We have designed the STABLE-2 RF pulse, a shorter, more B0-insensitive option for slice-selective adiabatic excitation than the previously proposed STABLE pulse. The shorter pulse duration increases the pulse’s applicability by enabling its use in a wider range of pulse sequences and at higher field strengths. Two variants of the STABLE-2 pulse were investigated: (1) a nonselective version that is designed to achieve robust excitation profiles in the presence of B1 and B0 inhomogeneity and (2) a selective version that is designed to provide effective fat suppression, while retaining B1-insensitive slice-selective excitation.
In vivo data at 3 and 7 T demonstrate that STABLE-2 performs adiabatically over approximately a 50% increase in the B1
-amplitude above the adiabatic threshold. The pulse is better suited than the previously proposed STABLE pulse (4
) for imaging at high magnetic fields, such as 7 T, due to increased robustness to B0
-shifts and a shorter pulse duration enabling shorter echo times.
Intrinsic nulls in the off-resonance profile were also leveraged to suppress signal from fat in the selective version of the pulse. Phantom and in vivo data at 3 T demonstrate that simultaneous adiabatic slice-selective excitation and fat suppression are achieved by this version of STABLE-2. To use STABLE-2 for fat-suppressed excitation at 7 T, the spectral profile would have to be redesigned to achieve a null at the scaled frequency difference between water and fat, which is ~1026 Hz.
STABLE-2 may provide a valuable B1
-insensitive alternative to existing fat suppression techniques. A common method for fat suppression is the use of frequency selective presaturation (FATSAT) (19
) pulses that are not adiabatic and susceptible to B1
). Water excitation using SPSP pulses results in more robust fat-suppression but loss in water excitation efficiency, as the B1
field varies (21
). Short tau inversion recovery (22
) utilizes an adiabatic 180° pulse for inversion of all spins and delays excitation until an inversion time when fat is passing through a null. Due to the use of an adiabatic pulse, short tau inversion recovery is inherently more resistant to changes in B1
. However, as fat suppression is achieved by exploiting the differential T1
value of tissues, SNR efficiency reduces as T1
values converge with increasing field strength. Spectrally selective adiabatic inversion recovery has been proposed, which does not depend on differential T1
’s but still requires waiting a inversion time prior to excitation (24
-Insensitive RF pulse trains may be used to achieve fat suppression over a limited range of B1
values, but still require a total duration of 77 ms (25
). STABLE-2 offers a B1
- and T1
-insensitive option for water only excitation and requires no additional preparation time. STABLE-2 does have the disadvantage of having greater SAR than a FATSAT pulse or conventional SPSP excitation pulse. However, the pulse functioned well within SAR limits at both 3 and 7 T when integrated into a GRE sequence with a 500-ms TR, as shown in the “Results” section. Also, narrowing of the lipid spectral stop band beyond a 30% increase in B1
-amplitude above the adiabatic threshold results in incomplete fat suppression for STABLE-2. Further exploration of new spectral envelopes with a wider stop band on lipids would make this a more robust fat suppression technique.
We investigated the 2D magnetization profile of STABLE pulses for flip angles below 90° and found that off-resonance immunity for all STABLE designs is reduced for excitation flip angles below 90°. shows the simulated slice profile versus off-resonance frequency for a 45° STABLE-2 pulse. The pulse was created by reducing the phase offset between adiabatic half-passage pulse segments to equal the desired flip angle. As the off-resonance frequency approaches ±60 Hz, distortion lobes appear in the 2D profile and the excited magnetization gradually increases to 100%, which deviates from the expected flip angle. Therefore, the pulse only provides the expected flip angle over a narrower frequency range of ~50 Hz.
Simulated slice profile versus off-resonance frequency for STABLE-2 RF Pulse designed for a 45° excitation flip angle. The desired flip angle is achieved for approximately a 50-Hz range in off-resonance.
Limited off-resonance immunity is an intrinsic disadvantage of STABLE pulses. The use of insert gradients may allow shorter pulse duration, thereby increasing the frequency range over which undistorted slice-selection is achieved. A higher gradient maximum amplitude and slew rate will also enable thinner minimum slice thickness. However, for some subjects, PNS may occur before reaching these limits. Due to the use of short subpulses to achieve slice selection in all STABLE designs, exciting thin slices remains a challenge. Slice selectivity may also be traded off with slice thickness, that is, subpulses with lower spatial bandwidth may be used to generate thinner slices with less sharp edges. This technique was employed in the version of STABLE-2 for fat suppressed excitation.
STABLE, STABLE-2, and the numerically optimized slice-selective composite pulses proposed by Moore et al. (5
) are all part of a family of SPSP-like nonlinear phase pulses, which exhibit adiabatic and slice-selective behavior. The pulses by Moore et al. achieve many of the same design goals as STABLE-2 and offer a powerful alternative for B1
-insensitive excitation. We expect that there probably exist more RF pulses that are members of this family that remain to be explored. Similar limits exist on the slice thickness that may be achieved by these pulses, as slice thickness is determined by the subpulse spatial bandwidth and the strength of the gradients used. Moore et al. explored gaussian and sinc subpulses and operated at gradient limits to achieve slices as thin as 2 mm. In the design of STABLE-2, different subpulse waveforms were not fully explored to decrease slice thickness. However, subpulses can easily be adjusted within this design framework to explore these trade offs. Moore et al. were successful in shortening pulse duration to 5 ms and achieved slice-selective behavior over a 200-Hz range with similar B1
-immunity but less selective slice profiles than STABLE-2. As STABLE-2 enforces an adiabatic envelope, it is challenging to reduce pulse duration to 5 ms without increasing peak RF amplitude to prohibitively high values. This may be an inherent disadvantage of STABLE-2 when compared to the composite pulses by Moore et al. However, the fact that STABLE-2 uses an adiabatic BIR-4 envelope, which is systematically generated by the adiabatic SLR algorithm (6
) makes it possible for the pulse designer to have greater control over the final shape of the spatial-spectral profile. The fat-nulling version of STABLE-2 presented in this work is an example of how pulse parameters may be adjusted to predictably change profile shape.
Contrast differences due to the spin-locked state of the magnetization during the STABLE-2 pulse were not explored in this study. These differences are less pronounced than those observed for the original STABLE pulse due to the shorter duration of the pulse. However, they remain a subject of future study.