Spin Lock Adiabatic Correction (SLAC) for B1-insensitive pulse design at 7T

https://doi.org/10.1016/j.jmr.2019.106595Get rights and content

Highlights

  • Pulse design framework for adiabatic pulses with improved B1 insensitivity.

  • The framework analytically derives a correction component for an arbitrary adiabatic pulse.

  • Experimental phantom images demonstrate 10% improvement when applied to BIR-4 pulse.

Abstract

A new framework for B1 insensitive adiabatic pulse design is proposed, denoted Spin Lock Adiabatic Correction (SLAC), which counteracts deviations from ideal behaviour through inclusion of an additional correction component during pulse design. SLAC pulses are theoretically derived, then applied to the design of enhanced BIR-4 and hyperbolic secant pulses to demonstrate practical utility of the new pulses. At 7T, SLAC pulses are shown to improve the flip angle homogeneity compared to a standard adiabatic pulse with validation in both simulations and phantom experiments, under SAR equivalent experimental conditions. The SLAC framework can be applied to any arbitrary adiabatic pulse to deliver excitation with increased B1 insensitivity.

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 B1 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 B1 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 B1 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 B1 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 B1 and B0 insensitivity [23], [24].

A new avenue for extending the B1 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 B1e,Ad(t), phase ϕAd(t), and duration Tp,B1Ad(t)=B1e,Ad(t)eiϕAd(t),0tTp.

Adiabatic pulses are known to be tolerant of both B0 and B1 inhomogeneity. An important factor in pulse design at high field is to maximise tolerance to B1 inhomogeneity. In the following, we will denote by ξ the B1 field weighting factor that relates the B1 strength at a reference point in space to the B1 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 ξ = 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 B1 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 B1 field amplitude threshold at which the desired magnetisation trajectory is maintained. When applied with a carefully chosen B1 field amplitude weighting parameter, ξref, 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)

Cited by (0)

1

These authors have contributed equally.

View full text