Analytic expressions for the ultimate intrinsic signal‐to‐noise ratio and ultimate intrinsic specific absorption rate in MRI

Abstract

The ultimate intrinsic signal-to-noise ratio is the highest possible signal-to-noise ratio, and the ultimate intrinsic specific absorption rate provides the lowest limit of the specific absorption rate for a given flip angle distribution. Analytic expressions for ultimate intrinsic signal-to-noise ratio and ultimate intrinsic specific absorption rate are obtained for arbitrary sample geometries. These expressions are valid when the distance between the point of interest and the sample surface is smaller than the wavelength, and the sample is homogeneous. The dependence on the sample permittivity, conductivity, temperature, size, and the static magnetic field strength is given in analytic form, which enables the easy evaluation of the change in signal-to-noise ratio and specific absorption rate when the sample is scaled in size or when any of its geometrical or electrical parameters is altered. Furthermore, it is shown that signal-to-noise ratio and specific absorption rate are independent of the permeability of the sample. As a practical case and a solution example, a uniform, circular cylindrically shaped sample is studied.

An analytic expression for the ultimate intrinsic SNR in a uniform sphere

“…The dielectric properties were set to values in the average brain tissue. Dashed lines show the approximate dependence of UISNR on ∼ , suggested by Kopanoglu et al for low field strengths…”

Specific absorption rate reduction using nonlinear gradient fields

“…Gt [3] where G denotes the amplitude of the LGF, and M (k) denotes the Fourier transform of the excitation profile (i.e., the excitation k-space), where k = γ 2π Gt and i is the imaginary unit; in this equation, a complete refocusing is assumed. In practice, the RF pulses are limited in time using apodization functions.…”

“…However, reducing the SAR below a certain level may impose restrictions on certain sequence parameters such as the maximum flip angle, the minimum repetition time, and the minimum pulse duration. The SAR varies quadratically with the field strength ; therefore, restrictions on the sequence parameters become more pronounced, as the field strength increases. Although homogeneous B 1 + fields are important in obtaining diagnostic‐quality images, homogeneous fields can only be obtained with additional increases in the SAR.…”

Modeling Endovascular MRI Coil Coupling With Transmit RF Excitation

“…The intrinsic signal to noise ratio (ISNR) ϕ s (9) is used to compare coil SNR performance, independent of imaging parameters [36]–[38]. In (9), ω is angular excitation frequency, M 0 is equilibrium magnetization per unit volume, k is Boltzman’s constant, T is absolute temperature, R load is sample loading, and B 1 + is excitation field value.…”