B1-Insensitive T2-Preparation Sequence with Outer Volume Suppression and Fat Saturation
David Y. Zeng1, Jieying Luo1, Dwight G. Nishimura1, and Adam B. Kerr1

1Electrical Engineering, Stanford University, Stanford, CA, United States


A B1-insensitive T2-weighted preparation sequence with integrated fat saturation and outer volume suppression for localized cardiac imaging is proposed. The sequence is composed of a BIR-4 90° tip-down pulse, two spectral-spatial adiabatic refocusing pulses and a BIR-4 -90° tip-up pulse. Outer volume suppression is achieved by the spatial selectivity of the first refocusing pulse in x and spatial selectivity of the second refocusing pulse in y. Fat suppression is achieved by spectral selectivity of the refocusing pulses. Numerical simulation and phantom experiments verify the performance of the sequence.


There have been several methods developed for outer volume suppression (OVS) and T2-Prep for coronary magnetic resonance angiography[1],[2]. This work presents an alternative OVS T2-Prep sequence that additionally provides fat saturation and B1-insensitivity.


The proposed preparation sequence is shown in Figure 1. A nonselective 90° B1-insensitive rotation-4 (BIR-4) pulse tips down all longitudinal magnetization into the transverse plane. The transverse magnetization then experiences T2 decay during this TE period. A spectral-spatial 180° pulse with an HSn adiabatic spectral weighting and sinc spatial weightingis applied at TE/4 with spatial selectivity in x. At 3TE/4, an identical spectral-spatial pulse is applied but with spatial selectivity in y. At TE, a -90° BIR-4 pulse tips up the T2-weighted magnetization back into the longitudinal axis. The minimum TE of this preparatory pulse is 39ms and including the spoiler gradients at the end, the entire sequence is 55ms.

The pulses were designed for robustness to B0 and B1 inhomogeneities at 1.5T. The BIR-4 pulses were designed with a hyperbolic tangent amplitude of β=10, a tangent frequency modulation of tan(λ)=10, max B1=0.15G, and pulse width=6ms[3]. The -90° tip up BIR-4 pulse is a time reversal of the 90° tip down BIR-4 pulse so that the phases of the two pulses cancel.

The true null spectral-spatial HSn pulse envelope was designed with n=2, β=3.2, bandwidth=260Hz, pulse width=15.12ms, and max B1=0.10G[4],[5]. The sinc subpulses have time-bandwidth (TBW)=4 and frequency FOV=1kHz. The HSn parameters were chosen for more robustness in the passband so that in Bloch simulations at ±1ppm there is 90% of the on-resonance signal (Figure 2b). The tradeoff is that fat is more sensitive to off-resonance for positive ΔB0. The sinc TBW and frequency FOV were chosen so that the HSn envelope is sampled often enough to maintain adiabatic spin-lock while also keeping gradient slew rates small enough to accommodate various FOVs. A higher TBW can reduce the transition band pin-cushion shape so that it is more rectangular (Figure 3a).

Outer volume suppression is achieved by spatial selectivity of the HSn pulses. The first HSn pulse is selective in x while the second HSn pulse is selective in y. Thus only spins in the intersection of these two regions are refocused while spins outside the region are spoiled. After tip up, only longitudinal magnetization from the inner volume remains. Although each HSn pulse has quadratic spectral phase and linear spatial phase, the spectral phase is cancelled out by the double refocusing so each voxel has linear phase after the two refocusing pulses.

We achieve fat saturation by the spectral selectivity of the HSn pulses. Fat is not affected by the refocusing pulses so it is consecutively spoiled by the four Gz pulses before any remaining transverse magnetization is tipped up. Bloch simulations show a longitudinal magnetization of 0.75%M0 at the end of the preparation sequence.


Bloch simulations of the proposed sequence in Figure 3a show Mz less than 0.2%M0 in the outer volume and an inner volume resembling the desired rectangle. Figures 3b-d show that at x=0cm, at an off resonance of ±1ppm, the worst-case Mz is 97%M0 and at B1/B1norm=0.8, the worst-case Mz is 92%M0.

A 1.5T Signa scanner (GE Healthcare, Waukesha, WI) and a single channel head coil were used to acquire phantom data with a single-slice Cartesian gradient-echo readout, FOV (24cm)2, RF excitation angle 30°, TE/TR 6.7/200ms, and T2-Prep TE 41ms. Figure 4 demonstrates T2-Prep and OVS; the suppression outside the passband is well appreciated when comparing the images with and without preparation. Figure 4c verifies the T2-Prep and by quantitative ROI analysis the stopband signal is below 7%M0. Figure 5 demonstrates OVS and fat saturation. Using quantitative ROI analysis, the image with preparation has a 13.1%M0 fat signal relative to the image without preparation.


From the results, we see that the proposed sequence has effective OVS, fat saturation, and T2-Prep while its B1-insensitivity comes from its adiabatic design. The key difference between the proposed sequence and existing sequences is the achievement of OVS and fat saturation by the spectral-spatial refocusing pulses. This alleviates the constraints on the 90° tip down and -90° tip up pulses so that we can choose adiabatic BIR-4 pulses for both, leading to an entirely adiabatic sequence. This method of OVS also provides a more uniform FOV than 2D spiral excitation, which produces a jinc-weighted FOV. Furthermore, the FOV can be designed as any volume that can be constructed by the intersection of two spatially selective pulses. This provides versatility of the sequence to adapt to various anatomical structures.


This research is supported by NIH R01 HL127039, GE Healthcare, Joseph W. and Hon Mai Goodman Stanford Graduate Fellowship, and the National Science Foundation Graduate Research Fellowship under Grant No. DGE-114747.


[1] Luo J et al. Combined Outer Volume Suppression and T2 Preparation Sequence for Coronary Angiography. MRM 2014.

[2] Coristine A, van Heeswijk R, Stuber M, Combined T2-Preparation and Two-Dimensional Pencil-Beam Inner Volume Selection. MRM 2015;74(2):529-536.

[3] Staewen R et al. 3-D FLASH imaging using a single surface coil and a new adiabatic pulse, BIR-4. Invest Radiol. 1990;25(5):559-567.

[4] Tannus A, Garwood M, Improved Performance of Frequency-Swept Pulses Using Offset-Independent Adiabaticity. JMR A 1996;120(1):133-137.

[5] Meyer C, Pauly J, Macovski A, Nishimura D, Simultaneous Spatial and Spectral Selective Excitation. MRM 1990:15(2):287-304.


Figure 1: Preparatory sequence timing diagram. The first pulse is a BIR-4 90° nonselective tip-down pulse. The second and third pulses are 180° spectral-spatial HSn refocusing pulses with sinc subpulses selective in x and y respectively. The fourth pulse is a BIR-4 -90° tip-up time reversal of the first pulse.

Figure 2: Bloch simulations of spectral-spatial HSn (n=2, β=3.2, bandwidth=260Hz) pulse with sinc subpulses (TBW=4, Frequency FOV=1kHz). a) Spatial and frequency inversion profile. b) Frequency inversion profile at x = 0cm. c) Spatial inversion profile at on-resonance.

Figure 3: Bloch simulation results at the end of the preparatory sequence. a) 2D distribution of longitudinal magnetization on-resonance. b) Off-resonance sensitivity of water. c) B1 sensitivity at on-resonance. d) Average longitudinal magnetization in the passband with B0 and B1 inhomogeneities.

Figure 4: Preparatory sequence performance in phantoms. Gradient-echo (TE/TR = 6.7/200ms, α=30°) reference image acquired without preparation and intended OVS FOV (red rectangle) (a) and image with preparatory sequence (b) are shown with the same display window. Passband and stopband agree well with theoretical values.

Figure5: Corn oil and water phantom (top) and water phantom (bottom) scans. Gradient-echo (TE/TR=6.7/200ms) reference image acquired without preparation and intended OVS FOV (red rectangle)(a) and image with preparatory sequence (b) are shown with different display windows. Fat signal reduced by 86.9%. OVS of bottom phantom has 95.0% reduced signal.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)