Adiabatic turbo spin echo in human applications at 7 T

Abstract

Nonuniform B(1) fields in ultrahigh-field MR imaging cause severe image artifacts, when conventional radiofrequency (RF) pulses are used. Particularly in MR sequences that encompass multiple RF pulses, e.g., turbo spin echo (TSE) sequences, complete signal loss may occur in certain areas. When using a surface coil for transmitting the RF pulses, these problems become even more challenging, as the spatial B(1) field variance is substantial. As an alternative to conventional TSE sequences, adiabatic TSE sequences can be applied, which have the benefit that these sequences are insensitive to B(1) nonuniformity. In this study, we investigate the potential of using adiabatic TSE at 7 T with surface coil transceivers in human applications. The adiabatic RF pulses were tuned to deal with the constraints in B(1) strength and RF power deposition, but remained in the superadiabatic regime. As a consequence, the dynamic range in B(1) is compromised, and signal modulation is obtained over the echo train. Multidimensional Bloch simulations over the echo train and phantom measurements were obtained to assess these limitations. Still, using proper k-space sampling, we demonstrate improved image quality of the adiabatic TSE versus conventional TSE in the brain, the neck (carotid artery) and in the pelvis (prostate) at 7 T.

Role of high‐field MR in studies of localized prostate cancer

“…The rapid decrease of B 1 + with increasing distance from the coil limits the advantage of peak B 1 + performance and severely compromises anatomical imaging with conventional RF pulses . Specifically tuned adiabatic pulses, insensitive to B 1 non‐uniformity and designed to remain in a so‐called superadiabatic regime, which provides substantially less constraints to the B 1 and RF power deposition, can be used to improve image quality with a transceiver ERC . The transceiver ERC is especially very suited for spectroscopic imaging studies because of the high B 1 + close to the coil.…”

2D radially compensating excitation pulse in combination with an internal transceiver antenna for 3D MRI of the rectum at 7 T

“…We used a BIR-4 pulse [Eq. (2)], 16,30 which behaved adiabatically on the domain of 20 − 50 µT (M x y > 90%),…”

“…However, the B 1 field of an antenna typically decreases fast with increasing distance from the antenna. While adiabatic RF pulses 16 have been proposed to provide uniform flip angles with such nonuniform B 1 fields, these RF pulses are very SAR demanding and consequently increase the scan time. Recently, promising results in rabbits have been demonstrated, using an internal antenna with nonadiabatic (composite) RF pulses.…”

Tilt optimized flip uniformity (TOFU) RF pulse for uniform image contrast at low specific absorption rate levels in combination with a surface breast coil at 7 Tesla

“…We compare the efficiency of the TOFU RF pulse with an adiabatic BIR‐4 pulse at a low B 1 strength of 5 µT , and, therefore, the pulse is against the limits of adiabaticity. Also a comparison between the TOFU and a SAR optimized spokes RF pulse is made.…”