Showing papers in "Magnetic Resonance in Medicine in 2006"
TL;DR: A new family of Riemannian metrics called Log‐Euclidean is proposed, based on a novel vector space structure for tensors, which can be converted into Euclidean ones once tensors have been transformed into their matrix logarithms.
Abstract: Diffusion tensor imaging (DT-MRI or DTI) is an emerging imaging modality whose importance has been growing considerably. However, the processing of this type of data (i.e., symmetric positive-definite matrices), called "tensors" here, has proved difficult in recent years. Usual Euclidean operations on matrices suffer from many defects on tensors, which have led to the use of many ad hoc methods. Recently, affine-invariant Riemannian metrics have been proposed as a rigorous and general framework in which these defects are corrected. These metrics have excellent theoretical properties and provide powerful processing tools, but also lead in practice to complex and slow algorithms. To remedy this limitation, a new family of Riemannian metrics called Log-Euclidean is proposed in this article. They also have excellent theoretical properties and yield similar results in practice, but with much simpler and faster computations. This new approach is based on a novel vector space structure for tensors. In this framework, Riemannian computations can be converted into Euclidean ones once tensors have been transformed into their matrix logarithms. Theoretical aspects are presented and the Euclidean, affine-invariant, and Log-Euclidean frameworks are compared experimentally. The comparison is carried out on interpolation and regularization tasks on synthetic and clinical 3D DTI data.
1,137 citations
TL;DR: Rapid T1‐weighted 3D spoiled gradient‐echo data sets were acquired in the abdomen of 23 cancer patients during a total of 113 separate visits to allow dynamic contrast‐enhanced MRI (DCE‐MRI) analysis of tumor microvasculature to indicate the potential for increased sensitivity to therapy‐induced change.
Abstract: Rapid T(1)-weighted 3D spoiled gradient-echo (GRE) data sets were acquired in the abdomen of 23 cancer patients during a total of 113 separate visits to allow dynamic contrast-enhanced MRI (DCE-MRI) analysis of tumor microvasculature. The arterial input function (AIF) was measured in each patient at each visit using an automated AIF extraction method following a standardized bolus administration of gadodiamide. The AIFs for each patient were combined to obtain a mean AIF that is representative for any individual. The functional form of this general AIF may be useful for studies in which AIF measurements are not possible. Improvements in the reproducibility of DCE-MRI model parameters (K(trans), v(e), and v(p)) were observed when this new, high-temporal-resolution population AIF was used, indicating the potential for increased sensitivity to therapy-induced change.
623 citations
TL;DR: The results suggest that λ∥ and λ⟂ may be useful in vivo surrogate markers of axonal and myelin damage in mouse CNS white matter and may be modulated in the presence ofAxonal damage during the early stage of demyelination at 4 weeks of cuprizone treatment.
Abstract: Previously, we tested the prediction that axonal damage results in decreased axial diffusivity (lambda(parallel)) while demyelination leads to increased radial diffusivity (lambda(perpendicular)). Cuprizone treatment of C57BL/6 mice was a highly reproducible model of CNS white matter demyelination and remyelination affecting the corpus callosum (CC). In the present study, six C57BL/6 male mice were fed 0.2% cuprizone for 12 weeks followed by 12 weeks of recovery on normal chow. The control mice were fed normal chow and imaged in parallel. Biweekly in vivo DTI examinations showed transient decrease of lambda(parallel) in CC at 2-6 weeks of cuprizone treatment. Immunostaining for nonphosphorylated neurofilaments demonstrated corresponding axonal damage at 4 weeks of treatment. Significant demyelination was evident from loss of Luxol fast blue staining at 6-12 weeks of cuprizone ingestion and was paralleled by increased lambda(perpendicular) values, followed by partial normalization during the remyelination phase. The sensitivity of lambda(perpendicular) to detect demyelination may be modulated in the presence of axonal damage during the early stage of demyelination at 4 weeks of cuprizone treatment. Our results suggest that lambda(parallel) and lambda(perpendicular) may be useful in vivo surrogate markers of axonal and myelin damage in mouse CNS white matter.
438 citations
TL;DR: A method is presented that uses accelerated parallel imaging to reduce image artifacts and acquire images at multiple TEs following a single excitation, with no need to increase TR.
Abstract: Functional MRI (fMRI) generally employs gradient-echo echo-planar imaging (GE-EPI) to measure blood oxygen level-dependent (BOLD) signal changes that result from changes in tissue relaxation time T(*) (2) between activation and rest. Since T(*) (2) strongly varies across the brain and BOLD contrast is maximal only where the echo time (TE) equals the local T(*) (2), imaging at a single TE is a compromise in terms of overall sensitivity. Furthermore, the long echo train makes EPI very sensitive to main field inhomogeneities, causing strong image distortion. A method is presented that uses accelerated parallel imaging to reduce image artifacts and acquire images at multiple TEs following a single excitation, with no need to increase TR. Sensitivity gains from the broadened T(*) (2) coverage are optimized by pixelwise weighted echo summation based on local T(*) (2) or contrast-to-noise ratio (CNR) measurements. The method was evaluated using an approach that allows differential BOLD CNR to be calculated without stimulation, as well as with a Stroop experiment. Results obtained at 3 T showed that BOLD sensitivity improved by 11% or more in all brain regions, with larger gains in areas typically affected by strong susceptibility artifacts. The use of parallel imaging markedly reduces image distortion, and hence the method should find widespread application in functional brain imaging.
422 citations
TL;DR: Using data from healthy human subjects, it is shown that different image segmentation approaches that are commonly used to account for partial volume effects lead to different estimates of metabolite levels, particularly in gray matter, owing primarily to variability in the estimates of the cerebrospinal fluid fraction.
Abstract: A strategy for using tissue water as a concentration standard in (1)H magnetic resonance spectroscopic imaging studies on the brain is presented, and the potential errors that may arise when the method is used are examined. The sensitivity of the method to errors in estimates of the different water compartment relaxation times is shown to be small at short echo times (TEs). Using data from healthy human subjects, it is shown that different image segmentation approaches that are commonly used to account for partial volume effects (SPM2, FSL's FAST, and K-means) lead to different estimates of metabolite levels, particularly in gray matter (GM), owing primarily to variability in the estimates of the cerebrospinal fluid (CSF) fraction. While consistency does not necessarily validate a method, a multispectral segmentation approach using FAST yielded the lowest intersubject variability in the estimates of GM metabolites. The mean GM and white matter (WM) levels of N-acetyl groups (NAc, primarily N-acetylaspartate), choline (Ch), and creatine (Cr) obtained in these subjects using the described method with FAST multispectral segmentation are reported: GM [NAc] = 17.16 +/- 1.19 mM; WM [NAc] = 14.26 +/- 1.38 mM; GM [Ch] = 3.27 +/- 0.47 mM; WM [Ch] = 2.65 +/- 0.25 mM; GM [Cr] = 13.98 +/- 1.20 mM; and WM [Cr] = 7.10 +/- 0.67 mM.
406 citations
TL;DR: MRI can detect single cells in vivo, homing to tissue, following cell labeling and transplantation, and opens the door to a number of experiments, such as monitoring rare cellular events, assessing the kinetics of stem cell homing, and achieving early detection of metastases.
Abstract: The use of high-relaxivity, intracellular contrast agents has enabled MRI monitoring of cell migration through and homing to various tissues, such as brain, spinal cord, heart, and muscle. Here it is shown that MRI can detect single cells in vivo, homing to tissue, following cell labeling and transplantation. Primary mouse hepatocytes were double-labeled with green fluorescent 1.63-μm iron oxide particles and red fluorescent endosomal labeling dye, and injected into the spleens of recipient mice. This is a common hepatocyte transplantation paradigm in rodents whereby hepatocytes migrate from the spleen to the liver as single cells. One month later the animals underwent in vivo MRI and punctuated, dark contrast regions were detected scattered through the livers. MRI of perfused, fixed samples and labeled hepatocyte phantoms in combination with histological evaluation confirmed the presence of dispersed single hepatocytes grafted into the livers. Appropriate controls were used to determine whether the observed contrast could have been due to dead cells or free particles, and the results confirmed that the contrast was due to disperse, single cells. Detecting single cells in vivo opens the door to a number of experiments, such as monitoring rare cellular events, assessing the kinetics of stem cell homing, and achieving early detection of metastases. Magn Reson Med, 2006. Published 2006 Wiley-Liss, Inc.
405 citations
TL;DR: Reconstruction of the q‐ball orientation distribution function (ODF) is reformulated in terms of spherical harmonic basis functions, yielding an analytic solution with useful properties of a frequency domain representation that brings the technique closer to clinical feasibility from the standpoint of total imaging time.
Abstract: Diffusion tensor imaging (DTI) accurately delineates white matter pathways when the Gaussian model of diffusion is valid. However, DTI yields erroneous results when diffusion takes on a more complex distribution, as is the case in the brain when fiber tracts cross. High angular resolution diffusion imaging (HARDI) overcomes this limitation of DTI by more fully characterizing the angular dependence of intravoxel diffusion. Among the various HARDI methods that have been proposed, QBI offers advantages such as linearity, model independence, and relatively easy implementation. In this work, reconstruction of the q-ball orientation distribution function (ODF) is reformulated in terms of spherical harmonic basis functions, yielding an analytic solution with useful properties of a frequency domain representation. The harmonic basis is parsimonious for typical b-values, which enables the ODF to be synthesized from a relatively small number of noisy measurements and thus brings the technique closer to clinical feasibility from the standpoint of total imaging time. The proposed method is assessed using Monte Carlo computer simulations and compared with conventional q-ball reconstruction using spherical RBFs. In vivo results from 3T whole-brain HARDI of adult volunteers are also provided to verify the underlying mathematical theory.
397 citations
TL;DR: An approach that is formulated as a quadratic optimization problem in the spatial domain and allows the use of arbitrary k‐space trajectories is presented, which allows for the specification of a region of interest (ROI), which improves excitation accuracy at high speedup factors.
Abstract: Parallel excitation has been introduced as a means of accelerating multidimensional, spatially-selective excitation using multiple transmit coils, each driven by a unique RF pulse. Previous approaches to RF pulse design in parallel excitation were either formulated in the frequency domain or restricted to echo-planar trajectories, or both. This paper presents an approach that is formulated as a quadratic optimization problem in the spatial domain and allows the use of arbitrary k-space trajectories. Compared to frequency domain approaches, the new design method has some important advantages. It allows for the specification of a region of interest (ROI), which improves excitation accuracy at high speedup factors. It allows for magnetic field inhomogeneity compensation during excitation. Regularization may be used to control integrated and peak pulse power. The effects of Bloch equation nonlinearity on the large-tip-angle excitation error of RF pulses designed with the method are investigated, and the utility of Tikhonov regularization in mitigating this error is demonstrated.
397 citations
TL;DR: It is demonstrated that the concept of CAIPIRINHA can be transferred to 3D imaging, where data reduction can be performed in two spatial dimensions simultaneously, resulting in a more robust parallel imaging reconstruction.
Abstract: The CAIPIRINHA (Controlled Aliasing In Parallel Imaging Results IN Higher Acceleration) concept in parallel imaging has recently been introduced, which modifies the appearance of aliasing artifacts during data acquisition in order to improve the subsequent parallel imaging reconstruction procedure. This concept has been successfully applied to simultaneous multi-slice imaging (MS CAIPIRINHA). In this work, we demonstrate that the concept of CAIPIRINHA can also be transferred to 3D imaging, where data reduction can be performed in two spatial dimensions simultaneously. In MS CAIPIRINHA, aliasing is controlled by providing individual slices with different phase cycles by means of alternating multi-band radio frequency (RF) pulses. In contrast to MS CAIPIRINHA, 2D CAIPIRINHA does not require special RF pulses. Instead, aliasing in 2D parallel imaging can be controlled by modifying the phase encoding sampling strategy. This is done by shifting sampling positions from their normal positions in the undersampled 2D phase encoding scheme. Using this modified sampling strategy, coil sensitivity variations can be exploited more efficiently in multiple dimensions, resulting in a more robust parallel imaging reconstruction. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc.
388 citations
TL;DR: A 32‐channel 3T receive‐only phased‐array head coil was developed for human brain imaging and showed SNR gains and the experimentally measured g‐factor performance of the helmet array showed significant improvement compared to the eight‐channel array.
Abstract: A 32-channel 3T receive-only phased-array head coil was developed for human brain imaging. The helmet-shaped array was designed to closely fit the head with individual overlapping circular elements arranged in patterns of hexagonal and pentagonal symmetry similar to that of a soccer ball. The signal-to-noise ratio (SNR) and noise amplification (g-factor) in accelerated imaging applications were quantitatively evaluated in phantom and human images and compared with commercially available head coils. The 32-channel coil showed SNR gains of up to 3.5-fold in the cortex and 1.4-fold in the corpus callosum compared to a (larger) commercial eight-channel head coil. The experimentally measured g-factor performance of the helmet array showed significant improvement compared to the eight-channel array (peak g-factor 59% and 26% of the eight-channel values for four- and fivefold acceleration). The performance of the arrays is demonstrated in high-resolution and highly accelerated brain images.
365 citations
TL;DR: A simple non‐iterative unfiltered backprojection algorithm that incorporates the idea of a composite image consisting of portions or all of the acquired data to constrain the back projection process is presented, which significantly reduces streak artifacts and increases the overall SNR.
Abstract: Recent work in k-t BLAST and undersampled projection angiography has emphasized the value of using training data sets obtained during the acquisition of a series of images. These techniques have used iterative algorithms guided by the training set information to reconstruct time frames sampled at well below the Nyquist limit. We present here a simple non-iterative unfiltered backprojection algorithm that incorporates the idea of a composite image consisting of portions or all of the acquired data to constrain the backprojection process. This significantly reduces streak artifacts and increases the overall SNR, permitting decreased numbers of projections to be used when acquiring each image in the image time series. For undersampled 2D projection imaging applications, such as cine phase contrast (PC) angiography, our results suggest that the angular undersampling factor, relative to Nyquist requirements, can be increased from the present factor of 4 to about 100 while increasing SNR per individual time frame. Results are presented for a contrast-enhanced PR HYPR TRICKS acquisition in a volunteer using an angular undersampling factor of 75 and a TRICKS temporal undersampling factor of 3 for an overall undersampling factor of 225.
TL;DR: A new method is presented for highly rapid B1+ magnitude mapping that combines the double angle method with a B1‐insensitive magnetization‐reset sequence such that the choice of repetition time (TR) is independent of T1 and with a multislice segmented (spiral) acquisition to achieve volumetric coverage with adequate spatial resolution in a few seconds.
Abstract: For in vivo magnetic resonance imaging at high field (> or =3 T) it is essential to consider the homogeneity of the active B(1) field (B(1)+), particularly if surface coils are used for RF transmission. A new method is presented for highly rapid B(1)+ magnitude mapping. It combines the double angle method with a B(1)-insensitive magnetization-reset sequence such that the choice of repetition time (TR) is independent of T(1) and with a multislice segmented (spiral) acquisition to achieve volumetric coverage with adequate spatial resolution in a few seconds. Phantom experiments confirmed the accuracy of this technique even when TR << T(1), with the side effect being lowered SNR. The speed of this method enabled B(1)+ mapping in the chest and abdomen within a single breath-hold. In human cardiac imaging, the method enabled whole-heart coverage within a single 16-s breath-hold. Results from phantoms and healthy volunteers at 1.5 T and 3 T are presented.
TL;DR: This work presents a streamlined technique to generate a sequence of refocusing flip angles on a per‐prescription basis that produces relatively high SNR and limits blurring in a wide range of materials encountered in vivo.
Abstract: Reducing and continuously varying the flip angle of the refocusing RF pulses in a rapid acquisition with relaxation enhancement (RARE; fast/turbo spin echo) sequence is a useful means of addressing high RF power deposition and modulation transfer function (MTF) distortion due to relaxation. This work presents a streamlined technique to generate a sequence of refocusing flip angles on a per-prescription basis that produces relatively high SNR and limits blurring in a wide range of materials encountered in vivo. Since the "effective TE" (traditionally defined as the time at which the center of k-space is sampled) no longer corresponds to the expected amount of spin-echo T2 contrast due to the mixing of stimulated and spin echoes, a "contrast-equivalent" TE is defined and experimentally demonstrated that allows annotation of a more accurate effective TE that matches the contrast produced by 180 degrees refocusing. Furthermore, contrast is shown to be manipulable by the addition of magnetization preparation pulse sequence segments, such as T2-prep, to produce clinically desirable contrast for routine head and body imaging.
TL;DR: The VFA method was optimized for both accuracy and precision by considering the influence of imperfect transmit fields, noise bias, and selection of flip angles and it was shown that three flip angles were the most efficient for maintaining accuracy and high precision over large ranges of T1.
Abstract: Rapid 3D mapping of T(1) relaxation times is valuable in diverse clinical applications. Recently, the variable flip angle (VFA) spoiled gradient recalled echo approach was shown to be a practical alternative to conventional methods, providing better precision and speed. However, the method is known to be sensitive to transmit field (B(1) (+)) inhomogeneity and can result in significant systematic errors in T(1) estimates, especially at high field strengths. The main challenge is to improve the accuracy of the VFA approach without sacrificing speed. In this article, the VFA method was optimized for both accuracy and precision by considering the influence of imperfect transmit fields, noise bias, and selection of flip angles. An analytic solution was developed for systematic B(1) (+)-induced T(1) errors and allows simple correction of T(1) measurements acquired with any imaging parameters. A noise threshold was also identified and provided a guideline for avoiding T(1) biases. Finally, it was shown that three flip angles were the most efficient for maintaining accuracy and high precision over large ranges of T(1). A rapid B(1) (+) mapping sequence was employed in all phantom experiments and high-field in vivo brain scans. Experimental results confirmed the theory and validated the accuracy of the proposed method.
TL;DR: The ability to measure proton exchange rates in tissue using MRI would be very useful for quantitative assessment of magnetization transfer properties, both in conventional MT imaging and in the more recent chemical exchange saturation transfer (CEST) approach.
Abstract: The ability to measure proton exchange rates in tissue using MRI would be very useful for quantitative assessment of magnetization transfer properties, both in conventional MT imaging and in the more recent chemical exchange saturation transfer (CEST) approach CEST is a new MR contrast mechanism that depends on several factors, including the exchange rate of labile protons in the agent in a pH-dependent manner Two new methods to monitor local exchange rate based on CEST are introduced The two MRI-compatible approaches to measure exchange are quantifying exchange using saturation time (QUEST) dependence and quantifying exchange using saturation power (QUESP) dependence These techniques were applied to poly-L-lysine (PLL) and a generation-5 polyamidoamine dendrimer (SPD-5) to measure the pH dependence of amide proton exchange rates in the physiologic range Data were fit both to an analytical expression and to numerical solutions to the Bloch equations Results were validated by comparison with exchange rates determined by two established spectroscopic methods The exchange rates determined using the four methods were pooled for the pH-calibration curve of the agents consisting of contributions from spontaneous (k0) acid catalyzed (ka), and base catalyzed (kb) exchange rate constants These constants were k0 = 689 Hz, ka = 121 Hz, kb = 192 x 10(9) Hz, and k0 = 1064 Hz, ka = 258 Hz, kb = 545 x 10(8) Hz for PLL and SPD-5, respectively, showing the expected predominance of base-catalyzed exchange for these amide protons
TL;DR: The first human APT data were acquired from patients with brain tumors and compared with T1‐ (T1w) and T2‐weighted (T2w), fluid‐attenuated inversion recovery (FLAIR), and diffusion images (fractional anisotropy (FA) and apparent diffusion coefficient).
Abstract: Amide proton transfer (APT) imaging is a technique in which the nuclear magnetization of water-exchangeable amide protons of endogenous mobile proteins and peptides in tissue is saturated, resulting in a signal intensity decrease of the free water. In this work, the first human APT data were acquired from 10 patients with brain tumors on a 3T whole-body clinical scanner and compared with T-1- (T(1)w) and T-2-weighted (T(2)w), fluid-attenuated inversion recovery (FLAIR), and diffusion images (fractional anisotropy (FA) and apparent diffusion coefficient (ADC)). The APT-weighted images provided good contrast between tumor and edema. The effect of APT was enhanced by an approximate 4% change in the water signal intensity in tumor regions compared to edema and normal-appearing white matter (NAWM). These preliminary data from patients with brain tumors show that the APT is a unique contrast that can provide complementary information to standard clinical MRI measures.
TL;DR: A magnetic resonance imaging technique is described that permits the tracking of breast cancer cells in a mouse model of brain metastasis at the single‐cell level, and to quantify and monitor the presence of solitary undivided cells.
Abstract: Metastasis (the spread of cancer from a primary tumor to secondary organs) is responsible for most cancer deaths. The ability to follow the fate of a population of tumor cells over time in an experimental animal would provide a powerful new way to monitor the metastatic process. Here we describe a magnetic resonance imaging (MRI) technique that permits the tracking of breast cancer cells in a mouse model of brain metastasis at the single-cell level. Cancer cells that were injected into the left ventricle of the mouse heart and then delivered to the brain were detectable on MR images. This allowed the visualization of the initial delivery and distribution of cells, as well as the growth of tumors from a subset of these cells within the whole intact brain volume. The ability to follow the metastatic process from the single-cell stage through metastatic growth, and to quantify and monitor the presence of solitary undivided cells will facilitate progress in understanding the mechanisms of brain metastasis and tumor dormancy, and the development of therapeutics to treat this disease.
TL;DR: The preliminary results demonstrate the feasibility of safe and successful human imaging at 9.4T, and facilitated compensation for RF artifacts attributed to destructive interference patterns, in order to achieve homogeneous 9.
Abstract: This work reports the preliminary results of the first human images at the new high-field benchmark of 9.4T. A 65-cm-diameter bore magnet was used together with an asymmetric 40-cm-diameter head gradient and shim set. A multichannel transmission line (transverse electromagnetic (TEM)) head coil was driven by a programmable parallel transceiver to control the relative phase and magnitude of each channel independently. These new RF field control methods facilitated compensation for RF artifacts attributed to destructive interference patterns, in order to achieve homogeneous 9.4T head images or localize anatomic targets. Prior to FDA investigational device exemptions (IDEs) and internal review board (IRB)-approved human studies, preliminary RF safety studies were performed on porcine models. These data are reported together with exit interview results from the first 44 human volunteers. Although several points for improvement are discussed, the preliminary results demonstrate the feasibility of safe and successful human imaging at 9.4T.
TL;DR: A model‐free arterial spin labeling (ASL) quantification approach for measuring cerebral blood flow (CBF) and arterial blood volume (aBV) is proposed, based on the acquisition of a train of multiple images following the labeling scheme.
Abstract: In this work a model-free arterial spin labeling (ASL) quantification approach for measuring cerebral blood flow (CBF) and arterial blood volume (aBV) is proposed. The method is based on the acquisition of a train of multiple images following the labeling scheme. Perfusion is obtained using deconvolution in a manner similar to that of dynamic susceptibility contrast (DSC) MRI. Local arterial input functions (AIFs) can be estimated by subtracting two perfusion-weighted images acquired with and without crusher gradients, respectively. Furthermore, by knowing the duration of the bolus of tagged arterial blood, one can estimate the aBV on a voxel-by-voxel basis. The maximum of the residue function obtained from the deconvolution of the tissue curve by the AIF is a measure of CBF after scaling by the locally estimated aBV. This method provides averaged gray matter (GM) perfusion values of 38 +/- 2 ml/min/100 g and aBV of 0.93% +/- 0.06%. The average CBF value is 10% smaller than that obtained on the same data set using the standard general kinetic model (42 +/- 2 ml/min/100 g). Monte Carlo simulations were performed to compare this new methodology with parametric fitting by the conventional model.
TL;DR: An algorithm is presented that generates the 3D Cones gradient waveforms given a desired field of view and resolution and the resulting trajectory is very signal‐to‐noise ratio (SNR) efficient and has excellent aliasing properties.
Abstract: The 3D Cones k-space trajectory has many desirable properties for rapid and ultra-short echo time magnetic resonance imaging. An algorithm is presented that generates the 3D Cones gradient waveforms given a desired field of view and resolution. The algorithm enables a favorable trade-off between increases in readout time and decreases in the total number of required readouts. The resulting trajectory is very signal-to-noise ratio (SNR) efficient and has excellent aliasing properties. A rapid high-resolution ultra-short echo time imaging sequence is used to compare the 3D Cones trajectory to 3D projection reconstruction (3DPR) sampling schemes. For equivalent scan times, the 3D Cones trajectory has better SNR performance and fewer aliasing artifacts as compared to the 3DPR trajectory.
TL;DR: It is demonstrated, for the first time, that single cells can be detected in mouse brain in vivo using magnetic resonance imaging (MRI), and the sensitivity of MRI for detecting single cells in small animals for a wide range of application from stem cell to cancer cell tracking.
Abstract: In the current work we demonstrate, for the first time, that single cells can be detected in mouse brain in vivo using magnetic resonance imaging (MRI). Cells were labeled with superparamagnetic iron oxide nanoparticles and injected into the circulation of mice. Individual cells trapped within the microcirculation of the brain could be visualized with high-resolution MRI using optimized MR hardware and the fast imaging employing steady state acquisition (FIESTA) pulse sequence on a 1.5 T clinical MRI scanner. Single cells appear as discrete signal voids on MR images. Direct optical validation was provided by coregistering signal voids on MRI with single cells visualized using high-resolution confocal microscopy. This work demonstrates the sensitivity of MRI for detecting single cells in small animals for a wide range of application from stem cell to cancer cell tracking.
TL;DR: While spectral resolution continues to increase with field strength, the absolute signal‐to‐noise ratio (SNR) in T1/T2‐based anatomical MRI quickly levels off beyond ∼7 T and may actually decrease at higher magnetic fields.
Abstract: Comprehensive and quantitative measurements of T1 and T2 relaxation times of water, metabolites, and macromolecules in rat brain under similar experimental conditions at three high magnetic field strengths (4.0 T, 9.4 T, and 11.7 T) are presented. Water relaxation showed a highly significant increase (T1) and decrease (T2) with increasing field strength for all nine analyzed brain structures. Similar but less pronounced effects were observed for all metabolites. Macromolecules displayed field-independent T2 relaxation and a strong increase of T1 with field strength. Among other features, these data show that while spectral resolution continues to increase with field strength, the absolute signal-to-noise ratio (SNR) in T1/T2-based anatomical MRI quickly levels off beyond approximately 7 T and may actually decrease at higher magnetic fields.
TL;DR: The application of 3D radial sampling of the free‐induction decay to proton ultrashort echo‐time (UTE) imaging is reported and a maximal signal‐to‐noise ratio (SNR) with negligible decay‐induced loss in spatial resolution is obtained.
Abstract: The application of 3D radial sampling of the free-induction decay to proton ultrashort echo-time (UTE) imaging is reported. The effects of T2 decay during signal acquisition on the 3D radial point-spread function are analyzed and compared to 2D radial and 1D sampling. It is found that in addition to the use of ultrashort TE, the proper choice of the acquisition-window duration TAQ is essential for imaging short-T2 components. For 3D radial sampling, a maximal signal-to-noise ratio (SNR) with negligible decay-induced loss in spatial resolution is obtained for an acquisition-window duration of TAQ ≈ 0.69 T2. For 2D and 1D sampling, corresponding values are derived as well. Phantom measurements confirm the theoretical findings and demonstrate the impact of different acquisition-window durations on SNR and spatial resolution for a given T2 component. In vivo scans show the potential of 3D UTE imaging with T2-adapted sampling for musculoskeletal imaging using standard MR equipment. The visualization of complex anatomy is demonstrated by extracting curved slices from the isotropically resolved 3D UTE image data. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc.
TL;DR: The theoretical developments provide a method for generating scalar maps of the diffusion tensor data, including novel fractional anisotropy maps that are color encoded for the mode of anisOTropy and directionally encoded colormaps of only linearly anisotropic structures, rather than of high fractionalAnisotropic structures.
Abstract: This paper outlines the mathematical development and application of two analytically orthogonal tensor invariants sets. Diffusion tensors can be mathematically decomposed into shape and orientation information, determined by the eigenvalues and eigenvectors, respectively. The developments herein orthogonally decompose the tensor shape using a set of three orthogonal invariants that characterize the magnitude of isotropy, the magnitude of anisotropy, and the mode of anisotropy. The mode of anisotropy is useful for resolving whether a region of anisotropy is linear anisotropic, orthotropic, or planar anisotropic. Both tensor trace and fractional anisotropy are members of an orthogonal invariant set, but they do not belong to the same set. It is proven that tensor trace and fractional anisotropy are not mutually orthogonal measures of the diffusive process. The results are applied to the analysis and visualization of diffusion tensor magnetic resonance images of the brain in a healthy volunteer. The theoretical developments provide a method for generating scalar maps of the diffusion tensor data, including novel fractional anisotropy maps that are color encoded for the mode of anisotropy and directionally encoded colormaps of only linearly anisotropic structures, rather than of high fractional anisotropy structures.
TL;DR: This paper presents an extensive study of high‐order models for apparent diffusion coefficient estimation and illustrates some of their applications and proposes a new regularization algorithm based on the Laplace–Beltrami operator.
Abstract: High angular resolution diffusion imaging has recently been of great interest in characterizing non-Gaussian diffusion processes. One important goal is to obtain more accurate fits of the apparent diffusion processes in these non-Gaussian regions, thus overcoming the limitations of classical diffusion tensor imaging. This paper presents an extensive study of high-order models for apparent diffusion coefficient estimation and illustrates some of their applications. Using a meaningful modified spherical harmonics basis to capture the physical constraints of the problem, a new regularization algorithm is proposed. The new smoothing term is based on the Laplace-Beltrami operator and its closed form implementation is used in the fitting procedure. Next, the linear transformation between the coefficients of a spherical harmonic series of order l and independent elements of a rank-l high-order diffusion tensor is explicitly derived. This relation allows comparison of the state-of-the-art anisotropy measures computed from spherical harmonics and tensor coefficients. Published results are reproduced accurately and it is also possible to recover voxels with isotropic, single fiber anisotropic, and multiple fiber anisotropic diffusion. Validation is performed on apparent diffusion coefficients from synthetic data, from a biological phantom, and from a human brain dataset.
TL;DR: A method was developed to quantify prostate metabolite concentrations using 1H high‐resolution magic angle spinning (HR‐MAS) spectroscopy and indicated that the degradation of Cho‐containing metabolites was minimized by acquiring HR‐MAS data at 1°C compared to 20°C.
Abstract: A method was developed to quantify prostate metabolite concentrations using (1)H high-resolution magic angle spinning (HR-MAS) spectroscopy. T(1) and T(2) relaxation times (in milliseconds) were determined for the major prostate metabolites and an internal TSP standard, and used to optimize the acquisition and repetition times (TRs) at 11.7 T. At 1 degrees C, polyamines (PAs; T(1mean) = 100 +/- 13, T(2mean) = 30.8 +/- 7.4) and citrate (Cit; T(1mean) = 237 +/- 39, T(2mean) = 68.1 +/- 8.2) demonstrated the shortest relaxation times, while taurine (Tau; T(1mean) = 636 +/- 78, T(2mean) = 331 +/- 71) and choline (Cho; T(1mean) = 608 +/- 60, T(2mean) = 393 +/- 81) demonstrated the longest relaxation times. Millimolal metabolite concentrations were calculated for 60 postsurgical tissues using metabolite and TSP peak areas, and the mass of tissue and TSP. Phosphocholine plus glycerophosphocholine (PC+GPC), total choline (tCho), lactate (Lac), and alanine (Ala) concentrations were higher in prostate cancer ([PC+GPC](mean) = 9.34 +/- 6.43, [tCho](mean) = 13.8 +/- 7.4, [Lac](mean) = 69.8 +/- 27.1, [Ala](mean) = 12.6 +/- 6.8) than in healthy glandular ([PC+GPC](mean) = 3.55 +/- 1.53, P < 0.01; [tCho](mean) = 7.06 +/- 2.36, P < 0.01; [Lac](mean) = 46.5 +/- 17.4, P < 0.01; [Ala](mean) = 8.63 +/- 4.91, P = 0.051) and healthy stromal tissues ([PC+GPC](mean) = 4.34 +/- 2.46, P < 0.01; [tCho](mean) = 7.04 +/- 3.10, P < 0.01; [Lac](mean) = 45.1 +/- 18.6, P < 0.01; [Ala](mean) = 6.80 +/- 2.95, P < 0.01), while Cit and PA concentrations were significantly higher in healthy glandular tissues ([Cit](mean) = 43.1 +/- 21.2, [PAs](mean) = 18.5 +/- 15.6) than in healthy stromal ([Cit](mean) = 16.1 +/- 5.6, P < 0.01; [PAs](mean) = 3.15 +/- 1.81, P < 0.01) and prostate cancer tissues ([Cit](mean) = 19.6 +/- 12.7, P < 0.01; [PAs](mean) = 5.28 +/- 5.44, P < 0.01). Serial spectra acquired over 12 hr indicated that the degradation of Cho-containing metabolites was minimized by acquiring HR-MAS data at 1 degree C compared to 20 degrees C.
TL;DR: A short‐echo‐time (TE) sequence for proton localized spectroscopy is developed by combining a 1D add‐subtract scheme with a doubly slice‐selective spin‐echo sequence, which preserves the full magnetization available from the selected volume of interest (VOI).
Abstract: We developed a short-echo-time (TE) sequence for proton localized spectroscopy by combining a 1D add-subtract scheme with a doubly slice-selective spin-echo (SE) sequence. The sequence preserves the full magnetization available from the selected volume of interest (VOI). By reducing the number of radiofrequency (RF) pulses acting on transverse magnetization, we were able to minimize the TE to the level that is achievable with the stimulated echo acquisition mode (STEAM) technique, and also gained a twofold increase in sensitivity. The use of an adiabatic pulse in the add-subtract localization improved the efficiency of excitation in spatially inhomogeneous RF fields, which are frequently encountered at high magnetic fields. The localization performance and sensitivity gains of this method, which is termed SPin ECho, full Intensity Acquired Localized (SPECIAL) spectroscopy, were demonstrated in vivo in rat brains. In conjunction with spectroscopic imaging, a 2-microl spatial resolution was accomplished with a signal-to-noise ratio (SNR) above 30, which is usually sufficient for reliable quantification of a large number of metabolites (neurochemical profile).
TL;DR: Current methods for ASL with velocity‐selective (VS) tags (termed VSASL) that do not require spatial selectivity and can thus provide quantitative measures of cerebral blood flow under slow and collateral flow conditions are described.
Abstract: In pathologies in which slow or collateral flow conditions may exist, conventional arterial spin labeling (ASL) methods that apply magnetic tags based on the location of arterial spins may not provide robust measures of cerebral blood flow (CBF), as the transit delay for the delivery of blood to target tissues may far exceed the relaxation time of the tag. Here we describe current methods for ASL with velocity-selective (VS) tags (termed VSASL) that do not require spatial selectivity and can thus provide quantitative measures of CBF under slow and collateral flow conditions. The implementation of a robust multislice VSASL technique is described in detail, and data obtained with this technique are compared with those obtained with conventional pulsed ASL (PASL). The technical considerations described here include the design of VS pulses, background suppression, anisotropy with respect to velocity-encoding directions, and CBF quantitation issues.
TL;DR: Numerical calculations are used to explore the limits of RF shimming in the human head and it is found that a 16‐element array can effectively shim a single slice at frequencies up to 600 MHz and the whole brain at up to 300 MHz.
Abstract: Several methods have been proposed for overcoming the effects of radiofrequency (RF) magnetic field inhomogeneity in high-field MRI. Some of these methods rely at least in part on the ability to independently control magnitude and phase of different drives in either one multielement RF coil or in different RF coils in a transmit array. The adjustment of these drive magnitudes and phases alone to create uniform RF magnetic (B1) fields has been called RF shimming, and has certain limits at every frequency as dictated by possible solutions to Maxwell’s equations. Here we use numerical calculations to explore the limits of RF shimming in the human head. We found that a 16-element array can effectively shim a single slice at frequencies up to 600 MHz and the whole brain at up to 300 MHz, while an 80-element array can shim the whole brain at up to 600 MHz. Magn Reson Med 56:918 –922, 2006. © 2006 WileyLiss, Inc.
TL;DR: A method is presented that measures and corrects rigid body motion and associated first‐order shim changes in real time, using a pulse sequence with embedded cloverleaf navigators and a feedback control mechanism, and demonstrates a consistent improvement in image quality if motion occurred during the acquisition.
Abstract: Subject motion during scanning can greatly reduce MRI image quality and is a major reason for discarding data in both clinical and research scanning. The quality of the high-resolution structural data used for morphometric analysis is especially compromised by subject movement because high-resolution scans are of longer duration. A method is presented that measures and corrects rigid body motion and associated first-order shim changes in real time, using a pulse sequence with embedded cloverleaf navigators and a feedback control mechanism. The procedure requires a 12-s preliminary mapping scan. A single-path, 4.2-ms cloverleaf navigator is inserted every repetition time (TR) after the readout of a 3D fast low-angle shot (FLASH) sequence, requiring no additional RF pulses and minimally impacting scan duration. Every TR, a rigid body motion estimate is made and a correction is fed back to adjust the gradients and shim offsets. Images are corrected and reconstructed on the scanner computer for immediate access. Correction for between-scan motion can be accomplished by using the same reference map for each scan repetition. Human and phantom tests demonstrated a consistent improvement in image quality if motion occurred during the acquisition.