Sidebands in CEST MR-How to recognize and avoid them.

PURPOSE: Clinical scanners require pulsed CEST sequences to maintain amplifier and specific absorption rate limits. During off-resonant RF irradiation and interpulse delay, the magnetization can accumulate specific relative phases within the pulse train. In this work, we show that these phases are important to consider, as they can lead to unexpected artifacts when no interpulse gradient spoiling is performed during the saturation train. METHODS: We investigated sideband artifacts using a CEST-3D snapshot gradient-echo sequence at 3 T. Initially, Bloch-McConnell simulations were carried out with Pulseq-CEST, while measurements were performed in vitro and in vivo. RESULTS: Sidebands can be hidden in Z-spectra, and their structure becomes clearly visible only at high sampling. Sidebands are further influenced by B0 inhomogeneities and the RF phase cycling within the pulse train. In vivo, sidebands are mostly visible in liquid compartments such as CSF. Multi-pulse sidebands can be suppressed by interpulse gradient spoiling. CONCLUSION: We provide new insights into sidebands occurring in pulsed CEST experiments and show that, similar as in imaging sequences, gradient and RF spoiling play an important role. Gradient spoiling avoids misinterpretations of sidebands as CEST effects especially in liquid environments including pathological tissue or for CEST resonances close to water. It is recommended to simulate pulsed CEST sequences in advance to avoid artifacts.

Hexagonal gradient scheme with RF spoiling improves spoiling performance for high-flip-angle fast gradient echo imaging.

PURPOSE: To present a framework in which time-varying gradients are applied with RF spoiling to reduce unwanted signal, particularly at high flip angles. METHODS: A time-varying gradient spoiler scheme compatible with RF spoiling is defined, in which spoiler gradients cycle through the vertices of a hexagon, which we call hexagonal spoiling. The method is compared with a traditional constant spoiling gradient both in the transition to and in the steady state. Extended phase graph (EPG) simulations, phantom acquisitions, and in vivo images were used to assess the method. RESULTS: Simulations, phantom and in vivo experiments showed that unwanted signal was markedly reduced by employing hexagonal spoiling, both in the transition to and in the steady state. For adipose tissue at 1.5 Tesla, the unwanted signal in the steady state with a 60 ° flip angle was reduced from 22% with constant spoiling to 2% with hexagonal spoiling. CONCLUSIONS: A time-varying gradient spoiler scheme that works with RF spoiling, called "hexagonal spoiling," has been presented and found to offer improved spoiling over the traditional constant spoiling gradient. Magn Reson Med 77:1231-1237, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Spoiling without additional gradients: Radial FLASH MRI with randomized radiofrequency phases.

PURPOSE: To develop a method for spoiling transverse magnetizations without additional gradients to minimize repetition times for radial fast low angle shot (FLASH) MRI. METHODS: Residual steady state transverse magnetizations and corresponding image artifacts were analyzed for radial gradient echo sequences with constant and randomized RF phases in comparison with a sequence with refocused frequency-encoding gradients, constant spoiler gradient, and conventional RF spoiling (gold standard). The spoiling performance was assessed for different radial trajectories using numerical simulations, phantom experiments, and in vivo MRI studies of the human brain. RESULTS: Simulations as well as phantom and in vivo measurements reveal a highly efficient spoiling capacity for randomized RF phases and radial FLASH sequences without the need for gradient rewinding and spoiler gradients. The data also demonstrate a strong dependence of the spoiling performance on the chosen radial trajectory (ie, the azimuthal angular increment between successive projections) with excellent results for an interleaved multiturn scheme. CONCLUSION: Effective spoiling of transverse magnetizations in radial FLASH MRI may be achieved by randomized RF phases without additional spoiler gradients. The technique allows for short repetition times as required for high-speed real-time MRI.

Estimation of effective b-value for a diffusion-weighted double-echo steady-state sequence with bipolar gradients.

Diffusion-weighted double-echo steady-state (dwDESS) MRI with bipolar diffusion gradients is a promising candidate to obtain diffusion weighted images (DWI) free of geometric distortions and with low motion sensitivity. However, a wider clinical application of dwDESS is currently hindered as no method is reported to explicitly calculate the effective b-value of the obtained DWI from the diffusion-gradients applied in the sequence. To this end, a previously described signal model was adapted for dwDESS with bipolar diffusion gradients, which allows to estimate an effective b-value, dubbed b'. Evaluation in phantom examinations was performed on a clinical 1.5 T MR system. Experimental results were compared with theoretical predictions, including the apparent diffusion coefficient (ADC) based on b-values from a standard EPI-DWI sequence and ADC' based on the effective b' from the dwDESS sequence. The adapted signal model was able to describe the experimental results, and the obtained values of ADC' were in line with conventional ADC measurements.