Synopsis

Keywords: Bone, Bone

$$$B_1^+$$$ field inhomogeneity is a source of metal artifacts in patients with orthopedic hardware. Local $$$B_1^+$$$ shimming can potentially decrease these artifacts and improve visualization of the bone-metal interface. Our proposed turbo-spin echo-based $$$B_1^+$$$ mapping technique enables accurate estimation of the $$$B_1^+$$$ field near the metal hardware. After optimization for in-vivo applications, the technique was successfully employed on a clinical 3.0 T parallel-transmit system aiming at $$$B_1^+$$$ shimming near the orthopedic hardware. Our results demonstrate significant improvement in visualization of the bone-metal interface compared to standard 1.5 and 3.0 T acquisitions.

Introduction

Susceptibility artifacts are a well-known source of image artifacts in MRI of patients with metallic orthopedic hardware. The other much less investigated source of artifacts is the perturbation in the transmit $$$B_1^+$$$ field, which is caused by metal-induced electromagnetic fields ^1-2. Such $$$B_1^+$$$ inhomogeneities may cause signal alterations at the bone-metal interface and be misinterpreted as an abnormality and obscure the underlying pathology. $$$B_1^+$$$ shimming localized to the area surrounding the metal hardware using a multi-channel transmit system can potentially provide a more homogeneous $$$B_1^+$$$ field and improve visualization of the bone-metal interface. A successful $$$B_1^+$$$ shim requires (1) obtaining reliable $$$B_1^+$$$ field maps in the presence of metal and (2) a subject-specific combination of the individual $$$B_1^+$$$ field maps from each transmit channel. This work aims to develop a turbo-spin echo (TSE)-based $$$B_1^+$$$ mapping technique for in-vivo applications and analyze the effect of local $$$B_1^+$$$ shimming on the image quality of patients with metallic orthopedic hardware using a clinical dual-transmit system.

Methods

$$$\it B_1^+$$$ mapping: We have recently developed and validated a TSE-based method of $$$B_1^+$$$ mapping which relies on the acquisition of multiple images with various excitation-refocusing (EX-Ref) flip angles (FA) ³. Briefly, with a $$$B_1^+$$$ amplitude scale factor of b₁ (b₁ = actual $$$B_1^+$$$ / nominal $$$B_1^+$$$), the signal intensity of a TSE sequence can be expressed as: $$$S(b_{1}) = f(b_{1}.\theta,b_{1}.\phi,\psi)$$$, where θ and φ represent excitation and refocusing flip angles, respectively, ψ other imaging parameters, and f(.) is the signal Bloch model. For different sets of Ex-Ref FA, b₁ can be obtained by solving the following optimization problem: $$\widehat{b_{1}} = \min_{b_1}\sum_i^n\parallel f(b_{1}.\theta_{i},b_{1}.\phi_{i},\psi)-\widehat{S}(\theta_{i},\phi_{i},\psi) \parallel_{2}$$ with $$$\widehat{S}(\theta_{i},\phi_{i},\psi)$$$ being the pixel signal obtained by the i^th set of Ex-Ref FA.
$$$\it B_1^+$$$ mapping optimization for in-vivo imaging: Monte-Carlo simulations were performed to study the sensitivity of $$$B_1^+$$$ relative error to the number and choice of Ex-Ref FA pairs. The TSE signal was simulated for a wide range of excitation (30-120°) and refocusing (60-180°) FA sets using Bloch equations. Gaussian noise was added to the simulated signal for each set and the mean $$$B_1^+$$$ map was estimated from one thousand realizations.
$$$\it B_1^+$$$ shimming: The composite $$$B_1^+$$$ field can be obtained by complex linear combination of the $$$B_1^+$$$ field maps of the two transmit channels using the $$$B_1^+$$$ amplitudes of the previous step and the relative $$$B_1^+$$$ phase distribution of the TSE images. For a given region-of-interest (ROI) near the metal, the shimming was performed by adjusting the relative amplitude, and phases of the two transmit elements, aiming at minimizing the difference between the $$$B_1^+$$$ in the ROI and that of the background.
MRI Experiments: After obtaining institutional review board approval and informed consent, a volunteer with a femoral intramedullary nail was imaged on a parallel-transmit 3T clinical system with the following parameters: TR/TE = 2000/29 ms, voxel size = 0.6 x 0.6 x 3.0 mm³, and turbo factor = 13. Standard and $$$B_1^+$$$ shimmed intermediate-weighted axial images were acquired, focusing on visualization of the bone-metal interface. For comparison purposes, $$$B_1^+$$$ maps were also obtained using a TurboFLASH (TFL) sequence equipped with a preceding RF pulse for magnetization preparation ⁴.

Results and Discussion

Monte-Carlo simulation results on the optimal number and choice of the Ex-Ref FA pairs are summarized in Figure 1. The lowest mean relative error in $$$B_1^+$$$ estimations were 27%, 19%, and 18% in 2,3, and 4 Ex-Ref FA sets, respectively. Considering the increased imaging time and specific absorption rate (SAR) values, these results suggest that acquiring more than three Ex-Ref FA sets will not result in improved $$$B_1^+$$$ estimation. In addition, the error in $$$B_1^+$$$ estimation is more pronounced at low $$$B_1^+$$$ values, as the overall low signal at low $$$B_1^+$$$ values reaches the noise floor. Using the optimal three Ex-Ref FA sets, the $$$B_1^+$$$ field was estimated in a volunteer and compared with those of the TFL method (Figure 2). As seen on magnified subplots, the $$$B_1^+$$$ field near the metal is better resolved with our technique. The effect of $$$B_1^+$$$ shimming on the visualization of the bone-metal interface is shown in Figure 3. $$$B_1^+$$$ shimming in a bone-metal containing ROI provides the optimal shim parameters, which were used for subsequent imaging. Compared with the standard 1.5 and 3.0 T images, the visibility of the bone-metal interface is significantly improved after $$$B_1^+$$$ shimming.

Conclusion

The proposed $$$B_1^+$$$ mapping technique provides promising results at the bone-metal interface invisible to other mapping sequences. Our initial results suggest that patient-specific local $$$B_1^+$$$ shimming is clinically feasible and can reduce $$$B_1^+$$$-related artifacts surrounding the metal.

Acknowledgements

No acknowledgement found.