Positive visualization of nitinol stent-grafts by post-processing

Introduction

The emergence of new MR-compatible nitinol-based stent-grafts [1] allows follow-up MRI of patients with these devices to assess stent-graft and wall apposition, or monitoring endovascular aneurysm repair. In particular, the soft-tissue contrast provided by MRI holds benefit over CT and was shown to be more sensitive to endoleak detection [2]. However, sometimes the susceptibility artifact caused by stent graft may be confused with other sources of negative contrast. Recently, Vonken et al. proposed positive visualization based on susceptibility gradient mapping (SGM) [3]. However SGM calculates the susceptibility gradient over neighboring pixels and thus requires MR-images with very high-resolution (0.3-0.7 mm isotropic) that cannot be achieved in clinical practice. In this work we investigated the use of a new positive contrast method that maintains the original resolution and allows selective visualization of devices from the clinical MR-data.

Purpose

This study focuses on the feasibility for positive visualization of nitinol stent-grafts by post-processing.

Methods

Our technique maps the echo-shift in k-space associated with the susceptibility difference from the stent-graft by applying a filter in a sequential manner in k-space. The k-space shift was determined by analyzing the drop in the Fourier transform of the filtered signal.

A GoreTAG^® endovascular stent-graft was deployed to treat aneurysm formation in chronic aortic dissection. MRI was conducted with blood-pool agent post-surgery on a 3 T clinical scanner (Achieva, Philips Healthcare): 3D TFE; FOV = 251 × 400 × 156 mm³; FA = 20°; TE/TR = 2.7/5.6 ms; resolution = 1 × 1.5 × 3.0 mm³. A positive contrast image was obtained by mapping the k-space shift in each direction and use of the magnitude at each pixel (Fig. 1). The positive contrast by post-processing was thus fused with MR images (OsiriX Imaging Software), without requiring registration. A rough mask was also applied based on the aorta to remove unrelated positive contrast.

Figure 1

The same MR-data from the negative contrast image (a) was used to calculated a positive contrast map (b) from the k-space shift due to susceptibility gradients. A mask is applied to the positive contrast based on the aorta (c) and then fused with the original image (d).

Results

Proximal apposition of the stent-graft to the wall is seen between the positive and negative contrast images (Fig. 2). The distal extent and length of the device may also be assessed easily.

Figure 2

(a) Saggital section demonstrating a good proximal seal with some residual filling of the false lumen. (b) Poor medial apposition of the stent-graft to the wall. (c) Positive contrast fused with data from MR-angiography allows 3D visulization.

Conclusion

Application of this positive contrast technique permitted improved visualization of the stent-graft as well as the aortic lumen and wall. The poor stent-graft - wall apposition proximally and the continued perfusion of the false lumen distally provided an early non-invasive assessment of the endovascular repair and guided the clinical workflow. The proposed post-processing method does not require any additional scan and provides better depiction of a post-implant stent-graft.