMed1999

Cardiac motion tracking using CINE harmonic phase (HARP) magnetic resonance imaging.

This article introduces a new image processing technique for rapid analysis of tagged cardiac magnetic resonance image sequences. The method uses isolated spectral peaks in SPAMM-tagged magnetic resonance images, which contain information about cardiac motion. The inverse Fourier transform of a spectral peak is a complex image whose calculated angle is called a harmonic phase (HARP) image. It is shown how two HARP image sequences can be used to automatically and accurately track material points through time. A rapid, semiautomated procedure to calculate circumferential and radial Lagrangian strain from tracked points is described. This new computational approach permits rapid analysis and visualization of myocardial strain within 5-10 min after the scan is complete. Its performance is demonstrated on MR image sequences reflecting both normal and abnormal cardiac motion. Results from the new method are shown to compare very well with a previously validated tracking algorithm. Magn Reson Med 42:1048-1060, 1999.

Algorithms↗
PubMed1999

Reconstruction of the human cerebral cortex from magnetic resonance images.

Reconstructing the geometry of the human cerebral cortex from MR images is an important step in both brain mapping and surgical path planning applications. Difficulties with imaging noise, partial volume averaging, image intensity inhomogeneities, convoluted cortical structures, and the requirement to preserve anatomical topology make the development of accurate automated algorithms particularly challenging. In this paper we address each of these problems and describe a systematic method for obtaining a surface representation of the geometric central layer of the human cerebral cortex. Using fuzzy segmentation, an isosurface algorithm, and a deformable surface model, the method reconstructs the entire cortex with the correct topology, including deep convoluted sulci and gyri. The method is largely automated and its results are robust to imaging noise, partial volume averaging, and image intensity inhomogeneities. The performance of this method is demonstrated, both qualitatively and quantitatively, and the results of its application to six subjects and one simulated MR brain volume are presented.

Algorithms↗
PubMed1999

Adaptive fuzzy segmentation of magnetic resonance images.

An algorithm is presented for the fuzzy segmentation of two-dimensional (2-D) and three-dimensional (3-D) multispectral magnetic resonance (MR) images that have been corrupted by intensity inhomogeneities, also known as shading artifacts. The algorithm is an extension of the 2-D adaptive fuzzy C-means algorithm (2-D AFCM) presented in previous work by the authors. This algorithm models the intensity inhomogeneities as a gain field that causes image intensities to smoothly and slowly vary through the image space. It iteratively adapts to the intensity inhomogeneities and is completely automated. In this paper, we fully generalize 2-D AFCM to three-dimensional (3-D) multispectral images. Because of the potential size of 3-D image data, we also describe a new faster multigrid-based algorithm for its implementation. We show, using simulated MR data, that 3-D AFCM yields lower error rates than both the standard fuzzy C-means (FCM) algorithm and two other competing methods, when segmenting corrupted images. Its efficacy is further demonstrated using real 3-D scalar and multispectral MR brain images.

Algorithms↗
PubMed1998

Cardiac material markers from tagged MR images.

Tagged magnetic resonance imaging (MRI) has shown great promise in non-invasive analysis of heart motion. To replace implanted markers as a gold standard, however, tagged MRI must be able to track a sparse set of material points, so-called material markers, with high accuracy. This paper presents a new method for generating accurate motion estimates over a sparse set of material points using standard, parallel-tagged MR images. Each tracked point is located at the intersection of three tag surfaces, each of which is estimated using a thin-plate spline. The intersections are determined by an iterative alternating projections algorithm for which a proof of convergence is provided. The resulting data sets are compatible with applications developed to exploit implanted marker data. One set of these material markers from a normal human volunteer is examined in detail using several methods to visualize the markers. Numerical results that include additional studies are also discussed. Finally, an error analysis is presented using a computer-simulated left ventricle for which material markers are tracked with an RMS error of approximately 0.2 mm for typical imaging parameters and noise levels.

Algorithms↗
PubMed1995

Optimization of MR pulse sequences for Bayesian image segmentation.

A method for optimizing MR imaging pulse sequence parameters in a statistical framework is presented. Parameters are defined to be optimal when the resulting scalar images yield optimal image segmentations using Bayesian pixel classification. Thus, Bayes risk is used as the objective function to minimize. Approximations are made to give a tractable solution in a four-step procedure. A sample calculation is carried out to determine the optimal TR and flip angle for scalar SPGR imaging of the brain. Overall, this paper gives a new approach to optimize MRI pulse sequences for the specific objective of improved image segmentation.

Bayes Theorem↗
PubMed1992

Measurement of radiotracer concentration in brain gray matter using positron emission tomography: MRI-based correction for partial volume effects.

Accuracy in in vivo quantitation of brain function with positron emission tomography (PET) has often been limited by partial volume effects. This limitation becomes prominent in studies of aging and degenerative brain diseases where partial volume effects vary with different degrees of atrophy. The present study describes how the actual gray matter (GM) tracer concentration can be estimated using an algorithm that relates the regional fraction of GM to partial volume effects. The regional fraction of GM was determined by magnetic resonance imaging (MRI). The procedure is designated as GM PET. In computer simulations and phantom studies, the GM PET algorithm permitted a 100% recovery of the actual tracer concentration in neocortical GM and hippocampus, irrespective of the GM volume. GM PET was applied in a test case of temporal lobe epilepsy revealing an increase in radiotracer activity in GM that was undetected in the PET image before correction for partial volume effects. In computer simulations, errors in the segmentation of GM and errors in registration of PET and MRI images resulted in less than 15% inaccuracy in the GM PET image. In conclusion, GM PET permits accurate determination of the actual radiotracer concentration in human brain GM in vivo. The method differentiates whether a change in the apparent radiotracer concentration reflects solely an alteration in GM volume or rather a change in radiotracer concentration per unit volume of GM.

Algorithms↗
PubMedDate not supplied

A computerized approach for morphological analysis of the corpus callosum.

OBJECTIVE: A new technique for analyzing the morphology of the corpus callosum is presented, and it is applied to a group of elderly subjects. MATERIALS AND METHODS: The proposed approach normalizes subject data into the Talairach space using an elastic deformation transformation. The properties of this transformation are used as a quantitative description of the callosal shape with respect to the Talairach atlas, which is treated as a standard. In particular, a deformation function measures the enlargement/shrinkage associated with this elastic deformation. Intersubject comparisons are made by comparing deformation functions. RESULTS: This technique was applied to eight male and eight female subjects. Based on the average deformation functions of each group, the posterior region of the female corpus callosum was found to be larger than its corresponding region in the males. The average callosal shape of each group was also found, demonstrating visually the callosal shape differences between the two groups in this sample. CONCLUSION: The proposed methodology utilizes the full resolution of the data, rather than relying on global descriptions such as area measurements. The application of this methodology to an elderly group indicated sex-related differences in the callosal shape and size.

Aged↗