Shimming denotes the technical procedure to improve the homogeneity of the magnetic field in the MRI system. This presentation will give an overview about why the magnetic field is inhomogeneous at all, and what the consequence is. Passive and superconducting shims as technical means to improve the shim in every MRI system are described. Practical information on when shimming is needed, and what the operator can do to optimize the shim of their MRI system will be given as well. Moreover, latest developments like high-order shim systems, shim arrays and dynamic shimming will be presented.
Technically, it is not possible to build a MRI system with perfectly homogenous B0 field. Therefore, during the installation process small pieces of iron or ferromagnetic pellets are deposited within the magnet bore to compensate the inhomogeneity, a process which is called passive shimming (1). This kind of shimming is usually only performed once during the scanner setup. Only atypical events like a magnet quench may require its repetition, otherwise it will not be touched later.
Active shimming uses electric current through dedicated shim coils to create an additional magnetic field which adds with the inhomogeneous B0 field and, in consequence, results in a static magnetic field with essentially better homogeneity than without the shims. Active shimming can either be performed using normal conducting coils (resistive shims) mounted on the same structure as the gradient coils, or using superconducting shim coils within the liquid-helium containing cryostat. The latter coils have become of increasing interest in ultrahigh field MRI systems recently (2).
Even if it would be possible to obtaine perfect B0 homogeneity by this means, further efforts would be needed because the magnetic properties of the tissue modifies the magnetic field distribution within the patient. Depending on the magnetic susceptibility constant c of the tissue, the spatial arrangement and shape of the tissue within the magnet bore will modify that internal magnetic field in an unpredictable and spatially dependent way. To compensate for this mainly object-induced inhomogeneity, user-adjustable resistive shims can be adapted to the individual object or patient using the scanner’s shim system menu.
Technically, this is performed by means of resistive shim coils mounted on the same structure as the gradient coils, i.e. within the scanner housing. A constant electric current is introduced into these shim coils which induces a magnetic field within the bore of the magnet. The strength and geometrical distribution of this magnetic field depends on the strength of the electric current in the shim coil and on the shape of the shim coil. Since the shape of the coil cannot be adapted to each object, another approach is selected: several shim (5-25) coils with different shapes and, thus, different generated geometrical magnetic field distributions are implemented. These additional shim coils are built such that they produce magnetic fields according to spherical harmonics such that their superposition can best compensate the inhomogeneity of the magnetic field in the patient.
Conventional MRI systems have hardware to compensate zeroth (0th), 1st and 2nd order terms. In research MRI systems and at ultrahigh fields (7T and beyond) 3rd order shim systems are available commercially, 4th order shims have been tested in a recent study (3). Shimming with 3rd order and more is denoted as higher order shimming.
Active shimming at the MRI system requires a three-step procedure: (i) First, the spatial distribution of B0 needs to be measured, e.g. using dual-echo pulse sequences. (ii) the correction field and the corresponding shim currents for the individual shim components need to be calculated before (iii) they are sent to the shim system electronics and hardware. Finding the optimal shim currents is a non-trivial problem (4), in particular for breast (5) and cardiac MRI (6).
Shim arrays take up the multiple coil concept from parallel imaging receive coil comprising multiple coil elements. Analogously, shim arrays comprise of multiple coils placed directly around the object but they are fed by static electric currents instead of radiofrequency. Thus, these shim arrays generate a complex magnetic field which can be shaped by modifying the electric current in the individual coil element, and by the varying the electric currents of the different coil elements. The total field of the shim array is the superposition of the magnetic fields of the individual fields, it is used to compensate for the B0 inhomogeneities. It has been shown in mouse (7) and in human brain (8) at 7T that this is a powerful concept to compensate for the strong B0 inhomogeneities in tissue near tissue-air-interfaces (such as the frontal lobe) (9). MR applications which are particularly sensitive to B0 inhomogeneities such as MRS (10) and echo-planar-imaging (11) will also benefit from this concept.
Combining the concept of shim arrays with multiple receive (12,13) and multiple transmit/receive coils (14,15) is a recent development. It promises reduced space demands of the combined device when compared with separate units.
Dynamic shimming denotes inline correction of B0 variations. For this purpose, measurement of the B0 field distribution with subsequent calculation of the compensating magnetic field and transmission of the calculated current values to the shim system needs to be performed. This has been shown recently in diffusion tensor imaging of brain (16) and neck (17).
Advanced Shimming in Other Organs than Brain
Most advanced shim applications have been developed and used in brain imaging. It has been demonstrated that shim array technology may also be used successfully in the spine at 3T (18). Moreover, analysis of the heart at 3T demonstrates that cardiac and pulmonary motion result in significant variation of the Larmor frequency over the cardiac cycle, and that at 3T higher order shimming (6,19) as well as cardiac-cycle specific shimming may be advantageous (20).Financial Support by the German Ministry for Education and Research (BMBF, grant numbers 01EO1004 and BMBF 01E1O1504) is appreciated.
1. Kong X, Zhu M, Xia L, Wang Q, Li Y, Zhu X, Liu F, Crozier S. Passive shimming of a superconducting magnet using the L1-norm regularized least square algorithm. J Magn Reson 2016;263:122-125.
2. Winkler SA, Schmitt F, Landes H, DeBever J, Wade T, Alejski A, Rutt BK. Gradient and shim technologies for ultra high field MRI. Neuroimage 2016.
3. Kim T, Lee Y, Zhao T, Hetherington HP, Pan JW. Gradient-echo EPI using a high-degree shim insert coil at 7 T: Implications for BOLD fMRI. Magn Reson Med 2016.
4. Fillmer A, Kirchner T, Cameron D, Henning A. Constrained image-based B0 shimming accounting for "local minimum traps" in the optimization and field inhomogeneities outside the region of interest. Magn Reson Med 2015;73(4):1370-1380.
5. Hancu I, Govenkar A, Lenkinski RE, Lee SK. On shimming approaches in 3T breast MRI. Magn Reson Med 2013;69(3):862-867.
6. Jaffer FA, Wen H, Balaban RS, Wolff SD. A method to improve the B0 homogeneity of the heart in vivo. Magn Reson Med 1996;36(3):375-383.
7. Juchem C, Brown PB, Nixon TW, McIntyre S, Rothman DL, de Graaf RA. Multicoil shimming of the mouse brain. Magn Reson Med 2011;66(3):893-900.
8. Juchem C, Nixon TW, McIntyre S, Boer VO, Rothman DL, de Graaf RA. Dynamic multi-coil shimming of the human brain at 7 T. J Magn Reson 2011;212(2):280-288.
9. Juchem C, Green D, de Graaf RA. Multi-coil magnetic field modeling. J Magn Reson 2013;236:95-104.
10. Juchem C, de Graaf RA. B0 magnetic field homogeneity and shimming for in vivo magnetic resonance spectroscopy. Anal Biochem 2016.
11. Juchem C, Umesh Rudrapatna S, Nixon TW, de Graaf RA. Dynamic multi-coil technique (DYNAMITE) shimming for echo-planar imaging of the human brain at 7 Tesla. Neuroimage 2015;105:462-472.
12. Stockmann JP, Witzel T, Keil B, Polimeni JR, Mareyam A, LaPierre C, Setsompop K, Wald LL. A 32-channel combined RF and B0 shim array for 3T brain imaging. Magn Reson Med 2016;75(1):441-451.
13. Truong TK, Darnell D, Song AW. Integrated RF/shim coil array for parallel reception and localized B0 shimming in the human brain. Neuroimage 2014;103:235-240.
14. Darnell D, Truong TK, Song AW. Integrated parallel reception, excitation, and shimming (iPRES) with multiple shim loops per radio-frequency coil element for improved B0 shimming. Magn Reson Med 2016.
15. Han H, Song AW, Truong TK. Integrated parallel reception, excitation, and shimming (iPRES). Magn Reson Med 2013;70(1):241-247.
16. Alhamud A, Taylor PA, van der Kouwe AJ, Meintjes EM. Real-time measurement and correction of both B0 changes and subject motion in diffusion tensor imaging using a double volumetric navigated (DvNav) sequence. Neuroimage 2016;126:60-71.
17. Gatidis S, Graf H, Weiss J, Stemmer A, Kiefer B, Nikolaou K, Notohamiprodjo M, Martirosian P. Diffusion-weighted echo planar MR imaging of the neck at 3 T using integrated shimming: comparison of MR sequence techniques for reducing artifacts caused by magnetic-field inhomogeneities. MAGMA 2017;30(1):57-63.
18. Topfer R, Starewicz P, Lo KM, Metzemaekers K, Jette D, Hetherington HP, Stikov N, Cohen-Adad J. A 24-channel shim array for the human spinal cord: Design, evaluation, and application. Magn Reson Med 2016;76(5):1604-1611.
19. Mattar W, Juchem C, Terekhov M, Schreiber L. Multi-Coil B0 shimming of the Human Heart: A Theoretical Assessment. 2016; Singapore. p 1151.
20. Kubach MR, Bornstedt A, Hombach V, Merkle N, Schar M, Spiess J, Nienhaus GU, Rasche V. Cardiac phase-specific shimming (CPSS) for SSFP MR cine imaging at 3 T. Phys Med Biol 2009;54(20):N467-478.