Spin Lock Adiabatic Correction (SLAC) for B1-insensitive pulse design at 7T
Graphical abstract
Introduction
The use of greater static magnetic field strength in magnetic resonance imaging (MRI) facilitates acquisition of images with higher signal-to-noise ratio (SNR). At 7T and beyond, however, this improvement is undermined by the excitation wavelength causing interference and dielectric resonances that result in an inhomogeneous field throughout the object being imaged [1], [2]. The heterogeneous excitation field causes inconsistency in image contrast as flip angles vary across the object. Efforts have been made to overcome inhomogeneity using post-processing [3], [4], multiple channel transmit array coils [5], [6] and pulse design [7], [8], [9].
Frequency swept pulses have been used for decades to overcome inhomogeneous fields. Much of this work has been in the form of adiabatic pulses, in which the magnetisation follows a sweeping effective field when the RF amplitude exceeds a given threshold at which the adiabatic condition is satisfied [10]. Developments over the years have increased the utility of adiabatic pulses such that they can be used for excitation with arbitrary flip angles [11], slice-selective inversion [12], refocusing pulses [13] and slice-selective excitation [14], [15], [16]. Further progress in frequency-modulated (FM) pulses has led to pulses which depend on the weighting given to their trajectory through excitation k-space to achieve insensitivity [17], [18], [19], [20]. With these developments, adiabatic pulses have extended the utility of a range of imaging sequences to be applicable to conditions with significant RF transmit inhomogeneity.
The performance of adiabatic pulses can be defined as how well magnetisation follows the trajectory of the effective field produced by the pulse. This may be improved by increasing pulse amplitude or increasing pulse duration, however this comes at the expense of greater specific absorption rate (SAR). Also crucial for optimising the performance of an adiabatic pulse is choice of amplitude and phase modulation functions. Examples of such functions are sech/tanh [12], tanh/tan [21] and sin/cos [22]. Modifications to these well known analytical functions can produce improvements in both and insensitivity [23], [24].
A new avenue for extending the insensitivity of adiabatic pulses is proposed, involving the introduction of an additional component to dynamically reduce the deviation from the desired trajectory by creating a spin lock in an excitation frame of reference. This approach of Spin Lock Adiabatic Correction (SLAC) thus defines a new class of pulses that by design lead to increased flip angle homogeneity in high field environments. We present a derivation of the SLAC principle and analyse its characteristics using the superadiabiticity framework [25]. We demonstrate SLAC performance in both simulation and experiment at 7T, building on the exemplars of a BIR-4 adiabatic pulse [11] and a hyperbolic secant (HS) pulse [12].
Section snippets
Theory
Consider an arbitrary adiabatic pulse defined by amplitude , phase , and duration ,
Adiabatic pulses are known to be tolerant of both and inhomogeneity. An important factor in pulse design at high field is to maximise tolerance to inhomogeneity. In the following, we will denote by the field weighting factor that relates the strength at a reference point in space to the strength at an arbitrary location in space.
In the
Methods
SLAC was applied to two widely used adiabatic pulses, the BIR-4 pulse, which enables plane rotations of arbitrary flip angle by locking magnetisation in a plane orthogonal to the effective field [11], and the hyperbolic secant pulse [12], a widely used adiabatic full passage (AFP) pulses, in order to assess its effect on pulse performance in simulations and experiments. Excitation performance was assessed using BIR-4, SLAC-BIR-4 and SAR-matched SLAC-BIR-4 pulses. Inversion performance was
Results
SLAC optimisation targets a reduction of the cone angle, (Fig. 1c). Superadiabatic analysis was performed to investigate this aperture in the first three adiabatic frames for BIR-4 and SLAC-BIR-4 (Fig. 4) and for HS and SLAC-HS at = 1. The unscaled and SAR-matched SLAC-BIR-4 result in smaller cone angles in the first two frames compared with standard BIR-4 pulse. The unscaled SLAC-BIR-4 pulse also produces a smaller maximal cone angle in the third adiabatic frame than a standard BIR-4
Discussion
SLAC is a new framework for improving the performance of adiabatic pulses. This method involves applying a correcting field to an adiabatic pulse in order to minimise the deviation which arises from restrictions on the rate at which the effective field is swept. The performance improvement was analysed in simulations and confirmed in experiment. SLAC pulses were found to outperform the BIR-4 and HS pulses upon which they were based and when rescaled to be equal in terms of SAR, SLAC pulses
Conclusion
A method for developing insensitive pulses based on an extension of adiabatic pulses has been demonstrated in theory, simulation and experiment. This method uses an additional field applied in concert with an arbitrary adiabatic pulse in order to lower the field amplitude threshold at which the desired magnetisation trajectory is maintained. When applied with a carefully chosen field amplitude weighting parameter, , this was shown to produce an increase in image intensity on the
Declaration of Competing Interest
None.
Acknowledgment
We acknowledge the facilities, and the scientific and technical assistance of the Australian National Imaging Facility at the Melbourne Brain Centre Imaging Unit.
References (30)
- et al.
B1-insensitive, single-shot localization and water suppression
J. Magn. Reson., Ser. B
(1996) - et al.
Feasibility of a fast method for B1-inhomogeneity correction for FSPGR sequences
Magn. Reson. Med.
(2015) - et al.
Composite RF pulses for -insensitive volume excitation at 7 Tesla
J. Magn. Reson.
(2010) - et al.
The return of the frequency sweep: designing adiabatic pulses for contemporary NMR
J. Magn. Reson.
(2001) - et al.
RF pulse methods for use with surface coils: Frequency-modulated pulses and parallel transmission pulse methods for use with surface coils: frequency-modulated pulses and parallel transmission
J. Magn. Reson.
(2018) - et al.
Highly selective π/2 and πpulse generation
J. Magn. Reson.
(1984) - et al.
A selective adiabatic spin-echo pulse
J. Magn. Reson.
(1989) - et al.
Slice selection with gradient-modulated adiabatic excitation despite the presence of large B1 inhomogeneities
J. Magn. Reson.
(1989) - et al.
Two-dimensional frequency-swept pulse with resilience to both B1 and B0 inhomogeneity
J. Magn. Reson.
(2019) - et al.
Symmetric pulses to induce arbitrary flip angles with compensation for RF inhomogeneity and resonance offsets
J. Magn. Reson.
(1991)
Uniform sample excitation with surface coils for in vivo spectroscopy by adiabatic rapid half passage
J. Magn. Reson.
Optimization of modulation functions to improve insensitivity of adiabatic pulses to variations in B1 magnitude
J. Magn. Reson.
Improved performance of frequency-swept pulses using offset-independent adiabaticity
J. Magn. Reson., Ser. A
Mapping of the radiofrequency field
J. Magn. Reson., Ser. A
Is the sech/tanh adiabatic pulse really adiabatic?
J. Magn. Reson.
Cited by (0)
- 1
These authors have contributed equally.