Parallel excitation for B ‐field insensitive fat‐saturation preparation

Small-tip fast recovery (STFR) imaging is a new steady-state imaging sequence that is a potential alternative to balanced steady-state free precession (bSSFP). Under ideal imaging conditions, STFR may provide comparable signal-to-noise ratio (SNR) and image contrast as bSSFP, but without signal variations due to resonance offset. STFR relies on a tailored “tip-up”, or “fast recovery”, RF pulse to align the spins with the longitudinal axis after each data readout segment. The design of the tip-up pulse is based on the acquisition of a separate off-resonance (B0) map. Unfortunately, the design of fast (a few ms) slice- or slab-selective RF pulses that accurately tailor the excitation pattern to the local B0 inhomogeneity over the entire imaging volume remains a challenging and unsolved problem. We introduce a novel implementation of STFR imaging based on non-slice-selective tip-up pulses, which simplifies the RF design problem significantly. Out-of-slice magnetization pathways are suppressed using RF-spoiling. Brain images obtained with this technique show excellent gray/white matter contrast, and point to the possibility of rapid steady-state T2/T1-weighted imaging with intrinsic suppression of cerebrospinal fluid, through-plane vessel signal, and off-resonance artifacts. In the future we expect STFR imaging to benefit significantly from parallel excitation hardware and high-order gradient shim systems.

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confidence: 99%

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

Nielsen¹,

Yoon²,

Noll³

2012

“…Moreover, the pulse length of this design was too long for steady‐state imaging sequences, and it does not handle B 1 inhomogeneity. With the advent of parallel excitation, Heilman et al proposed efficient pulses to uniformly saturate fat via multichannel transmitters by tuning the center frequency of each coil to match the variation of B 0 maps , but it is difficult for this approach to address complex B 0 patterns and to insure uniform tip angles. Recently, a parallel excitation pulse design for broadband slab selection with B 1 inhomogeneity correction demonstrates the feasibility of addressing some simple spectral profiles by tailored SPSP pulses in 3D k‐space ( k x − k y − k z ).…”

Section: Introductionmentioning

confidence: 99%

Four dimensional spectral‐spatial fat saturation pulse design

Zhang

Nielsen

Noll

2013

Purpose The conventional spectrally selective fat saturation pulse may perform poorly with inhomogeneous B0 and/or B1 fields. We propose a 4D spectral-spatial fat saturation pulse that is more robust to B0/B1 inhomogeneity and also shorter than the conventional fat saturation pulse. Theory The proposed pulse is tailored for local B0 inhomogeneity, which avoids the need of a sharp transition band in the spectral domain, so it improves both performance and pulse length. Furthermore, it can also compensate for B1 inhomogeneity. The pulse is designed sequentially by small-tip-angle approximation design and an automatic rescaling procedure. Methods The proposed method is compared to the conventional fat saturation in phantom experiments and in-vivo knee imaging at 3T for both single-channel and parallel excitation versions. Results Compared to the conventional method, the proposed method produces superior fat suppression in the presence of B0 and B1 inhomogeneity, and reduces pulse length by up to half of the standard length. Conclusion The proposed 4D spectral-spatial fat saturation suppresses fat more robustly with shorter pulse length than the conventional fat saturation in the presence of B0 and B1 inhomogeneity.

“…Multichannel transmit systems appear to be a promising solution to the B 1 nonuniformities (1–5) and increased specific absorption rate present at high field imaging ( B 0 > 3 T) (6–8). Additionally, transmit arrays with sufficient number of elements allow for selective excitation patterns (9, 10) and B 0 corrections (11, 12). These promising advantages have led to the design of transmit arrays with a high number of elements (10, 13, 14).…”

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confidence: 99%

On‐coil multiple channel transmit system based on class‐D amplification and pre‐amplification with current amplitude feedback

Gudino

Heilman

Riffe

et al. 2012

Self Cite

A complete high-efficiency transmit amplifier unit designed to be implemented in on-coil transmit arrays is presented. High power capability, low power dissipation, scalability and cost minimization were some of the requirements imposed to the design. The system is composed of a current mode class-D (CMCD) amplifier output stage and a voltage mode class-D (VMCD) preamplification stage. The amplitude information of the radio frequency pulse was added through a customized step-down DC-DC converter with current amplitude feedback that connects to the CMCD stage. Benchtop measurements and imaging experiments were carried out to analyze system performance. Direct control of B1 was possible and its load sensitivity was reduced to less than 10% variation from unloaded to full loaded condition. When using the amplifiers in an array configuration, isolation above 20 dB was achieved between neighboring coils by the amplifier decoupling method. High output current operation of the transmitter was proved on the benchtop through output power measurements and in a 1.5 T scanner through flip angle quantification. Finally, single and multiple channel excitations with the new hardware were demonstrated by receiving signal with the body coil of the scanner.