Slice profile effects on nCPMG SS‐FSE

Abstract

Purpose To determine the effects of the RF refocusing pulse profile on the magnitude of the transverse signal smoothness throughout the echo train in non‐Carr‐Purcell‐Meiboom‐Gill (nCPMG) single‐shot fast spin echo (SS‐FSE) imaging and to design an RF refocusing pulse that provides improved signal stability. Theory and Methods nCPMG SS‐FSE quadratic phase modulation requires sufficiently high and uniform refocusing flip angle to achieve a stable signal. Typically, refocusing pulses used in SS‐FSE sequences are designed for minimum duration to minimize echo spacing and as a consequence have poor selectivity. However, delay‐insensitive variable rate excitation Shinnar‐Le Roux (DV‐SLR) refocusing pulses can achieve both improved selectivity as well as a short duration. This class of RF pulse is compared against a traditional low time‐bandwidth refocusing pulse in a nCPMG SS‐FSE in simulation, phantom, and in vivo. Results DV‐SLR pulses achieve a more stable signal in simulation, phantom, and in vivo cases while maintaining an appropriately short duration as well as not dramatically increasing specific absorption rate (SAR) accumulation. Conclusion The nCPMG SS‐FSE method demonstrates improved robustness when a more selective refocusing pulse is used. Refocusing pulses that use a time‐varying excitation gradient can achieve this selectivity while maintaining short echo spacing. Magn Reson Med 79:430–438, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

A joint linear reconstruction for multishot diffusion weighted non‐Carr‐Purcell‐Meiboom‐Gill fast spin echo with full signal

“…Although the echo train is stable for $$$ 15{0}^{\circ } $$$, a nominal refocusing flip angle of $$$ 18{0}^{\circ } $$$ was prescribed to prevent B1 inhomogeneity from reducing the refocusing flip angle below $$$ 15{0}^{\circ } $$$. We also used a widened refocusing slice thickness to reduce ghosting of the non‐CPMG component from low flip‐angles in the transition band 28 . Delay Insensitive Variable Rate Selective Excitation (DI‐VERSE) pulses 45 employed by Gibbons et al 16 can reduce slice cross‐talk and SAR.…”

“…The slice thickness of the refocusing pulse with time bandwidth product 2 was widened to 2× the excited slice thickness to create a uniform 180 • slice profile and reduce ghosting from slice profile effects. 28 Fat suppression was achieved with standard fat-saturation, a nonselective 90 • pulse centered on fat. For the prostate, B1-insensitive fat suppression was achieved with a spectrally selective adiabatic 180 • inversion pulse centered on fat.…”

Body diffusion‐weighted imaging using magnetization prepared single‐shot fast spin echo and extended parallel imaging signal averaging

“…This loss of signal in the Y magnetization is the source of artifacts that are associated with CPMG SS‐FSE diffusion sequences. This is further elaborated on in .…”

“…This non-Carr-Purcell-Meiboom-Gill (nCPMG) phase modulation does preserve the full signal throughout the echo train. However, this sequence has strict requirements on the RF pulses used (8) and presents a complicated reconstruction (9) due to oscillation of the quadrature component of the magnetization, which requires doubling the number of phase encodes and increases image blurring and specific absorption rate (SAR) accumulation (10).…”

Robust Self-Calibrating nCPMG Acquisition: Application to Body Diffusion-Weighted Imaging

“…This sequence is seen in Figure 4. Variable excitation rate Shinnar-Le Roux (DV-SLR) pulses were used for refocusing according to [25]. In each case, image matrix sizes were 128 y × 192 x pixels.…”