PTB News 3/2004 (149 kB) PTB News 3/2004, English edition, Issue January 2005PTB has developed a 4-channel array coil for 3 tesla magnetic resonance (MR) imaging. The radio frequency (RF) transmit/receive array coil operates in a coherent mode and can actively control the RF magnetic field distribution in the investigated body region to produce more uniform MR images in high and ultra-high field MR scanners.
In present magnetic resonance systems for medical imaging a radio frequency magnetic field B1 is superimposed with a static magnetic field of up to 3 tesla. The trend is to go to much higher static magnetic fields of 7 tesla and more to increase sensitivity and resolution. To image the water protons in the human body the RF magnetic field B1 with frequencies between 125 MHz (3 tesla static field) and 500 MHz (11.7 tesla) is applied by a resonant coil system. In the human body the wavelength of this RF radiation is reduced to some 10 cm because of the high dielectric permittivity of water. This gives rise to wave-like propagation phenomena, which lead to severely degraded illumination uniformity in the MR images and so partially deteriorate the desired increase in sensitivity of ultra-high field MR systems.
A coherently driven assembly of several trans-mit/receive coils (TR-arrays) can alter and control the distribution of the RF B1-field within the human body. This requires that the individual coil elements are driven with adequate amplitudes and RF phases. Based on this principle PTB in cooperation with Bruker Biospin MRI developed a 4-channel TR head coil and established a measuring technique to determine the B1-field distribution. This project was funded by BMWA (German Federal Ministry for Economy and Labour). Further, the B1-distribution and the specific absorption rate (SAR) in a human’s head are determined in model calculations. The SAR must not exceed legal limits to protect patients from overheating. The numeric model is validated by comparison with phantom measurements. The driving conditions for the coil array is determined by the simulation calculations which yield the most homogenous distribution of the B1-field in the body and guarantee the highest patient safety.