Faculty Bibliography

A comparison of methods to decrease RF power dissipation and related heating in conductive samples using passive conductors surrounding a sample in a solenoid coil is presented. Full-Maxwell finite difference time domain numerical calculations were performed to evaluate the effect of the passive conductors by calculating conservative and magnetically-induced electric field and magnetic field distributions. To validate the simulation method, experimental measurements of temperature increase were conducted using a solenoidal coil (diameter 3 mm), a saline sample (10 mM NaCl) and passive copper shielding wires (50 microm diameter). The temperature increase was 58% lower with the copper wires present for several different input powers to the coil. This was in good agreement with simulation for the same geometry, which indicated 57% lower power dissipated in the sample with conductors present. Simulations indicate that some designs should be capable of reducing temperature increase by more than 85%

It is important to accurately characterize the heating of tissues due to the radiofrequency energy applied during MRI. This has led to an increase in the use of numerical methods to predict specific energy absorption rate distributions for safety assurance in MRI. To ensure these methods are accurate for actual MRI coils, however, it is necessary to compare to experimental results. Here, we report results of some recent efforts to experimentally map temperature change and specific energy absorption rate in a phantom and in vivo where the only source of heat is the radiofrequency fields produced by the imaging coil. Results in a phantom match numerical simulation well, and preliminary results in vivo show measurable temperature increase. With further development, similar methods may be useful for verifying numerical methods for predicting specific energy absorption rate distributions and in some cases for directly measuring temperature changes and specific energy absorption rate induced by the radiofrequency fields in MRI experiments

Numerical calculations of static, switched, and radiofrequency (RF) electromagnetic (EM) fields considering the geometry and EM properties of the human body are used increasingly in MRI to explain observed phenomena, explore the limitations of various approaches, engineer improved techniques and technology, and assure safety. As the static field strengths and RF field frequencies in MRI have increased in recent years, the value of these methods has become more pronounced and their use has become more widespread. With the recent growth in parallel reception techniques and the advent of transmit RF arrays, the utility of these calculations will become only more critical to continued progress of MRI. Proper relation of field calculation results to the MRI experiment can require significant understanding of MRI physics, EM field principles, MRI coil hardware, and EM field safety. Here some fundamental principles are reviewed and current approaches and applications are catalogued to aid the reader in finding resources valuable in beginning field calculations for their own applications in MR, with an eye to the current needs and future utility of numerical field calculations in MRI

This work presents a method to separately analyze the conservative electric fields (E(c), primarily originating with the scalar electric potential in the coil winding), and the magnetically-induced electric fields (E(i), caused by the time-varying magnetic field B1) within samples that are much smaller than one wavelength at the frequency of interest. The method consists of first using a numerical simulation method to calculate the total electric field (E(t)) and conduction currents (J), then calculating E(i) based on J, and finally calculating E(c) by subtracting E(i) from E(t). The method was applied to calculate electric fields for a small cylindrical sample in a solenoid at 600MHz. When a non-conductive sample was modeled, calculated values of E(i) and E(c) were at least in rough agreement with very simple analytical approximations. When the sample was given dielectric and/or conductive properties, E(c) was seen to decrease, but still remained much larger than E(i). When a recently-published approach to reduce heating by placing a passive conductor in the shape of a slotted cylinder between the coil and sample was modeled, reduced E(c) and improved B1 homogeneity within the sample resulted, in agreement with the published results

PURPOSE: To examine the thermal effects of the physiological response to heating during exposure to radiofrequency (RF) electromagnetic fields in magnetic resonance imaging (MRI) with a head-specific volume coil. MATERIALS AND METHODS: Numerical methods were used to calculate the temperature elevation in MRI of the human head within volume coils from 64-400 MHz at different power levels both with and without consideration of temperature-induced changes in rates of metabolism, perspiration, radiation, and perfusion. RESULTS: At the highest power levels currently allowed in MRI for head volume coils, there is little effect from the physiological response as predicted with existing methods. This study does not rule out the possibility that at higher power levels or in different types of coils (such as extremity or whole-body coils) the physiological response may have more significant effects. CONCLUSION: In modeling temperature increase during MRI of the human head in a head-sized volume coil at up to 3.0 W/kg head-average specific energy absorption rates, it may not be necessary to consider thermally induced changes in rates of metabolism, perfusion, perspiration, and radiation

Transmit coil arrays allowing independent control of individual coil drives facilitate adjustment of the B(1) field distribution, but when the B(1) field distribution is changed the electric field and SAR distributions are also altered. This makes safety evaluation of the transmit array a challenging problem because there are potentially an infinite number of possible field distributions in the sample. Local SAR levels can be estimated with numerical calculations, but it is not practical to perform separate full numerical calculations for every current distribution of interest. Here we evaluate superposition of separate electric field calculations-one for each coil-for predicting SAR in a full numerical calculation where all coils are driven simultaneously. It is important to perform such an evaluation because the effects of coil coupling may alter the result. It is shown that while there is good agreement between the superimposed and simultaneous drive results when using current sources in the simulations, the agreement is not as good when voltage sources are used. Finally, we compare maximum local SAR levels for B(1) field distributions that are either unshimmed or shimmed over one of three regions of interest. When B(1) field homogeneity is improved in a small region of interest without regard for SAR, the maximum local SAR can become very high.

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

Direct imaging of a histological slice is challenging. The vast difference in dimension between planar size and the thickness of histology slices would require an RF coil to produce a uniform RF magnetic (B1) field in a 2D plane with minimal thickness. In this work a novel RF coil designed specifically for imaging a histology slice was developed and tested. The experimental data demonstrate that the coil is highly sensitive and capable of producing a uniform B1 field distribution in a planar region of histological slides, allowing for the acquisition of high-resolution T2 images and T2 maps from a 60-microm-thick histological sample. The image intensity and T2 distributions were directly compared with histological staining of the relative iron concentration of the same slice. This work demonstrates the feasibility of using a microimaging histological coil to image thin slices of pathologically diseased tissue to obtain a precise one-to-one comparison between stained tissue and MR images

A number of methods to improve excitation homogeneity in high-field MRI have been proposed, and some of these methods rely on separate control of radiofrequency (RF) coils in a transmit array. In this work we combine accurate RF field calculations and the Bloch equation to demonstrate that by using a sequence of pulses with individually optimized current distributions (i.e., an array-optimized composite pulse), one can achieve remarkably homogeneous distributions of available signal intensity over the entire brain volume. This homogeneity is greater than that achievable using the same transmit array to produce either a single optimized (or RF shimmed) pulse or a single RF shimmed field distribution in a standard 90x-90y composite pulse arrangement. Simulations indicate that with a very simple array-optimized composite pulse, excellent whole-brain excitation homogeneity can be achieved at up to 600 MHz

Four 12-rung linear birdcage-type coils were built to experimentally examine the effects of the end-ring/shield configuration on radiofrequency magnetic field (B(1)) homogeneity and SNR at 125 MHz. The coil configurations include (a) a cylindrical shield (conventional), (b) a shield with annular extensions to closely shield the end-rings (surrounding shield), (c) a shield with annular extensions connected to the rungs (solid connection), and (d) a shield with radially oriented conductors connected to the rungs (radial connection). These coils were also modeled closely with finite difference time domain (FDTD) methods to corroborate experimental findings. Images of a human head were acquired, and the signal-to-noise ratio (SNR) was measured on the central axial, sagittal, and coronal slices. B(1) field homogeneity in the unloaded coils was assessed on images of an oil phantom. Among the four configurations, the solid connection configuration has a lower SNR than the conventional configuration and the surrounding shield configuration but a higher SNR than the radial connection. Although there is no significant difference between the overall SNR of the conventional configuration and the surrounding shield configuration, the surrounding shield configuration has the potential to be tuned to higher frequencies than the conventional configuration. The conventional birdcage coil results in the most homogeneous B(1) field in the oil phantom. Numerical results are also compared with the experimental results.