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The influence of gray and white matter tissue anisotropy on the human electroencephalogram (EEG) and magnetoencephalogram (MEG) was examined with a high resolution finite element model of the head of an adult male subject. The conductivity tensor data for gray and white matter were estimated from magnetic resonance diffusion tensor imaging. Simulations were carried out with single dipoles or small extended sources in the cortical gray matter. The inclusion of anisotropic volume conduction in the brain was found to have a minor influence on the topology of EEG and MEG (and hence source localization). We found a major influence on the amplitude of EEG and MEG (and hence source strength estimation) due to the change in conductivity and the inclusion of anisotropy. We expect that inclusion of tissue anisotropy information will improve source estimation procedures. [Copyright &y& Elsevier]

Image distortion due to field gradient eddy currents can create image artifacts in diffusion-weighted MR images. These images, acquired by measuring the attenuation of NMR signal due to directionally dependent diffusion, have recently been shown to be useful in the diagnosis and assessment of acute stroke and in mapping of tissue structure. This work presents an improvement on the spin-echo (SE) diffusion sequence that displays less distortion and consequently improves image quality. Adding a second refocusing pulse provides better image quality with less distortion at no cost in scanning efficiency or effectiveness, and allows more flexible diffusion gradient timing. By adjusting the timing of the diffusion gradients, eddy currents with a single exponential decay constant can be nulled, and eddy currents with similar decay constants can be greatly reduced. This new sequence is demonstrated in phantom measurements and in diffusion anisotropy images of normal human brain., (Copyright 2003 Wiley-Liss, Inc.)

Knowledge of the electrical conductivity properties of excitable tissues is essential for relating the electromagnetic fields generated by the tissue to the underlying electrophysiological currents. Efforts to characterize these endogenous currents from measurements of the associated electromagnetic fields would significantly benefit from the ability to measure the electrical conductivity properties of the tissue noninvasively. Here, using an effective medium approach, we show how the electrical conductivity tensor of tissue can be quantitatively inferred from the water self-diffusion tensor as measured by diffusion tensor magnetic resonance imaging. The effective medium model indicates a strong linear relationship between the conductivity and diffusion tensor eigenvalues (respectively, final sigma and d) in agreement with theoretical bounds and experimental measurements presented here (final sigma/d approximately 0.844 +/- 0.0545 S small middle dots/mm(3), r(2) = 0.945). The extension to other biological transport phenomena is also discussed.

The myoarchitecture of the tongue is comprised of a complex array of muscle fiber bundles, which form the structural basis for lingual deformations during speech and swallowing. We used magnetic resonance imaging of the water diffusion tensor to display the primary and secondary fiber architectural attributes of the excised bovine tongue. Fiber orientation mapping provides a subdivision of the tongue into its principal intrinsic and extrinsic muscular components. The anterior tongue consists of a central region of orthogonally oriented intrinsic fibers surrounded by an axially oriented muscular sheath. The posterior tongue consists principally of a central region of extrinsic fibers, originating at the inferior surface and projecting in a fan-like manner in the superior, lateral, and posterior directions, and lateral populations of extrinsic fibers directed posterior-inferior and posterior-superior. Analysis of cross-fiber anisotropy indicates a basic contrast of design between the extrinsic and the intrinsic fibers. Whereas the extrinsic muscles exhibit a uniaxial architecture typical of skeletal muscle, the intrinsic core muscles, comprised of the verticalis and the transversus muscles, show strong cross-fiber anisotropy. This pattern is consistent with the theory that the tongue's core functions as a muscular hydrostat in that conjoint contraction of the transverse and vertical fibers enable the tissue to expand at right angles to these fibers. These findings suggest that three-dimensional analysis of diffusion tensor magnetic resonance imaging provides a structural basis for understanding the micromechanics of the mammalian tongue.

Human white matter fiber crossings were investigated with use of the full eigenstructure of the magnetic resonance diffusion tensor. Intravoxel fiber dispersions were characterized by the plane spanned by the major and medium eigenvectors and depicted with three-dimensional graphics. This method improves the analysis of fiber orientations, beyond the principal fiber directions, to a broader range of complex fiber architectures.

Purpose: To use diffusion-sensitive magnetic resonance (MR) imaging to obtain images of fiber orientation in vivo and to map fiber shortening in humans by means of integrating such data with strain images., Materials and Methods: Images of fiber shortening for midventricular short-axis sections were acquired in eight healthy subjects. Fiber orientation maps obtained by means of diffusion-sensitive MR imaging were coregistered with systolic strain maps obtained by means of velocity-sensitive MR imaging. Fiber shortening was quantified by use of the component of systolic strain in the fiber direction., Results: The results were reproducible among subjects and were consistent with published values. MR imaging of myocardial fibers showed axisymmetric progression of fiber angles from -90 degrees epicardially to +90 degrees endocardially, with maxima near 0 degrees. Fiber shortening (mean, 0.12 +/- 0.01 [SD]) was more uniform than radial, circumferential, longitudinal, or cross-fiber strain or any principal strain. Fiber orientation coincided with the direction of maximum contraction epicardially, with that of minimum contraction endocardially, and varied between these extremes linearly with wall depth (r = 0.6)., Conclusion: Registered diffusion and strain MR imaging can be used quantitatively to map fiber orientation and its relations to myocardial deformation in humans.

Purpose: To (a) determine the optimal choice of a scalar metric of anisotropy and (b) determine by means of magnetic resonance imaging if changes in diffusion anisotropy occurred in acute human ischemic stroke., Materials and Methods: The full diffusion tensor over the entire brain was measured. To optimize the choice of a scalar anisotropy metric, the performances of scalar indices in simulated models and in a healthy volunteer were analyzed. The anisotropy, trace apparent diffusion coefficient (ADC), and eigenvalues of the diffusion tensor in lesions and contralateral normal brain were compared in 50 patients with stroke., Results: Changes in anisotropy in patients were quantified by using fractional anisotropy because it provided the best performance in terms of contrast-to-noise ratio as a function of signal-to-noise ratio in simulations. The anisotropy of ischemic white matter decreased (P = .01). Changes in anisotropy in ischemic gray matter were not significant (P = .63). The trace ADC decreased for ischemic gray matter and white matter (P < .001). The first and second eigenvalues decreased in both ischemic gray and ischemic white matter (P < .001). The third eigenvalue decreased in ischemic gray (P = .001) and white matter (P = .03)., Conclusion: Gray matter is mildly anisotropic in normal and early ischemic states. However, early white matter ischemia is associated with not only changes in trace ADC values but also significant changes in the anisotropy, or shape, of the water self-diffusion tensor.

Our goal was to quantify intramural mechanics in the tongue through an assessment of local strain during the physiological phases of swallowing. Subjects were imaged with an ultrafast gradient echo magnetic resonance imaging (MRI) pulse sequence after the application of supersaturated magnetized bands in the x and y directions. Local strain was defined through deformation of discrete triangular elements defined by these bands and was depicted graphically either as color-coded two-dimensional strain maps or as three-dimensional octahedra whose axes correspond to the principal strains for each element. During early accommodation, the anterior tongue showed positive strain (expansive) in the anterior-posterior direction (x), whereas the middle tongue showed negative strain (contractile) in the superior-inferior direction (y). During late accommodation, the anterior tongue displayed increased positive x-direction and y-direction strain, whereas the posterior tongue displayed increased negative y-direction strain. These findings were consistent with contraction of the anterior-located intrinsic muscles and the posterior-located genioglossus and hyoglossus muscles. During propulsion, posterior displacement of the tongue was principally associated with positive strain directed in the x and y directions. These findings were consistent with posterior passive stretch in the midline due to contraction of the laterally inserted styloglossus muscle, as well as contraction of the posterior located transversus muscle. We conclude that MRI of lingual deformation during swallowing resolves the synergistic contractions of the intrinsic and extrinsic muscle groups.

Cardiac diffusion MRI with diffusion encoding that spans a cardiac cycle is complicated by myocardial strains. This paper presents a method to obtain accurate diffusion data without strain correction. Owing to the synchrony of normal cardiac motion, there are time points in the cardiac cycle, "sweet spots," when the cardiac configuration approximates its temporal mean. If the diffusion is encoded then, the net effect of strain on the observed diffusion approximates zero. To test this, MRI diffusion and strain-rate movies are performed on cyclically deformed gel phantoms and in five normal subjects. In phantoms, the sweet spots predicted from the strain time curves agree with the times when the observed diffusion equals the true diffusion. In humans, the strain prediction of the sweet spots and the locations determined by the diffusion trace show a high correlation, r = 0.99. In all subjects, diffusion MRI presents a fiber orientation pattern comparable to that obtained from a stationary specimen. Magn Reson Med 42:393-403, 1999., (Copyright 1999 Wiley-Liss, Inc.)

The muscular anatomy of the tongue consists of a complex three-dimensional array of fibers, which together produce the variations of shape and position necessary for deglutition. To define the myoarchitecture of the intact mammalian tongue, we have utilized NMR techniques to assess the location and orientation of muscle fiber bundles through measurement of the direction-specific diffusional properties of water molecules. Whole sheep tongues were excised and imaged with a slice-selective stimulated-echo diffusion sequence in the midline sagittal plane, and three-dimensional diffusion tensors were determined for each voxel. The derived diffusion tensors were depicted graphically as octahedra whose long axes indicate local muscle fiber orientation. Two distinct groups of midline fibers were identified: 1) in-plane sagittal fibers originating in the posteroinferior region of the tongue, radiating with a fanlike projection anteriorly and superiorly and merging with vertically oriented fibers, and 2) cross-plane (transverse) fibers, oriented at right angles to the vertically aligned fibers, predominantly in the anterior and superior regions of the tongue. Regional comparison of diffusion anisotropy revealed uniform and parallel alignment (high anisotropy) in the posteroinferior region of the tongue, corresponding to the base of the genioglossus, and less uniform, orthogonally aligned fibers (low anisotropy) in the anterosuperior region of the tongue, corresponding to the core intrinsic muscles. These data indicate that lingual myoarchitecture, determined through direction-dependent mobility of water molecules, can be depicted as discrete regions of muscle fibers, whose orientation and extent of diffusion anisotropy predict local contractility.

Methods are presented to image the fiber architecture of the human myocardium in vitro and in vivo. NMR images are obtained of the diffusion anisotropy tensor, indicative of local myofiber orientation. Studies of cardiac necropsy specimens demonstrate classic features of ventricular myoarchitecture including the continuous endocardial to epicardial variation of fiber helix angles (angles to the ventricular circumferential direction) of approximately +1.3 to -1.3 radians. Cross-fiber anisotropy is also observed. In the beating heart, NMR diffusion data must be corrected for the effects of myocardial deformation during the cardiac cycle. This correction can be performed using an independent MRI method to map the strain-rate tensor field of the myocardium through time. Combining fiber orientation with local myocardial strain rate, local rates of myocardial fiber shortening may be computed.

We present methods to acquire and analyze NMR movies of myocardial strain rates in which cardiac motion is suppressed and the histories of strain rates are accurately defined for each voxel of myocardial tissue. By means of stimulated echoes, the myocardial strain-rate tensor is phase-encoded at progressive delays in the cardiac cycle while the slice-select and spatial encoding of the image acquisition are performed at a constant cardiac delay. In these data, every image shows the identical myocardial tissue, and the anatomic configuration of the heart appears motionless. The myocardial strain-rate data, however, indicate the state of motion which existed in this slice at the time of the velocity phase-encoding, and these data evolve with the progressive delay as a movie. Using echo-planar MRI, motionless movies of myocardial strain rate of four to eight cardiac delays are obtained in a breath-hold. As an application, a quantitative characterization of cardiac mechanical synchrony is accomplished by principal component analysis (PCA) of the time series of strain rates.

Existing magnetic resonance methods for diffusion imaging, including echo planar, are ineffective in the beating heart due to motion-induced signal attenuation. To overcome this problem, we used a diffusion-weighted stimulated echo-echo planar magnetic resonance imaging sequence. The two lobes of the diffusion-sensitizing gradient were synchronized to the same point in successive cardiac cycles in order to fix the cardiac position and avoid bulk motion effects. The apparent diffusion coefficients (ADCs) of the interventricular septum in 12 healthy subjects for diffusion gradients along the x-, y-, and z-directions were 1.40 +/- 0.27, 1.48 +/- 0.35, and 1.78 +/- 0.27 x 10(-3) mm2/s. The ADCs of the interventricular septum in a second group of 15 healthy subjects for diffusion gradients along the short axis, horizontal and vertical long axes were 0.92 +/- 0.15, 1.50 +/- 0.15, and 1.10 +/- 0.24 x 10(-3) mm2/s. Because the ADCs were less than the measured values for skeletal muscle and their standard deviations were low, it seems unlikely that bulk motion effects made the dominant contribution to the measured myocardial ADC for the interventricular septum, although motion and/or susceptibility artifacts frequently degraded measurements in the free wall of the left ventricle. Additional evidence that ADC was not predominantly determined by wall motion was obtained in a third group of patients with various cardiac abnormalities, in whom there was only a weak correlation between ADC and ejection fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

The process of MRI signal attenuation due to in-plane intravoxel velocity inhomogeneity is described. Given rigid rotation or linear shear, velocity phase-sensitivity will induce a phase distribution that varies linearly with position, which is exactly equivalent to the effect of a spatial phase encoding gradient pulse. It follows that the effect of such motion on the raw MRI signal is to displace it a fixed distance in kappa-space. Attenuation becomes marked when the center of the spin-echo reaches an edge of kappa-space, which happens when intravoxel phase shifts reach pi radian/voxel. Because spin echoes are typically peaked sharply at center, this attenuation usually is abrupt. Analytic and numerical simulations of linear and nonlinear velocity fields confirm abrupt MRI attenuation where phase dispersion exceeds pi radian/voxel. Examples of this phenomenon include the abrupt loss of blood signal adjacent the vessel wall in laminar flow, abrupt loss of subendocardial signal in early diastole, and sudden disappearance due to rotation of a kidney during a measurement of diffusion.

Detection and quantification of flow of the left anterior descending (LAD) coronary artery in healthy volunteers are demonstrated using echo-planar imaging (EPI). A time-of-flight (TOF) model was used to derive coronary flow velocities from wash-in curves, free of cardiac wall motion contamination. Short-axis cardiac studies were performed using a gated, gradient echo EPI technique to limit the effect of cardiac wall motion on coronary vessel imaging. A series of 10 to 20 single or multislice images were acquired within a single breath-hold. Real-time cine series showed the LAD coronary artery with a detectability of 91% (n = 23) and revealed beat-to-beat variability in vessel position of a magnitude equal to or greater than its diameter. Flow velocity was measured in the proximal portion of the artery at rest and during exercise. The data demonstrated the known phasic pattern of LAD flow: Vsystole < or = 5 cm/s and peak Vdiastole = 14 +/- 3 cm/s (n = 11, V = mean laminar flow velocity). During isometric exercise, a LAD flow velocity increase (52 +/- 24%) was detected in eight of nine subjects. The capacity of the EPI TOF method to detect flow velocity changes should prove clinically useful for future assessment of coronary flow reserve.

A magnetic resonance imaging (MRI) method is presented to detect, localize, and quantify myocardial kinematics by measuring the material rate-of-strain tensor at each pixel in gated NMR images of the heart. The immediate, local effect of muscular activity is self-deformation, and the strain tensor is the basic mathematical device by which such deformation may be quantified. The present method, called "strain-phase" MRI (SP-MRI), entails four steps: (1) the velocity of the myocardium is encoded by means of a set of motion-sensitive NMR image acquisitions, one image per velocity component; (2) the spatial derivatives of the velocity are computed at each pixel; (3) the velocity-derivative data are combined to compute an approximation of the strain-rate tensor of the myocardium at each pixel; and (4) the strain-rate tensor data are simplified to produce a color-coded functional image which represents strain-rate components which are of particular biomedical interest in the myocardium. We present a quantitative SP-MRI methodology suited to conventional MRI, and in addition present an "echo-planar" methodology, able to produce qualitative functional images of myocardial kinematics at almost real-time speeds. Two-dimensional strain-phase MRI data acquired in normal human subjects are presented. These data demonstrate the practicability of SP-MRI in vivo, that SP-MRI resolves myocardial kinematics at the single-pixel scale, having resolution comparable to that of conventional MRI, and that SP-MRI data may have a signal-to-noise ratio up to 50% as great as that of the conventional MRI data from which they are produced. SP-MRI measurements of the local instantaneous strain rates in the human left ventricular myocardium are quantitatively consistent with known transmural average values of myocardial strain.

Techniques for subtraction angiography with magnetic resonance imaging have been extended from two to three dimensions, and a novel method that reduces the expected data acquisition time by at least an order of magnitude is presented. Electrocardiogram-gated three-dimensional (3D) images are acquired by Fourier transform technique, and flow contrast is obtained by subtracting pairs of images acquired at different points in the cardiac cycle. The vascular tree is shown in 3D perspective by means of a surface detection and a 3D display program. Isotropic 3D angiography requires determining the disposition of the blood vessels in a matrix of cubical voxels. Using orthodox Fourier transform technique, for an image matrix with 256 voxels to the edge, a data acquisition with 256 X 256 = 65 K phase-encodings would be needed. If gated, this would require approximately 1 day. In this study we abbreviate the data acquisition by doing only 1/64 of the usual set of phase-encoding gradient pulses. Spatial resolution is undiminished, but aliasing or "wraparound" results in each of the two phase-encoded coordinates of the 3D image. This aliasing is rectified in a two stage process. First, 64 copies of the undersampled 3D arteriogram are juxtaposed in a two-dimensional grid pattern. This assembles many copies of the complete vascular tree. Because they occupy only a small fraction of ambient volume, these copies are unlikely to overlap or collide with one another. Second, a single copy of the vascular tree is isolated by a surface detection program that takes advantage of the fact that the vascular tree is topologically connected. Studies of the abdominal aorta are presented.

Projective MR images of vascular anatomy and flow are performed at 0.14 T by using phase contrast to suppress the signal contribution of the stationary background. The source of the contrast is the distinctive phase evolution of moving protons under the influence of the read-out gradient of a conventional two-dimensional Fourier transform (2D FT) spin-echo pulse sequence. By using short echo times, small phase shifts may be obtained. When phase shifts are less than about 45 degrees, the phase contrast assumes a simple and useful form. The flow image intensity at any pixel becomes proportional to the net flux or flow volume of protons which cross the corresponding voxel. This proportionality is demonstrated in images of flow phantoms as is the reproducibility of measured flow volume under a variety of transformations of imaging conditions and of the subject. Projective images gated in vivo produce angiographic views of arteries and veins, in systole and diastole, in the neck of a dog and in the lower extremities of a human subject.

Noninvasive angiography with magnetic resonance is demonstrated. Signal arising in all structures except vessels that carry pulsatile flow is eliminated by means of velocity-dependent phase contrast, electrocardiographic gating, and image subtraction. Background structures become in effect transparent, enabling the three-dimensional vascular tree to be imaged by projection to a two-dimensional image plane. Image acquisition and processing are accomplished with entirely conventional two-dimensional Fourier transform magnetic resonance imaging techniques. When imaged at 0.6 tesla, vessels 1 to 2 millimeters in diameter are routinely detected in a 50-centimeter field of view with data acquisition times less than 15 minutes. Studies of normal and pathologic anatomy are illustrated in human subjects.

Using a one-dimensional rapid imaging technique, we have found that injection of lanthanide chelates such as Gd(DTPA)2- leads to a significant decrease (50%) in rat brain signal intensity at 1.45 T using T2-weighted pulse sequences; however, no effect of comparable size is observed with T1-weighted pulse sequences. The transient effect and its kinetics were followed with a temporal resolution of between 1 and 8 s. Experiments with different lanthanide chelates show that the observed decrease in signal intensity correlates with the magnetic moment of each agent but not with their longitudinal relaxivity. Three-dimensional chemical-shift resolved experiments demonstrate significant line broadening in brain during infusion with Dy(DTPA)2-. Our results show that the cause of this effect is the difference in susceptibility between the capillaries, containing the contrast agent, and the surrounding tissue. As a result of these susceptibility differences, field gradients are produced in the tissue and diffusion of water through these gradients leads to a loss of spin phase coherence and thus a decrease in signal intensity. We propose this as a new type of contrast agent mechanism in NMR. The effect and its kinetics are likely to be related to important physiological parameters such as cerebral blood volume and cerebral blood flow, and do not depend on a breakdown of the blood-brain barrier as do conventional contrast agent techniques.

Most magnetic resonance imaging (MRI) techniques are subject to a "motional blurring" arising from the acquisition of data in the presence of a frequency-encoding gradient. The Fourier transform of the signal from a spin moving along a magnetic field gradient obeys an equation analogous to the free space Schrödinger equation. Computer simulations of the Bloch equations illustrate the implications of this motional blurring in MRI.

To establish the effect of the paramagnetic contrast agent gadolinium diethylenetriaminepentaacetic acid ([Gd]DTPA) on myocardial magnetic resonance relaxation parameters T1 and T2, and its relationship to myocardial perfusion, we administered [Gd] DTPA 0.2 mM/kg to two groups of dogs. Group I had severe, resting myocardial ischemia induced by coronary occlusion, followed in 2 min by [Gd]DTPA infusion and heart excision 1 min later. Group II had a variable reduction in blood flow. In Group II the coronary vasodilator dipyridamole was infused to enhance blood flow to the normal myocardium before [Gd]DTPA was given. In Group I [Gd]DTPA caused a significant difference in T1 between the normal and severely ischemic zones; changes in T1 correlated with the severity of myocardial ischemia. Although vasodilatation delivered more Gd-DTPA to the normal myocardium in Group II, the lack of further decrease in T1 suggested that it was cleared more rapidly. Thus, [Gd]DTPA permits the detection and characterization of severe, resting myocardial ischemia by magnetic resonance techniques. Using the experimental techniques described in this study, less severe flow differences caused by vasodilatation and resultant hyperemia are not detected.

The ability of the nuclear magnetic resonance signal to encode information about macroscopic motion has been recognized since the works of Hahn and Carr and Purcell. In the medical imaging setting this ability has led to a variety of ingenious magnetic resonance flow imaging schemes that ultimately may become competitive with X-ray angiography in sensitivity and specificity while remaining radically noninvasive. This work demonstrates that conventional spin-echo Fourier transform image acquisitions naturally encode a component of flow velocity that lies within the image plane. By displacing just the real part of the complex image data (phase display), the velocity distribution within the subject is revealed in image form. This method of flow imaging requires neither special pulse sequences nor image reconstruction and format software for its implementation. Further, images that intersect a flow channel longitudinally, demonstrating in-plane flow, yield an unusually large quantity of physiologic information per image. Phantom and in vivo flow images are presented. Also described is a phantom based on a rotating disk that enables calibration of the velocity/phase-shift constant for an untested pulse sequence.

The paramagnetic metal complex iron(III) ethylenebis-(2-hydroxyphenylglycine) [Fe(EHPG)-] is an effective hepatobiliary contrast agent for liver enhancement in magnetic resonance (MR) imaging. The intravenous administration of 0.2 mmol/kg of Fe(EHPG)- to rats yields a 200% increase in the signal intensity of the liver when using a T1-weighted inversion recovery pulse sequence on a 1.4 T imaging system. Biodistribution studies in rats and a rabbit, along with imaging studies in a dog at 0.6 T, confirm that the complex has significant hepatocellular uptake and appears to be excreted unaltered into the bile. Control experiments with a different iron complex, iron(III) diethylenetriaminepentaacetic acid, reveal little hepatic affinity and poor enhancement capability due to its extracellular distribution. This initial evaluation of Fe(EHPG)- demonstrates that paramagnetic metal complexes with hepatobiliary specificity are well suited for enhancement of normal liver parenchyma and may increase the sensitivity of MR in the detection of liver disease.

In current magnetic resonance imaging (MRI), valuable information must often be discarded because the NMR signal has greater dynamic range than the analog-to-digital converter (ADC) hardware. Typically, a small set of high-intensity data points near the center of the spin echo is responsible for most of the MRI data dynamic range. We predict that it is possible to reduce the dynamic range of the MRI spin echo by incorporating an identical nonlinear gradient pulse into each repetition of the imaging pulse sequence, prior to data sampling. This pulse converts the phase distribution of the subject, ordinarily a linear function of image coordinates, into a nonlinear function. A nonlinear phase distribution can have a negligible impact on image magnitude and yet a profound impact on spin-echo magnitude. Given a nonlinear phase distribution, there will no longer be a single data point at which all of the protons have an identical phase (the echo center). Instead, the protons become phase coherent on a piecemeal basis, the echo peak is smoothed out, and its maximum amplitude and dynamic range are greatly diminished. Using gradient pulses of quadratic spatial variation, we estimate that maximum echo amplitude and dynamic range can be reduced in most cases by an order of magnitude.