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1.
Magn Reson Med ; 83(2): 695-711, 2020 02.
Artículo en Inglés | MEDLINE | ID: mdl-31483521

RESUMEN

PURPOSE: Local specific absorption rate (SAR) cannot be measured and is usually evaluated by offline numerical simulations using generic body models that of course will differ from the patient's anatomy. An additional safety margin is needed to include this intersubject variability. In this work, we present a deep learning-based method for image-based subject-specific local SAR assessment. We propose to train a convolutional neural network to learn a "surrogate SAR model" to map the relation between subject-specific B1+ maps and the corresponding local SAR. METHOD: Our database of 23 subject-specific models with an 8-transmit channel body array for prostate imaging at 7 T was used to build 5750 training samples. These synthetic complex B1+ maps and local SAR distributions were used to train a conditional generative adversarial network. Extra penalization for local SAR underestimation errors was included in the loss function. In silico and in vivo validation were performed. RESULTS: In silico cross-validation shows a good qualitative and quantitative match between predicted and ground-truth local SAR distributions. The peak local SAR estimation error distribution shows a mean overestimation error of 15% with 13% probability of underestimation. The higher accuracy of the proposed method allows the use of less conservative safety factors compared with standard procedures. In vivo validation shows that the method is applicable with realistic measurement data with impressively good qualitative and quantitative agreement to simulations. CONCLUSION: The proposed deep learning method allows online image-based subject-specific local SAR assessment. It greatly reduces the uncertainty in current state-of-the-art SAR assessment methods, reducing the time in the examination protocol by almost 25%.


Asunto(s)
Aprendizaje Profundo , Procesamiento de Imagen Asistido por Computador/métodos , Imagen por Resonancia Magnética , Próstata/diagnóstico por imagen , Neoplasias de la Próstata/diagnóstico por imagen , Algoritmos , Simulación por Computador , Bases de Datos Factuales , Voluntarios Sanos , Humanos , Masculino , Modelos Estadísticos , Redes Neurales de la Computación , Fantasmas de Imagen , Reproducibilidad de los Resultados , Relación Señal-Ruido
3.
NMR Biomed ; 32(1): e4015, 2019 01.
Artículo en Inglés | MEDLINE | ID: mdl-30376201

RESUMEN

The differentiation grade of cervical cancer is histologically assessed by examining biopsies or surgical specimens. MRS is a highly sensitive technique that images tissue metabolism and can be used to increase the specificity of tissue characterization in a non-invasive manner. We aim to explore the feasibility of using in vivo 1 H-MRS at 7 T in women with cervical cancer to study tissue fatty acid composition. 10 women with histologically proven Stage IB1-IIB cervical cancer were scanned with a whole-body 7 T MR system with a multi-transmit system and an internal receive only monopole antenna. A STEAM sequence was used to obtain 1 H-MRS data. Fatty acid resonances were fitted with Lorentzian curves and the 2.1 ppm/1.3 ppm ratios were calculated. 1 H-MRS data showed fatty acid signals resonating at 2.1 ppm, 1.9 ppm, 1.5 ppm, 1.3 ppm and 0.9 ppm. Mean 2.1/1.3 ppm ratios were 0.019 ± 0.01, 0.021 ± 0.006, 0.12 ± 0.089 and 0.39 ± 0.27 for normal, Grade I, Grade II and Grade III groups respectively. Poorly differentiated tumor tissue (Grade III) showed elevated fatty acid ratios when compared with the well differentiated tumor (Grade I) or normal tissue. 1 H-MRS in cervical cancer at 7 T is feasible and individual fatty acid signals were detected. In addition, poorly differentiated tumors show more fatty acid unsaturation. The 2.1 ppm/1.3 ppm ratio has potential for tumor characterization in a non-invasive manner for uterine cervical cancer.


Asunto(s)
Espectroscopía de Protones por Resonancia Magnética , Neoplasias del Cuello Uterino/diagnóstico por imagen , Adulto , Anciano , Ácidos Grasos/metabolismo , Femenino , Humanos , Persona de Mediana Edad , Clasificación del Tumor , Neoplasias del Cuello Uterino/patología
4.
Magn Reson Med ; 78(6): 2449-2459, 2017 Dec.
Artículo en Inglés | MEDLINE | ID: mdl-28164362

RESUMEN

PURPOSE: We introduce a new MR-based method to determine the transfer function (TF) for radiofrequency (RF) safety assessment of active implantable medical devices. Transfer functions are implant-specific measures that relate the incident tangential electric field on an (elongated) implant to a scattered electric field at its tip. The proposed method allows for TF determination with a high spatial resolution in relatively fast measurements without requiring dedicated bench setups from MRI images. THEORY AND METHODS: The principle of reciprocity is used in conjunction with the potential to measure currents with MRI to determine TF. Low-flip angle 3D dual gradient echo MRI data are acquired with an implant as transceive antenna, which requires minimal hardware adaptations. The implant-specific TF is determined from the acquired MRI data, with two different postprocessing methods for comparison. RESULTS: TFs of linear and helical implants can be determined accurately (with a Pearson correlation coefficient R ≥ 0.7 between measurements and simulations, and a difference in field at the tip ΔEtip ≤ 19%) from relatively quick (t < 20 minutes) MRI acquisitions with (several) millimeter spatial resolution. CONCLUSION: Transfer function determination with MRI for RF safety assessment of implantable medical devices is possible. The proposed MR-based method allows for TF determination in more realistic exposure scenarios and solid media. Magn Reson Med 78:2449-2459, 2017. © 2017 International Society for Magnetic Resonance in Medicine.


Asunto(s)
Procesamiento de Imagen Asistido por Computador , Imagen por Resonancia Magnética , Prótesis e Implantes , Animales , Gráficos por Computador , Simulación por Computador , Estimulación Encefálica Profunda , Campos Electromagnéticos , Radiación Electromagnética , Humanos , Campos Magnéticos , Fantasmas de Imagen , Ondas de Radio , Reproducibilidad de los Resultados , Relación Señal-Ruido
5.
Magn Reson Med ; 77(1): 221-228, 2017 01.
Artículo en Inglés | MEDLINE | ID: mdl-26762855

RESUMEN

PURPOSE: Development of a passive respiratory motion sensor based on the noise variance of the receive coil array. METHODS: Respiratory motion alters the body resistance. The noise variance of an RF coil depends on the body resistance and, thus, is also modulated by respiration. For the noise variance monitoring, the noise samples were acquired without and with MR signal excitation on clinical 1.5/3 T MR scanners. The performance of the noise sensor was compared with the respiratory bellow and with the diaphragm displacement visible on MR images. Several breathing patterns were tested. RESULTS: The noise variance demonstrated a periodic, temporal modulation that was synchronized with the respiratory bellow signal. The modulation depth of the noise variance resulting from the respiration varied between the channels of the array and depended on the channel's location with respect to the body. The noise sensor combined with MR acquisition was able to detect the respiratory motion for every k-space read-out line. CONCLUSION: Within clinical MR systems, the respiratory motion can be detected by the noise in receive array. The noise sensor does not require careful positioning unlike the bellow, any additional hardware, and/or MR acquisition. Magn Reson Med 77:221-228, 2017. © 2016 Wiley Periodicals, Inc.


Asunto(s)
Imagen por Resonancia Magnética/métodos , Movimiento/fisiología , Respiración , Procesamiento de Señales Asistido por Computador , Algoritmos , Diafragma/diagnóstico por imagen , Diafragma/fisiología , Humanos , Ondas de Radio , Relación Señal-Ruido
6.
NMR Biomed ; 30(9)2017 Sep.
Artículo en Inglés | MEDLINE | ID: mdl-28574604

RESUMEN

The aim of this study was to investigate the signal-to-noise ratio (SNR) gain in early-stage cervical cancer at ultrahigh-field MRI (e.g. 7 T) using a combination of multiple external antennas and a single endorectal antenna. In particular, we used an endorectal monopole antenna to increase the SNR in cervical magnetic resonance imaging (MRI). This should allow high-resolution, T2 -weighted imaging and magnetic resonance spectroscopy (MRS) for metabolic staging, which could facilitate the local tumor status assessment. In a prospective feasibility study, five healthy female volunteers and six patients with histologically proven stage IB1-IIB cervical cancer were scanned at 7 T. We used seven external fractionated dipole antennas for transmit-receive (transceive) and an endorectally placed monopole antenna for reception only. A region of interest, containing both normal cervix and tumor tissue, was selected for the SNR measurement. Separated signal and noise measurements were obtained in the region of the cervix for each element and in the near field of the monopole antenna (radius < 30 mm) to calculate the SNR gain of the endorectal antenna in each patient. We obtained high-resolution, T2 -weighted images with a voxel size of 0.7 × 0.8 × 3.0 mm3 . In four cases with optimal placement of the endorectal antenna (verified on the T2 -weighted images), a mean gain of 2.2 in SNR was obtained at the overall cervix and tumor tissue area. Within a radius of 30 mm from the monopole antenna, a mean SNR gain of 3.7 was achieved in the four optimal cases. Overlap between the two different regions of the SNR calculations was around 24%. We have demonstrated that the use of an endorectal monopole antenna substantially increases the SNR of 7-T MRI at the cervical anatomy. Combined with the intrinsically high SNR of ultrahigh-field MRI, this gain may be employed to obtain metabolic information using MRS and to enhance spatial resolutions to assess tumor invasion.


Asunto(s)
Imagen por Resonancia Magnética/métodos , Recto/diagnóstico por imagen , Relación Señal-Ruido , Neoplasias del Cuello Uterino/diagnóstico por imagen , Neoplasias del Cuello Uterino/patología , Adulto , Anciano , Femenino , Humanos , Estadificación de Neoplasias
7.
NMR Biomed ; 29(9): 1122-30, 2016 09.
Artículo en Inglés | MEDLINE | ID: mdl-26278544

RESUMEN

Although the potential of dipole antennas for ultrahigh-field (UHF) MRI is largely recognized, they are still relatively unknown to the larger part of the MRI community. This article intends to provide electromagnetic insight into the general operating principles of dipole antennas by numerical simulations. The major part focuses on a comparison study of dipole antennas and loop coils at frequencies of 128, 298 and 400 MHz. This study shows that dipole antennas are only efficient radiofrequency (RF) coils in the presence of a dielectric and/or conducting load. In addition, the conservative electric fields (E-fields) at the ends of a dipole are negligible in comparison with the induced E-fields in the center. Like loop coils, long dipole antennas perform better than short dipoles for deeply located imaging targets and vice versa. When the optimal element is chosen for each depth, loop coils have higher B1 (+) efficiency for shallow depths, whereas dipole antennas have higher B1 (+) efficiency for large depths. The cross-over point depth decreases with increasing frequency: 11.6, 6.2 and 5.0 cm for 128, 298 and 400 MHz, respectively. For single elements, loop coils demonstrate a better B1 (+) /√SARmax ratio for any target depth and any frequency. However, one example study shows that, in an array setup with loop coil overlap for decoupling, this relationship is not straightforward. The overlapping loop coils may generate increased specific absorption rate (SAR) levels under the overlapping parts of the loops, depending on the drive phase settings. Copyright © 2015 John Wiley & Sons, Ltd.


Asunto(s)
Aumento de la Imagen/instrumentación , Imagen por Resonancia Magnética/instrumentación , Magnetismo/instrumentación , Modelos Teóricos , Transductores , Imagen de Cuerpo Entero/instrumentación , Animales , Simulación por Computador , Diseño Asistido por Computadora , Diseño de Equipo , Análisis de Falla de Equipo , Humanos , Campos Magnéticos , Fantasmas de Imagen , Dosis de Radiación , Reproducibilidad de los Resultados , Sensibilidad y Especificidad
8.
NMR Biomed ; 29(6): 709-20, 2016 06.
Artículo en Inglés | MEDLINE | ID: mdl-27037615

RESUMEN

Widespread use of ultrahigh-field (31) P MRSI in clinical studies is hindered by the limited field of view and non-uniform radiofrequency (RF) field obtained from surface transceivers. The non-uniform RF field necessitates the use of high specific absorption rate (SAR)-demanding adiabatic RF pulses, limiting the signal-to-noise ratio (SNR) per unit of time. Here, we demonstrate the feasibility of using a body-sized volume RF coil at 7 T, which enables uniform excitation and ultrafast power calibration by pick-up probes. The performance of the body coil is examined by bench tests, and phantom and in vivo measurements in a 7-T MRI scanner. The accuracy of power calibration with pick-up probes is analyzed at a clinical 3-T MR system with a close to identical (1) H body coil integrated at the MR system. Finally, we demonstrate high-quality three-dimensional (31) P MRSI of the human body at 7 T within 5 min of data acquisition that includes RF power calibration. Copyright © 2016 John Wiley & Sons, Ltd.


Asunto(s)
Imagen por Resonancia Magnética/instrumentación , Espectroscopía de Resonancia Magnética/instrumentación , Magnetismo/instrumentación , Imagen Molecular/instrumentación , Compuestos de Fósforo/metabolismo , Isótopos de Fósforo/farmacocinética , Imagen de Cuerpo Entero/instrumentación , Diseño de Equipo , Análisis de Falla de Equipo , Humanos , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , Transductores
9.
NMR Biomed ; 29(9): 1295-304, 2016 09.
Artículo en Inglés | MEDLINE | ID: mdl-25916399

RESUMEN

Thus far, blood flow velocity measurements with MRI have only been feasible in large cerebral blood vessels. High-field-strength MRI may now permit velocity measurements in much smaller arteries. The aim of this proof of principle study was to measure the blood flow velocity and pulsatility of cerebral perforating arteries with 7-T MRI. A two-dimensional (2D), single-slice quantitative flow (Qflow) sequence was used to measure blood flow velocities during the cardiac cycle in perforating arteries in the basal ganglia (BG) and semioval centre (CSO), from which a mean normalised pulsatility index (PI) per region was calculated (n = 6 human subjects, aged 23-29 years). The precision of the measurements was determined by repeated imaging and performance of a Bland-Altman analysis, and confounding effects of partial volume and noise on the measurements were simulated. The median number of arteries included was 14 in CSO and 19 in BG. In CSO, the average velocity per volunteer was in the range 0.5-1.0 cm/s and PI was 0.24-0.39. In BG, the average velocity was in the range 3.9-5.1 cm/s and PI was 0.51-0.62. Between repeated scans, the precision of the average, maximum and minimum velocity per vessel decreased with the size of the arteries, and was relatively low in CSO and BG compared with the M1 segment of the middle cerebral artery. The precision of PI per region was comparable with that of M1. The simulations proved that velocities can be measured in vessels with a diameter of more than 80 µm, but are underestimated as a result of partial volume effects, whilst pulsatility is overestimated. Blood flow velocity and pulsatility in cerebral perforating arteries have been measured directly in vivo for the first time, with moderate to good precision. This may be an interesting metric for the study of haemodynamic changes in aging and cerebral small vessel disease. © 2015 The Authors NMR in Biomedicine Published by John Wiley & Sons Ltd.


Asunto(s)
Velocidad del Flujo Sanguíneo/fisiología , Angiografía Cerebral/métodos , Arterias Cerebrales/fisiología , Circulación Cerebrovascular/fisiología , Aumento de la Imagen/métodos , Angiografía por Resonancia Magnética/métodos , Flujo Pulsátil/fisiología , Adulto , Arterias Cerebrales/anatomía & histología , Femenino , Humanos , Interpretación de Imagen Asistida por Computador/métodos , Campos Magnéticos , Masculino , Dosis de Radiación , Reproducibilidad de los Resultados , Sensibilidad y Especificidad
10.
Neuroradiology ; 58(9): 853-7, 2016 Sep.
Artículo en Inglés | MEDLINE | ID: mdl-27236563

RESUMEN

INTRODUCTION: Cerebellar cortical infarct cavities are a newly recognised entity associated with atherothromboembolic cerebrovascular disease and worse physical functioning. We aimed to investigate the relationship of cerebellar cortical infarct cavities with symptomatic vertebrobasilar ischaemia and with vascular risk factors. METHODS: We evaluated the MR images of 46 patients with a recent vertebrobasilar TIA or stroke and a symptomatic vertebral artery stenosis ≥50 % from the Vertebral Artery Stenting Trial (VAST) for the presence of cerebellar cortical infarct cavities ≤1.5 cm. At inclusion in VAST, data were obtained on age, sex, history of vertebrobasilar TIA or stroke, and vascular risk factors. Adjusted risk ratios were calculated with Poisson regression analyses for the relation between cerebellar cortical infarct cavities and vascular risk factors. RESULTS: Sixteen out of 46 (35 %) patients showed cerebellar cortical infarct cavities on the initial MRI, and only one of these 16 patients was known with a previous vertebrobasilar TIA or stroke. In patients with symptomatic vertebrobasilar ischaemia, risk factor profiles of patients with cerebellar cortical infarct cavities were not different from patients without these cavities. CONCLUSION: Cerebellar cortical infarct cavities are seen on MRI in as much as one third of patients with recently symptomatic vertebral artery stenosis. Since patients usually have no prior history of vertebrobasilar TIA or stroke, cerebellar cortical infarct cavities should be added to the spectrum of common incidental brain infarcts visible on routine MRI.


Asunto(s)
Enfermedades Cerebelosas/epidemiología , Infarto Cerebral/epidemiología , Angiografía por Resonancia Magnética/estadística & datos numéricos , Insuficiencia Vertebrobasilar/epidemiología , Insuficiencia Vertebrobasilar/cirugía , Distribución por Edad , Causalidad , Enfermedades Cerebelosas/diagnóstico por imagen , Enfermedades Cerebelosas/prevención & control , Infarto Cerebral/diagnóstico por imagen , Infarto Cerebral/prevención & control , Comorbilidad , Femenino , Humanos , Hallazgos Incidentales , Masculino , Persona de Mediana Edad , Países Bajos/epidemiología , Prevalencia , Factores de Riesgo , Distribución por Sexo , Stents/estadística & datos numéricos , Resultado del Tratamiento , Insuficiencia Vertebrobasilar/diagnóstico por imagen
11.
NMR Biomed ; 28(4): 514-22, 2015 Apr.
Artículo en Inglés | MEDLINE | ID: mdl-25802216

RESUMEN

Lactate levels are measurable by MRS and are related to neural activity. Therefore, it is of interest to accurately measure lactate levels in the basal ganglia networks. If sufficiently stable, lactate measurements may be used to investigate alterations in dopaminergic signalling in the striatum, facilitating the detection and diagnosis of metabolic deficits. The aim of this study is to provide a J-difference editing MRS technique for the selective editing of lactate only, thus allowing the detection of lactate without contamination of overlapping macromolecules. As a validation procedure, macromolecule nulling was combined with J-difference editing, and this was compared with J-difference editing with a new highly selective editing pulse. The use of a high-field (7T) MR scanner enables the application of editing pulses with very narrow bandwidth, which are selective for lactate. We show that, despite the sensitivity to B0 offsets, the use of a highly selective editing pulse is more efficient for the detection of lactate than the combination of a broad-band editing pulse with macromolecule nulling. Although the signal-to-noise ratio of uncontaminated lactate detection in healthy subjects is relatively low, this article describes the test-retest performance of lactate detection in the striatum when using highly selective J-difference editing MRS at 7 T. The coefficient of variation, σw and intraclass correlation coefficients for within- and between-subject differences of lactate were determined. Lactate levels in the left and right striatum were determined twice in 10 healthy volunteers. Despite the fact that the test-retest performance of lactate detection is moderate with a coefficient of variation of about 20% for lactate, these values can be used for the design of new studies comparing, for example, patient populations with healthy controls.


Asunto(s)
Cuerpo Estriado/química , Ácido Láctico/análisis , Espectroscopía de Resonancia Magnética , Adulto , Ácido Aspártico/análogos & derivados , Ácido Aspártico/análisis , Ganglios Basales/química , Colina/análisis , Creatina/análisis , Humanos , Imagen por Resonancia Magnética , Espectroscopía de Resonancia Magnética/estadística & datos numéricos , Persona de Mediana Edad , Reproducibilidad de los Resultados , Relación Señal-Ruido , Adulto Joven
12.
Mult Scler ; 21(2): 155-62, 2015 Feb.
Artículo en Inglés | MEDLINE | ID: mdl-25013150

RESUMEN

BACKGROUND: Virchow-Robin spaces (VRS) are associated with vascular and neurodegenerative disease. In multiple sclerosis (MS), VRS have been associated with neuroinflammation. Ultra-high field imaging may be used to gain insight in these contradictory findings. OBJECTIVE: The objective of this paper is to analyze VRS in MS patients using high-resolution 7 Tesla (T) MRI. Additionally, we investigated whether the widening of VRS is related to inflammatory or neurodegenerative aspects of MS. METHODS: Thirty-four MS patients and 11 healthy controls were examined at 7T. Number and size of VRS were measured on three-dimensional (3D) T1-weighted images, and 3D fluid-attenuated inversion recovery (FLAIR) images were used for MS lesion detection. Brain atrophy was quantified by computing supratentorial brain volume fraction (sBVF). VRS counts were correlated with clinical variables, lesion count and sBVF. RESULTS: MS patients displayed more VRS (median 11) than healthy controls (median four), p = 0.001. VRS size did not differ between both groups. VRS count in MS patients was associated with sBVF (rho = -0.40, p = 0.02), but not with lesion count (p = 0.22). CONCLUSIONS: The 7T MRI reveals increased numbers of VRS in MS. The finding that VRS are associated with supratentorial brain atrophy, but not with lesion count, suggests that VRS might rather serve as a neurodegenerative than an inflammatory marker in MS.


Asunto(s)
Encéfalo/patología , Imagen por Resonancia Magnética/métodos , Esclerosis Múltiple/patología , Enfermedades Neurodegenerativas/patología , Adulto , Atrofia/patología , Biomarcadores , Femenino , Humanos , Imagen por Resonancia Magnética/instrumentación , Masculino , Persona de Mediana Edad , Esclerosis Múltiple/diagnóstico , Enfermedades Neurodegenerativas/diagnóstico
13.
Breast Cancer Res Treat ; 144(3): 583-9, 2014 Apr.
Artículo en Inglés | MEDLINE | ID: mdl-24570008

RESUMEN

Phosphorus metabolite ratios are potential biomarkers in breast cancer diagnosis and treatment monitoring. Our purpose was to investigate the metabolite ratios phosphomonoester to phosphodiester, phosphoethanolamine (PE) to glycerophosphoethanolamine (GPE), and phosphocholine (PC) to glycerophosphocholine (GPC) in glandular breast tissue, and the potential effect of the menstrual cycle, using (31)P magnetic resonance spectroscopy (MRS) at 7T. Seven women with regular menstrual cycles each underwent four examinations using a 3D (31)P multi-echo magnetic resonance spectroscopic imaging sequence. Peak integrals were assessed using IDL and JMRUI software. First, T2 relaxation times were calculated using multi-echo data pooled across subjects and time points. Subsequent, metabolite ratios were calculated for each phase of the menstrual cycle using the calculated T2 values to account for when combining the free induction decay and all five echoes. The metabolite ratios were calculated both on group level and individually. T2 decay fits resulted in a T2 relaxation time for PE of 154 ms (95 % CI 144-164), for PC of 173 ms (95 % CI 148-205), for Pi of 188 ms (95 % CI 182-193), for GPE of 48 ms (95 % CI 44-53), and for GPC of 23 ms (95 % CI 21-26). The metabolite ratios analyzed on group level showed negligible variation throughout the menstrual cycle. Individual results did show an apparent intra-individual variation; however, not significant due to the measurements' uncertainty. To conclude, phospholipids in glandular tissue as measured with (31)P MRS at 7 T are not significantly affected by the menstrual cycle.


Asunto(s)
Mama/metabolismo , Espectroscopía de Resonancia Magnética/métodos , Ciclo Menstrual/metabolismo , Adulto , Neoplasias de la Mama/diagnóstico , Neoplasias de la Mama/metabolismo , Femenino , Voluntarios Sanos , Humanos , Metabolómica/métodos , Fósforo/metabolismo , Adulto Joven
14.
Magn Reson Med ; 71(4): 1641-9, 2014 Apr.
Artículo en Inglés | MEDLINE | ID: mdl-23788130

RESUMEN

PURPOSE: Magnetic resonance imaging of humans at high magnetic field strengths is strongly influenced by the interference of the radiofrequency (RF) electromagnetic field and the body. To minimize this effect, multiple RF sources could be used. A novel setup (called multimode, coaxial waveguide) is proposed that facilitates RF shimming based on the traveling waves. METHODS: The multimode, coaxial waveguide combines the coaxial waveguide, cylindrical waveguide, high dielectric permittivity lining, and eight radial stub antennas. Each antenna excites multiple waveguide modes. Based on modes orthogonality, a method was devised to decompose an excitation pattern of single stub antenna into waveguide modes. RESULTS: The number of modes present in the excitation pattern of a single stub antenna increased with the higher effective permittivity of the dielectric lining. Thus, RF shimming performance of the setup was improved. An average homogeneity of 10% was demonstrated for a single slice of each principle plane in the human head at 7 T. CONCLUSION: Traveling wave RF shimming is feasible both in axial and longitudinal directions and is improved with an increased amount of orthogonal waveguide modes. Nevertheless, with the currently available RF amplifiers at 7 T, the performance of the setup is limited to low flip angles.


Asunto(s)
Aumento de la Imagen/métodos , Imagen por Resonancia Magnética/instrumentación , Transductores , Imagen de Cuerpo Entero/instrumentación , Diseño de Equipo , Análisis de Falla de Equipo , Campos Magnéticos , Fantasmas de Imagen , Ondas de Radio , Reproducibilidad de los Resultados , Dispersión de Radiación , Sensibilidad y Especificidad
15.
NMR Biomed ; 27(10): 1248-55, 2014 Oct.
Artículo en Inglés | MEDLINE | ID: mdl-25146680

RESUMEN

Here, we show that the sensitivity of (31)P MRSI of (31)P spins J-coupled to protons can be increased by almost a factor of three when compared with an optimal direct detection free induction decay. By direct detection integrated with multi-echo polarization transfer (DIMEPT), multiple signals from polarization transfer and direct detection can be acquired in one repetition time, with minimal mutual interference, provided that the number of refocusing pulses in the multi-echo polarization transfer part is even. The DIMEPT sequence was implemented on a 7-T body scanner and tested on a phantom and on the breasts of five healthy volunteers. The in vivo signal-to-noise ratio (SNR) enhancement for the J-coupled phosphomonoesters was 270% when compared with an Ernst angle pulse-acquire sequence. However, the phosphodiester signals, presumably mainly mobile phospholipids, had T2 values that were too short to be enhanced. Uncoupled (31)P spins, with sufficiently long T2 values, such as inorganic phosphate, were SNR enhanced by a factor of 1.9 relative to an Ernst-angle excitation pulse-acquire sequence by multi-echo direct detection.


Asunto(s)
Mama/química , Espectroscopía de Resonancia Magnética/métodos , Adulto , Algoritmos , Etanolaminas/análisis , Femenino , Glicerilfosforilcolina/análisis , Humanos , Isótopos de Fósforo , Fosforilcolina/análisis , Valores de Referencia , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , Relación Señal-Ruido
16.
NMR Biomed ; 27(11): 1353-60, 2014 Nov.
Artículo en Inglés | MEDLINE | ID: mdl-25212868

RESUMEN

Spectral degradations as a result of temporal field variations are observed in MRSI of the human prostate. Moving organs generate substantial temporal and spatial field fluctuations as a result of susceptibility mismatch with the surrounding tissue (i.e. periodic breathing, cardiac motion or random bowel motion). Nine patients with prostate cancer were scanned with an endorectal coil (ERC) on a 7-T MR scanner. Temporal B0 field variations were observed with fast dynamic B0 mapping in these patients. Simulations of dynamic B0 corrections were performed using zero- to second-order shim terms. In addition, the temporal B0 variations were applied to simulated MR spectra causing, on average, 15% underestimation of the choline/citrate ratio. Linewidth distortions and frequency shifts (up to 30 and 8 Hz, respectively) were observed. To demonstrate the concept of observing local field fluctuations in real time during MRSI data acquisition, a field probe (FP) tuned and matched for the (19) F frequency was incorporated into the housing of the ERC. The data acquired with the FP were compared with the B0 field map data and used to correct the MRSI datasets retrospectively. The dynamic B0 mapping data showed variations of up to 30 Hz (0.1 ppm) over 72 s at 7 T. The simulated zero-order corrections, calculated as the root mean square, reduced the standard deviation (SD) of the dynamic variations by an average of 41%. When using second-order corrections, the reduction in the SD was, on average, 56%. The FP data showed the same variation range as the dynamic B0 data and the variation patterns corresponded. After retrospective correction, the MRSI data showed artifact reduction and improved spectral resolution. B0 variations can degrade the MRSI substantially. The simple incorporation of an FP into an ERC can improve prostate cancer MRSI without prior knowledge of the origin of the dynamic field distortions.


Asunto(s)
Adenocarcinoma/química , Imagen por Resonancia Magnética/métodos , Espectroscopía de Resonancia Magnética/métodos , Próstata/química , Neoplasias de la Próstata/química , Adenocarcinoma/diagnóstico , Adenocarcinoma/patología , Algoritmos , Artefactos , Colina/análisis , Citratos/análisis , Estudios de Factibilidad , Humanos , Imagen por Resonancia Magnética/instrumentación , Espectroscopía de Resonancia Magnética/instrumentación , Masculino , Próstata/patología , Neoplasias de la Próstata/diagnóstico , Neoplasias de la Próstata/patología , Recto , Factores de Tiempo
17.
Magn Reson Med ; 70(3): 885-94, 2013 Sep.
Artículo en Inglés | MEDLINE | ID: mdl-23044511

RESUMEN

Application of travelling wave MR to human body imaging is restricted by the limited peak power of the available RF amplifiers. Nevertheless, travelling wave MR advantages like a large field of view excitation and distant location of transmit elements would be desirable for whole body MRI. In this work, improvement of the B1+ efficiency of travelling wave MR is demonstrated. High permittivity dielectric lining placed next to the scanner bore wall effectively reduces attenuation of the travelling wave in the longitudinal direction and at the same time directs the radial power flow toward the load. First, this is shown with an analytical model of a metallic cylindrical waveguide with the dielectric lining next to the wall and loaded with a cylindrical phantom. Simulations and experiments also reveal an increase of B1+ efficiency in the center of the bore for travelling wave MR with a dielectric lining. Phantom experiments show up to a 2-fold gain in B1+ with the dielectric lining. This corresponds to a 4-fold increase in power efficiency of travelling wave MR. In vivo experiments demonstrate an 8-fold signal-to-noise ratio gain with the dielectric lining. Overall, it is shown that dielectric lining is a constructive method to improve efficacy of travelling wave MR.


Asunto(s)
Imagen por Resonancia Magnética/métodos , Humanos , Modelos Teóricos , Fantasmas de Imagen
18.
Magn Reson Med ; 69(4): 1186-93, 2013 Apr.
Artículo en Inglés | MEDLINE | ID: mdl-22760686

RESUMEN

A setup for 7T MRI of the carotid arteries in the neck was designed and constructed. Separate dedicated arrays were used for transmit and receive. For the transmit array, single-side adapted dipole antennas were mounted on a dielectric pillow, which was shown to serve as a leaky waveguide, efficiently distributing B1 into the neck. Risk assessment was performed by simulations. Phantom measurements were performed to establish optimal positions of the antennas on the pillow. Using two antennas, a dual transmit setup was created. In vivo B1 (+) maps with different shim configurations were acquired to assess transmit performance. This effective transmit array was used in combination with a dedicated 30 channel small element receive coil. High-resolution in vivo turbo spin echo images were acquired to demonstrate the excellent performance of the setup.


Asunto(s)
Arterias Carótidas/anatomía & histología , Aumento de la Imagen/instrumentación , Angiografía por Resonancia Magnética/instrumentación , Transductores , Diseño de Equipo , Análisis de Falla de Equipo , Humanos , Reproducibilidad de los Resultados , Sensibilidad y Especificidad
19.
NMR Biomed ; 26(11): 1596-601, 2013 Nov.
Artículo en Inglés | MEDLINE | ID: mdl-23893556

RESUMEN

A method to measure the T2 relaxation time of GABA with spectral editing techniques is proposed. Spectral editing techniques can be used to unambiguously extract signals of low concentration J-coupled spins such as γ-aminobutyric acid (GABA) from overlapping resonances such as creatine and macromolecules. These sequences, however, generally have fixed and relatively long echo times. Therefore, for the absolute quantification of the edited spectrum, the T2 relaxation time must be taken into account. To measure the T2 relaxation time, the signal intensity has to be obtained at multiple echo times. However, on a coupled spin system such as GABA this is challenging, since the signal intensity of the target resonances is modulated not only by T2 decay but also by the J-coupling, which strongly influences the shapes and amplitudes of the edited signals, depending on the echo time. Here, we propose to refocus the J-modulation of the edited signal at different echo times by using chemical shift selective refocusing. In this way the echo time can be arbitrarily extended while preserving the shape of the edited signal. The method was applied in combination with the MEGA-sLASER editing technique to measure the in vivo T2 relaxation time of GABA (87 ± 11 ms, n = 10) and creatine (109 ± 8 ms, n = 10) at 7 T. The T1 relaxation time of these metabolites in a single subject was also determined (GABA, 1334 ± 158 ms; Cr, 1753 ± 12 ms). The T2 decay curve of coupled spin systems can be sampled in an arbitrary fashion without the need for signal shape correction. Furthermore, the method can be applied with any spectral editing technique. The shortest echo time of the method is limited by the echo time of the spectral editing technique.


Asunto(s)
Imagen por Resonancia Magnética , Ácido gamma-Aminobutírico/metabolismo , Creatina/metabolismo , Humanos , Fantasmas de Imagen , Procesamiento de Señales Asistido por Computador , Factores de Tiempo
20.
NMR Biomed ; 26(10): 1299-307, 2013 Oct.
Artículo en Inglés | MEDLINE | ID: mdl-23553945

RESUMEN

An adiabatic multi-echo spectroscopic imaging (AMESING) sequence, used for (31) P MRSI, with spherical k-space sampling and compensated phase-encoding gradients, was implemented on a whole-body 7-T MR system. One free induction decay (FID) and up to five symmetric echoes can be acquired with this sequence. In tissues with low T2 and high T2 , this can theoretically lead to a potential maximum signal-to-noise ratio (SNR) increase of almost a factor of three, compared with a conventional FID acquisition with Ernst-angle excitation. However, with T2 values being, in practice, ≤400 ms, a maximum enhancement of approximately two compared with low flip Ernst-angle excitation should be feasible. The multi-echo sequence enables the determination of localized T2 values, and was validated with (31) P three-dimensional MRSI on the calf muscle and breast of a healthy volunteer, and subsequently applied in a patient with breast cancer. The T2 values of phosphocreatine, phosphodiesters (PDE) and inorganic phosphate in calf muscle were 193 ± 5 ms, 375 ± 44 ms and 96 ± 10 ms, respectively, and the apparent T2 value of γ-ATP was 25 ± 6 ms. A T2 value of 136 ± 15 ms for inorganic phosphate was measured in glandular breast tissue of a healthy volunteer. The T2 values of phosphomonoesters (PME) and PDE in breast cancer tissue (ductulolobular carcinoma) ranged between 170 and 210 ms, and the PME to PDE ratios were calculated to be phosphoethanolamine/glycerophosphoethanolamine = 2.7, phosphocholine/glycerophosphocholine = 1.8 and PME/PDE = 2.3. Considering the relatively short T2 values of the metabolites in breast tissue at 7 T, the echo spacing can be short without compromising spectral resolution, whilst maximizing the sensitivity.


Asunto(s)
Mama/anatomía & histología , Espectroscopía de Resonancia Magnética , Músculos/anatomía & histología , Fósforo/metabolismo , Adiposidad , Femenino , Humanos , Masculino , Persona de Mediana Edad , Relación Señal-Ruido , Factores de Tiempo , Agua
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