Faculty Bibliography

Experimental results for changes in brain temperature during functional activation show large variations. It is, therefore, desirable to develop a careful numerical model for such changes. Here, a three-dimensional model of temperature in the human head using the bioheat equation, which includes effects of metabolism, perfusion, and thermal conduction, is employed to examine potential temperature changes due to functional activation in brain. It is found that, depending on location in brain and corresponding baseline temperature relative to blood temperature, temperature may increase or decrease on activation and concomitant increases in perfusion and rate of metabolism. Changes in perfusion are generally seen to have a greater effect on temperature than are changes in metabolism, and hence active brain is predicted to approach blood temperature from its initial temperature. All calculated changes in temperature for reasonable physiological parameters have magnitudes <0.12 degrees C and are well within the range reported in recent experimental studies involving human subjects

A phantom design method suitable for high-field MRI based on the RF field wave characteristics of sample and experimental validations at 7.0 T and 3.0 T are presented. The RF field distribution in a phantom with a given RF coil system is primarily determined by the sample size relative to the wavelength inside the sample, and the ratio between the displacement and conduction currents. Experimental results demonstrate that the MR image intensity patterns associated with wave behavior in human samples at a given field strength can be reproduced with a phantom at the same or different field strengths once the dimension and penetration constant are scaled by the corresponding wavelength in the sample medium

PURPOSE: To examine relationships between specific energy absorption rate (SAR) and temperature distributions in the human head during radio frequency energy deposition in MRI. MATERIALS AND METHODS: A multi-tissue numerical model of the head was developed that considered thermal conductivity, heat capacity, perfusion, heat of metabolism, electrical properties, and density. Calculations of SAR and the resulting temperature increase were performed for different coils at different frequencies. RESULTS: Because of tissue-dependent perfusion rates and thermal conduction, there is not a good overall spatial correlation between SAR and temperature increase. When a volume coil is driven to induce a head average SAR level of either 3.0 or 3.2 W/kg, it is unlikely that a significant temperature increase in the brain will occur due to its high rate of perfusion, although limits on SAR in any 1 g of tissue in the head may be exceeded. CONCLUSION: Attempts to ensure RF safety in MRI often rely on assumptions about local temperature from local SAR levels. The relationship between local SAR and local temperature is not, however, straightforward. In cases where high SAR levels are required due to pulse sequence demands, calculations of temperature may be preferable to calculations of SAR because of the more direct relationship between temperature and safety

PURPOSE: To determine if gender is a significant variable for in vivo magnetic resonance imaging (MRI) T2-mapping of knee articular cartilage in young asymptomatic volunteers. MATERIALS AND METHODS: Cartilage MRI T2 mapping was performed in a young healthy population consisting of seven male and 10 female volunteers, 22 to 29 years of age. High-resolution in vivo T2 maps were obtained of patellar, tibial, and weight-bearing femoral articular cartilage. Spatial dependency of cartilage T2 between groups was evaluated through a comparison of cartilage T2 as a function of normalized distance from bone. RESULTS: Bulk cartilage T2 values were similar at all three anatomic sites, and between male and female volunteers. All volunteers demonstrated similar spatial variation in cartilage MRI T2 values, with a minimum located in the radial zone and increasing T2 values toward the articular surface. There was no difference in spatial dependency of cartilage T2 between males and females. CONCLUSION: In young, healthy volunteers, the magnitude and spatial dependency of cartilage T2 does not differ with gender

We modeled four different end-ring/shield configurations of a birdcage coil to examine their effects on field homogeneity and signal-to-noise ratio (SNR) at 64 MHz and 125 MHz. The configurations are defined as: 1) conventional: a conventional cylindrical shield; 2) surrounding shield: a shield with annular extensions to closely shield the end rings; 3) solid connection: a shield with annular extensions connected to the rungs; and 4) thin wire connection: a shield with thin wires connected to the rungs. At both frequencies, the coil with conventional end-ring/shield configuration produces the most homogeneous RF magnetic (B1) field when the coil is empty, but produces the least homogeneous B1 field when the coil is loaded with a human head. The surrounding shield configuration results in the most homogeneous B1 and highest SNR in the coil loaded with the human head at both frequencies, followed closely by the solid connection configuration

PURPOSE: To examine how fine a model resolution is necessary for calculation of specific energy absorption rate (SAR) for the human head in regions as small as 1 g. MATERIALS AND METHODS: Here we perform a simple study comparing the maximum SAR averaged over any 1 cm(3) and SAR averaged over the entire head for several models of the same human head within the same radiofrequency coil, but with spatial resolutions varying from 8-100 Yee cells per cm(3). RESULTS: Over the range of model resolutions from 8-100 Yee cells per cm(3), there is only a 16% variation in maximum SAR in any 1 cm(3) of tissue in the head, and only a 7% variation in SAR averaged over the entire head. CONCLUSION: While it is always desirable to perform SAR calculations with the greatest possible accuracy, in calculations of the maximum SAR levels in any 1 cm(3) of tissue, spatial resolutions greater than 5 mm may not yield notably different results than those with a spatial resolution of 5 mm

The use of detached endcaps for 3 T birdcage coils was investigated both theoretically and experimentally. Finite difference time domain analysis, along with workbench and MRI techniques, were used to map the radiofrequency (RF) B(1) distribution along the coil axis with and without an endcap. Without an endcap the measured B(1) value at the service end of the birdcage was only 45% of the value at the coil's center. This was improved to 85% with a detached endcap of maximum achievable diameter (375 mm), positioned 4 mm from the RF shield. The B(1) field distribution on the patient side of the coil was unaffected by the presence of the endcap. The dependence of the B(1) distribution as a function of endcap diameter was also investigated. Surprisingly, simulations and experiments show that there is an optimum ratio of endcap-to-birdcage coil diameter (approximately 1.08) that gives the best B(1) homogeneity. In the human head the optimized endcap, positioned 16 mm from the RF shield, improves the MRI signal amplitude from 55% to 85% of maximum toward the service end. This novel endcap design is easy to implement with existing birdcage coils, and could prove useful when flexibility in access to the RF coil is required

The RF field intensity distribution in the human brain becomes inhomogeneous due to wave behavior at high field. This is further complicated by the spatial distribution of RF field polarization that must be considered to predict image intensity distribution. An additional layer of complexity is involved when a quadrature coil is used for transmission and reception. To study such complicated RF field behavior, a computer modeling method was employed to investigate the RF field of a quadrature surface coil at 300 MHz. Theoretical and experimental results for a phantom and the human head at 7.0 T are presented. The results are theoretically important and practically useful for high-field quadrature coil design and application

Susceptibility-induced perturbation of the static magnetic field by the human body during magnetic resonance imaging (MRI) leads to undesirable artifacts as well as valuable physiological information, as in functional MRI. The ability to calculate these perturbations for a multi-tissue human body model provides a powerful tool in designing hardware and acquisition methods for reducing the artifacts, and in relating observed image contrast to physiological origins. We have developed a method for calculating the static field in arbitrary 3D magnetic susceptibility distributions and performed calculations in a complete model of the human head and shoulders. The accuracy of our method was validated in regular geometries with known analytical solutions and in comparison with experimental results acquired from the head of the same human subject used for computer modeling. Results are presented in parts per million (ppm) deviation from the applied field strength and are valid for any imaging or spectroscopy system

A simple method for birdcage coil design (high pass, low pass, and band pass) is presented. Rather than iteratively approaching the Larmor resonant frequency with known capacitances and calculated inductances, a more versatile approach to birdcage coil design is developed and validated, in which the necessary capacitances are calculated using a desired current pattern, a calculated inductance, and a predetermined resonant frequency. In order to expedite coil design for experienced and novice coil builders, a computer program (BirdcageBuilder) is also implemented based on this method. Experimental results show that the calculated capacitances and actual capacitances for several existing birdcage coils are in good agreement.