Timothy G. Reese

High angular resolution diffusion imaging reveals intravoxel white matter fiber heterogeneity

Magnetic resonance (MR) diffusion tensor imaging (DTI) can resolve the white matter fiber orientation within a voxel provided that the fibers are strongly aligned. However, a given voxel may contain a distribution of fiber orientations due to, for example, intravoxel fiber crossing. The present study sought to test whether a geodesic, high b‐value diffusion gradient sampling scheme could resolve multiple fiber orientations within a single voxel. In regions of fiber crossing the diffusion signal exhibited multiple local maxima/minima as a function of diffusion gradient orientation, indicating the presence of multiple intravoxel fiber orientations. The multimodality of the observed diffusion signal precluded the standard tensor reconstruction, so instead the diffusion signal was modeled as arising from a discrete mixture of Gaussian diffusion processes in slow exchange, and the underlying mixture of tensors was solved for using a gradient descent scheme. The multitensor reconstruction resolved multiple intravoxel fiber populations corresponding to known fiber anatomy. Magn Reson Med 48:577–582, 2002.

Reduction of eddy-current-induced distortion in diffusion MRI using a twice-refocused spin echo

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. Magn Reson Med 49:177–182, 2003.

Mapping complex tissue architecture with diffusion spectrum magnetic resonance imaging.

Methods are presented to map complex fiber architectures in tissues by imaging the 3D spectra of tissue water diffusion with MR. First, theoretical considerations show why and under what conditions diffusion contrast is positive. Using this result, spin displacement spectra that are conventionally phase‐encoded can be accurately reconstructed by a Fourier transform of the measured signals modulus. Second, studies of in vitro and in vivo samples demonstrate correspondence between the orientational maxima of the diffusion spectrum and those of the fiber orientation density at each location. In specimens with complex muscular tissue, such as the tongue, diffusion spectrum images show characteristic local heterogeneities of fiber architectures, including angular dispersion and intersection. Cerebral diffusion spectra acquired in normal human subjects resolve known white matter tracts and tract intersections. Finally, the relation between the presented model‐free imaging technique and other available diffusion MRI schemes is discussed. Magn Reson Med, 2005.

Diffusion MRI of Complex Neural Architecture

While functional brain imaging methods can locate the cortical regions subserving particular cognitive functions, the connectivity between the functional areas of the human brain remains poorly understood. Recently, investigators have proposed a method to image neural connectivity noninvasively using a magnetic resonance imaging method called diffusion tensor imaging (DTI). DTI measures the molecular diffusion of water along neural pathways. Accurate reconstruction of neural connectivity patterns from DTI has been hindered, however, by the inability of DTI to resolve more than a single axon direction within each imaging voxel. Here, we present a novel magnetic resonance imaging technique that can resolve multiple axon directions within a single voxel. The technique, called q-ball imaging, can resolve intravoxel white matter fiber crossing as well as white matter insertions into cortex. The ability of q-ball imaging to resolve complex intravoxel fiber architecture eliminates a key obstacle to mapping neural connectivity in the human brain noninvasively.

Diffusion Tensor Magnetic Resonance Imaging Mapping the Fiber Architecture Remodeling in Human Myocardium After Infarction Correlation With Viability and Wall Motion

Background— Diffusion tensor magnetic resonance imaging (DT-MRI) provides a means for nondestructive characterization of myocardial architecture. We used DT-MRI to investigate changes in direction-dependent water diffusivity to reflect alterations in tissue integrity (trace apparent diffusion coefficients [ADCs] and fractional anisotropy [FA]), as well as indicators of remodeling of fiber helix angles, in patients after myocardial infarction. Methods and Results— Thirty-seven patients (35 men, 2 women; median age, 59) after acute myocardial infarction (median interval from onset, 26 days) were enrolled. DT-MRI was performed at the midventricular level to measure trace ADC, FA, and helix angles of myofibers. Helix angles were grouped into left-handed helical fibers, circumferential fibers, and right-handed helical fibers. Measurements were correlated with viability and regional wall motion assessed by contrast-delay-enhancement and cine MRI, respectively. The infarct zone showed significantly increased trace ADC and decreased FA than the remote zone. The percentage of left-handed helical fibers increased from the remote zone (mean±SD, 13.3±5.8%) to the adjacent zone (19.2±9.7%) and infarct zone (25.8±18.4%) (MANOVA, P=0.004). The percentage of right-handed helical fibers decreased from the remote zone (35.0±9.0%) to the adjacent zone (25.5±11.5%) and infarct zone (15.9±9.2%) (P<0.001). Multiple linear regression showed that the percentage of left-handed helical fibers of the infarct zone was the strongest correlate of infarct size and predictor of ejection fraction. Conclusions— In vivo DT-MRI of postinfarct myocardium revealed a significant increase in trace ADC and a decrease in FA, indicating altered tissue integrity. The redistribution of fiber architecture correlated with infarct size and left ventricular function. This technique may help us understand structural correlates of functional remodeling after infarction.

Multislice Perfusion and Perfusion Territory Imaging in Humans With Separate Label and Image Coils

An arterial spin labeling technique using separate RF labeling and imaging coils was used to obtain multislice perfusion images of the human brain at 3 T. Continuous RF irradiation at a peak power of 0.3 W was applied to the carotid arteries to adiabatically invert spins. Labeling was achieved without producing magnetization transfer effects since the B1 field of the labeling coil did not extend into the imaging region or couple significant power into the imaging coil. Eliminating magnetization transfer allowed the acquisition of multislice perfusion images of arbitrary orientation. Combining surface coil labeling with a reduced RF duty cycle permitted significantly lower SAR than single coil approaches.

Diffusion tensor MRI of myocardial fibers and sheets: correspondence with visible cut-face texture.

To test the hypothesis that the primary, secondary, and tertiary eigenvectors of the diffusion tensor (DT) measured with DT‐MRI correspond to the fiber, sheet, and sheet normal directions, respectively, we compared DT‐MRI data with the texture visible in the cut face of fresh bovine myocardium.

Imaging myocardial fiber disarray and intramural strain hypokinesis in hypertrophic cardiomyopathy with MRI.

To examine the relationship between myofiber disarray and myocardial hypokinesis in human hypertrophic cardiomyopathy (HCM) using noninvasive cardiac diffusion and strain MRI.

MRI-Based Nonrigid Motion Correction in Simultaneous PET/MRI

Respiratory and cardiac motion is the most serious limitation to whole-body PET, resulting in spatial resolution close to 1 cm. Furthermore, motion-induced inconsistencies in the attenuation measurements often lead to significant artifacts in the reconstructed images. Gating can remove motion artifacts at the cost of increased noise. This paper presents an approach to respiratory motion correction using simultaneous PET/MRI to demonstrate initial results in phantoms, rabbits, and nonhuman primates and discusses the prospects for clinical application. Methods: Studies with a deformable phantom, a free-breathing primate, and rabbits implanted with radioactive beads were performed with simultaneous PET/MRI. Motion fields were estimated from concurrently acquired tagged MR images using 2 B-spline nonrigid image registration methods and incorporated into a PET list-mode ordered-subsets expectation maximization algorithm. Using the measured motion fields to transform both the emission data and the attenuation data, we could use all the coincidence data to reconstruct any phase of the respiratory cycle. We compared the resulting SNR and the channelized Hotelling observer (CHO) detection signal-to-noise ratio (SNR) in the motion-corrected reconstruction with the results obtained from standard gating and uncorrected studies. Results: Motion correction virtually eliminated motion blur without reducing SNR, yielding images with SNR comparable to those obtained by gating with 5–8 times longer acquisitions in all studies. The CHO study in dynamic phantoms demonstrated a significant improvement (166%–276%) in lesion detection SNR with MRI-based motion correction as compared with gating (P < 0.001). This improvement was 43%–92% for large motion compared with lesion detection without motion correction (P < 0.001). CHO SNR in the rabbit studies confirmed these results. Conclusion: Tagged MRI motion correction in simultaneous PET/MRI significantly improves lesion detection compared with respiratory gating and no motion correction while reducing radiation dose. In vivo primate and rabbit studies confirmed the improvement in PET image quality and provide the rationale for evaluation in simultaneous whole-body PET/MRI clinical studies.

Diffusion MR tractography of the heart.

Histological studies have shown that the myocardium consists of an array of crossing helical fiber tracts. Changes in myocardial fiber architecture occur in ischemic heart disease and heart failure, and can be imaged non-destructively with diffusion-encoded MR. Several diffusion-encoding schemes have been developed, ranging from scalar measurements of mean diffusivity to a 6-dimensional imaging technique known as diffusion spectrum imaging or DSI. The properties of DSI make it particularly suited to the generation of 3-dimensional tractograms of myofiber architecture. In this article we review the physical basis of diffusion-tractography in the myocardium and the attributes of the available techniques, placing particular emphasis on DSI. The application of DSI in ischemic heart disease is reviewed, and the requisites for widespread clinical translation of diffusion MR tractography in the heart are discussed.