User:Sprevrha/dielectric shading

Dielectric Shading ...

Definition
Dielectric Shading is the name for interference effects of radiofrequency waves traversing matter. When their wave length is similar to the diameter of the traversed object, standing waves, stationary zones of destructive and constructive interference, occur.

Magnetic Resonance Imaging (MRI)
Dielectric Shading is a cause for local signal loss and results in image artifacts in Magnetic Resonance Imaging at higher main field strengths such as generated by devices employing 3-Tesla magnets. The Larmor frequency for hydrogen at 3 Tesla is 128 MHz with a wavelength of 26 cm, which is closer to typical human body cross-sectional dimensions than the 52 cm of 1.5 Tesla magnets. This effect is sought to be mitigated by MRI devices with third-generation radiofrequency (RF) transmission systems, which employ two independent instead of one RF amplifier and thus allow controlling the phase and shape of the RF pulses. This leads to visibly reduced artifacts.