Rapid RF Field Mapping Using a Slice-Selective Pre-conditioning RF Pulse

Introduction: In MRI, radiofrequency (RF) field (B1) inhomogeneity can produce flip angle variations, which can ultimately lead to signal intensity variations and quantitative measurement errors. By independently calibrating the local spatial variation of B1, the RFrelated signal variations can be corrected. The most widely used B1 mapping method is the double angle method (DAM) [1], which uses the ratio of two images acquired at two different nominal flip angles. However, it is inherently inefficient, due to a need for a long repetition time, TR ≥ 5T1. In this study, we describe a novel and efficient method for rapid B1 mapping using a slice-selective preconditioning RF pulse (SS-Pre pulse) followed by an ultra-fast gradient echo (TurboFLASH) pulse sequence, and evaluate its accuracy and time efficiency against the DAM method at 3T. Methods: The SS-Pre with TurboFLASH readout pulse sequence was implemented on a 3T whole-body MRI scanner (Tim Trio, Siemens). The SS-Pre pulse was designed as a slice-selective sinc pulse with nominal flip angle, nom α = 60°, pulse duration = 2.8 ms, time-bandwidth product = 6, transmitter bandwidth = 2.1 kHz, and slice thickness = 6 times that of the imaging slice thickness [2]. Immediately after a SS-Pre pulse excitation, a TurboFLASH imaging sequence with centric k-space reordering was performed to image the residual longitudinal magnetization, with associated spoiler gradients to dephase the transversal magnetization (Figure 1). An effective saturation-recovery (SR) module [3] with recovery time (tSR≤T1) can be optionally used to achieve a constant magnetization prior to the SS-Pre pulse excitation, in order to accelerate data acquisition, as has been done with DAM [4]. The pulse sequence parameters of SS-Pre with TurboFLASH readout sequence include: acquisition matrix =64x48 (64x32 for pelvis), TR/TE = 2.6/1.3ms, flip angle = 10o, bandwidth = 1500Hz/pixel, and slice thickness = 8mm. For signal normalization, a proton density (PD) image was acquired with identical imaging parameters, except without the SS-Pre pulse. The ratio of two images, a SS-Pre and a PD image, gave the measurement of the B1 scale factor (κ) using the following equation: nom 1 / )) r ( PD / ) r ( e Pr SS ( cos ) r ( α = κ − , at a position r. The pulse sequence was evaluated by imaging a T1-doped water phantom (T1=0.3s) and three healthy volunteers for brain and pelvic imaging (T1=2s). To evaluate the sensitivity to off-resonance, phantom imaging was repeated with resonance offset ranging from 0-500 Hz (100 Hz steps). The total image acquisition times of the SS-Pre method were approximately 2s/0.6s (water) and 10s/2.3s (brain and pelvis) without/with using a SR module. For the reference measurements acquired by using DAM, the total image acquisition times were approximately 153.6s (water), 960s (brain), and 640s (pelvis). Results: Phantom experiments showed that the SS-Pre pulse sequence is insensitive to off-resonance, with less than 1.4% B1 measurement error up to 500Hz off-resonance. Figure 2 shows the reference DAM κ maps and the corresponding κ maps measured by the SS-Pre method, as well as their corresponding profiles of κ. The region-of-interest (ROI) was chosen within the whole region for the phantom and the brain, and within a rectangular region (green, drawn in Fig.2) to avoid the motion artifact in the reference DAM measurements. The κ maps measured by the SS-Pre and reference methods showed excellent agreement within the ROI (water, brains, pelvises; mean difference =0.05%, 1.27%, 1.3%; 95% limits of agreement were -1.54% and 1.63%, -2.6% and 5.2%, -1.7% and 4.5%; respectively). The root-mean-squared error (RMSE) of the κ map compared to the reference map was less than 3% in both the phantom and in vivo volunteers, for both the SS-Pre method and SR SS-Pre method. Discussion: This study demonstrates a new fast B1 mapping method that can be used for a variety of applications, including body imaging applications. The SS-Pre method was faster compared to DAM, and produced B1 measurements that were in excellent agreement with those measured by the DAM method. This method assumes ideal slice profile and B0 insensitivity of the SS-Pre pulse, so that κ gives an indirect measurement of the actual B1. This method can be used for quantitative MRI applications that require fast B1 calibration. References: 1. Insko, EK et al. JMR 1993; 103:82-85. 2. Breton, E et al. NMR Biomed (in press). 3. Kim, D et al. MRM, 2009; ; DOI: 10.1002/mrm.22140 4. Cunningham CH et al.MRM 2006;55(6):1326-1333. Grant Sponsor: NIH-5R01HL083309-02, AHA-0730143N Figure 1. Sequence diagram of the SS-Pre with TurboFLASH readout method.