AbstractThe move towards higher static magnetic field strengths in MRI has allowed improved imaging quality from increased signal to noise ratio. However challenges have arisen from increased inhomogeneity in the radio frequency (RF) fields required to create MR signals and greater RF energy deposition – known as the specific absorption rate (SAR) – within imaging subjects. These factors have prompted the use of parallel transmission (PTx) MRI; in which multiple independent channels are used to control the RF electromagnetic fields.
In this thesis the aim was to develop methods for controlling SAR using PTx and to assess the impact of RF safety in various scenarios. The electromagnetic behaviour of an 8-channel PTx RF coil was fully simulated which enabled the examination of differences between full simulations and a commonly modelled idealised situation. It was found that large discrepancies could result in the idealised model in certain situations. The full RF coil model was for producing SAR simulations of various adult male voxel models.
These SAR models were used to perform RF shimming, in which a complex weighting is applied to each channel of a PTx system to yield improved RF conditions. This was done for two scenarios: to perform lower SAR cardiac MRI with greater RF field homogeneity in vivo for optimised imaging; and to explore methods for decoupling the transmit coil from a simulated prosthetic hip implant embedded within an adult male whilst still producing a uniform imaging field. In both scenarios, reduced SAR configurations could be found that enabled improved imaging with greater RF safety.
A separate model of a 2-channel birdcage RF coil was developed to assess SAR deposition in neonates during MRI examinations. It was found that under normal operation at 3 T, local SAR constraints produced by the scanner are conservative by a factor of four.
|Date of Award||1 Dec 2015|
|Supervisor||Shaihan Malik (Supervisor) & Jo Hajnal (Supervisor)|