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  • Articles  (1,145)
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  • Articles  (1,145)
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  • 1
    Publication Date: 2018-03-16
    Description: The cover image, by Yue Zhang et al., is based on the Research Article Comparison of reproducibility of single voxel spectroscopy and whole-brain magnetic resonance spectroscopy imaging at 3T , DOI: 10.1002/nbm.3898 .
    Print ISSN: 0952-3480
    Electronic ISSN: 1099-1492
    Topics: Medicine
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  • 2
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    Wiley-Blackwell
    Publication Date: 2018-03-16
    Description: No abstract is available for this article.
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    Electronic ISSN: 1099-1492
    Topics: Medicine
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  • 3
    Publication Date: 2018-03-14
    Description: Hyperglycemia (blood glucose concentration 〉150 mg/dL) is common in extremely low gestational age newborns (ELGANs; birth at 〈28 week gestation). Hyperglycemia increases the risk of brain injury in the neonatal period. The long-term effects are not well understood. In adult rats, hyperglycemia alters hippocampal energy metabolism. The effects of hyperglycemia on the developing hippocampus were studied in rat pups. In Experiment 1, recurrent hyperglycemia of graded severity (moderate hyperglycemia (moderate-HG), mean blood glucose 214.6 ± 11.6 mg/dL; severe hyperglycemia (severe-HG), 338.9 ± 21.7 mg/dL; control, 137.7 ± 2.6 mg/dL) was induced from postnatal day (P) 3 to P12. On P30, the hippocampal neurochemical profile was determined using in vivo 1 H MR spectroscopy. Dendritic arborization in the hippocampal CA1 region was determined using microtubule-associated protein (MAP)-2 immunohistochemistry. In Experiment 2, continuous hyperglycemia (mean blood glucose 275.3 ± 25.8 mg/dL; control, 142.3 ± 2.6 mg/dL) was induced from P2 to P6 by injecting streptozotocin (STZ) on P2. The mRNA expression of glycogen synthase 1 ( Gys1 ), lactate dehydrogenase ( Ldh ), glucose transporters 1 ( Glut1 ) and 3 ( Glut3 ) and monocarboxylate transporters 1 ( Mct1 ), 2 ( Mct2 ) and 4 ( Mct4 ) in the hippocampus was determined on P6. In Experiment 1, MRS demonstrated lower lactate concentration and glutamate/glutamine (Glu/Gln) ratio in the severe-HG group, compared with the control group ( p 〈 0.05). Phosphocreatine/creatine ratio was higher in both hyperglycemia groups ( p 〈 0.05). MAP-2 histochemistry demonstrated longer apical segment length, indicating abnormal synaptic efficacy in both hyperglycemia groups ( p 〈 0.05). Experiment 2 showed lower Glut1 , Gys1 and Mct4 expression and higher Mct1 expression in the hyperglycemia group, relative to the control group ( p 〈 0.05). These results suggest that hyperglycemia alters substrate transport, lactate homeostasis, dendritogenesis and Glu-Gln cycling in the developing hippocampus. Abnormal neurochemical profile and dendritic structure due to hyperglycemia may partially explain the long-term hippocampus-mediated cognitive deficits in human ELGANs. In vivo 1 H MRS shows that recurrent neonatal hyperglycemia leads to lower lactate and glutamate/glutamine ratio, and higher phosphocreatine/creatine ratio, in the developing rat hippocampus. Histochemical analysis demonstrates longer apical dendritic length, indicating decreased synaptic efficacy in the formerly hyperglycemic hippocampus. Altered monocarboxylate and glucose transporter expression in the hippocampus during neonatal hyperglycemia suggests that decreased lactate availability during development may be responsible for the long-term neurochemical and structural changes
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    Topics: Medicine
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  • 4
    Publication Date: 2018-03-09
    Description: The aim of this work was to develop and evaluate a fast phase contrast magnetic resonance imaging (PC-MRI) technique with hybrid one- and two-sided flow encodings only (HOTFEO) for accurate blood flow and velocity measurements of three-directional velocity encoding PC-MRI. Four-dimensional (4D) PC-MRI acquires flow-compensated (FC) and three-directional flow-encoded (FE) echoes in an interleaved fashion. We hypothesize that the blood flow velocity direction (not magnitude) has minimal change between two consecutive cardiac phases. This assumption provides a velocity direction constraint that can achieve 4/3-fold acceleration using three-directional FE data to calculate FC data instead of acquiring them. The HOTFEO acquisition pattern can address the ill-conditioned constraint and improve the calculation accuracy. HOTFEO was evaluated in healthy volunteers and compared with conventional two-dimensional (2D) and 4D flow imaging techniques with FC and three-directional FE acquisitions (FC/3FE). Compared with FC/3FE, Bland–Altman tests showed that the 4/3-fold accelerated HOTFEO technique resulted in relatively small bias error for total volumetric flow (0.89% for prospective 2D data, –1.19% for retrospective 4D data and –3.40% for prospective 4D data) and maximum peak velocity (0.50% for prospective 2D data, –0.17% for retrospective 4D data and –2.00% for prospective 4D data) measurements in common carotid arteries. HOTFEO can accelerate three-directional velocity encoding PC-MRI whilst maintaining the measurement accuracy of the total volumetric flow and maximum peak velocity. We hypothesize that the blood flow velocity direction (not magnitude) shows minimum change between two consecutive cardiac phases. This assumption provides a velocity direction constraint that can achieve 4/3-fold acceleration using three-directional flow encoding data to calculate flow compensation data instead of acquiring them. Hybrid one- and two-sided flow encodings only (HOTFEO) can accelerate three-directional velocity encoding phase contrast magnetic resonance imaging (PC-MRI) whilst maintaining the measurement accuracy of the total volumetric flow and maximum total peak velocity.
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    Topics: Medicine
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  • 5
    Publication Date: 2018-03-06
    Description: Overhauser-enhanced MRI (OMRI) is an electron-proton double-resonance imaging technique of interest for its ability to non-invasively measure the concentration and distribution of free radicals. In vivo OMRI experiments are typically undertaken at ultra-low magnetic field (ULF), as both RF power absorption and penetration issues—a consequence of the high resonance frequencies of electron spins—are mitigated. However, working at ULF causes a drastic reduction in MRI sensitivity. Here, we report on the design, construction and performance of an OMRI platform optimized for high NMR sensitivity and low RF power absorbance, exploring challenges unique to probe design in the ULF regime. We use this platform to demonstrate dynamic imaging of TEMPOL in a rat model. The work presented here demonstrates improved speed and sensitivity of in vivo OMRI, extending the scope of OMRI to the study of dynamic processes such as metabolism. Overhauser-enhanced MRI is a powerful tool for imaging of free radicals in vivo however its application to dynamic processes has been limited by low signal-to-noise ratios. Here, we detail the development of a high-sensitivity OMRI platform that combines steady-state acquisition with a custom double-resonance probe. We use this platform to demonstrate dynamic imaging of TEMPOL in a rat model with 15 s time resolution.
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    Topics: Medicine
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  • 6
    Publication Date: 2018-03-06
    Description: Whole-brain radiotherapy is the standard of care for patients with breast cancer with multiple brain metastases and, although this treatment has been essential in the management of existing brain tumors, there are many known negative consequences associated with the irradiation of normal brain tissue. In our study, we used in vivo magnetic resonance imaging analysis to investigate the influence of radiotherapy-induced damage of healthy brain on the arrest and growth of metastatic breast cancer cells in a mouse model of breast cancer brain metastasis. We observed that irradiated, but otherwise healthy, neural tissue had an increased propensity to support metastatic growth compared with never-irradiated controls. The elucidation of the impact of irradiation on normal neural tissue could have implications in clinical patient management, particularly in patients with residual systemic disease or with residual radio-resistant brain cancer. Whole-brain radiotherapy is the standard of care for patients with breast cancer with multiple brain metastases, and there are negative consequences associated with the irradiation of normal tissue. In our study, we used in vivo magnetic resonance imaging to investigate the influence of radiotherapy-induced damage of healthy brain on the arrest and growth of cancer cells in experimental breast cancer brain metastasis. We observed that irradiated neural tissue had an increased propensity to support metastatic growth compared with never-irradiated controls.
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  • 7
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    Wiley-Blackwell
    Publication Date: 2018-02-28
    Description: Modern diffusion magnetic resonance imaging (dMRI) acquires intricate volume datasets and biological meaning can only be found in the relationship between its different measurements. Suitable strategies for visualizing these complicated data have been key to interpretation by physicians and neuroscientists, for drawing conclusions on brain connectivity and for quality control. This article provides an overview of visualization solutions that have been proposed to date, ranging from basic grayscale and color encodings to glyph representations and renderings of fiber tractography. A particular focus is on ongoing and possible future developments in dMRI visualization, including comparative, uncertainty, interactive and dense visualizations. This paper provides an overview of visualization solutions that have been proposed to date for diffusion MRI, ranging from basic grayscale and color encodings to glyph representations and renderings of fiber tractography. A particular focus is on ongoing and possible future developments in dMRI visualization, including comparative, uncertainty, interactive, and dense visualizations.
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    Topics: Medicine
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  • 8
    Publication Date: 2018-02-27
    Description: The purpose of this study was to measure the sodium transverse relaxation time T 2 * in the healthy human brain. Five healthy subjects were scanned with 18 echo times (TEs) as short as 0.17 ms. T 2 * values were fitted on a voxel-by-voxel basis using a bi-exponential model. Data were also analysed using a continuous distribution fit with a region of interest-based inverse Laplace transform. Average T 2 * values were 3.4 ± 0.2 ms and 23.5 ± 1.8 ms in white matter (WM) for the short and long components, respectively, and 3.9 ± 0.5 ms and 26.3 ± 2.6 ms in grey matter (GM) for the short and long components, respectively, using the bi-exponential model. Continuous distribution fits yielded results of 3.1 ± 0.3 ms and 18.8 ± 3.2 ms in WM for the short and long components, respectively, and 2.9 ± 0.4 ms and 17.2 ± 2 ms in GM for the short and long components, respectively. 23 Na T 2 * values of the brain for the short and long components for various anatomical locations using ultra-short TEs are presented for the first time. Healthy subjects were scanned using 23 Na ultra-short echo time (UTE) magnetic resonance imaging (MRI) with 18 TEs between 0.17 and 67 ms. T 2 * values were fitted on a voxel-by-voxel basis using a bi-exponential model and using a continuous distribution fit with a region of interest-based inverse Laplace transform. Detailed 23 Na T 2 * values of the brain for the short and long components for various anatomical locations using UTEs are presented for the first time.
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  • 9
    Publication Date: 2018-02-22
    Description: Quantification of magnetic resonance spectroscopy signals using the phantom replacement method requires an adequate correction of differences between the acquisition of the reference signal in the phantom and the measurement in vivo. Applying the principle of reciprocity, sensitivity differences can be corrected at low field strength by measuring the RF transmitter gain needed to obtain a certain flip angle in the measured volume. However, at higher field strength the transmit sensitivity may vary from the reception sensitivity, which leads to wrongly estimated concentrations. To address this issue, a quantification approach based on the principle of reciprocity for use at 3T is proposed and validated thoroughly. In this approach, the RF transmitter gain is determined automatically using a volume-selective power optimization and complemented with information from relative reception sensitivity maps derived from contrast-minimized images to correct differences in transmission and reception sensitivity. In this way, a reliable measure of the local sensitivity was obtained. The proposed method is used to derive in vivo concentrations of brain metabolites and tissue water in two studies with different coil sets in a total of 40 healthy volunteers. Resulting molar concentrations are compared with results using internal water referencing (IWR) and Electric REference To access In vivo Concentrations (ERETIC). With the proposed method, changes in coil loading and regional sensitivity due to B 1 inhomogeneities are successfully corrected, as demonstrated in phantom and in vivo measurements. For the tissue water content, coefficients of variation between 2% and 3.5% were obtained (0.6–1.4% in a single subject). The coefficients of variation of the three major metabolites ranged from 3.4–14.5%. In general, the derived concentrations agree well with values estimated with IWR. Hence, the presented method is a valuable alternative for IWR, without the need for additional hardware such as ERETIC and with potential advantages in diseased tissue. A quantification approach based on the phantom replacement method is implemented and validated in healthy volunteers. Using relative reception sensitivity maps and a volume-selective RF power optimization, a precise correction of coil loading and reception sensitivity differences between the acquisition of the reference signal in the phantom and the measurement in vivo is possible. The molal concentrations determined agree well with values derived using the tissue water signal as a reference, implying that the method presented is a valid alternative to internal water referencing in diseased tissue.
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    Topics: Medicine
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  • 10
    Publication Date: 2018-02-21
    Description: Chemical exchange saturation transfer (CEST) is an imaging method based on magnetization exchange between solutes and water. This exchange generates changes in the measured signal after off-resonance radiofrequency irradiation. Although the analytic solution for CEST with continuous wave (CW) irradiation has been determined, most studies are performed using pulsed irradiation. In this work, we derive an analytic solution for the CEST signal after pulsed irradiation that includes both short-time rotation effects and long-time saturation effects in a two-pool system corresponding to water and a low-concentration exchanging solute pool. Several approximations are made to balance the accuracy and simplicity of the resulting analytic form, which is tested against numerical solutions of the coupled Bloch equations and is found to be largely accurate for amides at high fields, but less accurate at the higher exchange rates, lower offsets and typically higher irradiation powers of amines. We derive an analytic solution for the chemical exchange saturation transfer (CEST) signal after pulsed irradiation that includes both short-time solute rotation effects and long-time saturation effects. Several approximations are made to balance the accuracy and simplicity of the resulting analytic form, which is tested against numerical solutions of the coupled Bloch equations and is found to be largely accurate for amides at high fields, but less accurate at the higher exchange rates, lower offsets and typically higher irradiation powers of amines.
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    Topics: Medicine
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