In Vivo Imaging of Venous Side Cerebral Small-Vessel Disease in Older Adults: An MRI Method at 7T.

BACKGROUND AND PURPOSE: Traditional neuroimaging markers of small-vessel disease focus on late-stage changes. We aimed to adapt a method of venular assessment at 7T for use in older adults. We hypothesized that poorer venular morphologic characteristics would be related to other small-vessel disease neuroimaging markers and a higher prevalence of small-vessel disease-Alzheimer disease risk factors. MATERIALS AND METHODS: Venules were identified in periventricular ROIs on SWI and defined as tortuous or straight. The tortuosity ratio was defined as total tortuous venular length divided by total straight venular length. White matter hyperintensity burden (visually rated from 0 to 3) and the number of microbleeds (0, 1, >1) were determined. Differences in tortuous and straight venular lengths were evaluated. Relationships with demographic variables, allele producing the e4 type of apolipoprotein E (APOE4), growth factors, pulse pressure, physical activity, and Modified Mini-Mental State Examination were assessed via Spearman correlations. RESULTS: Participants had 42% more tortuous venular tissue than straight (median, 1.42; 95% CI, 1.13-1.62). APOE4 presence was associated with a greater tortuosity ratio (ρ = 0.454, P = .001), and these results were robust to adjustment for confounders and multiple comparisons. Associations of the tortuosity ratio with sex and vascular endothelial growth factor did not survive adjustment. Associations of the tortuosity ratio with other variables of interest were not significant. CONCLUSIONS: Morphologic measures of venules at 7T could be useful biomarkers of the early stages of small-vessel disease and Alzheimer disease. Longitudinal studies should examine the impact of apolipoprotein E and vascular endothelial growth factor on the risk of venular damage.

Analysis of B1 field profiles and SAR values for multi-strut transverse electromagnetic RF coils in high field MRI applications.

In this work, the B1 field homogeneity and specific absorption rate (SAR) values were evaluated for three high-frequency (340 MHz) radio frequency coils designed for use in human magnetic resonance imaging at 8 tesla. Eight-, 16-, and 24-strut transverse electromagnetic (TEM) resonators were examined both experimentally and with the finite difference time domain numerical method. It was observed that increasing the number of TEM elements acted to lower the maximal achievable frequency of the coil and to increase the experimental complexities associated with tuning and matching. In addition, it is demonstrated from experiment and numerical analysis that the circularly polarized component of the B1 (B1+) field homogeneity in the head improved most from 8- to 16-strut coils. Numerical analysis revealed little difference in terms of SAR distribution between these coils; however, stronger tissue/coil coupling and consequently higher SAR peak values were obtained for the 8-strut case.

Dielectric resonances and B(1) field inhomogeneity in UHFMRI: computational analysis and experimental findings.

B(1) Field inhomogeneity and the relative effects of dielectric resonances are analyzed within the context of ultra high field MRI. This is accomplished by calculating the electromagnetic fields inside spherical phantoms and within a human head model in the presence and absence of an RF coil. These calculations are then compared to gradient echo and RARE images, respectively. For the spherical phantoms, plane incident wave analyses are initially presented followed by full wave finite difference time domain (FDTD) calculations. The FDTD methods are then utilized to examine the electromagnetic interactions between the TEM resonator and an anatomically detailed human head model. The results at 340 MHz reveal that dielectric resonances are most strongly excited in objects similar in size to the human head when the conducting medium has a high dielectric constant and a low conductivity. It is concluded that in clinical UFHMRI, the most important determinants of B(1) field homogeneity consist of 1) the RF coil design, 2) the interaction between the RF coil, the excitation source and the sample, and finally 3) the geometry and electrical properties of the sample.

B1 field homogeneity and SAR calculations for the birdcage coil.

The finite-difference time-domain (FDTD) method is used to model a birdcage resonator. All the coil components, including the wires, lumped capacitors and the source, are geometrically modelled together. As such, the coupling effects within the birdcage, including the interactions of coil, source and human head, are accurately computed. A study of the transverse magnetic (B1) field homogeneity and the specific absorption rate (SAR) is presented on an anatomically detailed human head model at 64 and 200 MHz representing 1.5 and 4.7 T MRI systems respectively. Unlike that at 64 MHz, the B1 field distribution is found to be inhomogeneous at 200 MHz. Also, high local SAR values are observed in the tissue near the source due to the coupling between the source and the head at 200 MHz.

Effect of RF coil excitation on field inhomogeneity at ultra high fields: a field optimized TEM resonator.

In this work, computational methods were utilized to optimize the field produced by the transverse electromagnetic (TEM) resonator in the presence of the human head at 8 Tesla. Optimization was achieved through the use of the classical finite difference time domain (FDTD) method and a TEM resonator loaded with an anatomically detailed human head model with a resolution of 2 mm x 2 mm x 2 mm. The head model was developed from 3D MR images. To account for the electromagnetic interactions between the coil and the tissue, the coil and the head were treated as a single system at all the steps of the model including, numerical tuning and excitation. In addition to 2, 3, 4, 6, and 10-port excitations, an antenna array concept was utilized by driving all the possible ports (24) of a 24-strut TEM resonator. The results show that significant improvement in the circularly polarized component of the transverse magnetic field could be obtained when using multiple ports and variable phase and fixed magnitude, or variable phase and variable magnitude excitations.

Computational analysis of the high pass birdcage resonator: finite difference time domain simulations for high-field MRI.

In this work, a finite difference time domain (FDTD) algorithm is validated at 1.5 tesla using the standard GE Signa quadrature head coil and a muscle phantom. The electrical characteristics of the birdcage head coil are then calculated for the linear and quadrature cases. Unlike previous computational analysis which assume idealized currents on the end rings and the struts of the resonator, a complete computational analysis is provided. This treatment considers the coupling between the resonator and the sample and includes a real coil excitation, a complete current derivation, and a thorough description of both B(1) fields and RF radiation. With this improvement, electromagnetic phenomena such as radiation, standing wave currents on the wires, and field inhomogeneities due to interactions between the coil and the load inside the coil are observed. At 200 MHz, it is demonstrated that this particular coil does not work well due to radiation and non-uniformities on the struts of the device. Also, at this frequency magnetic field inhomogeneities become large when the coil is loaded with a phantom.

Application of finite difference time domain method for the design of birdcage RF head coils using multi-port excitations.

A three-dimensional finite difference time domain model was developed where the high pass birdcage coil and the imaged object are analysed as a single unit. A study was performed comparing linear, conventional quadrature, and four-port excitation at 64 MHz and 200 MHz for different coil loadings, namely muscle phantoms and an anatomically detailed human head model. A phase array concept was utilized to excite the birdcage coil in four ports. Two phase conditions were analyzed, the simple fixed phase and the variable phase. At 200 MHz, compared to the conventional quadrature drive, the four-port drive reduces the effects of the tissue-coil interactions leading to more uniform currents on the coil legs and consequently to a better B(1) field homogeneity. Also at 200 MHz, driving the coil in four ports provides an SAR distribution with peak values that are significantly less than those with linear or quadrature excitations.