Robust slice-selective broadband refocusing pulses

Application of spin echoes in the regime of weak dephasing to T1 -mapping of the lung

PURPOSE

This work presents an approach to mapping the entire lung's proton density and T₁ within a single breath-hold and analyzes the apparent T₁ when exciting with a spin echo generating pulse in comparison to a standard gradient echo acquisition.

METHODS

An inversion-recovery SNAPSHOT-FLASH sequence with a stack-of-stars k-space readout with a golden angle increment was modified to use a spin echo generating radiofrequency-pulse for excitation. Data of five volunteers were acquired on a 3T scanner and image reconstruction was performed by an iterative algorithm adopted from MR-Fingerprinting.

RESULTS

The feasibility of acquiring quantitative maps of the entire lung with a resolution of 5 × 5 × 10 mm within 7.5 s is demonstrated. It is shown that the proposed spin echo forming radiofrequency-pulse increases the apparent proton density compared to a rectangular pulse. Further, the apparent T₁ is reduced in the spin echo case compared to the gradient echo sequence.

CONCLUSION

The proposed spin echo based method results in T₁ maps that are comparable to the ones that were acquired with ultra-short echo time sequences elsewhere. The T₁ shortening is believed to originate from increased signal contributions of the extra vascular compartment, which has a short T2∗ and T₁ . Magn Reson Med 79:960-967, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Wavelet Domain Radiofrequency Pulse Design Applied to Magnetic Resonance Imaging

A new method for designing radiofrequency (RF) pulses with numerical optimization in the wavelet domain is presented. Numerical optimization may yield solutions that might otherwise have not been discovered with analytic techniques alone. Further, processing in the wavelet domain reduces the number of unknowns through compression properties inherent in wavelet transforms, providing a more tractable optimization problem. This algorithm is demonstrated with simultaneous multi-slice (SMS) spin echo refocusing pulses because reduced peak RF power is necessary for SMS diffusion imaging with high acceleration factors. An iterative, nonlinear, constrained numerical minimization algorithm was developed to generate an optimized RF pulse waveform. Wavelet domain coefficients were modulated while iteratively running a Bloch equation simulator to generate the intermediate slice profile of the net magnetization. The algorithm minimizes the L2-norm of the slice profile with additional terms to penalize rejection band ripple and maximize the net transverse magnetization across each slice. Simulations and human brain imaging were used to demonstrate a new RF pulse design that yields an optimized slice profile and reduced peak energy deposition when applied to a multiband single-shot echo planar diffusion acquisition. This method may be used to optimize factors such as magnitude and phase spectral profiles and peak RF pulse power for multiband simultaneous multi-slice (SMS) acquisitions. Wavelet-based RF pulse optimization provides a useful design method to achieve a pulse waveform with beneficial amplitude reduction while preserving appropriate magnetization response for magnetic resonance imaging.

Spectroscopic localization by simultaneous acquisition of the double-spin and stimulated echoes

PURPOSE

To design a proton MR spectroscopy ((1) H-MRS) localization sequence that combines the signal-to-noise-ratio (SNR) benefits of point resolved spectroscopy (PRESS) with the high pulse bandwidths, low chemical shift displacements (CSD), low specific absorption rates (SAR), short echo times (TE), and superior radiofrequency transmit field (B1+) immunity of stimulated echo acquisition mode (STEAM), by simultaneously refocusing and acquiring both the double-spin and stimulated echo coherence pathways from the volume of interest.

THEORY AND METHODS

We propose a family of (1)H-MRS sequences comprising three orthogonal spatially selective pulses with flip angles 90° < α, β, γ < 128°. The stimulated and double-spin echo are refocused in-phase simultaneously by altering the pulses' phases, flip angles and timing, as well as the interpulse gradient spoiling moments. The ≈ 90° nutations of α, β, γ provide STEAM-like advantages (lower SAR, in-plane CSD and TE; greater B1+ immunity), but with SNRs comparable with PRESS.

RESULTS

Phantom and in vivo brain experiments show that 83-100% of the PRESS SNR (metabolite-dependent) is achieved at under 75% of the SAR and 66% lower in-plane CSD.

CONCLUSION

The advantages of STEAM can be augmented with the higher SNR of PRESS by combining the spin and stimulated echoes. Quantification, especially of J-coupled resonances and intermediate and long TEs, must be carefully considered.

Gradient-excitation encoding combined with frequency and phase encodings for three-dimensional ultra-low-field MRI

Ultra-low-field magnetic resonance imaging (ULF MRI) in microtesla fields is a new technology with features unseen in tesla-range MRI. Instead of induction coils as sensors, superconducting quantum interference device (SQUID) sensors are used, providing a frequency-independent signal-to-noise ratio (SNR). Owing to its tolerance for large relative imaging-field inhomogeneities, electromagnet shimming is not necessary. ULF MRI can also be combined with magnetoencephalography (MEG) to image the brain with close to millimetre-millisecond resolution. In this paper, the hybrid MEG-MRI device developed at Aalto University will be presented, as well as a 3D imaging scheme combining gradient-excitation encoding with frequency and phase and encodings. It is noteworthy that, regarding the presented gradient-excitation encoding in ULF MRI, the kilohertz-range Larmor frequencies allow MR signals to propagate unattenuated through tissue, which is not the case in tesla-range MRI with Larmor frequencies even above 100 MHz. Thus, the presented encoding method is especially compatible with ULF MRI, where the use of three different encoding mechanisms for three-dimensional imaging is possible. The feasibility of image reconstruction with the gradient-excitation-encoding method is demonstrated by simulations.