Digital subtraction MR angiography roadmapping for magnetic steerable catheter tracking

Abstract

Purpose: To develop a high temporal resolution MR imaging technique that could be used with magnetically assisted remote control (MARC) endovascular catheters.

Materials and methods: A technique is proposed based on selective intra-arterial injections of dilute MR contrast at the beginning of a fluoroscopic MR angiography acquisition. The initial bolus of contrast is used to establish a vascular roadmap upon which MARC catheters can be tracked. The contrast to noise ratio (CNR) of the achieved roadmap was assessed in phantoms and in a swine animal model. The ability of the technique to permit navigation of activated MARC catheters through arterial branch points was evaluated.

Results: The roadmapping mode proved effective in phantoms for tracking objects and achieved a CNR of 35.7 between the intra- and extra-vascular space. In vivo, the intra-arterial enhancement strategy produced roadmaps with a CNR of 42.0. The artifact produced by MARC catheter activation provided signal enhancement patterns on the roadmap that experienced interventionalists could track through vascular structures.

Conclusion: A roadmapping approach with intra-arterial contrast-enhanced MR angiography is introduced for navigating the MARC catheter. The technique mitigates the artifact produced by the MARC catheter, greatly limits the required specific absorption rate, permits regular roadmap updates due to the low contrast agent requirements, and proved effective in the in vivo setting. Inc.

Keywords: digital subtraction angiography; endovascular procedure; interventional MR; magnetic resonance angiography.

Figure 1

The MARC catheter construction is summarized. The schematic overview (A) indicates the presence of intra-luminal lead wires that connect the distal solenoidal coil on the catheter tip to the proximal ethernet cable at the catheter hub. The latter is subsequently connected to a current source for coil activation. The relative position of the balloon to the tip mounted coil can also be appreciated. The physical device (B) is shown, with a zoomed in view of the tip (C) assembly on the Sprinter catheter.

Figure 2

The roadmapping mode is demonstrated on a vascular phantom that mimics the distal aorta. IA contrast is administered at the start of the run to highlight the vasculature (A). This reference is subtracted from all subsequent scans and thus initially there is minimal contrast (B). The roadmap only appears once the contrast injection is washed out (C) and is durable for the remainder of the run. It is possible to track bright objects against this roadmap as demonstrated by inflating the balloon catheter with 1mM Gd (arrows in D-F).

Figure 3

The roadmapping approach is demonstrated in the distal aorta of a swine. The source images are shown on the top row and the roadmapping mode images are on the bottom row. An IA injection of Gd is initially used to highlight arterial anatomy (A) and establish the roadmap (E). The MARC catheter is then activated, producing a substantial signal void on the source images (arrows in B-D) and a signal enhancement pattern on the roadmap images that is superimposed on the arterial anatomy (arrows in F-H). The roadmap images can be used to track the device without losing visibility of the local arterial anatomy.

Figure 4

The effect of several MARC catheter current levels are demonstrated on the source (top row) and roadmapping (bottom row) images. Current levels of 300mA (A,E), 100mA (B,F), 50mA (C,G) and 10mA (D,H) are demonstrated in the distal aorta of a swine. The spatial extent of the artifact on the source images (top row arrows) scales with current and is still evident at very low current levels. The corresponding signal increase on the roadmap images (bottom row arrows) similarly localizes the catheter tip even at very low current levels.