Ali Aghaeifar1
1Siemens Healthineers, United Kingdom
Synopsis
Homogeneous static magnetic field (B0) is a prerequisite for many magnetic resonance imaging (MRI) applications. Scanners are equipped with a built-in spherical harmonic based shim setup to counteract B0 inhomogeneity. However, built-in shim setup might not be adequate for all applications, especially at UHF MRI where susceptibility induced B0 inhomogeneity is scaled with the strength of B0. Slice-wise shimming and multi-coil shim setup are some promising tactics that could significantly mitigate B0 inhomogeneity. This presentation will give an overview of the principle of SH-based, slice-wise, and multi-coil shimming and some details of each strategy will be discussed.
Introduction
Homogeneous static magnetic field
(B0) is a prerequisite for many magnetic resonance imaging (MRI) applications.
Inhomogeneities in B0 can affect the accuracy of excitation and signal
encoding. Geometric distortion, signal loss, disruption of frequency selective pulses,
and line broadening in spectroscopy are some of well-known consequences of
unwanted nonuniformity in B0 [1]. A
large portion of contaminations arises from magnet imperfection and environment
(e.g., bed, gradients, or lab equipment); however, these contaminations are
static and can be significantly rectified during scanner installation.
Installing small pieces of iron or ferromagnetic pellets within the magnet
(which is called passive shimming) and employing a dedicated shim coil (active
shimming) can limit static inhomogeneities to a sub-ppm range. Even if we can
completely eliminate static inhomogeneities, further efforts are required,
because the subject being scanned can distort the magnetic field as well. The
susceptibility differences between the materials (e.g., tissue and air) lead to
field distortions that are spatially variant and depend on the shape of the
tissue and its orientation with respect to the B0 field. To deal with this,
adjustable resistive shim coils are employed to superimpose additional
spatially varying magnetic fields to counteract the inhomogeneity. The optimal
driving currents for the shim coils are calculated based on adjustment scans
that are executed at the outset of the study.
Shim coils in clinical scanners can
generate spatially varying magnetic fields that resemble spherical harmonics
(SH). SH functions which are the solution of Laplace’s equation, ∇2B=0, are very popular for shimming since SH terms,
in principle, are orthogonal and in theory, any magnetic field in a source-free
region can be modelled by expansion into infinitive SH terms. However, clinical
scanners are only equipped with shim coils that can generate a limited number
of SH terms (up to 2nd order or 3rd order).
Ultrahigh field (UHF) MRI has been
increasingly used in the last decade by researchers because of the supralinear
gain in signal-to-noise ratio (SNR) [2].
However, inhomogeneities originated from the susceptibility differences scale
with strength of B0 and become a big challenge at UHF MRI. This will need extra
efforts to mitigate field nonuniformity. Some possible remedies would be 1)
breaking ROI into smaller sub-ROI, 2) increasing the number of shim coils, and
3) special coil design.Dynamic slice-wise shimming
B0 shimming problem is usually an
overdetermined system of linear equations. Smaller ROI can decrease the
complexity of the system and yields an improved uniformity. This can be the
case for 2D multi-slice sequences in MRI since every slice is excited and
encoded independently, thus, optimal shims can be calculated and applied on a
per-slice basis. Slice-wise shimming can achieve superior uniformity over
conventional global (entire ROI) shimming [3].
However, slice-wise shimming relies on amplifiers able to switch rapidly with a
short settling time that fits into dead-time between end of readout and start
of the next excitation. One should consider that abrupt current change in
built-in SH-based shims of scanner might result in eddy-current as they are
mounted in the body of scanner, close to conductive structure. Hence,
calibration of the pre-emphasis circuitry is crucial to compensate for temporal
eddy-current induced field contaminations. In this regard, including temporal variation
of shim current (i.e., shims change across slices) as a regularization term in calculation
of optimal currents can help to minimize eddy-current [4]. The application
of slice-wise shimming can be expanded to multi-band imaging too [5].Multi-coil shimming
Increasing number of the shim coil raises
degree of freedom for shimming and can improve field homogeneity. Generating
higher-order SH functions might not be always practical because of the space
required for special winding. As an alternative, the multi-coil (MC) shimming
method is proposed in which an array of small generic DC coils is placed around
the target [6, 7]. In
contrast to the SH-based method, MC employs many small low inductance local
coils. Since inductance is low and the coils are near to target and far from
scanners conductive structure, low voltage, and cost-effective amplifiers can
be employed for global and slice-wise shimming without worrying about
eddy-current. However, one should consider that the generated fields are no
longer orthogonal.
The application of local shim coils
is not limited to B0 shimming. Recent works have employed local B0 coils to improve
lipid suppression [8], image
encoding [9], and highly
accelerated acquisition [10].Integrated RF-B0 coils
Rx-coil element of the receiver can
share the structure with local B0 shim loops by utilizing inductive chokes to
block RF from the shim current and bridge tuning capacitor [11, 12]. In
this approach, the close proximity of shim coils to the body allows adequate
uniformity to be obtained with a small amount of current while local coils are
restricted to be single-turn.Optimized multi-coils
Having prior information about
shimming targets allows optimizing shim coils winding or the position of local
coils. The majority of magnetic field non-uniformities in humans are formed in
the interface of air and tissue. For example, ∆B0 inhomogeneities primarily are
observed near nasal cavities and ear canals in the human brain. Severity varies
with the geometry and size of individual heads; however, the overall pattern of
∆B0 distribution is consistent. Having a dataset of the B0 map of the human
brain as training data, one can optimize the size and position of local shim
coils [13]. This
allows achieving superior homogeneity or decreasing the number of local shim
coils without performance penalty in comparison to a regular multi-coil design.
The optimization can be performed on the winding path too. Set of B0 map can be
used as target field and stream function of electric current density will be
calculated, followed by singular value decomposition to define wire paths [14,15]. Acknowledgements
No acknowledgement found.References
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