Correction of Magnetic Field and Field Inhomogeneity Variations in MR Thermometry in an Open Permanent Magnet MRI System

To improve the practicability of the proton resonance frequency (PRF) thermometry in open permanent magnet systems, the field and field distribution variations in the permanent magnet system are characterized and a series of corrections are proposed to solve these problems. In order to correct the phase errors come from the variation of external electromagnetic field, a navigator was inserted in the standard GRE sequence. The phase of the navigator echo was used to correct the imaging data. Dynamic shimming was used to compensate for the variation of B_ο field distribution. Finally, a thermometry experiment was executed with the corrections mentioned above, and the result was compared with the data acquired by a thermometry without these corrections. With the proposed corrections, the image quality and the thermometry accuracy was improved significantly. The temperature uncertainties obtained from phantom is about ±3°C. The strategy proposed in this study for MR thermometry in MR scanner with an open permanent magnet can be used to correct the errors produced by the magnet´s spatial/temporal drifts in MR thermometry experiments and thus makes such systems suitable candidates for MR thermometry.