Radiofrequency Field Calculations for High Field MRI

Abstract

In MRI, increasing radiofrequency magnetic (B 1) field frequency is a consequence of employing higher static magnetic (B 0) field strengths in the drive to improve signal-to-noise ratio (SNR). Due to the direct proportionality between B 0 field strength and B 1 field frequency in MRI, B 1 field distributions become more complex at higher B 0 fields due in part to shorter wavelengths and penetration depths. Consequently, it becomes both more difficult to calculate RF field behavior and more important to do so accurately for high field MRI. In this chapter the basics of electromagnetic properties of tissue, the method of radiofrequency field calculation currently most prevalent in high field MRI (the FDTD method), and methods for relating calculation results to MRI are covered briefly before results from calculations are used to discuss current challenges in high field MRI including central brightening, SNR, power absorption by tissue, and image homogeneity.