Ali Aghaeifar

Human brain shimming at 9.4T with a multi-coil setup

The homogeneity of the main static magnetic field (B0) is a crucial prerequisite for MR imaging due to the manifold image artifacts that can arise from field perturbations. B0 inhomogeneity becomes more pronounced with increasing field strength and can lead to severe image distortions [1]. Usually, the scanner’s inbuilt shim system, which creates spherical harmonic (SH) field terms, is used for global static shimming. Recently, smaller local coils were proposed in the literature to achieve a higher shim performance [2]. We have made a multi-coil setup to achieve a superior homogeneity of the B0 field.

A multi-coil shim setup was constructed consisting of 16 circular coils, which were arranged in two rows. Each coil had 25 wire turns and a diameter of 100 mm. The coils were mounted on a fiber-glass cylinder having a diameter of 370 mm and a length of 310 mm. A custom-built amplifier was used to drive the 16 coils individually. Performance of the multi-coil setup was evaluated in terms of global and slice-wise dynamic shimming.

[1] S. Sengupta et al., “Dynamic B0 shimming at 7 T,” Magn. Reson. Imaging, vol. 29, no. 4, pp. 483–496, 2011.

[2] C. Juchem, T. W. Nixon, S. McIntyre, D. L. Rothman, and R. A. De Graaf, “Magnetic field modeling with a set of individual localized coils,” J. Magn. Reson., vol. 204, no. 2, pp. 281–289, 2010.

Advanced motion correction for high-resolution EPI imaging

Magnetic resonance imaging (MRI) is one of the most widely used non-invasive research tools for functional imaging. However, Image quality is highly sensitive to the subject motion, and undesired motions lead to artifacts in final image. EPI is one of the most favorite pulse sequences used in functional imaging since it almost owns the best scan time and enables to images rapid physiological functions in human brain. Despite lots of benefits, EPI suffer from high degree of sensitivity to B0 field inhomogeneity. Subject motion is one cause to motivate higher local field inhomogeneity.

To provide a high quality imaging capability, in one hand, accurate control of the radio-frequency (RF) pules and gradient fields in MRI are necessary. On the other hand, facing higher fields for higher image resolution, shimming is a substantial concept in imaging to achieve homogeneous slices.

The aim of this project is to address motion artifacts in EPI imaging through prospective motion correction. We will use position feedback signal from NMR field probes, installed on the subject, to discern subject position. Having new updated position, in a real-time framework the sequence will change the slice orientation by updating the gradients. Thereafter, to overcome field inhomogeneity resulted from subject motion, shimming coefficients will be reconfigured before the next excitation.