Showing papers in "Magnetic Resonance in Medicine in 2014"

ESPIRiT--an eigenvalue approach to autocalibrating parallel MRI: where SENSE meets GRAPPA.

Abstract: Purpose Parallel imaging allows the reconstruction of images from undersampled multicoil data. The two main approaches are: SENSE, which explicitly uses coil sensitivities, and GRAPPA, which makes use of learned correlations in k-space. The purpose of this work is to clarify their relationship and to develop and evaluate an improved algorithm. Theory and methods A theoretical analysis shows: (1) The correlations in k-space are encoded in the null space of a calibration matrix. (2) Both approaches restrict the solution to a subspace spanned by the sensitivities. (3) The sensitivities appear as the main eigenvector of a reconstruction operator computed from the null space. The basic assumptions and the quality of the sensitivity maps are evaluated in experimental examples. The appearance of additional eigenvectors motivates an extended SENSE reconstruction with multiple maps, which is compared to existing methods. Results The existence of a null space and the high quality of the extracted sensitivities are confirmed. The extended reconstruction combines all advantages of SENSE with robustness to certain errors similar to GRAPPA. Conclusion In this article the gap between both approaches is finally bridged. A new autocalibration technique combines the benefits of both.

Golden-angle radial sparse parallel MRI: combination of compressed sensing, parallel imaging, and golden-angle radial sampling for fast and flexible dynamic volumetric MRI

Abstract: Purpose To develop a fast and flexible free-breathing dynamic volumetric MRI technique, iterative Golden-angle RAdial Sparse Parallel MRI (iGRASP), that combines compressed sensing, parallel imaging, and golden-angle radial sampling. Methods Radial k-space data are acquired continuously using the golden-angle scheme and sorted into time series by grouping an arbitrary number of consecutive spokes into temporal frames. An iterative reconstruction procedure is then performed on the undersampled time series where joint multicoil sparsity is enforced by applying a total-variation constraint along the temporal dimension. Required coil-sensitivity profiles are obtained from the time-averaged data. Results iGRASP achieved higher acceleration capability than either parallel imaging or coil-by-coil compressed sensing alone. It enabled dynamic volumetric imaging with high spatial and temporal resolution for various clinical applications, including free-breathing dynamic contrast-enhanced imaging in the abdomen of both adult and pediatric patients, and in the breast and neck of adult patients. Conclusion The high performance and flexibility provided by iGRASP can improve clinical studies that require robustness to motion and simultaneous high spatial and temporal resolution. Magn Reson Med 72:707–717, 2014. © 2013 Wiley Periodicals, Inc.

Calibrationless parallel imaging reconstruction based on structured low‐rank matrix completion

Abstract: Purpose A calibrationless parallel imaging reconstruction method, termed simultaneous autocalibrating and k-space estimation (SAKE), is presented. It is a data-driven, coil-by-coil reconstruction method that does not require a separate calibration step for estimating coil sensitivity information. Methods In SAKE, an undersampled, multichannel dataset is structured into a single data matrix. The reconstruction is then formulated as a structured low-rank matrix completion problem. An iterative solution that implements a projection-onto-sets algorithm with singular value thresholding is described. Results Reconstruction results are demonstrated for retrospectively and prospectively undersampled, multichannel Cartesian data having no calibration signals. Additionally, non-Cartesian data reconstruction is presented. Finally, improved image quality is demonstrated by combining SAKE with wavelet-based compressed sensing. Conclusion Because estimation of coil sensitivity information is not needed, the proposed method could potentially benefit MR applications where acquiring accurate calibration data is limiting or not possible at all. Magn Reson Med 72:959–970, 2014. © 2013 Wiley Periodicals, Inc.

Saturation recovery single-shot acquisition (SASHA) for myocardial T1 mapping

Abstract: Purpose To validate a new saturation recovery single-shot acquisition (SASHA) pulse sequence for T1 mapping and to compare SASHA T1 values in heart failure patients and healthy controls. Theory The SASHA sequence consists of 10 electrocardiogram-triggered single-shot balanced steady-state free precession images in a breath-hold. The first image is acquired without magnetization preparation and the remaining nine images follow saturation pulses with variable saturation recovery times. Methods SASHA was validated through Bloch equation simulations, Monte Carlo simulations, and phantom experiments. Pre- and postcontrast myocardial and blood T1 values were measured in 29 healthy volunteers and 7 patients with heart failure. Results SASHA T1 values had excellent agreement (bias, 5 ± 5 ms) with spin echo experiments in phantoms with a wide range of physiologic T1 and T2 values and its accuracy was independent of flip angle, absolute T1, T2, and heart rate. The average baseline myocardial T1 in heart failure patients was higher than in healthy controls (1200 ± 32 vs. 1170 ± 9 ms, P < 0.05) at 1.5T, as was the calculated blood–tissue partition coefficient, λ, (0.42 ± 0.04 vs. 0.38 ± 0.02, P < 0.05), consistent with diffuse myocardial fibrosis. Conclusions The SASHA sequence is a simple and fast approach to in vivo T1 mapping with good accuracy in simulations and phantom experiments. Magn Reson Med 71:2082–2095, 2014. © 2013 Wiley Periodicals, Inc.

Interslice leakage artifact reduction technique for simultaneous multislice acquisitions

Abstract: Purpose Controlled aliasing techniques for simultaneously acquired EPI slices have been shown to significantly increase the temporal efficiency for both diffusion-weighted imaging (DWI) and fMRI studies. The “slice-GRAPPA” (SG) method has been widely used to reconstruct such data. We investigate robust optimization techniques for SG to ensure image reconstruction accuracy through a reduction of leakage artifacts.

Clinical cell therapy imaging using a perfluorocarbon tracer and fluorine-19 MRI.

Abstract: Purpose Cellular therapeutics are emerging as a treatment option for a host of serious human diseases. To accelerate clinical translation, noninvasive imaging of cell grafts in clinical trials can potentially be used to assess the initial delivery and behavior of cells. Methods The use of a perfluorocarbon (PFC) tracer agent for clinical fluorine-19 (19F) MRI cell detection is described. This technology was used to detect immunotherapeutic dendritic cells (DCs) delivered to colorectal adenocarcinoma patients. Autologous DC vaccines were labeled with a PFC MRI agent ex vivo. Patients received DCs intradermally, and 19F spin-density-weighted MRI at 3 Tesla (T) was used to observe cells. Results Spin-density-weighted 19F images at the injection site displayed DCs as background-free “hot-spot” images. 19F images were acquired in clinically relevant scan times (<10 min). Apparent DC numbers could be quantified in two patients from the 19F hot-spots and were observed to decrease by ∼50% at injection site by 24 h. From 3T phantom studies, the sensitivity limit for DC detection is estimated to be on the order of ∼105 cells/voxel in this study. Conclusion These results help to establish a clinically applicable means to track a broad range of cell types used in cell therapy. Magn Reson Med 72:1696–1701, 2014. © 2014 The Authors. Magnetic Resonance in Medicine Published by Wiley Periodicals, Inc. on behalf of International Society of Medicine in Resonance.

Liver fat quantification using a multi-step adaptive fitting approach with multi-echo GRE imaging

Abstract: Purpose The purpose of this study was to develop a multi-step adaptive fitting approach for liver proton density fat fraction (PDFF) and quantification, and to perform an initial validation on a broadly available hardware platform. Theory and Methods The proposed method uses a multi-echo three-dimensional gradient echo acquisition, with initial guesses for the fat and water signal fractions based on a Dixon decomposition of two selected echoes. Based on magnitude signal equations with a multi-peak fat spectral model, a multi-step nonlinear fitting procedure is then performed to adaptively update the fat and water signal fractions and values. The proposed method was validated using numeric phantoms as ground truth, followed by preliminary clinical validation of PDFF calculations against spectroscopy in 30 patients. Results The results of the proposed method agreed well with the ground truth of numerical phantoms, and were relatively insensitive to changes in field strength, field homogeneity, monopolar/bipolar readout, signal to noise ratio, and echo time selections. The in vivo patient study showed excellent consistency between the PDFF values measured with the proposed approach compared with spectroscopy. Conclusion This multi-step adaptive fitting approach performed well in both simulated and initial clinical evaluation, and shows potential in the quantification of hepatic steatosis. Magn Reson Med 72:1353–1365, 2014. © 2013 Wiley Periodicals, Inc.

A 16-channel dual-row transmit array in combination with a 31-element receive array for human brain imaging at 9.4 T

Abstract: Purpose Arranging transmit array elements in multiple rows provides an additional degree of freedom to correct B1+ field inhomogeneities and to achieve whole-brain excitation at ultrahigh field strengths. Receive arrays shaped to the contours of the anatomy increase the signal-to-noise ratio of the image. In this work, the advantages offered by the transmit and receive array techniques are combined for human brain imaging at 9.4 T. Methods A 16-element dual-row transmit array and a 31-element receive array were developed. Based on an accurate numerical model of the transmit array, the deposited power was calculated for different head sizes and positions. The influence of the receive array on the transmit field was characterized. Parallel imaging performance and signal-to-noise ratio of the receive array were evaluated. Results On average, a two fold increase in signal-to-noise ratio was observed in the whole-brain volume when compared with a 16-channel elliptic microstrip transceiver array. The benefits of combining the two arrays, B1+ shimming in three directions and high receive sensitivity, are demonstrated with high-resolution in vivo images. Conclusion The dual-row transmit array provides whole-brain coverage at 9.4 T, which, in combination with the helmet-shaped receive array, is a valuable radio frequency configuration for ultra-high field magnetic resonance imaging of the human brain. Magn Reson Med 71:870–879, 2014. © 2013 Wiley Periodicals, Inc.

Evaluations of extracellular pH within in vivo tumors using acidoCEST MRI

Abstract: Purpose A practical, noninvasive method is needed to measure the extracellular pH (pHe) within in vivo tumors to longitudinally monitor tumor acidosis. We have optimized a biomedical imaging method, termed acidoCEST MRI, to provide noninvasive assessments of tumor pHe in preclinical models of mammary carcinoma. Methods A CEST-FISP MRI method was optimized to detect the chemical exchange saturation transfer (CEST) of two amide protons of a clinically approved CT contrast agent, iopromide. The ratio of the two CEST effects was used to measure pH. Routes of administration of iopromide were evaluated to ensure sufficient delivery of the agent to the tumor. The optimized acidoCEST MRI method was then used to evaluate the change in tumor pHe following alkalinizing bicarbonate treatment. Results The acidoCEST MRI protocol measured pH between 6.2 and 7.2 pH units. Greater delivery of iopromide was shown to improve the precision of the measurement of tumor pHe, but the agent did not influence the tumor pHe. AcidoCEST MRI was used to longitudinally monitor the effect of bicarbonate treatment on the pHe of tumors and bladders. Conclusion This study demonstrates that an optimized acidoCEST MRI method is a practical, noninvasive method for assessing changes in tumor acidosis. Magn Reson Med 72:1408–1417, 2014. © 2013 Wiley Periodicals, Inc.

Mapping of amide, amine, and aliphatic peaks in the CEST spectra of murine xenografts at 7 T.

Abstract: Purpose To evaluate the performance of endogenous chemical exchange saturation transfer (CEST) spectra and derived maps in a longitudinal study of tumor xenografts to ascertain the role of CEST parameters in describing tumor progression and in distinguishing between tumor, muscle, and necrosis Methods CEST spectra of 24 mice with tumor xenografts (20 LLC and 4 MDA) were acquired at three time-points We employed a novel method of decomposing the CEST spectrum into a sum of four Lorentzian shapes, each with a corresponding measured amplitude, width and frequency offset This semi-quantitative method is an improvement over techniques which simply assess the asymmetry in the spectrum for the presence of CEST, due to the fact that it is not confounded by CEST peaks on opposing sides of the direct effect The CEST images were compared to several other commonly employed contrast mechanisms: T1 relaxation, T2 relaxation, diffusion (ADC), and magnetization transfer (MT) Results Tumor spectra had distinct CEST peaks corresponding to the presence of hydrogen exchange between free water and amide, amine, and aliphatic groups All three CEST peaks (amide, amine, and aliphatic) were larger in the tumor tissue as compared with the adjacent healthy muscle Conclusions CEST contrast (particularly the amine peak amplitude) performed especially well in distinguishing areas of apoptosis and/or necrosis from actively progressing tumor, as validated by histology Magn Reson Med 71:1841–1853, 2014 © 2013 Wiley Periodicals, Inc

Combined saturation/inversion recovery sequences for improved evaluation of scar and diffuse fibrosis in patients with arrhythmia or heart rate variability.

Abstract: Purpose To develop arrhythmia-insensitive inversion recovery sequences for improved visualization of myocardial scar and quantification of diffuse fibrosis. Methods A novel preparation pre-pulse, called saturation pulse prepared heart-rate-independent inversion recovery, is introduced, which consists of a combination of saturation and inversion pulses to remove the magnetization history in each heartbeat in late gadolinium enhancement (LGE) imaging and eliminate the need for rest periods in T1 mapping. The proposed LGE and T1 mapping sequences were evaluated against conventional LGE and modified Look-Locker inversion sequences using numerical simulations, phantom and imaging in healthy subjects and patients with suspected or known cardiovascular disease. Results Simulations and phantom experiments show that the saturation pulse prepared heart-rate-independent inversion recovery pre-pulse in LGE reduces ghosting artifacts and results in perfect nulling of the healthy myocardium in the presence of arrhythmia. In T1 mapping, saturation pulse prepared heart-rate-independent inversion recovery results in (a) reduced scan time (17 vs. 9 heartbeats), (b) insensitivity to heart rate for long T1, and (c) increased signal homogeneity for short T1. LGE images in a patient in atrial fibrillation during the scan show improved myocardial nulling. In vivo T1 maps demonstrate increased signal homogeneity in blood pools and myocardium. Conclusion The proposed sequences are insensitive to heart rate variability, yield improved LGE images in the presence of arrhythmias, as well as T1 mapping with shorter scan times. Magn Reson Med 71:1024–1034, 2014. © 2013 Wiley Periodicals, Inc.

Method for high‐resolution imaging of creatine in vivo using chemical exchange saturation transfer

Abstract: Purpose To develop a chemical exchange saturation transfer (CEST) based technique to measure free creatine (Cr) and to validate the technique by measuring the distribution of Cr in muscle with high spatial resolution before and after exercise.

Quantitative oxygenation venography from MRI phase

Abstract: Athinoula A. Martinos Center for Biomedical Imaging (Advanced Multimodal Neuroimaging Training Program; Grant number: R90-DA023427)

Background field removal using spherical mean value filtering and Tikhonov regularization.

Abstract: PurposeTo introduce a new method for removing background artifacts in field maps and apply it to enhance the accuracy of susceptibility mapping.MethodsA field artifact removal method is introduced that is based on the sophisticated harmonic artifact reduction for phase data (SHARP) method exploiting the harmonic mean value property. The new method uses Tikhonov regularization at the deconvolution stage and is referred to as regularization enabled SHARP (RESHARP). RESHARP was compared with SHARP in a field-forward susceptibility simulation and in human brain experiments, considering effects on both field maps and the resulting susceptibility maps.ResultsFrom the simulation, RESHARP was able to reduce error in the field map by 17.4% as compared with SHARP, resulting in a more accurate single-angle susceptibility map with 6.5% relative error (compared with 48.5% using SHARP). Using RESHARP in vivo, field and susceptibility maps of the brain displayed fewer artifacts particularly at the brain boundaries, and susceptibility measurements of iron-rich deep gray matter were also more consistent than SHARP across healthy subjects of similar age.ConclusionCompared with SHARP, RESHARP removes background field artifact more effectively, leading to more accurate susceptibility measurements in iron-rich deep gray matter. Magn Reson Med 71:1151-1157, 2014. (c) 2013 Wiley Periodicals, Inc.

Viscous energy loss in the presence of abnormal aortic flow

Abstract: Purpose To present a theoretical basis for noninvasively characterizing in vivo fluid-mechanical energy losses and to apply it in a pilot study of patients known to express abnormal aortic flow patterns. Methods Four-dimensional flow MRI was used to characterize laminar viscous energy losses in the aorta of normal controls (n = 12, age = 37 ± 10 yr), patients with aortic dilation (n = 16, age = 52 ± 8 yr), and patients with aortic valve stenosis matched for age and aortic size (n = 14, age = 46 ± 15 yr), using a relationship between the three-dimensional velocity field and viscous energy dissipation. Results Viscous energy loss was elevated significantly in the thoracic aorta in patients with dilated aorta (3.6 ± 1.3 mW, P = 0.024) and patients with aortic stenosis (14.3 ± 8.2 mW, P < 0.001) compared with healthy volunteers (2.3 ± 0.9 mW). The same pattern of significant differences was seen in the ascending aorta, where viscous energy losses in patients with dilated aortas (2.2 ± 1.1 mW, P = 0.021) and patients with aortic stenosis (10.9 ± 6.8 mW, P < 0.001) were elevated compared with healthy volunteers (1.2 ± 0.6 mW). Conclusion This technique provides a capability to quantify the contribution of abnormal laminar blood flow to increased ventricular afterload. In this pilot study, viscous energy loss in patient cohorts was significantly elevated and indicates that cardiac afterload is increased due to abnormal flow. Magn Reson Med 72:620–628, 2014. © 2013 Wiley Periodicals, Inc.

Adiabatic inversion pulses for myocardial T1 mapping.

Abstract: Purpose To evaluate the error in T1 estimates using inversion-recovery-based T1 mapping due to imperfect inversion and to perform a systematic study of adiabatic inversion pulse designs in order to maximize inversion efficiency for values of transverse relaxation (T2) in the myocardium subject to a peak power constraint. Methods The inversion factor for hyperbolic secant and tangent/hyperbolic tangent adiabatic full passage waveforms was calculated using Bloch equations. A brute-force search was conducted for design parameters: pulse duration, frequency range, shape parameters, and peak amplitude. A design was selected that maximized the inversion factor over a specified range of amplitude and off-resonance and validated using phantom measurements. Empirical correction for imperfect inversion was performed. Results The tangent/hyperbolic tangent adiabatic pulse was found to outperform hyperbolic secant designs and achieve an inversion factor of 0.96 within ±150 Hz over 25% amplitude range with 14.7 µT peak amplitude. T1 mapping errors of the selected design due to imperfect inversion was ∼4% and could be corrected to <1%. Conclusions Nonideal inversion leads to significant errors in inversion-recovery-based T1 mapping. The inversion efficiency of adiabatic pulses is sensitive to transverse relaxation. The tangent/hyperbolic tangent design achieved the best performance subject to the peak amplitude constraint. Magn Reson Med 71:1428–1434, 2014. © 2013 Wiley Periodicals, Inc.

Bone marrow fat quantification in the presence of trabecular bone: initial comparison between water-fat imaging and single-voxel MRS.

Abstract: Purpose The purpose of the present study was to test the relative performance of chemical shift-based water-fat imaging in measuring bone marrow fat fraction in the presence of trabecular bone, having as reference standard the single-voxel magnetic resonance spectroscopy (MRS). Methods Six-echo gradient echo imaging and single-voxel MRS measurements were performed on the proximal femur of seven healthy volunteers. The bone marrow fat spectrum was characterized based on the magnitude of measurable fat peaks and an a priori knowledge of the chemical structure of triglycerides, in order to accurately extract the water peak from the overlapping broad fat peaks in MRS. The imaging-based fat fraction results were then compared to the MRS-based results both without and with taking into consideration the presence of short water components in MRS. Results There was a significant underestimation of the fat fraction using the MRS model not accounting for short species with respect to the imaging-based fat fraction. A good equivalency was observed between the fat fraction using the MRS model accounting for short species and the imaging-based fat fraction (R2 = 0.87). Conclusion The consideration of the short water species effect on bone marrow fat quantification is essential when comparing MRS-based and imaging-based fat fraction results. Magn Reson Med 71:1158–1165, 2014. © 2013 Wiley Periodicals, Inc.

Different anesthesia regimes modulate the functional connectivity outcome in mice.

Abstract: Purpose The use of resting-state functional MRI (rsfMRI) in preclinical research is expanding progressively, with the majority of resting-state imaging performed in anesthetized animals. Since anesthesia may change the physiology and, in particular, the neuronal activity of an animal considerably, it may also affect rsfMRI findings. Therefore, this study compared rsfMRI data from awake mice with rsfMRI results obtained from mice anesthetized with α-chloralose (120 mg/kg), urethane (2.5 g/kg), or isoflurane (1%). Methods Functional connectivity (FC) was estimated using both independent component analysis (40 components) and ROI-based analysis to zoom in on the effect of different anesthetics on inter-hemispheric FC. Results The data revealed an important diminishment of cortical interhemispheric FC in both the α-chloralose and urethane groups in comparison with the isoflurane and awake groups. Conclusion When performing FC analysis in anesthetized mice, the impact of anesthetics must be taken into account. The required doses for stable anesthesia during MRI significantly decrease interhemispheric FC. Magn Reson Med 72:1103–1112, 2014. © 2013 Wiley Periodicals, Inc.

Correlations of noninvasive BOLD and TOLD MRI with pO2 and relevance to tumor radiation response

Abstract: Purpose To examine the potential use of BOLD (Blood Oxygenation Level Dependent) and TOLD (Tissue Oxygenation Level Dependent) contrast MRI to assess tumor oxygenation and predict radiation response.

A subspace approach to high-resolution spectroscopic imaging

Abstract: Purpose To accelerate spectroscopic imaging using sparse sampling of -space and subspace (or low-rank) modeling to enable high-resolution metabolic imaging with good signal-to-noise ratio. Methods The proposed method, called SPectroscopic Imaging by exploiting spatiospectral CorrElation, exploits a unique property known as partial separability of spectroscopic signals. This property indicates that high-dimensional spectroscopic signals reside in a very low-dimensional subspace and enables special data acquisition and image reconstruction strategies to be used to obtain high-resolution spatiospectral distributions with good signal-to-noise ratio. More specifically, a hybrid chemical shift imaging/echo-planar spectroscopic imaging pulse sequence is proposed for sparse sampling of -space, and a low-rank model-based algorithm is proposed for subspace estimation and image reconstruction from sparse data with the capability to incorporate prior information and field inhomogeneity correction. Results The performance of the proposed method has been evaluated using both computer simulations and phantom studies, which produced very encouraging results. For two-dimensional spectroscopic imaging experiments on a metabolite phantom, a factor of 10 acceleration was achieved with a minimal loss in signal-to-noise ratio compared to the long chemical shift imaging experiments and with a significant gain in signal-to-noise ratio compared to the accelerated echo-planar spectroscopic imaging experiments. Conclusion The proposed method, SPectroscopic Imaging by exploiting spatiospectral CorrElation, is able to significantly accelerate spectroscopic imaging experiments, making high-resolution metabolic imaging possible. Magn Reson Med 71:1349–1357, 2014. © 2014 Wiley Periodicals, Inc.

Fast quantitative susceptibility mapping with L1-regularization and automatic parameter selection.

Abstract: Purpose To enable fast reconstruction of quantitative susceptibility maps with total variation penalty and automatic regularization parameter selection. Methods l1-Regularized susceptibility mapping is accelerated by variable splitting, which allows closed-form evaluation of each iteration of the algorithm by soft thresholding and fast Fourier transforms. This fast algorithm also renders automatic regularization parameter estimation practical. A weighting mask derived from the magnitude signal can be incorporated to allow edge-aware regularization. Results Compared with the nonlinear conjugate gradient (CG) solver, the proposed method is 20 times faster. A complete pipeline including Laplacian phase unwrapping, background phase removal with SHARP filtering, and l1-regularized dipole inversion at 0.6 mm isotropic resolution is completed in 1.2 min using MATLAB on a standard workstation compared with 22 min using the CG solver. This fast reconstruction allows estimation of regularization parameters with the L-curve method in 13 min, which would have taken 4 h with the CG algorithm. The proposed method also permits magnitude-weighted regularization, which prevents smoothing across edges identified on the magnitude signal. This more complicated optimization problem is solved 5 times faster than the nonlinear CG approach. Utility of the proposed method is also demonstrated in functional blood oxygen level–dependent susceptibility mapping, where processing of the massive time series dataset would otherwise be prohibitive with the CG solver. Conclusion Online reconstruction of regularized susceptibility maps may become feasible with the proposed dipole inversion. Magn Reson Med 72:1444–1459, 2014. © 2013 Wiley Periodicals, Inc.

Tumor perfusion‐related parameter of diffusion‐weighted magnetic resonance imaging: Correlation with histological microvessel density

Abstract: Purpose We obtained intravoxel incoherent motion (IVIM) parameters through biexponential analysis on diffusion-weighted MR imaging (DWI) using multiple b values. Correlation was evaluated between these parameters and histological microvessel density (MVD) for the possibility of noninvasive evaluation of MVD with DWI. Methods Twenty-five nude mice with the HT29 colorectal cancer cells implanted were analyzed after undergoing DWI with multiple b values (0, 50, 100, 300, 500, 700, and 1000 s/mm2). Tissue diffusivity (Dt), pseudo-diffusion coefficient (Dp), and perfusion fraction (fp) were calculated using a biexponential analysis, and these parameters were correlated with MVD. The MVD was determined with the CD31 stain. For statistical analysis, Spearman's rank correlation was applied. Results The mean value and correlation coefficient with MVD for each IVIM parameter were as follows: Dt = 0.98 ± 0.06 × 10−3 mm2/s with r = 0.139 (P = 0.508); Dp = 23.70 ± 7.94 × 10−3 mm2/s with r = 0.782 (P < 0.001); and fp = 15.58 ± 5.7% with r = 0.749 (P < 0.001). Dp and fp showed significant correlation with MVD, but Dt did not. Conclusion The IVIM parameters, Dp and fp, on DWI might be used in the noninvasive evaluation of MVD. Magn Reson Med 71:1554–1558, 2014. © 2013 Wiley Periodicals, Inc.

MR elastography of the liver and the spleen using a piezoelectric driver, single-shot wave-field acquisition, and multifrequency dual parameter reconstruction

Abstract: Purpose Viscoelastic properties of the liver are sensitive to fibrosis. This study proposes several modifications to existing magnetic resonance elastography (MRE) techniques to improve the accuracy of abdominal MRE. Methods The proposed method comprises the following steps: (i) wave generation by a nonmagnetic, piezoelectric driver suitable for integration into the patient table, (ii) fast single-shot 3D wave-field acquisition at four drive frequencies between 30 and 60 Hz, and (iii) single-step postprocessing by a novel multifrequency dual parameter inversion of the wave equation. The method is tested in phantoms, healthy volunteers, and patients with portal hypertension and ascites. Results Spatial maps of magnitude and phase of the complex shear modulus were acquired within 6–8 min. These maps are not subject to bias from inversion-related artifacts known from classic MRE. The spatially averaged modulus for healthy liver was 1.44 ± 0.23 kPa with ϕ = 0.492 ± 0.064. Both parameters were significantly higher in the spleen (2.29 ± 0.97 kPa, P = 0.015 and 0.749 ± 0.144, P = 6.58·10−5, respectively). Conclusion The proposed method provides abdominal images of viscoelasticity in a short time with spatial resolution comparable to conventional MR images and improved quality without being compromised by ascites. The new setup allows for the integration of abdominal MRE into the clinical workflow. Magn Reson Med 71:267–277, 2014. © 2013 Wiley Periodicals, Inc.

Flow compensated quantitative susceptibility mapping for venous oxygenation imaging.

Abstract: Purpose Venous blood oxygen saturation is an indicator of brain oxygen consumption and can be measured directly from quantitative susceptibility mapping (QSM) by deconvolving the MR phase signal. However, accurate estimation of the susceptibility of blood may be affected by flow induced phase in the presence of imaging gradient and the inhomogeneous susceptibility field gradient. The purpose of this study is to correct the flow induced error in QSM for improved venous oxygenation quantification. Methods Flow compensation is proposed for QSM by using a fully flow compensated multi-echo gradient echo sequence for data acquisition. A quadratic fit of the phase with respect to echo time is employed for the flow phase in the presence of inhomogeneity field gradients. Phantom and in vivo experiments were carried out to validate the proposed method. Results Phantom experiments demonstrated reduced error in the estimated field map and susceptibility map. Initial data in in vivo human imaging demonstrated improvements in the quantitative susceptibility map and in the estimated venous oxygen saturation values. Conclusion Flow compensated multi-echo acquisition and an adaptive-quadratic fit of the phase images improves the quantitative susceptibility map of blood flow. The improved vein susceptibility enables in vivo measurement of venous oxygen saturation throughout the brain. Magn Reson Med 72:438–445, 2014. © 2013 Wiley Periodicals, Inc.

Whole-heart coronary MRA with 100% respiratory gating efficiency: self-navigated three-dimensional retrospective image-based motion correction (TRIM).

Abstract: Purpose To develop a three-dimensional retrospective image-based motion correction technique for whole-heart coronary MRA with self-navigation that eliminates both the need to setup a diaphragm navigator and gate the acquisition. Methods The proposed technique uses one-dimensional self-navigation to track the superior–inferior translation of the heart, with which the acquired three-dimensional radial k-space data is segmented into different respiratory bins. Respiratory motion is then estimated in image space using an affine transform model and subsequently this information is used to perform efficient motion correction in k-space. The performance of the proposed technique on healthy volunteers is compared with the conventional navigator gating approach as well as data binning using diaphragm navigator. Results The proposed method is able to reduce the imaging time to 7.1±0.5 min from 13.9±2.6 min with conventional navigator gating. The scan setup time is reduced as well due to the elimination of the navigator. The proposed method yields excellent image quality comparable with either conventional navigator gating or the navigator binning approach. Conclusion We have developed a new respiratory motion correction technique for coronary MRA that enables 1 mm3 isotropic resolution and whole-heart coverage with 7 min of scan time. Further tests on patient population are needed to determine its clinical usage. Magn Reson Med 71:67–74, 2014. © 2013 Wiley Periodicals, Inc.

ECG and navigator‐free four‐dimensional whole‐heart coronary MRA for simultaneous visualization of cardiac anatomy and function

Abstract: Purpose To develop a cardiac and respiratory self-gated four-dimensional (4D) coronary MRA technique for simultaneous cardiac anatomy and function visualization. Methods A contrast-enhanced, ungated spoiled gradient echo sequence with self-gating (SG) and 3DPR trajectory was used for image acquisition. Data were retrospectively binned into different cardiac and respiratory phases based on information extracted from SG projections using principal component analysis. Each cardiac phase was reconstructed using a respiratory motion-corrected self-calibrating SENSE framework, and those belong to the quiescent period were retrospectively combined for coronary visualization. Healthy volunteer studies were conducted to evaluate the efficacy of the SG method, the accuracy of the left ventricle (LV) function parameters and the quality of coronary artery visualization. Results SG performed reliably for all subjects including one with poor electrocardiogram (ECG). The LV function parameters showed excellent agreement with those from a conventional cine protocol. For coronary imaging, the proposed method yielded comparable apparent signal to noise ratio and coronary sharpness and lower apparent contrast to noise ratio on three subjects compared with an ECG and navigator-gated Cartesian protocol and an ECG-gated, respiratory motion-corrected 3DPR protocol. Conclusion A fully self-gated 4D whole-heart imaging technique was developed, potentially allowing cardiac anatomy and function assessment from a single measurement. Magn Reson Med 72:1208–1217, 2014. © 2014 Wiley Periodicals, Inc.

Variable delay multi-pulse train for fast chemical exchange saturation transfer and relayed-nuclear overhauser enhancement MRI.

Abstract: Purpose Chemical exchange saturation transfer (CEST) imaging is a new MRI technology allowing the detection of low concentration endogenous cellular proteins and metabolites indirectly through their exchangeable protons. A new technique, variable delay multi-pulse CEST (VDMP-CEST), is proposed to eliminate the need for recording full Z-spectra and performing asymmetry analysis to obtain CEST contrast. Methods The VDMP-CEST scheme involves acquiring images with two (or more) delays between radiofrequency saturation pulses in pulsed CEST, producing a series of CEST images sensitive to the speed of saturation transfer. Subtracting two images or fitting a time series produces CEST and relayed-nuclear Overhauser enhancement CEST maps without effects of direct water saturation and, when using low radiofrequency power, minimal magnetization transfer contrast interference. Results When applied to several model systems (bovine serum albumin, crosslinked bovine serum albumin, l-glutamic acid) and in vivo on healthy rat brain, VDMP-CEST showed sensitivity to slow to intermediate range magnetization transfer processes (rate < 100–150 Hz), such as amide proton transfer and relayed nuclear Overhauser enhancement-CEST. Images for these contrasts could be acquired in short scan times by using a single radiofrequency frequency. Conclusions VDMP-CEST provides an approach to detect CEST effect by sensitizing saturation experiments to slower exchange processes without interference of direct water saturation and without need to acquire Z-spectra and perform asymmetry analysis. Magn Reson Med 71:1798–1812, 2014. © 2013 Wiley Periodicals, Inc.

Slab‐selective, BOLD‐corrected VASO at 7 Tesla provides measures of cerebral blood volume reactivity with high signal‐to‐noise ratio

Abstract: Purpose MRI methods sensitive to functional changes in cerebral blood volume (CBV) may map neural activity with better spatial specificity than standard functional MRI (fMRI) methods based on blood oxygen level dependent (BOLD) effect. The purpose of this study was to develop and investigate a vascular space occupancy (VASO) method with high sensitivity to CBV changes for use in human brain at 7 Tesla (T). Methods To apply 7T VASO, several high-field-specific obstacles must be overcome, e.g., low contrast-to-noise ratio (CNR) due to convergence of blood and tissue T1, increased functional BOLD signal change contamination, and radiofrequency field inhomogeneities. In the present method, CNR was increased by keeping stationary tissue magnetization in a steady-state different from flowing blood, using slice-selective saturation pulses. Interleaved acquisition of BOLD and VASO signals allowed correction for BOLD contamination. Results During visual stimulation, a relative CBV change of 28% ± 5% was measured, confined to gray matter in the occipital lobe with high sensitivity. Conclusion By carefully considering all the challenges of high-field VASO and filling behavior of the relevant vasculature, the proposed method can detect and quantify CBV changes with high CNR in human brain at 7T. Magn Reson Med 72:137–148, 2014. © 2013 Wiley Periodicals, Inc.

Improved intravoxel incoherent motion analysis of diffusion weighted imaging by data driven Bayesian modeling.

Abstract: In addition to the diffusion coefficient, fitting the intravoxel incoherent motion model to multiple b-value diffusion-weighted MR data gives pseudo-diffusion measures associated with rapid signal attenuation at low b-values that are of use in the assessment of a number of pathologies. When summary measures are required, such as the average parameter for a region of interest, least-squares based methods give adequate estimation accuracy. However, using least-squares methods for pixelwise fitting typically gives noisy estimates, especially for the pseudo-diffusion parameters, which limits the applicability of the approach for assessing spatial features and heterogeneity. In this article, a Bayesian approach using a shrinkage prior model is proposed and is shown to substantially reduce estimation uncertainty so that spatial features in the parameters maps are more clearly apparent. The Bayesian approach has no user-defined parameters, so measures of parameter variation (heterogeneity) over regions of interest are determined by the data alone, whereas it is shown that for the least-squares estimates, measures of variation are essentially determined by user-defined constraints on the parameters. Use of a Bayesian shrinkage prior approach is, therefore, recommended for intravoxel incoherent motion modeling. Magn Reson Med 71:411–420, 2014. © 2013

On random walks and entropy in diffusion‐weighted magnetic resonance imaging studies of neural tissue

Abstract: Purpose In diffusion-weighted MRI studies of neural tissue, the classical model assumes the statistical mechanics of Brownian motion and predicts a monoexponential signal decay. However, there have been numerous reports of signal decays that are not monoexponential, particularly in the white matter. Theory We modeled diffusion in neural tissue from the perspective of the continuous time random walk. The characteristic diffusion decay is represented by the Mittag-Leffler function, which relaxes a priori assumptions about the governing statistics. We then used entropy as a measure of the anomalous features for the characteristic function. Methods Diffusion-weighted MRI experiments were performed on a fixed rat brain using an imaging spectrometer at 17.6 T with b-values arrayed up to 25,000 s/mm2. Additionally, we examined the impact of varying either the gradient strength, q, or mixing time, Δ, on the observed diffusion dynamics. Results In white and gray matter regions, the Mittag-Leffler and entropy parameters demonstrated new information regarding subdiffusion and produced different image contrast from that of the classical diffusion coefficient. The choice of weighting on q and Δ produced different image contrast within the regions of interest. Conclusion We propose these parameters have the potential as biomarkers for morphology in neural tissue. Magn Reson Med 71:617–627, 2014. © 2013 Wiley Periodicals, Inc.