Several methods have been proposed for overcoming the effects of radiofrequency (RF) magnetic field inhomogeneity in high-field MRI. Some of these methods rely at least in part on the ability to independently control magnitude and phase of different drives in either one multielement RF coil or in different RF coils in a transmit array. (1)(2)(3)(4)(5)(6). This is a valuable development, in part because distinct current distributions in the coil or array used for excitation will likely be required to achieve homogeneous B 1 fields on different planes (7,8) or in different subjects (9) in human MRI at increasingly high field strengths. These hardware developments are also fueled in part by the expectation that multicoil tailored RF pulses (coordinating simultaneous application of specially-shaped RF and gradient pulses for spatially-selective excitation), such as those described as "transmit SENSE" (2) will be able to achieve volumeselective homogeneous excitation with shorter pulse durations than their single-coil counterparts (2,10,11). A few other multicoil approaches to achieving homogeneous images besides RF shimming and transmit SENSE, requiring differing amounts of foreknowledge of the RF field and varying degrees of pulse sequence manipulation, have also been proposed and discussed (12)(13)(14).While many of these methods can theoretically produce more homogeneous excitations than RF shimming alone, they can require more foreknowledge of the field distributions (as for designing tailored pulses), longer-duration RF excitations and higher SAR (as for adiabatic and composite pulses), and restrictions on sequence design beyond the excitation portion. Although RF shimming by itself shows much promise, it will also inevitably have limits. While Maxwell's equations may allow for existence of an almost perfectly homogeneous RF magnetic (B 1 ) field distribution on any one plane in an object as large as the human head at frequencies of hundreds of MHz, achieving this may require a large number of excitation sources. And achieving perfect homogeneity throughout an entire volume, even with a very large number of excitation coils, may not be possible due to the constraints of Maxwell's equations (7). Here we simulate a large number (16 -80) of excitation coils using numerical calculations to examine the limits of B 1 field homogeneity that can be achieved by RF shimming alone on various single slices and over the whole brain volume at frequencies from 300 to 600 MHz. MATERIALS AND METHODSA 3D digital human head model was adapted from previous studies (15). It consisted of 23 different tissue types with a 5-mm isotropic resolution. Tissue electrical properties were assigned appropriately at each frequency of interest. Two elliptical, stripline coil arrays (16-element and 80-element) were modeled and driven at 300, 400, 500, and 600 MHz (see Fig. 1). The 16-element array geometry was based on designs from the University of Minnesota (5). Each of the arrays' elements was modeled as a 2 cm wide and 15 cm (for the 16-ele...

Purpose:To investigate the effects of high dielectric material padding on RF field distribution in the human head at 7.0 T, and demonstrate the feasibility and effectiveness of RF passive shimming and focusing with such an approach. Materials and Methods:The intensity distribution changes of gradient-recalled-echo (GRE) and spin-echo (SE) images of a human head acquired with water pads (dielectric constant ϭ 78) placed in specified configurations around the head at 7.0 T were evaluated and compared with computer simulation results using the finite difference time domain (FDTD) method. The contributions to the B 1 field distribution change from the displacement current and conductive current of a given configuration of dielectric padding were determined with computer simulations.Results: MR image intensity distribution in the human head with an RF coil at 7.0 T can be changed drastically by placing water pads around the head. Computer simulations reveal that the high permittivity of water pads results in a strong displacement current that enhances image intensity in the nearby region and alters the intensity distribution of the entire brain. Conclusion:The image intensity distribution in the human head at ultra-high field strengths can be effectively manipulated with high permittivity padding. Utilizing this effect, the B 1 field inside the human head of a given RF coil can be adjusted to reduce the B 1 field inhomogeneity artifact associated with the wave behavior (RF passive shimming) or to locally enhance the signal-to-noise ratio (SNR) in targeted regions of interest (ROIs; RF field focusing). INTERACTIONS BETWEEN THE HUMAN BODY and RF field in ultra-high-field human MRI systems (7.0 -9.4 T) present interesting challenges for RF engineering (1,2). In the corresponding RF field frequency regime, the RF magnetic field inside a human head exhibits prominent wave behavior that destroys the otherwise homogeneous field produced by an unloaded RF coil (3-7). The electrical properties, geometry, and relative position of the sample in the coil become important factors in determining the RF field distribution inside the sample (6). In addition to sample size with respect to the wavelength of the RF field, the high dielectric permittivity (dielectric constant) of human tissues is a key physical parameter that contributes to the wave phenomenon. Consequently, adjustment of RF field distribution inside the sample and the coupling between the sample and coil can be facilitated with high permittivity materials. Studies performed at 3.0 and 4.0 T indicate that the RF field distribution can be altered by changing the coil loading (8,9). With increasing RF field frequency (up to 300 MHz at 7.0 T), this effect is drastically amplified. In this study, experimental results and computer simulations demonstrate that the RF field distribution at 300 MHz in a human head can be drastically altered with water padding. The results provide experimental and theoretical evidence that the RF field inside the human body at high fields can be...

Purpose: To present and discuss numerical calculations of the specific absorption rate (SAR) and temperature in comparison to regulatory limits. While it is possible to monitor whole-body or whole-head average SAR and/or core body temperature during MRI in practice, this is not generally true for local SAR values or local temperatures throughout the body. While methods of calculation for SAR and temperature are constantly being refined, methods for interpreting results of these calculations in light of regulatory limits also warrant discussion. Materials and Methods:Numerical calculations of SAR and temperature for the human head in a volume coil for MRI at several different frequencies are presented.Results: Just as the field pattern changes with the frequency, so do the temperature distribution and the ratio of maximum local SAR (in 1-g or 10-g regions) to whole-head average SAR. In all of the cases studied here this ratio is far greater than that in the regulatory limits, indicating that existing limits on local SAR will be exceeded before limits on whole-body or whole-head average SAR are reached. Conclusion:Calculations indicate that both SAR and temperature distributions vary greatly with B 1 field frequency, that temperature distributions do not always correlate well with SAR distributions, and that regulatory limits on local temperature may not be exceeded as readily as those on local SAR.

A conserved guanine-rich sequence could be a new target for anti–hepatitis C virus drug development.

To understand the various effects that influence actual flip angles, and correct for these effects, it is important to precisely quantify the MRI parameters (such as T 1 , T 2 , and perfusion). In this paper actual flip angle maps are calculated using a conventional gradient-echo (

PfAgo-mediated Nucleic acid Detection (PAND) distinguishes single-nucleotide mutants and accomplishes multiplexed detection by a second round of cleavage.

Metformin has been used to treat type 2 diabetes for over 50 years. Epidemiological, preclinical and clinical studies suggest that metformin treatment reduces cancer incidence in diabetes patients. Due to its potential as an anti-cancer agent and its low cost, metformin has gained intense research interest. Its traditional anti-cancer mechanisms involve both indirect and direct insulin-dependent pathways. Here, we discussed the anti-tumor mechanism of metformin from the aspects of cell metabolism and epigenetic modifications. The effects of metformin on anti-cancer immunity and apoptosis were also described. Understanding these mechanisms will shed lights on application of metformin in clinical trials and development of anti-cancer therapy.

In this study, we have developed a tunable Lloyd-mirror interferometer with two degrees of freedom, in contrast to a traditional system with one degree of freedom. This new Lloyd-mirror interferometer allows an angular rotation of the mirror independently from that of a sample stage, resulting in an increased pattern coverage area with tunable pattern periodicity. Both theoretical and experimental results verify that the tunable characteristic of the modified configuration enhances the nanopatterning capabilities of the Lloyd-mirror interference lithography system especially in achieving greater pattern coverage area for larger pattern periodicities.