With the availability of high-field, whole-body magnetic resonance systems increasing, problems of obtaining uniform excitation have been discovered. These problems are well documented and drove the major manufacturers to fall back to 3T when determining which high field product to push for clinical neuroimaging. Solutions to the non-uniform excitation will need to be developed prior to clinical acceptance of high field body imaging (3T and greater) or neuroimaging (4T and higher). Some solutions are being investigated such as TEM, multiple bird cage coils and multi-transmit arrays. The first two solutions provide a uniform excitation field in the absence of the sample and then adjustments are made using a model of the average sample (size, shape and dielectric constant). However, the ability to make adjustments to the excitation profile on individual subjects has not been investigated. Multi-coil transmitter array technology has suffered from the inability to sufficiently decouple the individual coil elements from each other. In this application, we propose to investigate a possible solution, using a 3T, whole body, multi-array transmit coil with sufficient decoupling while allowing the current in each element to be independently adjustable. In addition, a procedure for selecting the currents that produce the most homogeneous excitation on each subject will be developed. To do this, we will test a negative feedback transmitter design to determine its ability to provide sufficient decoupling between two array elements when transmitting (R21 Phase). In the R33 Phase, we will design and build an 8-element body RF transmit array for 3T, develop and implement a pulse sequence and analysis method for "shimming" the excitation field, and build 6 more feedback systems to drive the 8-element coil. At the end of this four year project, we will have demonstrated a possible solution to the inhomogenous transmit problem at 3T, which will be transferable to higher fields with the appropriate array coil designs.
NIH ExPorter
NIH ExPorter