Simulation ofB1Field Distribution and Intrinsic Signal-to-Noise in Cardiac MRI as a Function of Static Magnetic Field
✍ Scribed by R.W Singerman; T.J Denison; H Wen; R.S Balaban
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 767 KB
- Volume
- 125
- Category
- Article
- ISSN
- 1090-7807
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✦ Synopsis
Two issues that pertain to the optimal static magnetic field for metry artifacts and reduces RF power requirements has also cardiac MRI were addressed: intrinsic signal-to-noise ratio (ISNR) been addressed. Model data on the effects of size and electriand radiofrequency power deposition. From 1.5 to 9.5 T, proton cal parameters are also available and directly compared with Larmor frequencies of 63 to 400 MHz, numerical simulations were experimental phantom data (6). Models introducing increasperformed of the RF fields from a surface coil and a body coil ing levels of complexity, such as skin depth, dielectric efloaded by a heterogeneous, three-dimensional, symmetric model fects, and surface charges, have been considered . The of the human chest. The RF field distribution, the power required RF field (up to 400 MHz) for a surface coil above a conductto produce the RF field, and the ISNR at the center of the heart ing dielectric sphere has been determined analytically (9). In were computed. The model was validated by comparison with contrast to homogeneous body models, more physiological experimental data up to 4 T. The RF field distortion was quantified heterogeneous body models have been proposed (10) and and found to increase linearly up to 6 T due mostly to dielectric resonance modes. Body coil simulations beyond 6 T showed the studied using finite element methods (11). Simulations have onset of higher-order modes at the center of the heart. A range of been performed on two-dimensional chest models (12, 13).
expected RF power requirements was constructed as a function
The intrinsic signal-to-noise ratio (ISNR), which is indeof field up to 9.5 T for surface coils and up to 6.8 T for body coils. pendent of relaxation properties, imaging artifacts, or instru-Over this range of static field, ISNR for a constant coil geometry ment characteristics, has been proposed ( ) as an objective was bracketed by an upper limit that was slightly greater than method of field strength comparison for MRI. Additionally, linear with field and a lower limit that was slightly less than linear other imaging parameters that influence the choice of magwith field. The RF power and ISNR showed a strong dependence netic field strength, such as relaxation times and chemical on chest thickness at 1.5 and 4.0 T. Additionally, independent of shifts, have been discussed (15).
chest thickness, the model predicts a lower limit of a factor of 5
In this study we build on previous work by performing increase in RF power as the static field is increased from 1.5 to 4 numerical simulations of the RF fields in axially symmetric, T. Implications for imaging with other nuclei are discussed. Methods for checking the self-consistency of electrodynamic simula-heterogeneous, three-dimensional models which were scaled tions are presented. ᭧ 1997 Academic Press to be appropriate for cardiac MRI. This was done to gain insight into the static field dependence of the ISNR, B 1 distribution, and required power in cardiac MRI. Simulations were conducted between 1.5 and 9.5 T (proton Larmor frequen-