Localizing intracavitary brachytherapy applicators from cone-beam CT x-ray projections via a novel iterative forward projection matching algorithm

Abstract

Purpose: To present a novel method for reconstructing the 3D pose (position and orientation) of radio-opaque applicators of known but arbitrary shape from a small set of 2D x-ray projections in support of intraoperative brachytherapy planning.

Methods: The generalized iterative forward projection matching (gIFPM) algorithm finds the six degree-of-freedom pose of an arbitrary rigid object by minimizing the sum-of-squared-intensity differences (SSQD) between the computed and experimentally acquired autosegmented projection of the objects. Starting with an initial estimate of the object's pose, gIFPM iteratively refines the pose parameters (3D position and three Euler angles) until the SSQD converges. The object, here specialized to a Fletcher-Weeks intracavitary brachytherapy (ICB) applicator, is represented by a fine mesh of discrete points derived from complex combinatorial geometric models of the actual applicators. Three pairs of computed and measured projection images with known imaging geometry are used. Projection images of an intrauterine tandem and colpostats were acquired from an ACUITY cone-beam CT digital simulator. An image postprocessing step was performed to create blurred binary applicators only images. To quantify gIFPM accuracy, the reconstructed 3D pose of the applicator model was forward projected and overlaid with the measured images and empirically calculated the nearest-neighbor applicator positional difference for each image pair.

Results: In the numerical simulations, the tandem and colpostats positions (x,y,z) and orientations (alpha, beta, gamma) were estimated with accuracies of 0.6 mm and 2 degrees, respectively. For experimentally acquired images of actual applicators, the residual 2D registration error was less than 1.8 mm for each image pair, corresponding to about 1 mm positioning accuracy at isocenter, with a total computation time of less than 1.5 min on a 1 GHz processor.

Conclusions: This work describes a novel, accurate, fast, and completely automatic method to localize radio-opaque applicators of arbitrary shape from measured 2D x-ray projections. The results demonstrate approximately 1 mm accuracy while compared against the measured applicator projections. No lateral film is needed. By localizing the applicator internal structure as well as radioactive sources, the effect of intra-applicator and interapplicator attenuation can be included in the resultant dose calculations. Further validation tests using clinically acquired tandem and colpostats images will be performed for the accurate and robust applicator/sources localization in ICB patients.

Figure 1

(a) Close-up photograph of the Fletcher–Weeks CT-compatible afterloadable colpostats and one of the tandems used in this study. (b) Computed CBCT projections of the 3D tandem and right colpostat models, where the image background represent uniform elliptical water cylinder. The image intensity values represent an arbitrary integer number assigned to each material in the model.

Figure 2

An example illustrating postprocessing of experimentally acquired ACUITY projection images. (a) Raw projection image, (b) top-hat filtered image, (c) binary image, and (d) blurred grayscale image used as an input to the generalized IFPM algorithm.

Figure 3

An illustration of the iterative convergence process for a simulated implant consisting of tandem and bilateral colpostats for a 25 mm colpostat separation (see convergence rate graph in Fig. 4). (a) Initial estimate of the applicator configuration, (b) computed images at convergence, (c) the true∕synthetic measured images, and (d) difference between (b) and (c), where the rows represents different gantry angles. The horizontal line in the third row shows the qualitative comparison of an initial estimate in part (a) with computed and measured images in part (b) and (c), respectively. The generalized IFPM algorithm was able to reproduce each applicator pose, as well as overlapping components.

Figure 4

The similarity metric score vs iteration number for the generalized IFPM algorithm for the three simulated full ICB applicator configurations.

Figure 5

Point-by-point overlay of the reconstructed applicators with the true∕synthetic measured applicators, demonstrating near coincidence achieved by the 3D∕2D gIFPM registration and applicator reconstruction as a unified process.

Figure 6

Superposition of experimentally acquired binary images (white) with automatically reconstructed applicators positions (black) projected onto the detector planes. (a) 0° gantry angle, (b) −30° gantry angle, and (c) +30° gantry angle, respectively, when using 40 mm colpostat separation. The applicator registration error was less than 1 mm for the intrauterine tandem and about 1.5 mm for the bilateral colpostats on each image plane.