Positive contrast spiral imaging for visualization of commercial nitinol guidewires with reduced heating

Abstract

Background: CMR-guidance has the potential to improve tissue visualization during cardiovascular catheterization procedures and to reduce ionizing radiation exposure, but a lack of commercially available CMR guidewires limits widespread adoption. Standard metallic guidewires are considered to be unsafe in CMR due to risks of RF-induced heating. Here, we propose the use of RF-efficient gradient echo (GRE) spiral imaging for reduced guidewire heating (low flip angle, long readout), in combination with positive contrast for guidewire visualization.

Methods: A GRE spiral sequence with 8 interleaves was used for imaging. Positive contrast was achieved using through-slice dephasing such that the guidewire appeared bright and the background signal suppressed. Positive contrast images were interleaved with anatomical images, and real-time image processing was used to produce a color overlay of the guidewire on the anatomy. Temperature was measured with a fiber-optic probe attached to the guidewire in an acrylic gel phantom and in vivo.

Results: Left heart catheterization was performed on swine using the real-time color overlay for procedural guidance with a frame rate of 6.25 frames/second. Using our standard Cartesian real-time imaging (flip angle 60°), temperature increases up to 50 °C (phantom) and 4 °C (in vivo) were observed. In comparison, spiral GRE images (8 interleaves, flip angle 10°) generated negligible heating measuring 0.37 °C (phantom) and 0.06 °C (in vivo).

Conclusions: The ability to use commercial metallic guidewires safely during CMR-guided catheterization could potentially expedite clinical translation of these methods.

Fig. 1

a Pulse sequence diagram for spiral gradient echo imaging demonstrating change in slice refocusing gradient to transition from standard anatomical contrast (1) to positive contrast (2). b Resulting in vivo anatomical (1) and positive contrast (2) images with red arrowheads indicating nitinol guidewire in the aorta of a Yorkshire swine

Fig. 2

Image processing algorithm used to isolate the guidewire signal from the background in the positive contrast image. Thresholding (Steps 1, 2 and 3) and selection of elongated connected structures (Step 4 and 5) are displayed. The device signal is overlaid in green on the anatomical image and displayed to the interventionist for procedural guidance

Fig. 3

Example left heart catheterization data set showing anatomical images (a), positive contrast images (b) and resulting color overlay (c) for three different insertion lengths: guidewire tip in descending aorta (left), guidewire tip at aortic arch (middle) and guidewire in left ventricle (right)

Fig. 4

Temperature increase observed in an acrylic gel phantom during 2 minutes of continuous scanning (scanning period indicated by vertical black lines). Temperature is measured at the tip of the nitinol guidewire for the guidewire and catheter co-located (a), guidewire extended 10 cm distal to catheter (b) and guidewire alone (c). Right-hand plots show reduced temperature scale (−0.2 °C to 1.2 °C ) to demonstrate subtle heating during GRE imaging

Fig. 5

In vivo temperature measurements at the tip of the nitinol guidewire and catheter at three insertion lengths: guidewire tip within the introducer sheath (a), in the descending aorta (b) and at the aortic arch (c)