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PURPOSE: To develop fixative agents for high-field MRI with suitable dielectric properties and measure MR properties in immersion-fixed brain tissue. METHODS: Dielectric properties of formalin-based agents were assessed (100 MHz-4.5 GHz), and four candidate fixatives with/without polyvinylpyrrolidone (PVP) and different salt concentrations were formulated. B1 field and MR properties (T1 , R2∗ , R2 , R2' , and magnetic susceptibility [QSM]) were observed in white and gray matter of pig brain samples during 0.5-35 days of immersion fixation. The kinetics were fitted using exponential functions. The immersion time required to reach maximum R2∗ values at different tissue depths was used to estimate the Medawar coefficient for fixative penetration. The effect of replacing the fixatives with Fluoroinert and phosphate-buffered saline as embedding media was also evaluated. RESULTS: The dielectric properties of formalin were nonlinearly modified by increasing amounts of additives. With 5% PVP and 0.04% NaCl, the dielectric properties and B1 field reflected in vivo conditions. The highest B1 values were found in white matter with PVP and varied significantly with tissue depth and embedding media, but not with immersion time. The MR properties depended on PVP yielding lower T1 , higher R2∗ , more paramagnetic QSM values, and a lower Medawar coefficient (0.9 mm/h ; without PVP: 1.5). Regardless of fixative, switching to phosphate-buffered saline as embedder caused a paramagnetic shift in QSM and decreased R2∗ that progressed during 1 month of storage, whereas no differences were found with Fluorinert. CONCLUSION: In vivo-like B1 fields can be achieved in formalin fixatives using PVP and a low salt concentration, yielding lower T1 , higher R2∗ , and more paramagnetic QSM than without additives. The kinetics of R2∗ allowed estimation of fixative tissue penetration.
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Formaldehído , Imagen por Resonancia Magnética , Animales , Encéfalo/diagnóstico por imagen , Fijadores , Neuroimagen , Porcinos , Fijación del TejidoRESUMEN
PURPOSE: To evaluate the homogeneity of the radiofrequency magnetic field (B1+ ) and signal intensity using different arm positions during 3T thoracolumbar spinal imaging. MATERIALS AND METHODS: Twenty volunteers were scanned with a four-channel radiofrequency (RF) transmit coil at 3T, with arms on the bed (conventional), arms elevated by 100 mm (arm lift), or with the arms-up position (elevated arm). Axial B1+ maps and sagittal T1 -weighted image (T1 WI)-performed RF shimming were obtained for each arm position. The mean and standard deviation (SD) of the flip angle (FA) at the center of the vertebra on each B1+ map, and contrast noise ratios (CNRs) between the spinal cord and cerebrospinal fluid of sagittal T1 WI, were calculated and compared among the different arm positions. RESULTS: Mean FA values (degrees) for the arm lift and elevated arm positions were significantly larger than for the conventional position (P < 0.001 for both) at the twelfth thoracic vertebra (Th12). FA SD values for the arm lift and elevated arm position were significantly smaller than for the conventional position (P < 0.001 for both) at Th12. CNR for the arm lift and elevated arm position were significantly higher than for the conventional position (P = 0.007 and 0.002, respectively). The mean and SD of the FA and the CNR did not differ significantly for the arm lift and elevated arm positions (P = 0.591, 0.958, and 0.927, respectively). CONCLUSION: Inhomogeneities of B1+ and signal intensities were improved by simply changing the arm position in 3T thoracolumbar spinal imaging. LEVEL OF EVIDENCE: 3 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:123-130.
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Imagen por Resonancia Magnética , Médula Espinal/diagnóstico por imagen , Columna Vertebral/diagnóstico por imagen , Vértebras Torácicas/diagnóstico por imagen , Adulto , Voluntarios Sanos , Humanos , Interpretación de Imagen Asistida por Computador , Campos Magnéticos , Masculino , Persona de Mediana Edad , Estudios Prospectivos , Ondas de Radio , Reproducibilidad de los ResultadosRESUMEN
Improvements are proposed for practical design and use of high permittivity materials in high field neuroimaging in three different areas: (i) a simple formula to predict the permittivity of tri-component aqueous-based perovskite suspensions with relative permittivities between 110 and 300, (ii) characterization of addition of a hydroxyethyl-cellulose gelling agent to improve the long-term stability and material properties of "dielectric pads", and (iii) investigation of the integration of, for example, headphones into the dielectric pads to increase patient comfort within tightly-fitting receive coil arrays.
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Neuroimagen , Humanos , Aumento de la Imagen , Imagen por Resonancia MagnéticaRESUMEN
Background Magnetic resonance imaging (MRI) using a 3-T MRI scanner is now widely used for clinical examinations. However, B1 inhomogeneity becomes larger with MRI scanners using 3-T and higher. It especially becomes a problem in the breast. To improve B1 homogeneity, a RF shimming technique has been developed. Purpose To evaluate the B1 homogeneity of breast MRI using RF shimming with individual specific values for subjects after mastectomy. Material and Methods The subjects are healthy female volunteers who underwent normal breast imaging, followed by imaging of one breast while the other breast was bound tightly to the chest by bleached cotton cloths (simulating volunteers after mastectomy). B1 mappings were performed with RF shimming using two techniques: (i) optimized fixed value; and (ii) individual specific values using a 3-T MRI scanner. The means and standard deviations of the B1 maps for all slices in the breast were measured and compared between the fixed value and the individual specific value cases. Results For normal volunteers, the breast B1 variation was not statistically significantly different between the RF shimming techniques. For volunteers after simulated surgery, the breast B1 variation was (1.02 ± 0.29) with the fixed value and (0.98 ± 0.22) with the individual specific value ( P < 0.01). With the individual specific optimization, B1 variation for all slices in the breast was improved for volunteers after simulated surgery. Conclusion RF shimming with individual specific values has the potential to improve the B1 homogeneity of breast MRI in patients after mastectomy.
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Artefactos , Mama/diagnóstico por imagen , Mama/cirugía , Aumento de la Imagen/métodos , Imagen por Resonancia Magnética/métodos , Mastectomía , Adulto , Mama/anatomía & histología , Femenino , Humanos , Interpretación de Imagen Asistida por Computador/métodos , Persona de Mediana Edad , Simulación de Paciente , Valores de Referencia , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , Resultado del Tratamiento , Adulto JovenRESUMEN
PURPOSE: To design parallel transmission spokes pulses with time-shifted profiles for joint mitigation of intensity variations due to B1+ effects, signal loss due to through-plane dephasing, and the specific absorption rate (SAR) at 7T. METHODS: We derived a slice-averaged small tip angle (SA-STA) approximation of the magnetization signal at echo time that depends on the B1+ transmit profiles, the through-slice B0 gradient and the amplitude and time-shifts of the spoke waveforms. We minimize a magnitude least-squares objective based on this signal equation using a fast interior-point approach with analytical expressions of the Jacobian and Hessian. RESULTS: Our algorithm runs in less than three minutes for the design of two-spoke pulses subject to hundreds of local SAR constraints. On a B0/B1+ head phantom, joint optimization of the channel-dependent time-shifts and spoke amplitudes allowed signal recovery in high-B0 regions at no increase of SAR. Although the method creates uniform magnetization profiles (ie, uniform intensity), the flip angle varies across the image, which makes it ill-suited to T1-weighted applications. CONCLUSIONS: The SA-STA approach presented in this study is best suited to T2*-weighted applications with long echo times that require signal recovery around high B0 regions. Magn Reson Med 76:540-554, 2016. © 2015 Wiley Periodicals, Inc.
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Algoritmos , Artefactos , Encéfalo/anatomía & histología , Aumento de la Imagen/métodos , Interpretación de Imagen Asistida por Computador/métodos , Imagen por Resonancia Magnética/métodos , Procesamiento de Señales Asistido por Computador , Absorción de Radiación , Humanos , Imagen por Resonancia Magnética/instrumentación , Fantasmas de Imagen , Reproducibilidad de los Resultados , Sensibilidad y EspecificidadRESUMEN
This article reviews the principle of dual-channel transmit MRI and highlights current clinical applications which are performed primarily at 3 Tesla. The main benefits of dual-channel transmit compared with single-transmit systems are the increased image contrast homogeneity and the decreased scanning time due to the more accurate local specific absorption ratio estimation, meaning that less conservative safety limits are needed. The dual-transmit approach has been particularly beneficial in body imaging applications, and is also promising in terms of cardiac, spine, and fetal imaging. Future advances in transmit SENSE, the combination of dual-channel transmit with high permittivity pads, as well as the potential increase in the number of transmit channels are also discussed.