The use of high‐permittivity materials has been shown to be an effective method for increasing transmit and receive sensitivity in areas of low‐signal intensity in the brain at high field. Results in this article show that the use of these materials does not increase the intercoil coupling for a phased array receive coil, does not have any detrimental effects on the B₀ homogeneity within the brain, and does not affect the specific absorption rate distribution within the head. Areas of the brain close to the pads exhibit significant increases (>100%) in transmit field efficiency, but areas further away show a less pronounced (∼10%) decrease due to the homogenization of the transmit field and the loss introduced by the dielectric pads. Magn Reson Med, 2012.

High permittivity “dielectric pads” have been shown to increase image quality at high magnetic fields in regions of low radiofrequency transmit efficiency. This article presents a series of electromagnetic simulations to determine the effects of pad size and geometry, relative permittivity value, as well as thickness on the transmit radiofrequency fields for neuroimaging at 7 T. For a 5‐mm thick pad, there is virtually no effect on the transmit field for relative permittivity values lower than ∼90. Significant improvements are found for values between 90 and ∼180. If the relative permittivity is increased above ∼180 then areas of very low transmit efficiency are produced. For a 1‐cm thick pad, the corresponding numbers are ∼60 and ∼120, respectively. Based upon the findings, a new material (barium titanate, relative permittivity ∼150) is used to produce thin (∼5 mm) dielectric pads which can easily be placed within a standard receive head array. Experimental measurements of transmit sensitivities, as well as acquisition of T₂‐ and T‐weighted images show the promise of this approach. Magn Reson Med, 2012.

Purpose: To improve image quality and reduce specific absorption rate in functional cardiac imaging at 3 T.Methods: Two high permittivity dielectric pads on the anterior and posterior sides of the thorax were numerically designed and implemented using an aqueous suspension of barium titanate. The effects on the average transmit efficiency, B₁ homogeneity, reception sensitivity, and contrast‐to‐noise ratio were verified in vivo on a dual‐transmit system with the body coil driven in conventional quadrature and radiofrequency‐shimmed mode.Results: Statistically significant improvements in average transmit efficiency, B₁ homogeneity, and contrast‐to‐noise ratio were measured in healthy volunteers (n = 11) with body mass indices between 20.3 and 34.9. Simulations show that no radiofrequency hot spots are introduced by the dielectric material.Conclusion: High permittivity pads are shown to reduce specific absorption rate, improve B₁ homogeneity, and increase contrast‐to‐noise ratio in functional cardiac magnetic resonance at 3 T. The results presented in this work show that the current approach is more effective than dual‐channel radiofrequency shimming. Magn Reson Med 71:1632–1640, 2014. © 2013 Wiley Periodicals, Inc.

The objective of this study was to evaluate the use of dielectric pads for improving high spatial resolution imaging of the inner ear at 7 T. Two sets of dielectric pads were designed using electromagnetic simulations and implemented using a deuterated suspension of barium titanate. Their effect on transmit efficiency, contrast homogeneity, and diagnostic image quality was evaluated in vivo (N = 10). In addition, their effect on the specific absorption rate was evaluated numerically. Statistically significant improvements (P < 0.001) in several measures of the image quality were obtained by using dielectric pads. The dielectric pads lead to an increase in the transmit efficiency and uniformity at the location of the inner ear, which is reflected in both an increased contrast homogeneity and an increased diagnostic value. Simulations show that the dielectric pads do not increase the peak local specific absorption rate. Using geometrically tailored dielectric pads enables high spatial resolution magnetic resonance imaging of the human inner ear at 7 T. The high spatial resolution improves the depiction of the fine inner ear structures, showing the benefit of magnetic resonance imaging at ultrahigh fields.

Electrical properties tomography (EPT) is an imaging method that uses a magnetic resonance (MR) system to non-invasively determine the spatial distribution of the conductivity and permittivity of the imaged object. This manuscript starts by providing clear definitions about the data required for, and acquired in, EPT, followed by comprehensively formulating the physical equations underlying a large number of analytical EPT techniques. This thorough mathematical overview of EPT harmonizes several EPT techniques in a single type of formulation and gives insight into how they act on the data and what their data requirements are. Furthermore, the review describes machine learning-based algorithms. Matlab code of several differential and iterative integral methods is available upon request.

The appearance of severe signal drop‐outs in abdominal imaging at 3 T arises primarily from areas of very low B transmit field in the body, and is problematic in both obese as well as very thin subjects. In this study, we show how thin patient‐friendly pads containing new high permittivity materials can be designed and optimized, and when placed around the subject increase substantially the B uniformity and the image quality. Results from nine healthy volunteers show that inclusion of these dielectric pads results in statistically significant decreases in the coefficient of variance of the B field, with stronger and more uniform fields being produced. In addition there are statistically significant decreases in time‐averaged power required for scanning. These differences are present in both quadrature‐mode operation (coefficient of variance decrease, P < 0.0001, mean 25.4 ± 10%: power decrease, P = 0.005, mean 14 ± 14%) and also for the RF‐shimmed case (coefficient of variance decrease, P = 0.01, mean 16 ± 13%: power decrease, P = 0.005, mean 22 ± 11%) of a dual‐transmit system. Magn Reson Med, 2012.