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1.
Adv Exp Med Biol ; 1192: 95-115, 2019.
Artigo em Inglês | MEDLINE | ID: mdl-31705491

RESUMO

Cerebral small vessel disease is associated with late-life depression, cognitive impairment, executive dysfunction, distress, and loss of life for older adults. Late-life depression is becoming a substantial public health burden, and a considerable number of older adults presenting to primary care have significant clinical depression. Even though white matter hyperintensities are linked with small vessel disease, white matter hyperintensities are nonspecific to small vessel disease and can co-occur with other brain diseases. Advanced neuroimaging techniques at the ultrahigh field magnetic resonance imaging are enabling improved characterization, identification of cerebral small vessel disease and are elucidating some of the mechanisms that associate small vessel disease with late-life depression.


Assuntos
Envelhecimento , Doenças de Pequenos Vasos Cerebrais/diagnóstico por imagem , Imagem por Ressonância Magnética/métodos , Neuroimagem/métodos , Idoso , Encéfalo , Depressão , Humanos , Substância Branca
2.
Med Sci Sports Exerc ; 51(8): 1684-1691, 2019 08.
Artigo em Inglês | MEDLINE | ID: mdl-30817709

RESUMO

Identifying promoters of cerebral small vein integrity is important to counter vascular contributions to cognitive impairment and dementia. PURPOSE: In this preliminary investigation, the effects of a randomized 24-month physical activity (PA) intervention on changes in cerebral small vein integrity were compared to those of a health education (HE) control. METHODS: Cerebral small vein integrity was measured in 24 older adults (n = 8, PA; n = 16, HE) using ultra-high field MRI before and at the end of the 24-month intervention. Deep medullary veins were defined as straight or tortuous; percent change in straight length, tortuous length, and tortuosity ratio were computed. Microbleed count and white matter hyperintensities were also rated. RESULTS: Accelerometry-based values of PA increased by 17.2% in the PA group but declined by 28.0% in the HE group. The PA group, but not the HE group, had a significant increase in straight vein length from baseline to 24-month follow-up (P = 0.02 and P = 0.21, respectively); the between-group difference in percent change in straight length was significant (increase: median, 93.6%; interquartile range, 112.9 for PA; median, 28.4%; interquartile range, 90.6 for HE; P = 0.07). Between group differences in other markers were nonsignificant. CONCLUSIONS: Increasing PA in late-life may promote cerebral small vein integrity. This should be confirmed in larger studies.


Assuntos
Veias Cerebrais/fisiologia , Exercício/fisiologia , Idoso , Idoso de 80 Anos ou mais , Hemorragia Cerebral/diagnóstico por imagem , Hemorragia Cerebral/fisiopatologia , Veias Cerebrais/diagnóstico por imagem , Veias Cerebrais/patologia , Circulação Cerebrovascular , Disfunção Cognitiva/patologia , Disfunção Cognitiva/fisiopatologia , Disfunção Cognitiva/prevenção & controle , Demência/patologia , Demência/fisiopatologia , Demência/prevenção & controle , Feminino , Educação em Saúde , Humanos , Angiografia por Ressonância Magnética , Masculino , Substância Branca/irrigação sanguínea , Substância Branca/diagnóstico por imagem
3.
PLoS One ; 14(1): e0209663, 2019.
Artigo em Inglês | MEDLINE | ID: mdl-30629618

RESUMO

A variety of 7 Tesla RF coil systems have been proposed to produce spin excitation (B1+ field) and MR image acquisition. Different groups have attempted to mitigate the challenges at high and ultra-high field MRI by proposing novel hardware and software solutions to obtain uniformly high spin excitation at acceptable RF absorption levels. In this study, we extensively compare the designs of two distributed-circuit based RF coils: the Tic-Tac-Toe (TTT) head coil and TEM head coil on multiple anatomically detailed head models and in-vivo. Bench measurements of s-parameters and experimental B1+ field distribution were obtained in volunteers and compared with numerical simulations. RF absorption, quantified by both average and peak SAR, and B1+ field intensity and homogeneity, calculated/measured in terms of maximum over minimum and coefficient of variation (CV) in the region of interest (ROI), are presented for both coils. A study of the RF consistency of both coils across multiple head models for different RF excitation strategies is also presented.


Assuntos
Imagem por Ressonância Magnética/instrumentação , Imagem por Ressonância Magnética/métodos , Simulação por Computador , Desenho de Equipamento , Análise de Falha de Equipamento , Cabeça , Humanos , Aumento da Imagem/métodos , Imagens de Fantasmas , Ondas de Rádio , Software
4.
Artigo em Inglês | MEDLINE | ID: mdl-31892441

RESUMO

OBJECTIVE: We sought to determine whether the aspects of white matter connectivity implicated in major depression also relate to mild depressive symptoms in family dementia caregivers (dCGs). METHODS: Forty-one dCGs (average age=69 years, standard deviation=6.4) underwent a 7 Tesla 64-direction (12-minute) diffusion-weighted imaging sequence. We compared the fractional anisotropy (FA) of 11 white matter features between dCGs with (n=20) and without (n=21) depressive symptoms (Patient Health Questionnaire-9 scores ≥5). RESULTS: Caregivers reporting depression symptoms had lower FA in tracts connecting to the posterior cingulate cortex (Cohen's d = -0.9) and connecting dorsolateral prefrontal with rostral cingulate regions (Cohen's d = -1.2). CONCLUSIONS: Posterior cingulate and dorsolateral prefrontal-to-rostral cingulate white matter, implicated in prior studies of major depression, appear relevant to mild depression in dCGs.

5.
PLoS One ; 13(11): e0206127, 2018.
Artigo em Inglês | MEDLINE | ID: mdl-30481187

RESUMO

Radio-frequency (RF) field inhomogeneities and higher levels of specific absorption rate (SAR) still present great challenges in ultrahigh-field (UHF) MRI. In this study, an in-depth analysis of the eigenmodes of a 20-channel transmit Tic-Tac-Toe (TTT) RF array for 7T neuro MRI is presented. The eigenmodes were calculated for five different Z levels (along the static magnetic field direction) of the coil. Four eigenmodes were obtained for each Z level (composed of 4 excitation ports), and they were named based on the characteristics of their field distributions: quadrature, opposite-phase, anti-quadrature, and zero-phase. Corresponding finite-difference time-domain (FDTD) simulations were performed and experimental B1+ field maps were acquired using a homogeneous spherical phantom and human head (in-vivo). The quadrature mode is the most efficient and it excites the central brain regions; the opposite-phase mode excites the brain peripheral regions; anti-quadrature mode excites the head periphery; and the zero-phase mode excites cerebellum and temporal lobes. Using this RF array, up to five eigenmodes (from five different Z levels) can be simultaneously excited. The superposition of these modes has the potential to produce homogeneous excitation with full brain coverage and low levels of SAR at 7T MRI.


Assuntos
Cerebelo/diagnóstico por imagem , Simulação por Computador , Imagem por Ressonância Magnética/métodos , Lobo Temporal/diagnóstico por imagem , Cerebelo/efeitos da radiação , Campos Eletromagnéticos , Cabeça/diagnóstico por imagem , Cabeça/efeitos da radiação , Humanos , Campos Magnéticos , Cavidade Nasal/diagnóstico por imagem , Cavidade Nasal/efeitos da radiação , Imagens de Fantasmas , Ondas de Rádio , Lobo Temporal/efeitos da radiação
6.
Am J Geriatr Psychiatry ; 26(6): 690-699, 2018 06.
Artigo em Inglês | MEDLINE | ID: mdl-29628321

RESUMO

OBJECTIVES: Hippocampal hyperactivation marks preclinical dementia pathophysiology, potentially due to differences in the connectivity of specific medial temporal lobe structures. Our aims were to characterize the resting-state functional connectivity of medial temporal lobe sub-structures in older adults, and evaluate whether specific substructural (rather than global) functional connectivity relates to memory function. METHODS: In 15 adults (mean age: 69 years), we evaluated the resting state functional connectivity of medial temporal lobe substructures: dentate/Cornu Ammonis (CA) 4, CA1, CA2/3, subiculum, the molecular layer, entorhinal cortex, and parahippocampus. We used 7-Tesla susceptibility weighted imaging and magnetization-prepared rapid gradient echo sequences to segment substructures of the hippocampus, which were used as structural seeds for examining functional connectivity in a resting BOLD sequence. We then assessed correlations between functional connectivity with memory performance (short and long delay free recall on the California Verbal Learning Test [CVLT]). RESULTS: All the seed regions had significant connectivity within the temporal lobe (including the fusiform, temporal, and lingual gyri). The left CA1 was the only seed with significant functional connectivity to the amygdala. The left entorhinal cortex was the only seed to have significant functional connectivity with frontal cortex (anterior cingulate and superior frontal gyrus). Only higher left dentate-left lingual connectivity was associated with poorer CVLT performance (Spearman r = -0.81, p = 0.0003, Benjamini-Hochberg false discovery rate: 0.01) after multiple comparison correction. CONCLUSIONS: Rather than global hyper-connectivity of the medial temporal lobe, left dentate-lingual connectivity may provide a specific assay of medial temporal lobe hyper-connectivity relevant to memory in aging.


Assuntos
Mapeamento Encefálico , Hipocampo/fisiopatologia , Memória/fisiologia , Idoso , Feminino , Humanos , Imagem por Ressonância Magnética , Masculino , Rememoração Mental/fisiologia , Vias Neurais/fisiopatologia
7.
8.
Magn Reson Imaging ; 45: 1-6, 2018 01.
Artigo em Inglês | MEDLINE | ID: mdl-28893660

RESUMO

A four-channel Tic-Tac-Toe (TTT) transmit RF coil was designed and constructed for foot and ankle imaging at 7T MRI. Numerical simulations using an in-house developed FDTD package and experimental analyses using a homogenous phantom show an excellent agreement in terms of B1+ field distribution and s-parameters. Simulations performed on an anatomically detailed human lower leg model demonstrated an B1+ field distribution with a coefficient of variation (CV) of 23.9%/15.6%/28.8% and average B1+ of 0.33µT/0.56µT/0.43µT for 1W input power (i.e., 0.25W per channel) in the ankle/calcaneus/mid foot respectively. In-vivo B1+ mapping shows an average B1+ of 0.29µT over the entire foot/ankle. This newly developed RF coil also presents acceptable levels of average SAR (0.07W/kg for 10g per 1W of input power) and peak SAR (0.34W/kg for 10g per 1W of input power) over the whole lower leg. Preliminary in-vivo images in the foot/ankle were acquired using the T2-DESS MRI sequence without the use of a dedicated receive-only array.


Assuntos
Pé/anatomia & histologia , Imagem por Ressonância Magnética/métodos , Imagens de Fantasmas , Tornozelo/anatomia & histologia , Tornozelo/diagnóstico por imagem , Desenho de Equipamento , Pé/diagnóstico por imagem , Humanos , Modelos Biológicos , Ondas de Rádio
9.
PLoS One ; 12(8): e0183168, 2017.
Artigo em Inglês | MEDLINE | ID: mdl-28806768

RESUMO

OBJECTIVE: The purpose of this study is to design an anthropomorphic heterogeneous head phantom that can be used for MRI and other electromagnetic applications. MATERIALS AND METHODS: An eight compartment, physical anthropomorphic head phantom was developed from a 3T MRI dataset of a healthy male. The designed phantom was successfully built and preliminarily evaluated through an application that involves electromagnetic-tissue interactions: MRI (due to it being an available resource). The developed phantom was filled with media possessing electromagnetic constitutive parameters that correspond to biological tissues at ~297 MHz. A preliminary comparison between an in-vivo human volunteer (based on whom the anthropomorphic head phantom was created) and various phantoms types, one being the anthropomorphic heterogeneous head phantom, were performed using a 7 Tesla human MRI scanner. RESULTS: Echo planar imaging was performed and minimal ghosting and fluctuations were observed using the proposed anthropomorphic phantom. The magnetic field distributions (during MRI experiments at 7 Tesla) and the scattering parameter (measured using a network analyzer) were most comparable between the anthropomorphic heterogeneous head phantom and an in-vivo human volunteer. CONCLUSION: The developed anthropomorphic heterogeneous head phantom can be used as a resource to various researchers in applications that involve electromagnetic-biological tissue interactions such as MRI.


Assuntos
Imagem por Ressonância Magnética/métodos , Imagens de Fantasmas , Impressão Tridimensional/instrumentação , Imagem Ecoplanar , Desenho de Equipamento , Cabeça , Humanos , Processamento de Imagem Assistida por Computador
10.
PLoS One ; 12(6): e0175629, 2017.
Artigo em Inglês | MEDLINE | ID: mdl-28662061

RESUMO

OBJECTIVE: The purpose of this study was to explore the efficacy of Ultra-high field [UHF] 7 Tesla [T] MRI as compared to 3T MRI in non-contrast enhanced [nCE] imaging of structural anatomy in the elbow, forearm, and hand [upper extremity]. MATERIALS AND METHOD: A wide range of sequences including T1 weighted [T1] volumetric interpolate breath-hold exam [VIBE], T2 weighted [T2] double-echo steady state [DESS], susceptibility weighted imaging [SWI], time-of-flight [TOF], diffusion tensor imaging [DTI], and diffusion spectrum imaging [DSI] were optimized and incorporated with a radiofrequency [RF] coil system composed of a transverse electromagnetic [TEM] transmit coil combined with an 8-channel receive-only array for 7T upper extremity [UE] imaging. In addition, Siemens optimized protocol/sequences were used on a 3T scanner and the resulting images from T1 VIBE and T2 DESS were compared to that obtained at 7T qualitatively and quantitatively [SWI was only qualitatively compared]. DSI studio was utilized to identify nerves based on analysis of diffusion weighted derived fractional anisotropy images. Images of forearm vasculature were extracted using a paint grow manual segmentation method based on MIPAV [Medical Image Processing, Analysis, and Visualization]. RESULTS: High resolution and high quality signal-to-noise ratio [SNR] and contrast-to-noise ratio [CNR]-images of the hand, forearm, and elbow were acquired with nearly homogeneous 7T excitation. Measured [performed on the T1 VIBE and T2 DESS sequences] SNR and CNR values were almost doubled at 7T vs. 3T. Cartilage, synovial fluid and tendon structures could be seen with higher clarity in the 7T T1 and T2 weighted images. SWI allowed high resolution and better quality imaging of large and medium sized arteries and veins, capillary networks and arteriovenous anastomoses at 7T when compared to 3T. 7T diffusion weighted sequence [not performed at 3T] demonstrates that the forearm nerves are clearly delineated by fiber tractography. The proper digital palmar arteries and superficial palmar arch could also be clearly visualized using TOF nCE 7T MRI. CONCLUSION: Ultra-high resolution neurovascular imaging in upper extremities is possible at 7T without use of renal toxic intravenous contrast. 7T MRI can provide superior peripheral nerve [based on fiber anisotropy and diffusion coefficient parameters derived from diffusion tensor/spectrum imaging] and vascular [nCE MRA and vessel segmentation] imaging.


Assuntos
Imagem de Tensor de Difusão/métodos , Nervos Periféricos/diagnóstico por imagem , Feminino , Humanos , Masculino
11.
NMR Biomed ; 30(1)2017 01.
Artigo em Inglês | MEDLINE | ID: mdl-27859861

RESUMO

In ultrahigh-field MRI, such as 7 T, the signal-to-noise ratio (SNR) increases while transmit (Tx) field (B1+ ) can be degraded due to inhomogeneity and elevated specific absorption rate (SAR). By applying new array coil concepts to both Tx and receive (Rx) coils, the B1+ homogeneity and SNR can be improved. In this study, we developed and tested in vivo a new RF coil system for 7 T breast MRI. An RF coil system composed of an eight-channel Tx-only array based on a tic-tac-toe design (can be combined to operate in single-Tx mode) in conjunction with an eight-channel Rx-only insert was developed. Characterizations of the B1+ field and associated SAR generated by the developed RF coil system were numerically calculated and empirically measured using an anatomically detailed breast model, phantom and human breasts. In vivo comparisons between 3 T (using standard commercial solutions) and 7 T (using the newly developed coil system) breast imaging were made. At 7 T, about 20% B1+ inhomogeneity (standard deviation over the mean) was measured within the breast tissue for both the RF simulations and 7 T experiments. The addition of the Rx-only array enhances the SNR by a factor of about three. High-quality MR images of human breast were acquired in vivo at 7 T. For the in vivo comparisons between 3 T and 7 T, an approximately fourfold increase of SNR was measured with 7 T imaging. The B1+ field distributions in the breast model, phantom and in vivo were in reasonable agreement. High-quality 7 T in vivo breast MRI was successfully acquired at 0.6 mm isotropic resolution using the newly developed RF coil system.


Assuntos
Mama/anatomia & histologia , Mama/diagnóstico por imagem , Aumento da Imagem/instrumentação , Imagem por Ressonância Magnética/instrumentação , Magnetismo/instrumentação , Transdutores , Adulto , Desenho de Equipamento , Análise de Falha de Equipamento , Feminino , Humanos , Aumento da Imagem/métodos , Interpretação de Imagem Assistida por Computador/instrumentação , Interpretação de Imagem Assistida por Computador/métodos , Campos Magnéticos , Imagens de Fantasmas , Doses de Radiação , Reprodutibilidade dos Testes , Sensibilidade e Especificidade
12.
Am J Geriatr Psychiatry ; 25(3): 209-217, 2017 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-27986412

RESUMO

BACKGROUND: Greater hippocampal volume is observed in healthy older adults after short-term structured exercise. Whether long-term exposure to real-world physical activity (PA) programs has similar effects for sedentary older adults with impaired mobility and comorbid conditions is not known. HYPOTHESIS: A long-term moderate intensity regimen of PA is related to larger volume of the hippocampus in older adults at risk for mobility disability. We further explore whether these associations are modified by factors known to be related to dementia. METHODS: Twenty-six sedentary adults at risk for mobility disability participated in a 24-month randomized intervention program of physical activity (PA, N = 10, age: 74.9 years, 7 women) or health education (HE, N = 16, age: 76.8 years, 14 women). Volumes of total hippocampus, dentate gyrus, and cornu ammonis were measured at baseline and at 24-month follow-up using 7-Tesla magnetic resonance imaging. Between-group volumetric differences at 24 months were adjusted for sessions attended and baseline volumes. The contribution of each dementia-related factor was tested separately for education, APOE, diabetes, cardiovascular diseases, white matter hyperintensities, and brain atrophy. RESULTS: Between-group differences were significant for left hippocampus, left cornu ammonis, and right hippocampus. Adjustment for regional baseline volume attenuated the associations to statistically nonsignificant for right hippocampus and left conru ammonis; associations for left hippocampus were robust for all adjustments. Results were similar after adjustment for dementia-related factors. CONCLUSIONS: In this group of sedentary older adults there was a hippocampal response to a long-term program of moderate-intensity PA. Future studies should examine whether hippocampal response could explain the beneficial effects of PA on cognition for vulnerable older adults.


Assuntos
Terapia por Exercício/métodos , Exercício/fisiologia , Hipocampo/diagnóstico por imagem , Comportamento Sedentário , Idoso , Idoso de 80 Anos ou mais , Feminino , Educação em Saúde/métodos , Humanos , Imagem por Ressonância Magnética , Masculino , Fatores de Tempo
13.
NMR Biomed ; 29(12): 1768-1779, 2016 12.
Artigo em Inglês | MEDLINE | ID: mdl-27809383

RESUMO

The purpose of this study is to develop and evaluate a custom-designed 7  T MRI coil and explore its use for upper extremity applications. An RF system composed of a transverse electromagnetic transmit coil and an eight-channel receive-only array was developed for 7  T upper extremity applications. The RF system was characterized and evaluated using scattering parameters and B1+ mapping. Finite difference time domain simulations were performed to evaluate the B1+ field distribution and specific absorption rate for the forearm region of the upper extremity. High-resolution 7  T images were acquired and compared with those at 3 T. The simulation and experimental results show very good B1+ field homogeneity across the forearm. High-resolution images of musculotendinous, osseocartilaginous, and neurovascular structures in the upper extremity are presented with T1 volumetric interpolated breath-hold examination, T2 double-echo steady state, T2 * susceptibility weighted imaging (SWI), diffusion tensor imaging, and time-of-flight sequences. Comparison between 3  T and 7  T is shown. Intricate contextual anatomy can be delineated in synovial, fibrocartilaginous, interosseous, and intraosseous trabecular structures of the forearm, as well as palmar and digital vascular anatomy (including microvascular detail in SWI). Ultra-high-field 7  T imaging holds great potential in improving the sensitivity and specificity of upper extremity imaging, especially in wrist and hand pathology secondary to bone, ligament, nerve, vascular, and other soft or hard tissue etiology.


Assuntos
Aumento da Imagem/instrumentação , Imagem por Ressonância Magnética/instrumentação , Magnetismo/instrumentação , Transdutores , Extremidade Superior/anatomia & histologia , Projeto Auxiliado por Computador , Desenho de Equipamento , Análise de Falha de Equipamento , Humanos , Reprodutibilidade dos Testes , Sensibilidade e Especificidade
14.
J Magn Reson ; 269: 55-64, 2016 08.
Artigo em Inglês | MEDLINE | ID: mdl-27240143

RESUMO

Developing a radiofrequency (RF) coil system that produces a uniform B1(+) field (circularly polarized component of the transverse magnetic field responsible for excitation) and low specific absorption rate (SAR) is critical for high performance ultrahigh field human imaging. In this study, we provide the design of a new eight channel radiofrequency (RF) transmit (Tx) array for breast MRI at 7T. A numerical analysis utilizing an in-house finite difference time domain (FDTD) package was carried out in (1) four breast models, (2) homogeneous spherical model and (3) full body model to calculate the B1(+) intensity (µT) and homogeneity represented by coefficient of variation (CoV=standard deviation/mean) in the proposed RF array design. The numerical results were compared with that measured in breast phantom (Bphantom) and homogeneous spherical phantom at 7T MRI and showed very good agreement. Average and peak SARs were also calculated in the four breast models and the temperature rises due to the operation of the RF array were also measured in the Bphantom. The proposed RF array; which can operate in a single or multi transmit modes, demonstrates homogeneous RF field excitation with acceptable local/average SAR levels for breast MRI at 7T.


Assuntos
Mama/diagnóstico por imagem , Imagem por Ressonância Magnética , Humanos , Campos Magnéticos , Imagens de Fantasmas , Ondas de Rádio
15.
Psychiatry Res ; 233(1): 18-22, 2015 Jul 30.
Artigo em Inglês | MEDLINE | ID: mdl-26002434

RESUMO

Sickle cell anaemia (SCA) is a hereditary hemoglobinopathy characterised by extensive vascular dysfunction that stems from inflammation, thrombosis and occlusion of post-capillary venules. Cognitive impairment is a neurological complication of SCA whose pathogenesis is unknown. We hypothesised that cerebral venular abnormalities are linked to cognitive impairment in SCA. Thus, we employed 7T magnetic resonance imaging (MRI) to examine the association between venular density and cognitive function in homozygous SCA. We quantified the density of total, long, and short venules in pre-defined regions of interest between the frontal and occipital cornu on each hemisphere. Cognitive function was assessed using the Hopkins Verbal Learning Test - Revised (HVLT-R) test of learning and memory. Patients (n=11) were compared with race, age and gender-equated controls (n=7). Compared to controls, patients had an overall venular rarefaction, with significantly lower density of long venules and greater density of short venules which was inversely related to HVLT-R performance and haemoglobin. To our knowledge, this is the first 7T MRI study in SCA and first report of associations between cerebral venular patterns and cognitive performance and haemoglobin. Future studies should examine whether these novel neuroimaging markers predict cognitive impairment longitudinally and are mechanistically linked to severity of anaemia.


Assuntos
Anemia Falciforme/patologia , Encéfalo/patologia , Transtornos Cognitivos/patologia , Hemoglobinas/análise , Memória/fisiologia , Vênulas/patologia , Adulto , Anemia Falciforme/complicações , Anemia Falciforme/psicologia , Encéfalo/irrigação sanguínea , Cognição/fisiologia , Transtornos Cognitivos/etiologia , Transtornos Cognitivos/psicologia , Feminino , Humanos , Imagem por Ressonância Magnética , Masculino , Pessoa de Meia-Idade , Testes Neuropsicológicos , Aprendizagem Verbal/fisiologia , Adulto Jovem
16.
Magn Reson Med ; 74(5): 1461-9, 2015 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-25367703

RESUMO

PURPOSE: To optimize the design of radiofrequency (RF) shielding of transmit coils at 7T and reduce eddy currents generated on the RF shielding when imaging with rapid gradient waveforms. METHODS: One set of a four-element, 2 × 2 Tic-Tac-Toe head coil structure was selected and constructed to study eddy currents on the RF coil shielding. The generated eddy currents were quantitatively studied in the time and frequency domains. The RF characteristics were studied using the finite difference time domain method. Five different kinds of RF shielding were tested on a 7T MRI scanner with phantoms and in vivo human subjects. RESULTS: The eddy current simulation method was verified by the measurement results. Eddy currents induced by solid/intact and simple-structured slotted RF shielding significantly distorted the gradient fields. Echo-planar images, B1+ maps, and S matrix measurements verified that the proposed slot pattern suppressed the eddy currents while maintaining the RF characteristics of the transmit coil. CONCLUSION: The presented dual-optimization method could be used to design RF shielding and reduce the gradient field-induced eddy currents while maintaining the RF characteristics of the transmit coil.


Assuntos
Imagem por Ressonância Magnética/instrumentação , Imagem por Ressonância Magnética/métodos , Encéfalo/anatomia & histologia , Simulação por Computador , Cobre , Campos Eletromagnéticos , Desenho de Equipamento , Humanos , Imagens de Fantasmas , Ondas de Rádio
17.
PLoS One ; 9(7): e103945, 2014.
Artigo em Inglês | MEDLINE | ID: mdl-25079941

RESUMO

Implantable Brain Computer Interfaces (BCIs) are designed to provide real-time control signals for prosthetic devices, study brain function, and/or restore sensory information lost as a result of injury or disease. Using Radio Frequency (RF) to wirelessly power a BCI could widely extend the number of applications and increase chronic in-vivo viability. However, due to the limited size and the electromagnetic loss of human brain tissues, implanted miniaturized antennas suffer low radiation efficiency. This work presents simulations, analysis and designs of implanted antennas for a wireless implantable RF-powered brain computer interface application. The results show that thin (on the order of 100 micrometers thickness) biocompatible insulating layers can significantly impact the antenna performance. The proper selection of the dielectric properties of the biocompatible insulating layers and the implantation position inside human brain tissues can facilitate efficient RF power reception by the implanted antenna. While the results show that the effects of the human head shape on implanted antenna performance is somewhat negligible, the constitutive properties of the brain tissues surrounding the implanted antenna can significantly impact the electrical characteristics (input impedance, and operational frequency) of the implanted antenna. Three miniaturized antenna designs are simulated and demonstrate that maximum RF power of up to 1.8 milli-Watts can be received at 2 GHz when the antenna implanted around the dura, without violating the Specific Absorption Rate (SAR) limits.


Assuntos
Interfaces Cérebro-Computador , Humanos , Implantes Experimentais , Imagens de Fantasmas , Tecnologia sem Fio
18.
J Magn Reson Imaging ; 39(2): 475-84, 2014 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-23913474

RESUMO

PURPOSE: To evaluate the effect of different cylindrical and close-conforming receive-only array designs on spin excitation and specific absorption rate (SAR) of a 7 T transmit-only head coil. MATERIALS AND METHODS: We developed finite difference time domain (FDTD) models of different receive-only array geometries. Cylindrical and close-fitting helmet arrays with varying copper trace widths, a TEM Tx coil model, and two head models were used in numerical simulations. Tx coil coupling was experimentally measured and validated with FDTD modeling. RESULTS: Changing copper trace width of loops in array models caused subtle changes in radiofrequency (RF) absorption (<5%). Changes in SAR distribution were observed in the head models with Rx-only inserts. Peak SAR increased (-1 to +15%) in different tissues for a mean B1 (+) in the brain of 2 µT. Total absorption in the head models for 1 W forward power increased (5%-21%) in the heads with Rx-only inserts. Changes in RF absorption with different Rx inserts indicate a change in RF radiation of the Tx coil even when changes in B1 (+) and coupling between ports of Tx coil were minimal. CONCLUSION: Changes in local/global SAR and subtle changes in B1 (+) field distributions were observed in the presence of Rx-only inserts. Thus, incorporation of the receive-only array effects are needed when evaluating SAR and designing RF transmit pulse waveform parameters for shimming and/or Tx-SENSE for 7 T MRI.


Assuntos
Encéfalo/anatomia & histologia , Aumento da Imagem/instrumentação , Imagem por Ressonância Magnética/instrumentação , Magnetismo/instrumentação , Modelos Biológicos , Simulação por Computador , Projeto Auxiliado por Computador , Desenho de Equipamento , Análise de Falha de Equipamento , Humanos , Aumento da Imagem/métodos , Imagem por Ressonância Magnética/métodos , Imagens de Fantasmas , Reprodutibilidade dos Testes , Sensibilidade e Especificidade
19.
PLoS One ; 8(11): e77759, 2013.
Artigo em Inglês | MEDLINE | ID: mdl-24223123

RESUMO

Implantable neural interfaces are designed to provide a high spatial and temporal precision control signal implementing high degree of freedom real-time prosthetic systems. The development of a Radio Frequency (RF) wireless neural interface has the potential to expand the number of applications as well as extend the robustness and longevity compared to wired neural interfaces. However, it is well known that RF signal is absorbed by the body and can result in tissue heating. In this work, numerical studies with analytical validations are performed to provide an assessment of power, heating and specific absorption rate (SAR) associated with the wireless RF transmitting within the human head. The receiving antenna on the neural interface is designed with different geometries and modeled at a range of implanted depths within the brain in order to estimate the maximum receiving power without violating SAR and tissue temperature elevation safety regulations. Based on the size of the designed antenna, sets of frequencies between 1 GHz to 4 GHz have been investigated. As expected the simulations demonstrate that longer receiving antennas (dipole) and lower working frequencies result in greater power availability prior to violating SAR regulations. For a 15 mm dipole antenna operating at 1.24 GHz on the surface of the brain, 730 uW of power could be harvested at the Federal Communications Commission (FCC) SAR violation limit. At approximately 5 cm inside the head, this same antenna would receive 190 uW of power prior to violating SAR regulations. Finally, the 3-D bio-heat simulation results show that for all evaluated antennas and frequency combinations we reach FCC SAR limits well before 1 °C. It is clear that powering neural interfaces via RF is possible, but ultra-low power circuit designs combined with advanced simulation will be required to develop a functional antenna that meets all system requirements.


Assuntos
Interfaces Cérebro-Computador , Próteses e Implantes , Absorção , Algoritmos , Encéfalo/fisiopatologia , Humanos , Modelos Biológicos , Ondas de Rádio , Temperatura Ambiente
20.
J Magn Reson ; 228: 37-44, 2013 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-23348046

RESUMO

T(2)(∗) weighted fMRI at high and ultra high field (UHF) is often hampered by susceptibility-induced, through-plane, signal loss. Three-dimensional tailored RF (3DTRF) pulses have been shown to be an effective approach for mitigating through-plane signal loss at UHF. However, the required RF pulse lengths are too long for practical applications. Recently, parallel transmission (PTX) has emerged as a very effective means for shortening the RF pulse duration for 3DTRF without sacrificing the excitation performance. In this article, we demonstrate a RF pulse design strategy for 3DTRF based on the use of multi-slice PTX 3DTRF to simultaneously and precisely recover signal with whole-brain coverage. Phantom and human experiments are used to demonstrate the effectiveness and robustness of the proposed method on three subjects using an eight-channel whole body parallel transmission system.


Assuntos
Mapeamento Encefálico/métodos , Imagem Tridimensional/métodos , Imagem por Ressonância Magnética/métodos , Imagem Ecoplanar , Humanos , Aumento da Imagem/métodos , Interpretação de Imagem Assistida por Computador/métodos , Imagens de Fantasmas , Ondas de Rádio , Processamento de Sinais Assistido por Computador
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