Localization of cardiac volume and patient features in inverse geometry x-ray fluoroscopy

Abstract

The scanning-beam digital x-ray (SBDX) system is an inverse geometry x-ray fluoroscopy technology that performs real-time tomosynthesis at planes perpendicular to the source-detector axis. The live display is a composite image which portrays sharp features (e.g. coronary arteries) extracted from a 16 cm thick reconstruction volume. We present a method for automatically determining the position of the cardiac volume prior to acquisition of a coronary angiogram. In the algorithm, a single non-contrast frame is reconstructed over a 44 cm thickness using shift-and-add digital tomosynthesis. Gradient filtering is applied to each plane to emphasize features such as the cardiomediastinal contour, diaphragm, and lung texture, and then sharpness vs. plane position curves are generated. Three sharpness metrics were investigated: average gradient in the bright field, maximum gradient, and the number of normalized gradients exceeding 0.5. A model correlating the peak sharpness in a non-contrast frame and the midplane of the coronary arteries in a contrast-enhanced frame was established using 37 SBDX angiographic loops (64-136 kg human subjects, 0-30° cranial-caudal). The average gradient in the bright field (primarily lung) and the number of normalized gradients >0.5 each yielded peaks correlated to the coronary midplane. The rms deviation between the predicted and true midplane was 1.57 cm. For a 16 cm reconstruction volume and the 5.5-11.5 cm thick cardiac volumes in this study, midplane estimation errors of 2.25-5.25 cm were tolerable. Tomosynthesis-based localization of cardiac volume is feasible. This technique could be applied prior to coronary angiography, or to assist in isocentering the patient for rotational angiography.

Keywords: interventional cardiology; inverse geometry; scanning-beam digital x-ray; tomosynthesis; x-ray fluoroscopy.

Figure 1

The SBDX system (A) uses a scanning x-ray tube, multihole collimator, high speed detector, and real-time reconstructor (B). Each displayed fluoroscopic frame is a composite of a stack of tomosynthesis images (C) generated by unfiltered backprojection (shift-add). Each tomosynthesis image portrays in-plane and near-plane objects in focus.

Figure 2

A: Example stack of 32 tomosynthesis images spaced by 5 mm, shown from lowest (top left) to highest (lower right) position relative to the x-ray tube. B: Composite image resulting from a properly centered tomosynthesis reconstruction volume. C: Effect of shifting the reconstruction volume 5 cm towards the x-ray tube, demonstrating loss of contrast and sharpness (yellow arrow) of an anterior coronary artery that falls outside the reconstruction volume. Contrast equalization has been applied to the images to facilitate visualization.

Figure 3

Sharpness vs. plane position, calculated using the average gradient in the bright lung field (green curve), the number of normalized image gradients exceeding 0.5 (blue curve), and the maximum gradient in the image (red curve). Vertical dashed lines indicate peak locations in the sharpness curves. Example tomosynthesis images and gradient images corresponding to the sharpness peaks are shown at right. From top to bottom, there are peaks corresponding to sternal wires (51 cm), cardiomediastinal contour and diaphragm (44.5 cm), lung texture and guide catheter (38 cm), and the posterior ribs (33 cm) (see yellow arrows). The green outline on the upper right of the images encloses the bright lung field segmented by Otsu’s thresholding method.

Figure 4

Position of peak sharpness measured in a non-contrast frame of each angiographic loop. Sharpness metrics measured by average gradient in the bright lung field (green) and the number of gradients > 0.5 (blue) were correlated to the true coronary midplane.

Figure 5

MIP of gradient-filtered planes (A) and depth localization of 10x10 pixel regions (B,C). 3D spline curve fits (red) are shown for points localized along the cardiac contour, diaphragm, and catheter.