Transmit Array Spatial Encoding (TRASE) using broadband WURST pulses for RF spatial encoding in inhomogeneous B0 fields

Abstract

Transmit Array Spatial Encoding (TRASE) is a promising new MR encoding method that uses transmit RF (B1+) phase gradients over the field-of-view to perform Fourier spatial encoding. Acquisitions use a spin echo train in which the transmit coil phase ramp is modulated to jump from one k-space point to the next. This work extends the capability of TRASE by using swept radiofrequency (RF) pulses and a quadratic phase removal method to enable TRASE where it is arguably most needed: portable imaging systems with inhomogeneous B0 fields. The approach is particularly well-suited for portable MR scanners where (a) inhomogeneous B0 fields are a byproduct of lightweight magnet design, (b) heavy, high power-consumption gradient coil systems are a limitation to siting the system in non-conventional locations and (c) synergy with the use of spin echo trains is required to overcome intra-voxel dephasing (short T2∗) in the inhomogeneous field. TRASE does not use a modulation of the B0 field to encode, but it does suffer from secondary effects of the inhomogeneous field. Severe artifacts arise in TRASE images due to off-resonance effects when the RF pulse does not cover the full bandwidth of spin resonances in the imaging FOV. Thus, for highly inhomogeneous B0 fields, the peak RF power needed for high-bandwidth refocusing hard pulses becomes very expensive, in addition to requiring RF coils that can withstand thousands of volts. In this work, we use swept WURST RF pulse echo trains to achieve TRASE imaging in a highly inhomogeneous magnetic field (ΔB0/B0 ~ 0.33% over the sample). By accurately exciting and refocusing the full bandwidth of spins, the WURST pulses eliminate artifacts caused by the limited bandwidth of the hard pulses used in previous realizations of TRASE imaging. We introduce a correction scheme to remove the unwanted quadratic phase modulation caused by the swept pulses. Also, a phase alternation scheme is employed to mitigate artifacts caused by mixture of the even and odd-echo coherence pathways due to defects in the refocusing pulse. In this paper, we describe this needed methodology and demonstrate the ability of TRASE to Fourier encode in an inhomogeneous field (ΔB0/B0 ~ 1% over the full FOV).

Design of a high power PIN‐diode controlled switchable RF transmit array for TRASE RF imaging

“…The Transmit Array Spatial Encoding (TRASE) MR imaging method achieves spatial encoding by the use of RF (B 1 ) phase gradients. The goal of designing practical low‐cost, lightweight MR systems motivates the development of TRASE imaging and the consequent elimination of the entire conventional B 0 switched gradient system.…”

Multienergy element‐resolved cone beam CT (MEER‐CBCT) realized on a conventional CBCT platform

“…Yet, it is difficult to achieve contrast-enhanced CBCT for liver tumor image guidance because of poor CBCT image quality. 36 Current patient positioning under CBCT yields substantial uncertainty. The proposed method may enable accurate liver tumor positioning under the iodine contrast.…”

New concept on an integrated interior magnetic resonance imaging and medical linear accelerator system for radiation therapy

“…A rotating spatial encoding magnetic field (rSEM) method was employed to create generalized projections of the unknown MRI image. 17,18 This rotational data acquisition method naturally fits to our proposed iMRI-LINAC system as the CBCT data acquisition requires gantry rotation as well. This rSEM method has also been recently proposed in a study on simultaneous CT and MRI data acquisition.…”

“…[16][17][18]45 With carefully designed magnets, it is possible to achieve a constant gradient and, hence, permit projectionbased reconstruction. 46 In either way, mechanically rotational data acquisition, which naturally fits to the iMRI-LINAC system due to the required CBCT data acquisition, is used.…”