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-dependant 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 aid in discussion about the mechanism for central brightening in high field magnetic resonance imaging (MRI), especially regarding the appropriateness of using the term dielectric resonance to describe the central brightening seen in images of the human head. Materials and Methods:We present both numerical calculations and experimental images at 3 T of a 35-cm-diameter spherical phantom of varying salinity both with one surface coil and with two surface coils on opposite sides, and further numerical calculations at frequencies corresponding to dielectric resonances for the sphere. STRONG CENTRAL BRIGHTENING has been observed in magnetic resonance imaging (MRI) of the human head using volume coils at high B 0 field strength and high B 1 field frequency (1-3). Recently there has been some discussion in the literature about the appropriateness of attributing this brightening to dielectric resonance (4 -8), as it has been previously (1,2). While it is certain that B 1 wavelengths are on the order of dimensions of the human body at these frequencies, relatively high tissue conductivity inhibits the creation of strong resonances (4 -8), and alternative explanations for the observed central brightening are warranted. ResultsA true dielectric resonance is characterized by relatively large oscillating electromagnetic fields in and around an object with a frequency of oscillation near a natural frequency for the object, and elicited by a relatively small stimulus near that natural frequency. The natural resonant frequencies for an object are determined by the geometry and electrical properties of the object, as well as the electrical properties of the surrounding medium. A single object typically has several different resonant modes at several different frequencies, each with a characteristic electromagnetic field pattern (8 -11). For some simple shapes, these frequencies and the characteristic field patterns can be calculated with analytical methods (11). But classically, the term resonance refers to field intensity as a function of frequency more than as a function of position.Although other authors have pointed out the lack of strong true resonances in biological tissues, they have not offered an easily understandable alternative explanation for observed central brightening. An enhancement of magnetic field strength near the center of an object can be created by a number of ways, and is not by itself evidence of a dielectric resonance. For example, midway between sources of traveling waves with currents in opposing directions (or 180°out of phase) is a location of constructive magnetic field interference at any frequency, provided the waves from each source travel through the same media for the same distance. This can occur even in an empty coil if the frequency is high enough for the wavelength to be on the order of the coil dimensions. We suggest that it is the combination of multiple current-carrying elements in the coil and wavelength effects in the sample leading to constructive and destructive interfe...

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.

Image inhomogeneity related to high radiofrequencies is one of the major challenges for high field imaging. This inhomogeneity can be thought of as having 2 radiofrequency-field related contributors: the transmit field distribution and the reception field