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.

Radiofrequency magnetic fields are critical to nuclear excitation and signal reception in magnetic resonance imaging. The interactions between these fields and human tissues in anatomical geometries results in a variety of effects regarding image integrity and safety of the human subject. In recent decades, numerical methods of calculation have been used increasingly to understand the effects of these interactions and aid in engineering better, faster, and safer equipment and methods. As magnetic resonance imaging techniques and technology have evolved through the years, so to have the requirements for meaningful interpretation of calculation results. Here, we review the basic physics of radiofrequency electromagnetics in magnetic resonance imaging and discuss a variety of ways radiofrequency field calculations are used in magnetic resonance imaging in engineering and safety assurance from simple systems and sequences through advanced methods of development for the future. Key words: MRI; electromagnetics; radiofrequency; signal; noise; SAR; safety Only 10 years ago, the task of reviewing the literature on radiofrequency (RF) field calculations performed with an attempt to consider the human subject in magnetic resonance imaging (MRI) would be a seemingly manageable task, and could even be comprehensive without being prohibitively long. The combination of increasing reliance on numerical calculations and increasing complexity of RF techniques and technology in MRI makes this much more challenging today. In this work, the fundamental roles of the RF fields in MRI are discussed before a basic example calculation of signal-to-noise ratio (SNR) and specific energy absorption rate (SAR) for a particular imaging sequence in a sphere of biological tissue is presented using analytical methods. Following a brief transition, an attempt to review the literature on current applications with emphasis on breadth more than depth is given. It is hoped that the end result will provide enough basic understanding to the reader that with further study of the references provided he or she may be comfortable pursuing calculations for new purposes, continuing to advance the frontiers of MRI. RF MAGNETIC FIELDS FOR MRI: NUCLEAR EXCITATION AND SIGNAL RECEPTIONIn MRI, a variety of magnetic fields is applied to the human body to manipulate the magnetic moments of nuclei in the tissue so that they produce a detectable RF signal. An equation often used to describe the behavior of the net nuclear magnetization vector (M) in the presence of a general applied magnetic field (B) at a single location is the Bloch equation, sometimes written aswhere g is the gyromagnetic ratio, T 1 and T 2 are the longitudinal and transverse relaxation rates, respectively, M 0 is the magnitude of the net nuclear magnetization at rest, proportional to the strength of a given z-oriented static magnetic field (B 0 ), and a i , a j , and a k are unit vectors in the x, y, and z directions. The first term on the right-hand-side (including the cross product) indicates t...

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The subtropical forested catchments are important for retention of atmospheric mercury deposition in southwest China. a r t i c l e i n f o a b s t r a c tForested catchments are an important part of the mercury (Hg) cycle and a link between the atmospheric and the aquatic environments. In this study, Hg input and output fluxes and its retention were investigated at subtropical forested catchments in southwest China. Significantly enhanced atmospheric Hg inputs were observed, and the contribution of litterfall Hg plays a more important role at these subtropical forested catchments. The ratios of Hg output fluxes from stream water to total input were 2.5% and 1.2% for LGS and TSP, which were markedly lower than those reported from Europe and North America. The current annual input Hg only accounted for 0.8 and 1.8 per mille to the Hg stored in the upper 90 cm of soil in LGS and TSP. These suggest that subtropical forested catchments are important for retention of atmospheric mercury deposition in southwest China.