Small‐tip fast recovery imaging using non‐slice‐selective tailored tip‐up pulses and radiofrequency‐spoiling

et al. 2015

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Purpose Small-tip fast recovery (STFR) imaging has been proposed recently as a potential alternative to balanced steady-state free precession (bSSFP). STFR relies on a tailored “tip-up” RF pulse to achieve comparable signal level as bSSFP, but with reduced banding artifacts and transient oscillations, and is compatible with magnetization-preparation pulses. Previous STFR implementations used 2D or 3D pulses spatially tailored to the accumulated phase calculated from a B0 field map, making the steady-state STFR signal contain some T2* weighting. Here we propose to replace the spatially tailored pulse with a recently introduced spectrally selective “pre-winding” pulse that is precomputed to a target frequency range. The proposed “spectral-STFR” sequence produces T2/T1-weighted images similar to bSSFP, but with reduced banding and potentially other benefits. Theory and Methods We investigated the steady-state signal properties of spectral-STFR using simulations, and phantom and human volunteer experiments. Results Our simulation and experimental results showed that the spectral-STFR sequence has similar signal level and tissue contrast as bSSFP, but has a wider passband and more consistent banding profiles across different tissues (e.g., less hyperintense signal at band edges for low flip angles). Care is needed in designing the spectral RF pulse to ensure that the small tip angle approximation holds during RF transmission. Conclusion Spectral-STFR has similar tissue contrast as bSSFP but a wider passband and more consistent CSF/brain tissue contrast across the passband. The spectral-STFR sequence is a potential alternative to bSSFP in some applications. Compared to a spatially tailored STFR sequence, spectral-STFR can be pre-computed, is easier to implement in practice, and potentially has more uniform image contrast and minimal T2* weighting.

Section: Discussionmentioning

confidence: 94%

Section: Discussionmentioning

confidence: 99%

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Balanced SSFP‐like steady‐state imaging using small‐tip fast recovery with a spectral prewinding pulse

et al. 2015

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“…Here, we assume that the tip‐up pulse is ideal, i.e.,

ϕ = θ_{f}

and

β = α

. For comparison, the calculated signals for bSSFP and spoiled STFR are also shown, using analytic results from and , respectively. Notice, we use twice the flip angle of STFR sequences in the calculation of bSSFP signals.…”

Section: Theorymentioning

confidence: 99%

“…Predicted tissue signal for unspoiled STFR (Eq. ), spoiled STFR and bSSFP . These calculations assumed T 1 / T 2 = 4000/2000, 1470/71, and 1110/56 ms for CSF, gray matter, and white matter, respectively .…”

Section: Theorymentioning

confidence: 99%

Strategies for improved 3D small‐tip fast recovery imaging

et al. 2013

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Purpose Small-tip fast recovery (STFR) imaging is a recently proposed steady-state sequence that has similar image contrast as balanced steady-state free precession (bSSFP) but has the potential to simultaneously remove banding artifacts and transient fluctuation. STFR relies on a “tip-up” radiofrequency (RF) pulse tailored to the accumulated phase during the free precession (data acquisition) interval, designed to bring spins back to the longitudinal axis, thereby preserving transverse magnetization as longitudinal magnetization for the next TR. We recently proposed an RF-spoiled STFR sequence suitable for thin slab imaging, however in many applications, e.g., functional MRI or isotropic-resolution structural imaging, 3D steady-state imaging is desirable. Unfortunately, 3D STFR imaging is challenging due to the need for 3D tailored RF pulses. Here we propose new strategies for improved 3D STFR imaging, based on (i) unspoiled imaging, and (ii) joint design of non-slice-selective tip-down/tip-up RF pulses. Theory and Methods We derive an analytic signal model for the proposed unspoiled STFR sequence, and propose two strategies for designing the 3D tailored tip-down/tip-up RF pulses. We validate the analytic results using phantom and in-vivo imaging experiments. Results Our analytic model and imaging experiments demonstrate that the proposed unspoiled STFR sequence is less sensitive to tip-up excitation error compared to the corresponding spoiled sequence, and may therefore be an attractive candidate for 3D imaging. The proposed “joint” RF pulse design method, in which we formulate the tip-down/tip-up RF pulse design task as a magnitude least squares problem, produces modest improvement over a simpler “separate” design approach. Using the proposed unspoiled sequence and joint RF pulse design, we demonstrate proof-of-principle 3D STFR brain images with bSSFP-like signal properties but with reduced banding. Conclusion Using the proposed unspoiled sequence and joint RF pulse design, STFR brain images in a 3D region of interest (ROI) with bSSFP-like signal properties but with reduced banding can be obtained.

Steady‐state functional MRI using spoiled small‐tip fast recovery imaging

et al. 2014

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Purpose To determine whether a recently-proposed steady-state magnetic resonance imaging (MRI) sequence, “small-tip fast recovery” (STFR), can be used for functional brain imaging. Compared to existing functional MRI (fMRI) based on T2*-contrast and long echo time (TE), STFR has the potential for high-resolution imaging with reduced B0 artifacts such as geometric distortions, blurring, or local signal dropout. Methods We used Monte Carlo Bloch simulations to calculate the voxel-averaged steady-state signal during rest and activation, for BOLD and STFR. STFR relies on a tailored “tip-up” radiofrequency (RF) pulse to align the spins with the longitudinal axis after each data readout segment, and here we performed proof-of-concept in vivo STFR fMRI experiments using a tip-up pulse tailored to a 2D region-of-interest (ROI) in motor cortex. Experiments were performed on multiple subjects to test reliability of the functional activation maps. Results Bloch simulations predict a detectable functional signal that depends mainly on intra-voxel dephasing, and only weakly on spin diffusion. STFR produces similar activation maps and signal change as BOLD in finger-tapping experiments, and shows reliability comparable to BOLD. Conclusion STFR can produce functional contrast (even with short TE), and is a potential alternative to long-TE (T2*) fMRI. The functional contrast arises primarily from the interaction between T2*-like dephasing and the tailored tip-up pulse, and not from spin diffusion.