Wyger M. Brink

Quantitative assessment of the effects of high‐permittivity pads in 7 Tesla MRI of the brain

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.

Simulations of high permittivity materials for 7 T neuroimaging and evaluation of a new barium titanate-based dielectric

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  2* ‐weighted images show the promise of this approach. Magn Reson Med, 2012.

High permittivity pads reduce specific absorption rate, improve B1 homogeneity, and increase contrast-to-noise ratio for functional cardiac MRI at 3 T.

To improve image quality and reduce specific absorption rate in functional cardiac imaging at 3 T.

Increasing signal homogeneity and image quality in abdominal imaging at 3 T with very high permittivity materials

The appearance of severe signal drop‐outs in abdominal imaging at 3 T arises primarily from areas of very low B  1+ 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  1+ 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  1+ 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.

High permittivity dielectric pads improve high spatial resolution magnetic resonance imaging of the inner ear at 7 T.

ObjectivesThe 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. Materials and MethodsTwo 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. ResultsStatistically 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. ConclusionsUsing 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.

Clinical applications of dual-channel transmit MRI: A review.

This article reviews the principle of dual‐channel transmit MRI and highlights current clinical applications which are performed primarily at 3 Tesla. The main benefits of dual‐channel transmit compared with single‐transmit systems are the increased image contrast homogeneity and the decreased scanning time due to the more accurate local specific absorption ratio estimation, meaning that less conservative safety limits are needed. The dual‐transmit approach has been particularly beneficial in body imaging applications, and is also promising in terms of cardiac, spine, and fetal imaging. Future advances in transmit SENSE, the combination of dual‐channel transmit with high permittivity pads, as well as the potential increase in the number of transmit channels are also discussed. J. Magn. Reson. Imaging 2015;42:855–869.

A theoretical approach based on electromagnetic scattering for analysing dielectric shimming in high-field MRI.

In this study, we analyzed dielectric shimming by formulating it as an electromagnetic scattering problem using integral equations.

Passive radiofrequency shimming in the thighs at 3 Tesla using high permittivity materials and body coil receive uniformity correction.

To explore the effects of high permittivity dielectric pads on the transmit and receive characteristics of a 3 Tesla body coil centered at the thighs, and their implications on image uniformity in receive array applications.

Diffusion-prepared neurography of the brachial plexus with a large field-of-view at 3T

To study diffusion‐prepared neurography optimized for a large field‐of‐view (FOV) to include the neck and both shoulders. In a large FOV poor homogeneity of the magnetic field (B0) often leads to poor image quality and possibly to poor diagnostic accuracy. The aim was therefore to find an optimal (combination of) shimming method(s) for diffusion‐prepared neurography in a large FOV.

Improved image quality and reduced power deposition in the spine at 3 T using extremely high permittivity materials

To explore the effect of using extremely high permittivity (εr∼1,000) materials on image quality and power requirements of spine imaging at 3 T.