Shimming: Superconducting & Passive Shims; Higher-Order Shims, Shim Arrays & Dynamic Shimming
Vincent Oltman Boer1

1Danish Research Centre for Magnetic Resonance, Funktions- og Billeddiagnostisk Enhed, Copenhagen University Hospital Hvidovre, Denmark


Magnetic resonance imaging (MRI) and spectroscopy (MRS) rely on a strong and highly homogeneous magnetic field inside the scanner. Although magnets have a highly sophisticated design, there are still several techniques used to homogenize the field. Secondly, all biological samples will induce (dynamic) distortion in the field due to the tissue magnetic susceptibility.

In this part of the course both passive shimming of a magnet, as well as active shimming with a subject in the magnet, will be discussed. Furthermore, several new advanced shimming strategies have emerged recently, some of the most promising ones will be discussed.


Scientists and clinicians working in the field of MRI and are interested in techniques for shimming, and how this impacts MRI sequences e.g. EPI, T2*, spectroscopy.


In this course, sources of magnetic field inhomogeneity are explained, as well as the most common ways of correction, as well as a few emerging advanced compensation strategies.


Magnetic resonance imaging (MRI) and spectroscopy (MRS) rely on a strong and highly homogeneous magnetic field inside the scanner. Although magnets have a highly sophisticated design process in order to deliver a magnetic field that is as homogeneous as possible, there can still be a lot of improvements on the bare magnet design. Secondly, any biological sample (e.g. the human body) will still distort the magnetic field in an unpredictable way, due to the fact that there is a lot of variation in the magnetic susceptibility of tissue (1). Imperfections in the homogeneity of the magnetic field can lead to a range of issues, including spatial distortions, blurring, signal loss and in spectroscopy specifically; line broadening. Some of these can be corrected for in retrospect (2), but most are best addressed by a better homogenized magnetic field; a process known as ‘shimming’.

First, magnet shimming techniques are discussed, aimed at designing a magnet with a homogeneity as good as possible. Typically this is expressed as the field deviation in a sphere in the center, for example “the homogeneity is below 5 ppm in a 30 cm sphere”. Despite the highly sophisticated design strategies of the magnets, the magnetic field homogeneity can be degraded. This happens either by inaccuracies in fabrication, or by influences of the surroundings at the place where the magnet is sited. To improve the homogeneity, typically one or more steps of shimming are performed on a magnet once it is in place. For one, passive shim elements (typically iron rods) can be placed on strategic points in the bore (3). Also, most new magnets have superconducting shim coils built into the magnet, which can be used to further optimize the homogeneity of the field in the bore. As they are superconductive, they are not switched, but only set once.

Secondly, when a subject is placed in the magnet, the magnetic field is distorted due to the susceptibility differences in the different materials (e.g. tissue, air). Room-temperature shimming coils are used to compensate the magnetic field on a subject-to-subject or even slice-to-slice basis (4,5). This is performed with electromagnetic coils that are built into the bore of the magnet. The fields of such shim coils are traditionally shaped as spherical harmonic fields of a certain order (2nd, 3rd or even up to 5th (6)).

To find the optimal driving currents for these shim coils, several methods exist. The basics of B0 mapping and frequently used techniques will be discussed. Field mapping can be performed using gradient echo imaging with varying echo times, where the phase difference between two echoes is related to the local frequency, and thus field. This way a map of the magnetic field can be established. Alternatively, the field can be measured along strategically chosen projections (7). The measured field can then be decomposed into spherical harmonic compounds, and the magnetic field can be shimmed.

Emerging methods try to move away from the traditional large shim coils with spherical harmonic basis, as the number of coils that can be used is limited and the efficiency decreases dramatically with higher order. This started with a few strategically placed pieces of magnetic material (8) or coils (9), but has led to the development of arrays of small coils around the head (10). This was shown to be more efficient, and perform better than traditional shimming with spherical harmonic coils. Recently, this multi-coil approach has even been merged with the receiver array, as to not take up any extra space in the bore (11). And although in most applications we still keep striving for the most homogeneous field, a severe distortion of the magnetic field can also have advantages. For example, areas that cause an unwanted MRI signals can be removed from the image e.g. for lipid suppression in the brain (12).

Thirdly, as an MRI scanner is a dynamic environment, where not only the hardware components experiences heating, but also the patient can move, there is a significant dynamic component to shimming. Several approaches exist to compensate for the different dynamic sources of field change, that can be caused by hardware instabilities over time (13), as well as physiologically induced field distortions as for example by breathing (14).


No acknowledgement found.


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Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)