Monoplane Stereoscopic Imaging Method for Inverse Geometry X-ray Fluoroscopy

Abstract

Scanning Beam Digital X-ray (SBDX) is a low-dose inverse geometry fluoroscopic system for cardiac interventional procedures. The system performs x-ray tomosynthesis at multiple planes in each frame period and combines the tomosynthetic images into a projection-like composite image for fluoroscopic display. We present a novel method of stereoscopic imaging using SBDX, in which two slightly offset projection-like images are reconstructed from the same scan data by utilizing raw data from two different detector regions. To confirm the accuracy of the 3D information contained in the stereoscopic projections, a phantom of known geometry containing high contrast steel spheres was imaged, and the spheres were localized in 3D using a previously described stereoscopic localization method. After registering the localized spheres to the phantom geometry, the 3D residual RMS errors were between 0.81 and 1.93 mm, depending on the stereoscopic geometry. To demonstrate visualization capabilities, a cardiac RF ablation catheter was imaged with the tip oriented towards the detector. When viewed as a stereoscopic red/cyan anaglyph, the true orientation (towards vs. away) could be resolved, whereas the device orientation was ambiguous in conventional 2D projection images. This stereoscopic imaging method could be implemented in real time to provide live 3D visualization and device guidance for cardiovascular interventions using a single gantry and data acquired through normal, low-dose SBDX imaging.

Keywords: cardiac interventional procedures; inverse geometry; stereoscopic x-ray fluoroscopy.

Figure 1

Stereoscopic XRF methods can be adapted from conventional XRF geometries (A) by using two complete gantries (B) or a special dual-anode x-ray tube with a common detector (C), but both methods have drawbacks.

Figure 2

SBDX produces a sequence of overlapping narrow beams (beamlets) using a raster scanned focal spot and multihole collimator. The full field-of-view is imaged at up to 30 frame/sec. Up to 100x100 positions are used in a frame period.

Figure 3

(A) During SBDX image formation, each beamlet emerging from the collimator illuminates part of the field of view. The grayed region represents a single beamlet. Black dots represent two objects at different distances from the source. (B) Shift-and-add reconstruction is performed at planes using pixel dimensions that increase linearly toward the source plane. The objects appear blurred at planes above and below the object. (C) The composite image uses pixel values that fall along rays based on local contrast and sharpness. Geometry is exaggerated for clarity.

Figure 4

The SBDX stereoscopic projection pairs can be directed anywhere in the full field of view. (A) Stereoscopic projection pair with zero parallax at the virtual detector plane. (B) is the same as (A), but with the virtual sources closer together which decreases the amount of parallax versus distance from the source. (C) Offsetting the two virtual detectors creates zero parallax near isocenter, which is useful for visualization.

Figure 5

The 3D localization method computes the position of a point (x,y,z) using its image coordinates in the two projections, (u₁,v₁) and (u₂,v₂), the image coordinates of the perpendicular ray of one of the projections, (u₀,v₀), the separation between the two virtual sources, b, and the source to image (i.e. detector) distance, SID.

Figure 6

(A) Red/cyan stereoscopic anaglyph of the helical phantom. There is zero parallax at the center of the helix. The 15 spheres within the dashed line box were used for error calculation. (B) Localized sphere positions (circles) are plotted against the true phantom geometry (line) after registration.

Figure 7

An RF ablation catheter was oriented toward the true x-ray detector, imaged with SBDX, and reconstructed using standard and stereo methods. In the standard reconstructions (A,B), the orientation is ambiguous. The true orientation of the catheter can be resolved in the stereo reconstructions (C,D) using red/cyan glasses. Without glasses, the change in parallax (i.e. difference in depth) between the shaft and the tip of the catheter is still prominent. Z coordinates are in millimeters above the true x-ray source. Note figure colors are optimized for viewing in electronic formats.