Signal‐to‐noise ratio and absorbed power as functions of main magnetic field strength, and definition of “90°” RF pulse for the head in the birdcage coil

Abstract

Calculations of the RF magnetic (B 1 ) field as a function of frequency between 64 and 345 MHz were performed for a head model in an idealized birdcage coil. Absorbed power (P abs ) and SNR were calculated at each frequency with three different methods of defining excitation pulse amplitude: maintaining 90°fl ip angle at the coil center (center ␣ Predictions of trends in signal-to-noise ratio (SNR) and specific energy absorption rate (SAR) with increasing static magnetic field (B 0 ) strength based on MR theory, the principle of reciprocity, and analytical RF magnetic field (B 1 ) calculations have been shown to be reasonably accurate at frequencies up to 64 MHz in head-and body-sized samples (1,2). MRI experiments are currently performed at static magnetic field (B 0 ) strengths as high as 8.0 Tesla, where the frequency of the RF magnetic field (B 1 ) for imaging with 1 H is about 340 MHz. At these frequencies, significant interaction between the applied B 1 field and human tissues is expected. The effects of this interaction on SNR and the total absorbed power are complicated, and are dependent on the experiment being performed, RF coil type and performance, and even on the specific subject geometry and position in the coil (2,3).‫؍‬In this study we performed calculations of SAR in the head, the total absorbed power (P abs ) in the head and shoulders, and SNR on an axial plane of the head at several B 1 frequencies between 64 and 345 MHz for an anatomically-accurate model in an idealized birdcage coil. The head position and orientation, and the coil behavior are kept constant so that B 1 frequency and definition of the excitation pulse are the only variables. Electrical properties of all tissues are set appropriately at each frequency. The excitation pulse amplitude is defined with three different methods at each frequency.Since our interest was primarily in the effects of the high-frequency RF fields on the imaging experiment, we chose to ignore several factors that complicated both the calculation and interpretation of the results. We chose to consider signal from protons in water only, and to ignore T 1 and T 2 relaxation effects in this work. This simplifies the presentation of results, making them independent of TE and TR, but it also removes some realism from the simulation. We also neglected many other experimental effects, such as those of B 0 inhomogeneity, inevitable variation in sample and coil geometry, signal filtering, and signal amplifier integrity and performance (4). Thus, the findings concerning signal in the images, FID amplitude, and SNR presented here should be considered predictions of the types of phenomena that may be seen at high frequency due to behavior of the RF fields. Manifestation of these phenomena in experiment should not be expected to occur exactly as in these calculations. METHODSThe finite difference time domain (FDTD) numerical method for electromagnetics was used to calculate all electrical and magnetic fields throughout a head model in an idealized birdcage coil. This me...

Temperature and SAR calculations for a human head within volume and surface coils at 64 and 300 MHz

“…In agreement with previous publications (3,4), the results in Table 2 indicate that the nonsignificant risk FDA limit on SAR in any 1 g region in the head (8 W/kg) will be exceeded before the nonsignificant risk FDA limit on head-average SAR (3 W/kg) is exceeded for both volume and surface coils intended for head imaging. Similarly, the results in Table 4 indicate that the normal mode IEC limit on SAR in any 10 g region in the head (10 W/kg) will be exceeded before the normal mode IEC limit on head-average SAR (3.2 W/kg) is exceeded, except in the TEM coil at 300 MHz.…”

“…A head model resolution of 5.0 mm in each direction was used for calculations with a 300 MHz TEM coil. The geometries and modeling methods for the 64 MHz birdcage coil and the 300 MHz TEM resonator have been described elsewhere (4,8). The methods for modeling the birdcage coil and TEM resonator have been proven accurate by agreement with experiments at frequencies of up to 128 MHz (9) and 300 MHz (8), respectively.…”

“…All electromagnetic field calculations were performed using the finite difference time domain (FDTD) method for electromagnetics (11) using commercially available software (xfdtd; Remcom, State College, PA). The SAR distributions were extracted from the electromagnetic field calculations as described previously (3,4). The maximum local SAR was defined as the average SAR in the 1 cm 3 (FDA) or 10 cm 3 (IEC) volume of tissue having the highest average SAR (3,4).…”

“…The SAR distributions were extracted from the electromagnetic field calculations as described previously (3,4). The maximum local SAR was defined as the average SAR in the 1 cm 3 (FDA) or 10 cm 3 (IEC) volume of tissue having the highest average SAR (3,4). The highest local SAR in the brain was determined by performing the same analysis, but considering only 1 cm 3 or 10 cm 3 regions with a center in white or gray matter.…”

“…Due to the complicated induced current patterns in the heterogeneous human head, calculations or measurements of the maximum local SAR in simple geometries are not likely to be indicative of the maximum local SAR levels in the head (3). Furthermore, existing limits on the maximum local SAR in any 1 g region of tissue are likely to be exceeded before the limits on average SAR are exceeded (3,4), especially when using local coils, such as surface coils.…”