Dynamic shimming of the human brain at 7 T

Abstract

Dynamic shim updating (DSU) of the zero-to second-order spherical harmonic field terms has previously been shown to improve the magnetic field homogeneity in the human brain at 4 Tesla. The increased magnetic field inhomogeneity at 7 Tesla can benefit from inclusion of third-order shims during DSU. However, pulsed higher-order shims can generate a multitude of temporally varying magnetic fields arising from eddy-currents that can strongly degrade the magnetic field homogeneity.The first realization of zero-to third-order DSU with full preemphasis and B 0 compensation enabled improved shimming of the human brain at 7 Tesla not only in comparison with global (i.e. static) shimming, but also when compared to state-of-the-art zero-to second-order DSU. Temporal shim-to-shim interactions were measured for each of the 16 zero-to third-order shim coils along 1D column projections on a spherical phantom. The decomposition into up to 3 exponentials allowed full preemphasis and B 0 compensation of all 16 shims covering 67 potential shim-to-shim interactions. Despite the significant improvements achievable with DSU, the magnetic field homogeneity is still not perfect even when updating all zero-through third-order shims. This is because DSU is still inherently limited by the shallowness of the low order spherical harmonic fields and their inability to compensate the higher-order inhomogeneities encountered in vivo. However, DSU maximizes the usefulness of conventional shim coil systems and provides magnetic field homogeneity that is adequate for a wide range of applications.

In vivo B 0 field shimming methods for MRI at 7 T

“…Compared to 1st–3rd order static shimming, Sengupta et al (2011) demonstrate performance gains from combining 1st–2nd order dynamic shimming with static 3rd order shims, achieving reduced distortion and through-slice dephasing in EPI slices. Juchem et al (2010) report significant gains for 0th–3rd order dynamic shimming as compared with 0th–2nd order dynamic shimming.…”

Functional Magnetic Resonance Imaging of the Spinal Cord: Current Status and Future Developments

“…Although it generally is not possible to obtain a perfectly homogeneous B 0 over a large volume using standard second or third order shimming systems-especially when using a large FOV, such as in brain-spine protocols-the shim can be improved by assigning specific shim coefficients to each imaging slice. This technique is called dynamic shimming and has shown promising application in the brain at 7 T. 59 Because most fMRI acquisitions of the spinal cord are done axially with thick slices (typically, 3-5 mm), Finsterbusch et al proposed to update only the z-gradient as a way to compensate for intravoxel dephasing 60 and showed promising results for simultaneous brain-spine fMRI studies. 56 Figure 5 shows an example of image quality improvement when using this approach on T 2 *-weighted EPI of the spinal cord.…”

Proton magnetic resonance spectroscopy and its diagnostically important metabolites in the brain

“…However, several technological challenges have to be solved at higher fields (Ugurbil et al 2003). The development of new hardware takes place in the field of radiofrequency coils (Fujita 2007) and shimming (Juchem et al 2010). New advances in the pulse sequences push the usage of MRS and MRI to applications previously considered impossible (Zhang et al 2010).…”