Accelerated susceptibility-based positive contrast imaging of MR compatible metallic devices based on modified fast spin echo sequences

Rapid variable flip angle positive susceptibility contrast imaging for clinical metal seeds

Positive susceptibility contrast imaging (PSCI) based on susceptibility mapping exhibits excellent efficacy for visualizing magnetic resonance (MR)-compatible metallic devices because of their high magnetic susceptibility compared to that of human tissues. However, the long-acquisition time required by the two-dimensional fast spin echo (2D FSE)-based PSCI approach, impedes its practical applications in 3D imaging. In this study, a three-dimensional (3D) susceptibility-based variable flip angle (vFA) FSE sequence was proposed to accelerate data acquisition in the clinical radiotherapy applications of ex vivo and in vivo rapid 3D PSCI for the imaging of metal seeds. Here, the proposed scheme applied a 3D modulated vFA technique for refocused imaging with an extended echo-train sequence for sampling data. The scheme integrated the projection-onto-dipole fields (PDF) to remove the background field and accelerate PSCI by using a compressive sensing framework with a variable-densitysampling mask. The experiments involved some gelatin phantoms, porcine tissues and patients with scapular tumors and brachytherapy seeds. All of the experimental results showed that the proposed scheme could accelerate data acquisition of 3D PSCI at the reduction factors of 2 ∼ 5 while accurately localizing the actual positions of the brachytherapy seeds in the ex vivo and in vivo applications. The results were compared with those of the existing methods, including susceptibility gradient mapping using the original resolution (SUMO) and gradient echo acquisition for superparamagnetic particle (GRASP).

Positive Contrast Susceptibility MR Imaging Using GPU-based Primal-Dual Algorithm

The susceptibility-based positive contrast MR technique was applied to estimate arbitrary magnetic susceptibility distributions of the metallic devices using a kernel deconvolution algorithm with a regularized L-1 minimization. Previously, the first-order primal-dual (PD) algorithm could provide a faster reconstruction time to solve the L-1 minimization, compared with other methods. Here, we propose to accelerate the PD algorithm of the positive contrast image using the multi-core multi-thread feature of graphics processor units (GPUs). The some experimental results showed that the GPU-based PD algorithm could achieve comparable accuracy of the metallic interventional devices in positive contrast imaging with less computational time. And the GPU-based PD approach was 4~15 times faster than the previous CPU-based scheme.Clinical Relevance-This can estimate arbitrary magnetic susceptibility distributions of the metallic devices with the processing efficacy of 4~15 times faster than before.

Three Dimensional Positive Contrast Susceptibility Fast Spin Echo MR Imaging with Variable Excitation Pulses and PD Algorithm

A susceptibility-based positive contrast MR technique is applied to image the MR compatible metallic devices by solving a regularized ℓ1 minimization problem. However, the previous SE/FSE sequence is used for the data acquisition which can result in high SAR and low sampling efficiency in 3D imaging. Therefore, a 3D single slab 3D FSE sequence with slab selective and variable excitation pulse is proposed to implement 3D positive contrast MR imaging for low SAR and acquiring high-resolution 3D images within a shorter timeframe. Furthermore, in order to achieve faster reconstruction and better imaging quality of the 3D positive contrast MRI, the primal-dual iteration algorithm is also used to solve the regularized ℓ1 minimization problem. The visualization of the positive contrast and convergence behaviour of the proposed reconstruction framework base on the first-order PD algorithm were tested and validated on phantom experiments, compared with the previous nonlinear conjugate gradient (NLCG), fast iterative soft thresholding (FISTA) and alternating direction method of multipliers (ADMM) algorithms.

Susceptibility-based MR Imaging of Nitinol Stent

Conventional MR techniques have difficulty to accurately localize the stent position and access the stent restenosis because of the effects of susceptibility and radiofrequency (RF) shielding artifacts caused by stent mesh. Previous studies have demonstrated that a susceptibility-based positive contrast MR method exhibits excellent efficacy for visualizing MR compatible metal devices by taking advantage of their high magnetic susceptibility. However, the method is not evaluated in the visualization of stents. Therefore, the purpose of this study is to prospectively assess whether the susceptibility-based positive contrast method can be used to visualize the nitinol stents, with the comparison of two typical MR positive contrast techniques, i.e., susceptibility gradient mapping using the original resolution (SUMO) and the gradient echo acquisition for super-paramagnetic particles with positive contrast (GRASP). The experiment results showed that the susceptibility-based method provided better visualization and more precise localization of the stent than SUMO and GRASP.

MARBLES - Metal Artifact Based Landmark Enhanced Susceptibility Weighted Imaging For Interventional Device Localization In MRI

Susceptibility Weighted Imaging (SWI) is a method extensively studied for its application to improve contrast in MR imaging modality. The method enhances the visualization of magnetically susceptible content such as iron, calcium and zinc in the tissues by using the susceptibility differences in tissues to generate a unique image contrast. In this study, we propose an SWI based approach to improve the visualization of interventional devices in MRI data. Results obtained from two datasets (biopsy needle and brachytherapy seeds), indicate SWI to be suitable for visualization of the interventional devices, while also being computationally faster when compared with quantitative susceptibility mapping (QSM).

Positive-contrast susceptibility imaging based on first-order primal-dual optimization

PURPOSE

To achieve faster reconstruction and better imaging quality of positive-contrast MRI based on the susceptibility mapping by incorporating a primal-dual (PD) formulation.

METHODS

The susceptibility-based positive contrast MR technique was applied to estimate arbitrary magnetic susceptibility distributions of the metallic devices using a kernel deconvolution algorithm with a regularized ℓ 1 minimization. The regularized positive-contrast inversion problem and its PD formulation were derived. The visualization of the positive contrast and convergence behavior of the PD algorithm were compared with those of the nonlinear conjugate gradient algorithm, fast iterative soft-thresholding algorithm, and alternating direction method of multipliers. These methods were tested and validated on computer simulations and phantom experiments.

RESULTS

The PD approach could provide a faster reconstruction time compared with other methods. Experimental results showed that the PD algorithm could achieve comparable or even better visualization and accuracy of the metallic interventional devices in positive-contrast imaging with different SNRs and orientations to the B₀ field.

CONCLUSION

A susceptibility-based positive-contrast imaging technique by PD algorithm was proposed. The PD approach has more superior performance than other algorithms in terms of reconstruction time and accuracy for imaging the metallic interventional devices.

Positive visualization of MR-compatible nitinol stent using a susceptibility-based imaging technique

Background

MR-compatible metallic stents have been widely used for the treatment of arterial occlusive diseases. However, conventional MR techniques have difficulty in accurately localizing the stent position and access the stent restenosis because of the susceptibility and radiofrequency (RF) shielding artifacts caused by the stent mesh. Previous studies have demonstrated that a susceptibility-based positive contrast MR method exhibits excellent efficacy for visualizing MR compatible metal devices. However, the method had not been evaluated in the visualization of stents and for the assessment of stent restenosis.

Methods

The susceptibility-based positive contrast MR method was used to visualize the nitinol stents and assess the stent restenosis by comparing two typical MR positive contrast techniques, i.e., susceptibility gradient mapping using the original resolution (SUMO) and the gradient echo acquisition for super-paramagnetic particles (GRASP) with positive contrast.

Results

Three sets of experiments were respectively performed to investigate the influence of stent orientation and spatial resolution on the susceptibility-based method, and to demonstrate the feasibility of the susceptibility-based method in evaluating the stent restenosis comparing to the two typical MR positive contrast methods, GRASP and SUMO.

Conclusions

The susceptibility-based method provides better visualization and localization of the stent than SUMO and GRASP and has the capability of assessing the stent restenosis.

Accelerated positive contrast MRI of interventional devices using parallel compressed sensing imaging

Susceptibility-based magnetic resonance imaging (MRI) method can image small MR-compatible devices with positive contrast. However, the relatively long data acquisition time required by the method hinders its practical applications. This study presents a parallel compressive sensing technique with a modified fast spin echo to accelerate data acquisition for the susceptibility-based positive contrast MRI. The method integrates the generalized autocalibrating partially parallel acquisitions and the compressive sensing techniques in the reconstruction algorithm. MR imaging data acquired from several phantoms containing interventional devices such as biopsy needles, stent, and brachytherapy seeds, used for validating the proposed technique. The results show that it can speed up data acquisition by a factor of about five while preserving the quality of the positive contrast images.