This subproject is one of many research subprojects utilizing theresources provided by a Center grant funded by NIH/NCRR. The subproject andinvestigator (PI) may have received primary funding from another NIH source,and thus could be represented in other CRISP entries. The institution listed isfor the Center, which is not necessarily the institution for the investigator.Mapping the brain neuronal activity by fMRI using fast non-spin-echo pulse sequences usually has difficulty obtaining accurate data in the inferior portion of the frontal lobe, where the magnetic field is seriously distorted due to the difference in the magnetic susceptibility across boundaries of the brain and air. The field distortion results in MRI signal loss and image artifacts. Therefore, correcting the field distortion is essential to obtain accurate fMRI activation maps. The harmonic shimming system equipped in modern MRI scanner is efficient for overall symmetrical or uniform objects. However, field inhomogeneity created by local structure such as the cavities and sinuses in the head has much smaller scale and requires shim coils of very high orders. This is not efficient.
The aim of the present project is to develop active shimming methods that are capable of correcting locally the field in the region of interest. The shimming device will have adjustable parameters to obtain optimal results on a patient by patient basis.Since, approximately, the z-component of the magnetic field in the frontal lobe has axial symmetry and is nearly matching the shape of a dipolar field, the primary shimming field can be generated by currents in circular coils held in the mouth. In the present implementation, the mouth shim-coil set contains two concentric circular coils (diameter 1.6 cm, 80 turns and 2.2 cm, 50 turns). To determine the shim currents, a test current (60 mA) was input to one of the circular coils and a B0 map was acquired; then the procedure was repeated for the other coil. The B0 maps were subtracted by a third B0 map obtained without any current so each subtracted map represents the shimming field generated by the test current. Then the required shim currents are solved by least-square fitting (using SVD) to eliminate the third B0 map. At 1.5 T on phantom and human subjects, the present method has been shown to be effective for fast imaging [Hsu and Glover, 11th ISMRM, 734 (2003)]. In the region of interest, the field inhomogeneity (root-mean-square) is reduced by at least 55%. Tests on breath-holding fMRI show considerable signal recovery in the activation maps [Hsu and Glover, 12th ISMRM (2004) (accepted)]. Based on the success, a more sophisticated shim coil is currently under study to further improve the field homogeneity.
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