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  • 1
    Publication Date: 2018-03-16
    Description: Purpose To evaluate the feasibility of an improved motion and flow robust methodology for imaging the pulmonary vasculature using non-contrast-enhanced, free-breathing, golden-angle radial MRI. Methods Healthy volunteers ( n  = 10, age 46 ± 11 years, 50% female) and patients ( n  = 2, ages 27 and 84, both female) were imaged at 1.5 T using a Cartesian and golden-angle radial 2D balanced SSFP pulse sequence. The acquisitions were made under free breathing without contrast agent enhancement. The radial acquisitions were reconstructed at 3 temporal footprints. All series were scored from 1 to 5 for perceived diagnostic quality, artifact level, and vessel sharpness in multiple anatomical locations. In addition, vessel sharpness and blood-to-blood clot contrast were measured. Results Quantitative measurements showed higher vessel sharpness for golden-angle radial ( n  = 76, 0.79 ± 0.11 versus 0.71 ± 0.16, p  〈 .05). Blood-to-blood clot contrast was found to be 23% higher in golden-angle radial in the 2 patients. At comparable temporal footprints, golden-angle radial was scored higher for diagnostic quality (mean ± SD, 2.3 ± 0.7 versus 2.2 ± 0.6, p  〈 .01) and vessel sharpness (2.2 ± 0.8 versus 2.1 ± 0.5, p  〈 .01), whereas the artifact level did not differ (3.0 ± 0.9 versus 3.0 ± 1.0, p  = .80). The ability to retrospectively choose a temporal resolution and perform sliding-window reconstructions was demonstrated in patients. Conclusion In pulmonary artery imaging, the motion and flow robustness of a radial trajectory does both improve image quality over Cartesian trajectory in healthy volunteers, and allows for flexible selection of temporal footprints and the ability to perform real-time sliding window reconstructions, which could potentially provide further diagnostic insight.
    Print ISSN: 0740-3194
    Electronic ISSN: 1522-2594
    Topics: Medicine
    Published by Wiley-Blackwell
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  • 2
    Publication Date: 2018-03-14
    Description: Purpose To build and evaluate a small-footprint, lightweight, high-performance 3T MRI scanner for advanced brain imaging with image quality that is equal to or better than conventional whole-body clinical 3T MRI scanners, while achieving substantial reductions in installation costs. Methods A conduction-cooled magnet was developed that uses less than 12 liters of liquid helium in a gas-charged sealed system, and standard NbTi wire, and weighs approximately 2000 kg. A 42-cm inner-diameter gradient coil with asymmetric transverse axes was developed to provide patient access for head and extremity exams, while minimizing magnet-gradient interactions that adversely affect image quality. The gradient coil was designed to achieve simultaneous operation of 80-mT/m peak gradient amplitude at a slew rate of 700 T/m/s on each gradient axis using readily available 1-MVA gradient drivers. Results In a comparison of anatomical imaging in 16 patients using T 2 -weighted 3D fluid-attenuated inversion recovery (FLAIR) between the compact 3T and whole-body 3T, image quality was assessed as equivalent to or better across several metrics. The ability to fully use a high slew rate of 700 T/m/s simultaneously with 80-mT/m maximum gradient amplitude resulted in improvements in image quality across EPI, DWI, and anatomical imaging of the brain. Conclusions The compact 3T MRI system has been in continuous operation at the Mayo Clinic since March 2016. To date, over 200 patient studies have been completed, including 96 comparison studies with a clinical 3T whole-body MRI. The increased gradient performance has reliably resulted in consistently improved image quality.
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    Electronic ISSN: 1522-2594
    Topics: Medicine
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  • 3
    Publication Date: 2018-03-14
    Description: Purpose The ultimate intrinsic signal-to-noise ratio (UISNR) represents an upper bound for the achievable SNR of any receive coil. To reach this threshold a complete basis set of equivalent surface currents is required. This study systematically investigated to what extent either loop- or dipole-like current patterns are able to reach the UISNR threshold in a realistic human head model between 1.5 T and 11.7 T. Based on this analysis, we derived guidelines for coil designers to choose the best array element at a given field strength. Moreover, we present ideal current patterns yielding the UISNR in a realistic body model. Methods We distributed generic current patterns on a cylindrical and helmet-shaped surface around a realistic human head model. We excited electromagnetic fields in the human head by using eigenfunctions of the spherical and cylindrical Helmholtz operator. The electromagnetic field problem was solved by a fast volume integral equation solver. Results At 7 T and above, adding curl-free current patterns to divergence-free current patterns substantially increased the SNR in the human head (locally 〉20%). This was true for the helmet-shaped and the cylindrical surface. On the cylindrical surface, dipole-like current patterns had high SNR performance in central regions at ultra-high field strength. The UISNR increased superlinearly with B0 in most parts of the cerebrum but only sublinearly in the periphery of the human head. Conclusion The combination of loop and dipole elements could enhance the SNR performance in the human head at ultra-high field strength.
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    Electronic ISSN: 1522-2594
    Topics: Medicine
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  • 4
    Publication Date: 2018-03-14
    Description: Purpose To characterize and suppress stripe artifact associated with velocity-selective (VS) magnetization for unenhanced MRA. Methods Extended phase graph formalism was used to show that the stripe artifact contains multiples of the fundamental frequency that is determined by the area of unipolar VS gradient. Four VS preparation pulses whose excitation profiles are spatially shifted by quarter the fundamental period of the stripes, were applied alternately. For further suppression of the artifact, k-space data at k z  = 0 were averaged over the 4 VS preparations. The proposed schemes were tested in a chicken breast phantom and healthy human subjects. Results When the standard VS preparation scheme was used, stripe artifact was shown in all the reconstructed images and appeared as artifactual peaks in k-space that corresponded to the first and second order harmonics of the fundamental frequency. Alternate application of the 4 phase-shifted VS preparation pulses suppressed the stripes, but not completely, as evidenced by residual erroneous peaks in k-space. After the k-space averaging, the stripe artifact was nearly eliminated. Conclusion Stripe artifact in VS-MRA consists of multiples of the fundamental frequency and can be effectively suppressed through alternate application of phase-shifted VS preparations along with k-space averaging.
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    Electronic ISSN: 1522-2594
    Topics: Medicine
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  • 5
    Publication Date: 2018-03-12
    Description: Purpose To improve 2D noncontrast-enhanced MRA by using a helical time-of-flight (TOF) acquisition technique and a slice-super-resolution reconstruction. Methods The TOF technique is combined with a helical trajectory with golden-angle–based radial projection reordering. A continuous spatial shift in slice direction is realized by adjusting the frequency of the excitation pulse between the individual projections. The limited resolution along the shift direction is improved by a deconvolution with simulated slice profile. The helical TOF (hTOF) was compared in vivo with a conventional 2D and 3D TOF. Results Results from in vivo experiments on the carotid show that the visual resolution in slice direction can be improved by using hTOF and the slice-super-resolution reconstruction. The vessels appear up to 1.5 times sharper and can be better separated from each other. Compared to 2D TOF images, the stair step artifacts are strongly reduced in reformatted hTOF images, whereas measurement time is decreased by at least 35%. Compared to 3D TOF, the hTOF offers a higher blood-to-background contrast, better visualization of smaller vessels, and reduced measurement time. Conclusion The hTOF benefits from a 2D acquisition and a 3D reconstruction, which makes it a promising technique for the noncontrast-enhanced imaging of the carotid.
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    Electronic ISSN: 1522-2594
    Topics: Medicine
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  • 6
    Publication Date: 2018-03-12
    Description: Purpose Deranged metabolism is now recognized as a key causal factor in a variety of heart diseases, and is being studied extensively. However, invasive methods may alter metabolism, and conventional imaging techniques measure tracer uptake but not downstream metabolism. These challenges may be overcome by hyperpolarized MR, a noninvasive technique currently crossing the threshold into human trials. The aim of this study was to image metabolic changes in the heart in response to endogastric glucose bolus and to acute hypertension. Methods Five postprandial pigs were scanned with hyperpolarized [1- 13 C]pyruvate cardiac MR at baseline, after oral glucose bolus, and after infusion of angiotensin-II. Results No effect of glucose bolus was seen using hyperpolarized [1- 13 C]pyruvate MR despite changes in circulating substrates. During angiotensin-II infusion, blood pressure increased 179% ( P  = 0.008) and ejection fraction decreased from 54 ± 2% to 47 ± 6% ( P  = 0.03) The hemodynamic changes were accompanied by increases in the hyperpolarized [1- 13 C]pyruvate MR derived ratios of lactate/alanine (from 0.58 ± 0.13 to 0.78 ± 0.06, P  = 0.03) and bicarbonate/alanine (from 0.55 ± 0.12 to 0.91 ± 0.14, P  = 0.007). Conclusion Glucose loading did not alter cardiac metabolism, but during acute hypertensive stress, cardiac aerobic, carbohydrate metabolism, and pyruvate-lactate exchange was altered. Hyperpolarized MR allows noninvasive evaluation of acute changes in cardiac metabolism. However, hemodynamics must be taken into account when interpreting the results.
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    Electronic ISSN: 1522-2594
    Topics: Medicine
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  • 7
    Publication Date: 2018-03-12
    Description: Purpose To demonstrate a computationally efficient and theoretically artifact-free method to calculate static field (B 0 ) inhomogeneity in a volume of interest induced by an arbitrary voxelated susceptibility distribution. Methods Our method computes B 0 by circular convolution between a zero-filled susceptibility matrix and a shifted, voxel-integrated dipolar field kernel on a grid of size N S +N T – 1 in each dimension, where N S and N T are the sizes of the susceptibility source and B 0 target grids, respectively. The computational resource requirement is independent of source-target separation. The method, called generalized susceptibility voxel convolution, is demonstrated on three susceptibility models: an ellipsoid, MR-compatible screws, and a dynamic human heartbeat model. Results B 0 in an ellipsoid calculated by generalized susceptibility voxel convolution matched an analytical solution nearly exactly. The method also calculated screw-induced B 0 in agreement with experimental data. Dynamic simulation demonstrated its computational efficiency for repeated B 0 calculations on time-varying susceptibility. On the contrary, conventional and alias-subtracted k-space-discretized Fourier convolution methods showed nonnegligible aliasing and Gibbs ringing artifacts in the tested models. Conclusion Generalized susceptibility voxel convolution can be a fast and reliable way to compute susceptibility-induced B 0 when the susceptibility source is not colocated with the B 0 target volume of interest, as in modeling B 0 variations from motion and foreign objects.
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    Topics: Medicine
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  • 8
    Publication Date: 2018-03-12
    Description: Purpose 2D turbo-spin-echo (TSE) is widely used in the clinic for neuroimaging. However, the long refocusing radiofrequency pulse train leads to high specific absorption rate (SAR) and alters the contrast compared to conventional spin-echo. The purpose of this work is to develop a robust 2D spiral TSE technique for fast T 2 -weighted imaging with low SAR and improved contrast. Methods A spiral-in/out readout is incorporated into 2D TSE to fully take advantage of the acquisition efficiency of spiral sampling while avoiding potential off-resonance-related artifacts compared to a typical spiral-out readout. A double encoding strategy and a signal demodulation method are proposed to mitigate the artifacts because of the T 2 -decay-induced signal variation. An adapted prescan phase correction as well as a concomitant phase compensation technique are implemented to minimize the phase errors. Results Phantom data demonstrate the efficacy of the proposed double encoding/signal demodulation, as well as the prescan phase correction and concomitant phase compensation. Volunteer data show that the proposed 2D spiral TSE achieves fast scan speed with high SNR, low SAR, and improved contrast compared to conventional Cartesian TSE. Conclusion A robust 2D spiral TSE technique is feasible and provides a potential alternative to conventional 2D Cartesian TSE for T 2 -weighted neuroimaging.
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  • 9
    Publication Date: 2018-03-12
    Description: Purpose Proton MRSI is a noninvasive modality capable of generating volumetric maps of in vivo tissue metabolism without the need for ionizing radiation or injected contrast agent. Magnetic resonance spectroscopic imaging has been shown to be a viable imaging modality for studying several neuropathologies. However, a key hurdle in the routine clinical adoption of MRSI is the presence of spectral artifacts that can arise from a number of sources, possibly leading to false information. Methods A deep learning model was developed that was capable of identifying and filtering out poor quality spectra. The core of the model used a tiled convolutional neural network that analyzed frequency-domain spectra to detect artifacts. Results When compared with a panel of MRS experts, our convolutional neural network achieved high sensitivity and specificity with an area under the curve of 0.95. A visualization scheme was implemented to better understand how the convolutional neural network made its judgement on single-voxel or multivoxel MRSI, and the convolutional neural network was embedded into a pipeline capable of producing whole-brain spectroscopic MRI volumes in real time. Conclusion The fully automated method for assessment of spectral quality provides a valuable tool to support clinical MRSI or spectroscopic MRI studies for use in fields such as adaptive radiation therapy planning.
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    Topics: Medicine
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  • 10
    Publication Date: 2018-03-12
    Description: Purpose Magnetic resonance imaging has been used extensively to track in vivo implanted cells that have been previously labeled with relaxation enhancers. However, this approach is not suitable to track multiple cell populations, as it may lead to confounding results in case the contrast agent is released from the labeled cells. This paper demonstrates how the use of CEST agents can overcome these issues. After encapsulating paramagnetic lanthanide shift reagents, we may shift the absorption frequency of the intracellular water resonance (δ In ), thus generating frequency-encoding CEST responsive cells that can be visualized in the MR image by applying the proper RF irradiation. Methods Eu-HPDO3A, Dy-HPDO3A, and Tm-HPDO3A were used as shift reagents for labeling murine breast cancer cells and murine macrophages by hypotonic swelling and pinocytosis. The CEST-MR images were acquired at 7 T, and the saturation transfer effect was measured. Samples at different dilution of cells were analyzed to quantify the detection threshold. In vitro experiments of cell proliferation were carried out. Finally, murine breast cancer cells were injected subcutaneously in mice, and MR images were acquired to assess the proliferation index in vivo. Results It was found that entrapment of the paramagnetic complexes into endosomes (i.e., using the pinocytosis route) leads to an enhanced shift of the intracellular water resonance. δ In appears to be proportional to the effective magnetic moment (μ eff ) and to the concentration of the loaded lanthanide complex. Moreover, a higher shift is present when the complexes are entrapped in the endosomes. The cell proliferation index was assessed both in vitro and in vivo by evaluating the reduction of δ In value in the days after the cell labeling. Conclusion Cells can be visualized by CEST MRI after loading with paramagnetic shift reagent, by exploiting the large ensemble of the properly shifted intracellular water molecules. A better performance is obtained when the complexes are entrapped inside the endosomes. The observed (δ In ) value is strongly correlated to the chemical nature of the probe, and to its concentration and cellular localization. Two applications of this method are reported in this paper: (1) for in vivo cell visualization and (2) for the monitoring of the cellular proliferation process, as this method is accompanied by a change in δ In that may be exploited as a longitudinal reporter of the proliferation rate.
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    Topics: Medicine
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