Showing papers in "Magnetic Resonance in Medicine in 2000"
TL;DR: Fiber tract trajectories in coherently organized brain white matter pathways were computed from in vivo diffusion tensor magnetic resonance imaging (DT‐MRI) data, and the method holds promise for elucidating architectural features in other fibrous tissues and ordered media.
Abstract: Fiber tract trajectories in coherently organized brain white matter pathways were computed from in vivo diffusion tensor magnetic resonance imaging (DT-MRI) data. First, a continuous diffusion tensor field is constructed from this discrete, noisy, measured DT-MRI data. Then a Frenet equation, describing the evolution of a fiber tract, was solved. This approach was validated using synthesized, noisy DT-MRI data. Corpus callosum and pyramidal tract trajectories were constructed and found to be consistent with known anatomy. The method's reliability, however, degrades where the distribution of fiber tract directions is nonuniform. Moreover, background noise in diffusion-weighted MRIs can cause a computed trajectory to hop from tract to tract. Still, this method can provide quantitative information with which to visualize and study connectivity and continuity of neural pathways in the central and peripheral nervous systems in vivo, and holds promise for elucidating architectural features in other fibrous tissues and ordered media.
3,053 citations
TL;DR: The RETROICOR method is found to perform well for both respiration‐ and cardiac‐induced noise without imposing spatial filtering on the correction.
Abstract: Respiration effects and cardiac pulsatility can induce signal modulations in functional MR image time series that increase noise and degrade the statistical significance of activation signals. A simple image-based correction method is described that does not have the limitations of k-space methods that preclude high spatial frequency correction. Low-order Fourier series are fit to the image data based on time of each image acquisition relative to the phase of the cardiac and respiratory cycles, monitored using a photoplethysmograph and pneumatic belt, respectively. The RETROICOR method is demonstrated using resting-state experiments on three subjects and compared with the k-space method. The method is found to perform well for both respiration- and cardiac-induced noise without imposing spatial filtering on the correction. Magn Reson Med 44:162‐167, 2000. © 2000 Wiley-Liss, Inc.
1,913 citations
TL;DR: A new computational approach permits rapid analysis and visualization of myocardial strain within 5–10 min after the scan is complete, and its performance is demonstrated on MR image sequences reflecting both normal and abnormal cardiac motion.
Abstract: The present invention relates to a method of measuring motion of an object such as a heart by magnetic resonance imaging. A pulse sequence is applied to spatially modulate a region of interest of the object and at least one first spectral peak is acquired from the Fourier domain of the spatially modulated object. The inverse Fourier transform information of the acquired first spectral-peaks is computed and a computed first harmonic phase image is determined from each spectral peak. The process is repeated to create a second harmonic phase image from each second spectral peak and the strain is determined from the first and second harmonic phase images. In a preferred embodiment, the method is employed to determine strain within the myocardium and to determine change in position of a point at two different times which may result in an increased distance or reduced distance. The method may be employed to determine the path of motion of a point through a sequence of tag images depicting movement of the heart. The method may be employed to determine circumferential strain and radial strain.
741 citations
TL;DR: A novel technique called Prospective Acquisition CorrEction (PACE) for reducing motion‐induced effects on magnetization history is described and they showed a significant decrease of variance between successively acquired datasets compared to retrospective correction algorithms.
Abstract: In functional magnetic resonance imaging (fMRI) head motion can corrupt the signal changes induced by brain activation. This paper describes a novel technique called Prospective Acquisition CorrEction (PACE) for reducing motion-induced effects on magnetization history. Full three-dimensional rigid body estimation of head movement is obtained by image-based motion detection to a high level of accuracy. Adjustment of slice position and orientation, as well as regridding of residual volume to volume motion, is performed in real-time during data acquisition. Phantom experiments demonstrate a high level of consistency (translation < 40 microm; rotation < 0.05 degrees ) for detected motion parameters. In vivo experiments were carried out and they showed a significant decrease of variance between successively acquired datasets compared to retrospective correction algorithms.
644 citations
TL;DR: Echo planar diffusion tensor imaging permits in vivo identification of the orientation and coherence of brain white matter tracts but suffers from field inhomogeneity‐induced geometric distortion.
Abstract: Echo planar (EP) diffusion tensor imaging (DTI) permits in vivo identification of the orientation and coherence of brain white matter tracts but suffers from field inhomogeneity-induced geometric distortion. To reduce spatial distortion, polynomial warping corrections were applied and the effects tested on measures of fractional anisotropy (FA) in the genu and splenium of corpus callosum. Implementation entailed spatially warping EP images obtained without diffusion weighting (b = 0) to long-echo T(2)-weighted fast spin echo images, collected for anatomical delineation, tissue segmentation, and coregistration with the diffusion images. Using the optimal warping procedure (third-order polynomial), the effects of age on FA and a quantitative measure of intervoxel coherence (C) in the genu, splenium, centrum semiovale, and frontal and parietal pericallosal white matter were examined in 31 healthy men (23-76 years). FA declined significantly with age in all regions except the splenium, whereas intervoxel coherence positively correlated with age in the genu. Magn Reson Med 44:259-268, 2000.
613 citations
TL;DR: Improvements in B0 mapping and shimming were achieved by measuring the static field information in multiple subsequent echoes generated by an asymmetric echo‐planar readout gradient train, reducing the time required for field mapping by two‐fold and the sensitivity was simultaneously optimized irrespective of the prevalent T*2 present.
Abstract: Improvements in B(o) mapping and shimming were achieved by measuring the static field information in multiple subsequent echoes generated by an asymmetric echo-planar readout gradient train. With careful compensation, eddy current effects were shown to affect the adjustment of the shim coils minimally. In addition to reducing the time required for field mapping by two-fold, the sensitivity was simultaneously optimized irrespective of the prevalent T2/* present, thereby minimizing the error of the static field measurement to below 0.1 Hz. With adiabatic low flip-angle excitation, the time required for field mapping was below 1 second. (C) 2000 Wiley-Liss, Inc.
544 citations
TL;DR: Both delay and dispersion were found to introduce significant underestimation of cerebral blood flow (CBF) and overestimation of mean transit time (MTT) in DSC quantification.
Abstract: Dynamic susceptibility contrast (DSC) MRI is now increasingly used for measuring perfusion in many different applications. The quantification of DSC data requires the measurement of the arterial input function (AIF) and the deconvolution of the tissue concentration time curve. One of the most accepted deconvolution methods is the use of singular value decomposition (SVD). Simulations were performed to evaluate the effects on DSC quantification of the presence of delay and dispersion in the estimated AIF. Both delay and dispersion were found to introduce significant underestimation of cerebral blood flow (CBF) and overestimation of mean transit time (MTT). While the error introduced by the delay can be corrected by using the information of the arrival time of the bolus, the correction for the dispersion is less straightforward and requires a model for the vasculature.
467 citations
TL;DR: Values obtained for the fast diffusion coefficient, including differences between white and gray matter and the degree of anisotropy are compatible with the predictions of extracellular diffusion of water based on tortuosity models and the diffusion of tetramethylammonium ions in rat brain.
Abstract: Biexponential diffusion decay is demonstrated in the human brain in vivo using b factors up to 4000 sec mm(-2). Fitting of the signal decay data yields values for the slow and fast diffusion components and volume fractions in agreement with previous studies in rat and human brain. In addition, differences in the fitted parameters are demonstrated in the white and gray matter and diffusion anisotropy is demonstrated in both the slow and fast diffusing components. Apparent anisotropy in the component fractions is discussed in terms of directionally dependent exchange rates between the compartments. The lack of a relationship between the estimated contribution to the signal of the fast and slow components and echo time appears to rule out T(2) differences in the observed water compartments. Values obtained for the fast diffusion coefficient, including differences between white and gray matter and the degree of anisotropy are compatible with the predictions of extracellular diffusion of water based on tortuosity models and the diffusion of tetramethylammonium ions in rat brain.
450 citations
TL;DR: The results suggest that ADCs can be used as measures of cell density and necrotic fraction of some but not of all tumors, depending on whether the individual necrotics regions are large enough to be differentiated from the viable tissue with the obtained spatial resolution of the DW images.
Abstract: The aim of this study was to investigate whether apparent diffusion coefficients (ADCs) could be used as measures of cell density and necrotic fraction of tumors. Tumors of four human melanoma xenograft lines were subjected to diffusion-weighted magnetic resonance imaging (DWI). ADCs were calculated from the images and related to cell density and necrotic fraction, as determined from histological sections. A significant correlation was found between the ADC of the viable tissue and cell density, regardless of whether tumors of different lines or different regions within individual tumors were considered. Necrosis was found in two of the lines. A single region of massive necrosis that could be differentiated from the viable tissue in ADC maps was found in one line, whereas a number of smaller necrotic regions that could not be identified in ADC maps were found in the other line. Tumor ADC was significantly correlated with the necrotic fraction of the former, but not of the latter line. Our results suggest that ADCs can be used as measures of cell density and necrotic fraction of some but not of all tumors, depending on whether the individual necrotic regions are large enough to be differentiated from the viable tissue with the obtained spatial resolution of the DW images. Magn Reson Med 43:828–836, 2000. © 2000 Wiley-Liss, Inc.
440 citations
TL;DR: Undersampled projection reconstruction (PR) is investigated as an alternative method for MRA (MR angiography), where bright, contrast‐filled vessels dominate, artifacts are often acceptable and the greater resolution per unit time provided by undersampled PR can be realized.
Abstract: Undersampled projection reconstruction (PR) is investigated as an alternative method for MRA (MR angiography). In conventional 3D Fourier transform (FT) MRA, resolution in the phase-encoding direction is proportional to acquisition time. Since the PR resolution in all directions is determined by the readout resolution, independent of the number of projections (Np), high resolution can be generated rapidly. However, artifacts increase for reduced Np. In X-ray CT, undersampling artifacts from bright objects like bone can dominate other tissue. In MRA, where bright, contrast-filled vessels dominate, artifacts are often acceptable and the greater resolution per unit time provided by undersampled PR can be realized. The resolution increase is limited by SNR reduction associated with reduced voxel size. The hybrid 3D sequence acquires fractional echo projections in the k(x)-k(y) plane and phase encodings in k(z). PR resolution and artifact characteristics are demonstrated in a phantom and in contrast-enhanced volunteer studies.
399 citations
TL;DR: A ratiometric approach was developed that permitted pH determinations that were independent of water T1 or exchange site concentration and several useful pH‐sensitive proton chemical exchange agents were found.
Abstract: Solution pH was measured using water proton NMR via chemical exchange dependent saturation transfer (CEST) with selected chemical exchange sites. Several useful pH-sensitive proton chemical exchange agents were found: 5,6-dihydrouracil, 5-hydroxytryptophan, and a combination of 5-hydroxytryptophan and 2-imidazolidinethione. A ratiometric approach was developed that permitted pH determinations that were independent of water T(1) or exchange site concentration.
TL;DR: It is demonstrated that the incorporation of these coil parameters into a localized Fourier transform allows reconstruction of full FOV images in each of the component coils from data sets acquired with a reduced number of phase encoding steps compared to conventional imaging techniques.
Abstract: In this study a novel partially parallel acquisition method is presented, which can be used to accelerate image acquisition using an RF coil array for spatial encoding. In this technique, Parallel Imaging with Localized Sensitivities (PILS), it is assumed that the individual coils in the array have localized sensitivity patterns, in that their sensitivity is restricted to a finite region of space. Within the PILS model, a detailed, highly accurate RF field map is not needed prior to reconstruction. In PILS, each coil in the array is fully characterized by only two parameters: the center of coil's sensitive region in the FOV and the width of the sensitive region around this center. In this study, it is demonstrated that the incorporation of these coil parameters into a localized Fourier transform allows reconstruction of full FOV images in each of the component coils from data sets acquired with a reduced number of phase encoding steps compared to conventional imaging techniques. After the introduction of the PILS technique, primary focus is given to issues related to the practical implementation of PILS, including coil parameter determination and the SNR and artifact power in the resulting images. Finally, in vivo PILS images are shown which demonstrate the utility of the technique.
TL;DR: This work helps elucidate how background noise introduces statistical artifacts in the distribution of the sorted eigenvalues and eigenvectors in diffusion tensor MRI (DT‐MRI) data and develops a statistical framework to enhance the ability to characterize microstructure and architecture of healthy tissue.
Abstract: This work helps elucidate how background noise introduces statistical artifacts in the distribution of the sorted eigenvalues and eigenvectors in diffusion tensor MRI (DT-MRI) data. Although it was known that sorting eigenvalues (principal diffusivities) by magnitude introduces a bias in their sample mean within a homogeneous region of interest (ROI), here it is shown that magnitude sorting also introduces a significant bias in the variance of the sample mean eigenvalues. New methods are presented to calculate the mean and variance of the eigenvectors of the diffusion tensor, based on a dyadic tensor representation of eigenvalue‐ eigenvector pairs. Based on their use it is shown that sorting eigenvalues by magnitude also introduces a bias in the mean and the variance of the sample eigenvectors (principal directions). This required the development of new methods to calculate the mean and variance of the eigenvectors of the diffusion tensor, based on a dyadic tensor representation of eigenvalue‐ eigenvector pairs. Moreover, a new approach is proposed to order these pairs within an ROI. To do this, a correspondence between each principal axis of the diffusion ellipsoid, an eigenvalue‐ eigenvector pair, and a dyadic tensor constructed from it is exploited. A measure of overlap between principal axes of diffusion ellipsoids in different voxels is defined that employs projections between these dyadic tensors. The optimal eigenvalue assignment within an ROI maximizes this overlap. Bias in the estimate of the mean and of the variance of the eigenvalues and of their corresponding eigenvectors is reduced in DT-MRI experiments and in Monte Carlo simulations of such experiments. Improvement is most significant in isotropic regions, but some is also observed in anisotropic regions. This statistical framework should enhance our ability to characterize microstructure and architecture of healthy tissue, and help to assess its changes in development, disease, and degeneration. Mitigating these artifacts should also improve the characterization of diffusion anisotropy and the elucidation of fiber-tract trajectories in the brain and in other fibrous tissues. Magn Reson Med 44:41‐50, 2000. Published 2000 Wiley-Liss, Inc.†
TL;DR: Hyperpolarized 3He gas MRI was used to form maps of the effective diffusivity of gas in human lungs, finding the mean value of De in the emphysematous lungs is found to be about 2.5 times that of healthy lungs, although still smaller than the unrestricted diffuivity of 3He in free air.
Abstract: Hyperpolarized (3)He gas MRI was used to form maps of the effective diffusivity of gas in human lungs Images of diffusion as well as spin density are presented from a study of 11 healthy volunteers and 5 patients with severe emphysema The effective rate of diffusion, D(e), of the gas is reduced by the alveolar walls; tissue destruction in emphysema is hypothesized to result in larger D(e) Indeed, the mean value of D(e) in the emphysematous lungs is found here to be about 25 times that of healthy lungs, although still smaller than the unrestricted diffusivity of (3)He in free air Histograms of D(e) values across coronal slices are presented The results are discussed in terms of spatial variations, variations among individuals, healthy and diseased, and variations due to changes in lung volume Magn Reson Med 44:174-179, 2000
TL;DR: The work presented here compared CBF values measured using steady‐state arterial spin tagging with CBFvalues measured in the same group of human subjects using the H215O IV bolus PET method.
Abstract: Steady-state arterial spin tagging approaches can provide quantitative images of CBF, but have not been validated in humans. The work presented here compared CBF values measured using steady-state arterial spin tagging with CBF values measured in the same group of human subjects using the H2 15 O IV bolus PET method. Blood flow values determined by H2 15 O PET were corrected for the known effects of incomplete extraction of water across the blood brain barrier. For a cortical strip ROI, blood flow values determined using arterial spin tagging (64 6 12 cc/100g/min) were not statistically different from corrected blood flow values determined using H2 15 O PET (67 6 13 cc/100g/min). However, for a central white matter ROI, blood flow values determined using arterial spin tagging were significantly underestimated compared to corrected blood flow values determined using H2 15 O PET. This underestimation could be caused by an underestimation of the arterial transit time for white matter regions. Magn Reson Med 44:450 ‐ 456, 2000. Published 2000 Wiley-Liss, Inc.†
TL;DR: An algorithm for the automatic alignment of MRI volumes of the human brain was developed, based on techniques adopted from the computer vision literature for image motion estimation, which automatically ignores voxels where the intensities are sufficiently different in the two volumes.
Abstract: An algorithm for the automatic alignment of MRI volumes of the human brain was developed, based on techniques adopted from the computer vision literature for image motion estimation. Most image registration techniques rely on the assumption that corresponding voxels in the two volumes have equal intensity, which is not true for MRI volumes acquired with different coils and/or pulse sequences. Intensity normalization and contrast equalization were used to minimize the differences between the intensities of the two volumes. However, these preprocessing steps do not correct perfectly for the image differences when using different coils and/or pulse sequences. Hence, the alignment algorithm relies on robust estimation, which automatically ignores voxels where the intensities are sufficiently different in the two volumes. A multiresolution pyramid implementation enables the algorithm to estimate large displacements. The resulting algorithm is used routinely to align MRI volumes acquired using different protocols (3D SPGR and 2D fast spin echo) and different coils (surface and head) to subvoxel accuracy (better than 1 mm).
TL;DR: Multiple‐inversion background suppression techniques, which suppress phase noise due to interacquisition fluctuations in the static magnetic field, reduced the temporal standard deviation of true 3D ΔM images acquired using arterial spin tagging approaches by ∼50%.
Abstract: Phase-encoded multishot SPIRAL approaches were used to acquire true 3D cerebral blood flow images of the human head using arterial spin tagging approaches. Multiple-inversion background suppression techniques, which suppress phase noise due to interacquisition fluctuations in the static magnetic field, reduced the temporal standard deviation of true 3D delta M images acquired using arterial spin tagging approaches by approximately 50%. Background suppressed arterial spin tagging (ASSIST) approaches were used to obtain high-resolution isotropic true 3D cerebral blood flow images, and to obtain true 3D activation images during cognitive (working memory) tasks. Magn Reson Med 44:92-100, 2000. Published 2000 Wiley-Liss, Inc.
TL;DR: A new parallel imaging technique was implemented which can result in reduced image acquisition times in MRI, and permits the arbitrary choice of the set of k‐space lines used in the reconstruction and lends itself to parallel reconstruction, hence allowing for real‐time rendering.
Abstract: A new parallel imaging technique was implemented which can result in reduced image acquisition times in MRI. MR data is acquired in parallel using an array of receiver coils and then reconstructed simultaneously with multiple processors. The method requires the initial estimation of the 2D sensitivity profile of each coil used in the receiver array. These sensitivity profiles are then used to partially encode the images of interest. A fraction of the total number of k-space lines is consequently acquired and used in a parallel reconstruction scheme, allowing for a substantial reduction in scanning and display times. This technique is in the family of parallel acquisition schemes such as simultaneous acquisition of spatial harmonics (SMASH) and sensitivity encoding (SENSE). It extends the use of the SMASH method to allow the placement of the receiver coil array around the object of interest, enabling imaging of any plane within the volume of interest. In addition, this technique permits the arbitrary choice of the set of k-space lines used in the reconstruction and lends itself to parallel reconstruction, hence allowing for real-time rendering. Simulated results with a 16-fold increase in temporal resolution are shown, as are experimental results with a 4-fold increase in temporal resolution. Magn Reson Med 44:301-308, 2000.
TL;DR: Findings indicate that the antibody‐targeted agent enhances MR signal intensity in the capillary bed in a corneal micropocket model of angiogenesis, and is selectively retained within the angiogenic region via specific interaction with the αvβ3 epitope.
Abstract: Site-directed contrast enhancement of angiogenic vessels in vivo was demonstrated using antibody targeting of an MRI contrast agent to the αvβ3 integrin, a molecular marker characteristic of angiogenic endothelium. The agent was tested in a rabbit corneal micropocket model, in which neovasculature is induced in the cornea using basic fibroblast growth factor. The targeted contrast agent consists of Gd-perfluorocarbon nanoparticles linked to αvβ3 integrin antibody DM101. The animal group receiving the targeted contrast agent displayed a 25% increase in the average MR signal intensity after 90 min. Control groups in which the nanoparticles are either used alone, linked to an isotype-matched antibody, or linked to DM101 and administered following receptor blocking did not display MR contrast enhancement at similar dose levels. These findings indicate that the antibody-targeted agent enhances MR signal intensity in the capillary bed in a corneal micropocket model of angiogenesis, and is selectively retained within the angiogenic region via specific interaction with the αvβ3 epitope. Magn Reson Med 44:433–439, 2000. © 2000 Wiley-Liss, Inc.
TL;DR: It is demonstrated that resonant heating does occur and that it can be significantly reduced by avoiding a resonant length of cable and by including coaxial chokes on the cable.
Abstract: The incorporation of RF coils into the tips of intravascular devices has been shown to enable the localization of catheters and guidewires under MR guidance. Furthermore, such coils can be used for endoluminal imaging. The long cable required to connect the coil with the scanner input inadvertently acts as a dipole antenna which picks up RF energy from the body coil during transmit. Currents are induced on the cable which can lead to localized heating of surrounding tissue. Cables of various lengths were measured to determine if a resonance in the heating as a function of cable length could be found. Coaxial chokes with a length of lambda/4 were added to coaxial cables to reduce the amplitude of the currents induced on the cable shield. A 0.7-mm diameter triaxial cable, small enough to fit into a standard intravascular device, was developed and measured both with and without a coaxial choke. It is demonstrated that resonant heating does occur and that it can be significantly reduced by avoiding a resonant length of cable and by including coaxial chokes on the cable.
TL;DR: DTI with a phase navigated interleaved echo planar imaging technique may be used to detect abnormalities of isotropic and anisotropic diffusion in the NAWM and selected fiber tracts of patients with MS throughout the entire brain, and it demonstrates substantial differences between various types of focal lesions.
Abstract: High-resolution diffusion tensor imaging (DTI) was performed in 14 patients with clinically definite multiple sclerosis (MS) and the trace of the diffusion tensor (〈D〉) and the fractional anisotropy (FA) were determined in normal appearing white matter (NAWM) and in different types of focal MS lesions. A small but significant increase of the 〈D〉 in NAWM compared to control white matter ((840 ± 85) × 10–6 mm2/sec vs. (812 ± 59) × 10–6 mm2/sec; P < 0.01) was found. In addition, there was a significant decrease in the FA of normal-appearing regions containing well-defined white matter tracts, such as the genu of the internal capsule. In non-acute lesions, the 〈D〉 of T1-hypointense areas was significantly higher than that of T1-isointense lesions ((1198 ± 248) × 10–6 mm2/sec vs. (1006 ± 142) × 10–6 mm2/sec; P < 0.001), and there was a corresponding inverse relation of FA. Diffusion characteristics of active lesions with different enhancement patterns were also significantly different. DTI with a phase navigated interleaved echo planar imaging technique may be used to detect abnormalities of isotropic and anisotropic diffusion in the NAWM and selected fiber tracts of patients with MS throughout the entire brain, and it demonstrates substantial differences between various types of focal lesions. Magn Reson Med 44:583–591, 2000. © 2000 Wiley-Liss, Inc.
TL;DR: Spatial specificities of the calcium‐dependent synaptic activity, hemodynamic‐based blood oxygenation level‐dependent (BOLD) and cerebral blood flow (CBF) fMRI were quantitatively compared in the same animals, suggesting that hemodynamic-based fMRI techniques can be used to accurately map the spatial loci of synaptic activity.
Abstract: Spatial specificities of the calcium-dependent synaptic activity, hemodynamic-based blood oxygenation level-dependent (BOLD) and cerebral blood flow (CBF) fMRI were quantitatively compared in the same animals. Calcium-dependent synaptic activity was imaged by exploiting the manganese ion (Mn 11 ) as a calcium analog and an MRI contrast agent at 9.4 T. Following forepaw stimulation in a-chloralose anesthetized rat, water T1 of the contralateral forepaw somatosensory cortex (SI) was focally and markedly reduced from 1.99 6 0.03 sec to 1.30 6 0.18 sec (mean 6 SD, N 5 7), resulting from the preferential intracellular Mn 11 accumulation. Based on an in vitro calibration, the estimated contralateral somatosensory cortex [Mn 11 ] was ;100mM, which was 2‐5-fold higher than the neighboring tissue and the ipsilateral SI. Regions with the highest calcium activities were localized around cortical layer IV. Stimulus-induced BOLD and CBF changes were 3.4 6 1.6% and 98 6 33%, respectively. The T1 synaptic activity maps extended along the cortex, whereas the hemodynamic-based activation maps extended radially along the vessels. Spatial overlaps among the synaptic activity, BOLD, and CBF activation maps showed excellent co-registrations. The center-of-mass offsets between any two activation maps were less than 200 mm, suggesting that hemodynamic-based fMRI techniques (at least at high field) can be used to accurately map the spatial loci of synaptic activity. Magn Reson Med 43:383‐392, 2000. © 2000 Wiley-Liss, Inc.
TL;DR: Simulations, phantom images and in vivo cardiac images show that the variable‐density sampling method can significantly reduce the total energy of aliasing artifacts and can be applied to all types of k‐space sampling trajectories.
Abstract: A variable-density k-space sampling method is proposed to reduce aliasing artifacts in MR images. Because most of the energy of an image is concentrated around the k-space center, aliasing artifacts will contain mostly low-frequency components if the k-space is uniformly undersampled. On the other hand, because the outer k-space region contains little energy, undersampling that region will not contribute severe aliasing artifacts. Therefore, a variable-density trajectory may sufficiently sample the central k-space region to reduce low-frequency aliasing artifacts and may undersample the outer k-space region to reduce scan time and to increase resolution. In this paper, the variable-density sampling method was implemented for both spiral imaging and two-dimensional Fourier transform (2DFT) imaging. Simulations, phantom images and in vivo cardiac images show that this method can significantly reduce the total energy of aliasing artifacts. In general, this method can be applied to all types of k-space sampling trajectories.
TL;DR: An optimized sequence of quadratic phase pulses is introduced to provide very selective spatial suppression with improved B1 and T1 insensitivity in 3D MRSI exams of brain tumors and prostate cancer.
Abstract: Focal three-dimensional magnetic resonance spectroscopic imaging (3D MRSI) methods based on conventional point resolved spectroscopy (PRESS) localization are compromised by the geometric restrictions in volume prescription and by chemical shift registration errors. Outer volume saturation (OVS) pulses have been applied to address the geometric limits, but conventional OVS pulses do little to overcome chemical shift registration error, are not particularly selective, and often leave substantial signals that can degrade the spectra of interest. In this paper, an optimized sequence of quadratic phase pulses is introduced to provide very selective spatial suppression with improved B1 and T1 insensitivity. This method was then validated in volunteer studies and in clinical 3D MRSI exams of brain tumors and prostate cancer.
TL;DR: The results suggest the potential for sensitive and specific detection of microthrombi that form on the intimal surfaces of unstable atherosclerotic plaque and targeted contrast agent dramatically improved the detectability of all clots.
Abstract: In this study, the sensitivity of a novel fibrin-targeted contrast agent for fibrin detection was defined in vitro on human thrombus. The contrast agent was a lipid-encapsulated perfluorocarbon nanoparticle with numerous Gd-DTPA complexes incorporated into the outer surface. After binding to fibrin clots, scanning electron microscopy of treated clots revealed dense accumulation of nanoparticles on the clot surfaces. Fibrin clots with sizes ranging from 0.5‐7.0 mm were imaged at 4.7 T with or without treatment with the targeted contrast agent. Regardless of sizes, untreated clots were not detectable by T1-weighted MRI, while targeted contrast agent dramatically improved the detectability of all clots. Decreases in T1 and T2 relaxation times (20 ‐ 40%) were measured relative to the surrounding media and the control clots. These results suggest the potential for sensitive and specific detection of microthrombi that form on the intimal surfaces of unstable atherosclerotic plaque. Magn Reson Med 44:867‐ 872, 2000. © 2000 Wiley-Liss, Inc.
TL;DR: MRI measurements of total blood volume fraction (corrected for agent extravasation) and ΔR2*/ΔR2, as opposed to maps of microvascular volume, may prove to be the most appropriate markers for the evaluation of tumor angiogenesis (the induction of new blood vessels) and antiangiogenic therapies.
Abstract: An interleaved gradient-echo (GE) / spin-echo (SE) EPI sequence was used to acquire images during the first pass of a susceptibility contrast agent, in patients with brain tumors. Maps of 1) GE (total) rCBV (relative cerebral blood volume), 2) SE (microvascular) rCBV, both corrected for T(1) leakage effects, and 3) (DeltaR(2)*/DeltaR(2)), a potential marker of averaged vessel diameter, were determined. Both GE rCBV and DeltaR(2)*/DeltaR(2) correlated strongly with tumor grade (P = 0.01, P = 0.01, n = 15), while SE rCBV did not (P = 0.24, n = 15). When the GE rCBV data were not corrected for leakage effects, the correlation with tumor grade was no longer significant (P = 0.09, n = 15). These findings suggest that MRI measurements of total blood volume fraction (corrected for agent extravasation) and DeltaR(2)*/DeltaR(2), as opposed to maps of microvascular volume, may prove to be the most appropriate markers for the evaluation of tumor angiogenesis (the induction of new blood vessels) and antiangiogenic therapies. Magn Reson Med 43:845-853, 2000.
TL;DR: The minimum SNR required to detect an expected MR signal change is determined using computer simulations for typical fMRI experimental designs, independent of manufacturer, site environment, field strength, coil type, or type of cognitive task used.
Abstract: Functional magnetic resonance imaging (fMRI) has recently been adopted as an investigational tool in the field of neuroscience. The signal changes induced by brain activations are small ( approximately 1-2%) at 1.5T. Therefore, the signal-to-noise ratio (SNR) of the time series used to calculate the functional maps is critical. In this study, the minimum SNR required to detect an expected MR signal change is determined using computer simulations for typical fMRI experimental designs. These SNR results are independent of manufacturer, site environment, field strength, coil type, or type of cognitive task used. Sensitivity maps depicting the minimum detectable signal change can be constructed. These sensitivity maps can be used as a mask of the activation map to help remove false positive activations as well as identify regions of the brain where it is not possible to confidently reject the null hypothesis due to a low SNR.
TL;DR: A range of liquids suitable as quality control test objects for measuring the accuracy of clinical MRI diffusion sequences (both apparent diffusion coefficient and tensor) has been identified and characterized.
Abstract: A range of liquids suitable as quality control test objects for measuring the accuracy of clinical MRI diffusion sequences (both apparent diffusion coefficient and tensor) has been identified and characterized. The self-diffusion coefficients for 15 liquids (3 cyclic alkanes: cyclohexane to cyclooctane, 9 n-alkanes: n-octane to n-hexadecane, and 3 n-alcohols: ethanol to 1-propanol were measured at 15-30 degrees C using an NMR spectrometer. Values at 22 degrees C range from 0.36 to 2.2 10(-9) m(2)s(-1). Typical 95% confidence limits are +/-2%. Temperature coefficients are 1.7-3.2% degrees C. T1 and T2 values at 1.5 T and proton density are given. n-tridecane has a diffusion coefficient close to that of normal white matter. The longer n-alkanes may be useful T2 standards. Measurements from a spin-echo MRI sequence agreed to within 2%.
TL;DR: The results of this study indicate that the Gd‐DTPA relaxivity in tissue extracellular compartment could be as much as 30–70% higher than that of Gd•DTPA in saline.
Abstract: Gd-DTPA T1 relaxivity of water protons was measured at 1.5 T and room temperature as a function of macromolecular content in model systems. Gd-DTPA relaxivity was found to increase with macromolecular concentration. The results of this study indicate that the Gd-DTPA relaxivity in tissue extracellular compartment could be as much as 30–70% higher than that of Gd-DTPA in saline. Quantitative MR analyses that use T1 as an estimation of local Gd-DTPA concentration require a priori determination of the Gd relaxivity in tissue. Magn Reson Med 44:665–667, 2000. © 2000 Wiley-Liss, Inc.
TL;DR: A new method for representing the diffusion tensor shape, called the three‐phase (3P) plot, is described, which is a graphical technique based upon a barycentric coordinate system, which weights the Tensor shape by a combination of linear, cylindrical, and spherical shape factors.
Abstract: The degree of diffusion tensor anisotropy is often associated with the organization of structural tissues such as white matter. Numerous measures of diffusion anisotropy have been proposed, which could lead to confusion in interpreting and comparing results from different studies. In this study, a new method for representing the diffusion tensor shape, called the three-phase (3P) plot, is described. This is a graphical technique based upon a barycentric coordinate system, which weights the tensor shape by a combination of linear, cylindrical, and spherical shape factors. This coordinate system can be used to map and potentially segment different tissues based upon the tensor shape. In addition, the 3P plot can be used to examine the shape properties of existing measures of diffusion anisotropy. In this paper, the 3P plot is used to compare four well-known anisotropy measures: the anisotropy index, the fractional anisotropy, the relative anisotropy, and the volume fraction. Computer simulations and diffusion tensor images of normal brains were obtained to study the properties of this new mapping technique. Magn Reson Med 44:283–291, 2000. © 2000 Wiley-Liss, Inc.