Ultrasound-fluoroscopy registration for prostate brachytherapy dosimetry

Abstract

Prostate brachytherapy is a treatment for prostate cancer using radioactive seeds that are permanently implanted in the prostate. The treatment success depends on adequate coverage of the target gland with a therapeutic dose, while sparing the surrounding tissue. Since seed implantation is performed under transrectal ultrasound (TRUS) imaging, intraoperative localization of the seeds in ultrasound can provide physicians with dynamic dose assessment and plan modification. However, since all the seeds cannot be seen in the ultrasound images, registration between ultrasound and fluoroscopy is a practical solution for intraoperative dosimetry. In this manuscript, we introduce a new image-based nonrigid registration method that obviates the need for manual seed segmentation in TRUS images and compensates for the prostate displacement and deformation due to TRUS probe pressure. First, we filter the ultrasound images for subsequent registration using thresholding and Gaussian blurring. Second, a computationally efficient point-to-volume similarity metric, an affine transformation and an evolutionary optimizer are used in the registration loop. A phantom study showed final registration errors of 0.84 ± 0.45 mm compared to ground truth. In a study on data from 10 patients, the registration algorithm showed overall seed-to-seed errors of 1.7 ± 1.0 mm and 1.5 ± 0.9 mm for rigid and nonrigid registration methods, respectively, performed in approximately 30s per patient.

Figure 1

The workflow for ultrasound-fluoroscopy registration, (a): Several transverse images of the prostate, (b): A volume created from the slices, (c): Several C-arm images taken from different angles, (d): Seeds reconstructed in 3D, (e): Registered seeds overlaid on the US volume. Seeds are shown as capsules.

Figure 2

Preprocessing steps. (a): One slice from VOI, (b): The same slice after thresholding, (c): After application of distance transform, (d): After application of the Gaussian function.

Figure 3

The ground truth phantom. The registration between CT and TRUS volumes is provided using fiducial markers on the phantom box. Both markers and TRUS probe are tracked using an optical tracker.

Figure 4

Two transverse slices of the phantom overlaid with ground truth seeds (squares) and registered seeds (diamonds)

Figure 5

Clinical results: Registered seeds overlaid on mid-gland slices of the prostate for 9 patients

Figure 6

Two transverse slices from the second (left) and fifth (right) patients, showing the seeds as a result of affine (squares) and rigid (diamonds) registration. The true position of some identifiable seeds are shown with arrows.

Figure 7

Deformably registered seeds (blue stars) and manually selected seeds (red circles)

Figure 8

Prostate volume segmented in Day-1 CT and postimplant TRUS

Figure 9

(a): Volume of the prostate covered with 100% of the prescribed dose, (b): Dose delivered to the 90% of the prostate volume (bottom). V₁₀₀ is shown in percentage of prostate volume and D₉₀ is shown in percentage of the prescribed dose.