Ultrasound–fluoroscopy registration for prostate brachytherapy dosimetry
Graphical abstract
Highlights
► A nonrigid intensity-based ultrasound–fluoroscopy registration method for prostate brachytherapy dosimetry is introduced. ► The affine registration compensates for the probe pressure effects on the prostate. ► The registration algorithm obviates the need for manual seed segmentation in TRUS. ► Seed-to-seed registration errors of 1.5 ± 0.9 mm are achieved.
Introduction
With an estimated number of 240,890 new cases in 2011, prostate cancer is the most common cancer among men in the United States, accounting for 29% of their cancers (Siegel et al., 2011). It is also the second highest cause of cancer death among men in the United States (Siegel et al., 2011). Radical prostatectomy, external-beam radiation therapy and brachytherapy are established treatments for prostate cancer. Low-dose-rate brachytherapy (hereafter, brachytherapy) is an effective and minimally invasive treatment for localized prostate cancer that can achieve outcomes at least equal to the other treatment options while showing less severe side-effects (Merrick et al., 2001, Blasko et al., 2002, Morris et al., 2009). In brachytherapy, the cancer is eradicated by internal radiation from permanently implanted radio-active sources (seeds) of 125I, 103Pd, or 137Cs. Generally, the physician implants 40–130 seeds using needles, based on the size of the prostate and the type and activity of the seeds. Before the operation, the seed positions are planned using a transrectal ultrasound (TRUS) volume. The goal of the planning is to cover the target gland with a prescribed dose of radiation, while sparing the healthy surrounding tissue such as urethra and rectum. In contemporary brachytherapy, seed placement is performed under visual guidance from TRUS.
In practice, the actual delivered implant geometry is different from the plan for several reasons including intraoperative tissue swelling (Yamada et al., 2003), prostate motion and deformation caused by needle insertion (Lagerburg et al., 2005), needle deflection (Nath et al., 2000) and seed migration. Deviation of the seeds from their planned positions results in a sub-optimal dose distribution. Over-radiation of the healthy surrounding tissue may lead to complications such as sexual and urinary dysfunction, and rectal ulceration. Excessive under-radiation of the cancerous gland may result in treatment failure. Traditionally, the delivered dose is quantitatively assessed using CT after the patient is released from the operating room.
Dynamic dosimetry—the ability to calculate the delivered dose intraoperatively, based on the actual position of the implanted seeds—enables the physician to adjust the plan and ensure sufficient dose coverage before the completion of the operation. Moreover, a dynamic dosimetry system may render postimplant CT-based dose evaluation unnecessary, as the delivered dose can be quantitatively assessed at the end of the operation. This will significantly reduce brachytherapy quality assurance complexity and cost.
In order to have accurate intraoperative dosimetry, one should be able to localize the deposited seeds in relation to the prostate. A variety of methods have been tried to localize the seeds in ultrasound images (Han et al., 2003, Holmes and Robb, 2004, Feleppa et al., 2002, McAleavey et al., 2003, Mitri et al., 2004, Ding et al., 2006, Xue et al., 2005, Wei et al., 2006, Wen et al., 2010). However, US-only seed localization is not considered a reliable tool for dose calculation as this method suffers from missing seeds in ultrasound images and presence of false positives—seed-like artifacts caused by calcification and air bubbles (Han et al., 2003).
An alternative technique used in brachytherapy treatment and available by commercialized products entails estimation of the seed positions based on the position of the needle visible in sagittal images. This method was further refined so that the seeds are manually localized in sagittal ultrasound images at the time of deposition (Potters et al., 2003, Meijer et al., 2006, Nag et al., 2001, Polo et al., 2010). Although this method showed improvements in the treatment outcome (Nath et al., 2009, Nag et al., 2001), it cannot account for intraoperative seed displacement after implantation caused by edema, tissue motion and/or seed migration.
The secondary imaging modality often utilized in brachytherapy operating rooms is C-arm fluoroscopy, which is more reliable than TRUS for seed visualization. However, a C-arm image shows a 2D projection of the implant geometry with no soft-tissue detail. Nonetheless, brachytherapists frequently use C-arm images for gross implant evaluation based on their mental 3D visualization of the seeds.
Although C-arm images do not have sufficient soft-tissue contrast to show the prostate boundaries, three or more C-arm images can be used to reconstruct the seeds in 3D (Amols and Rosen, 1981, Su et al., 2004, Narayanan et al., 2004, Lam et al., 2004, Jain et al., 2005b, Brunet-Benkhoucha et al., 2009, Lee et al., 2009, Lee et al., 2011b, Lee et al., 2011a, Dehghan et al., 2011a, Dehghan et al., 2011c). The seeds reconstructed from C-arm images can be registered to the prostate delineated in TRUS images to calculate the delivered dose. Therefore, ultrasound–fluoroscopy fusion can provide a practical solution for dynamic dosimetry and has demonstrated its benefits in limited clinical trials (Orio et al., 2007, Song et al., 2011).
Registration of the reconstructed seeds to the TRUS coordinate system is a necessary step for ultrasound–fluoroscopy fusion and has been extensively studied (Todor et al., 2003, Jain et al., 2012, Song et al., 2011, French et al., 2005, Su et al., 2007b, Orio et al., 2007, Tutar et al., 2008, Fallavollita et al., 2010, Dehghan et al., 2011b). Lead markers on the TRUS probe or radio-opaque fiducials were used for ultrasound–fluoroscopy registration (Todor et al., 2003, Jain et al., 2012). French et al. (French et al., 2005) used the probe as a fiducial for ultrasound–fluoroscopy registration.
In order to avoid image occlusion by the probe, it is necessary to retract the probe, at least partially, before the C-arm image acquisition. Since the physicians usually press the probe against the prostate to achieve a good acoustic coupling and improve the TRUS image quality, probe retraction results in prostate motion in the posterior direction and sometimes deformation (Wallner et al., 2001). The marker- and fiducial-based registration methods cannot account for this motion and deformation.
As a remedy, Su et al. (2007b), Orio et al. (2007) and Tutar et al. (2008) used a point-to-point registration method between ultrasound and fluoroscopy. In this method, the physician manually localizes some seeds in the sagittal TRUS images. This point set is then registered to the seeds reconstructed from the C-arm images. Manual seed localization in TRUS images is a difficult and time-consuming task and is entirely dependent on accurate seeds determined, rather subjectively, by the physician. Therefore, registration methods that rely on manual seed segmentation are not appropriate for wide-scale practical implementation.
Fallavollita et al. were the first to propose an intensity-based registration between CT or fluoroscopy and TRUS (Fallavollita et al., 2010). They reported successful registration results between CT and TRUS on a ground truth phantom. They also reported qualitative agreement between TRUS and fluoroscopy for a single patient data set. Since a rigid registration method was used in (Su et al., 2007b, Orio et al., 2007, Tutar et al., 2007, Tutar et al., 2008, Fallavollita et al., 2010), they could only account for rigid motion of the prostate due to probe retraction but not for the likely deformation.
For a more comprehensive review on intraoperative imaging and dosimetry techniques for prostate brachytherapy, we refer the readers to (Polo et al., 2010).
Despite considerable research and development efforts, dynamic dosimetry is not yet available for clinical use in brachytherapy (Nath et al., 2009). A practical method for TRUS-fluoroscopy fusion is a much sought-after solution to surmount the last standing obstacle in the road toward dynamic dosimetry.
In this paper, we introduce a new image-based TRUS-fluoroscopy registration algorithm. Our method obviates the need for manual seed segmentation in TRUS images and is robust to missing seeds and false positives in the TRUS images. In addition, by employing a deformation model based on the observed nature of the probe-prostate interaction, our algorithm is capable of compensating for the effects of prostate motion and deformation caused by probe retraction. To the best of our knowledge, this is the first nonrigid registration method for TRUS-fluoroscopy fusion for prostate brachytherapy.
We employ thresholding to prepare the TRUS images for registration, without any attempt to remove the false positives or identify the missing seeds. We also apply Gaussian blurring to increase the capture range of our registration method that exploits a robust evolutionary optimizer (Hansen, 2006). We examined our algorithm on a phantom and on patient data. In addition to excellent visual agreement, our registration method shows errors that are smaller than clinically acceptable levels (customarily, less than 2 mm). We also examined our registration method in prediction of dose parameters compared to CT-based dosimetry. Since rapid computations are essential for practical dynamic dosimetry, our method runs in approximately 30 s per patient. With fast and accurate C-arm-based seed reconstruction methods available (Lee et al., 2011b, Lee et al., 2011a, Dehghan et al., 2011a, Dehghan et al., 2011c), our algorithm can be readily integrated into a practical system to provide dynamic dosimetry for prostate brachytherapy in clinical application.
Compared to the work of Fallavollita et al. (2010), we introduce a deformable registration method that compensates for the effects of probe pressure. Our method also shows lower registration errors and faster computational speed. In addition, we use different preprocessing steps, similarity metric and optimizer to enhance the robustness of our algorithm. The underlying idea of our method for a rigid registration was presented in (Dehghan et al., 2011b). This manuscript significantly extends our previous work by presenting a deformable registration method, and by providing a more detailed description of the methodology and performance analysis on 10 clinical data sets.
This paper is organized as follows: In Section 2 we discuss the components of the registration algorithm and explain the experimental method on phantom and clinical data sets. The results are presented in Section 3, followed by the discussions in Section 4. At the end, conclusions and the future work are outlined in Section 5.
Section snippets
Workflow
We envision the following workflow for providing intraoperative dosimetry analysis and optimization using ultrasound–fluoroscopy registration. At some point during the operation or immediately at the end, the physician acquires a series of transverse TRUS images of the prostate by continuously retracting the probe from the prostate base toward its apex (see Fig. 1a). Since some of the seeds may be located superior to the base or inferior to the apex, it is recommended that the whole range be
Phantom study
We tested our registration algorithm on a CIRS-053 prostate brachytherapy training phantom (CIRS Inc., VA, USA) implanted with 48 dummy seeds.
In order to establish a ground truth, the phantom was imaged using CT. The phantom box was equipped with 6 fiducial beads visible in CT images (see Fig. 3). The seed clusters were segmented in the CT volume by thresholding. The center of mass of the seed clusters were grouped into a set of points similar to the outcome of seed reconstruction from C-arm
Accuracy in prediction of dose parameters
We compared our dosimetry results from ultrasound and fluoroscopy registration to postimplant CT dosimetry which, currently, is the standard quality assessment method. The comparison gives an overall accuracy assessment of our algorithm and shows its potential as a dynamic dosimetry system. The difference between our results and CT-based dosimetry can be caused by multiple factors as discussed below.
Su et al. (Su et al., 2007a) showed that seed localization uncertainties of less than 2 mm are
Conclusions and future work
In this paper, we introduced a new nonrigid image-based ultrasound–fluoroscopy registration method to provide a practical solution for dynamic dosimetry in prostate brachytherapy. We employed thresholding and Gaussian blurring to enhance the quality of the TRUS images and prepare them for registration. We used a computationally efficient point-to-volume similarity metric and a stochastic evolutionary optimizer within our registration loop.
Our trials on a ground truth phantom showed registration
Acknowledgments
Ehsan Dehghan was supported by an Ontario Ministry of Research and Innovation post-doctoral fellowship. Gabor Fichtinger was supported as Cancer Care Ontario Research Chair. This work was also supported by National Institutes of Health/National Cancer Institute (NIH/NCI) under Grants 2R44CA099374 and 1R01CA151395, and by the Idea to Innovation Program of the Natural Sciences and Engineering Research Council of Canada.
References (51)
- et al.
Brachytherapy for carcinoma of the prostate: techniques, patient selection, and clinical outcomes
Semin. Radiat. Oncol.
(2002) - et al.
A real-time freehand ultrasound calibration system with automatic accuracy feedback and control
Ultrasound Med. Biol.
(2009) - et al.
American brachytherapy society consensus guidelines for transrectal ultrasound-guided permanent prostate brachytherapy
Brachytherapy
(2012) - et al.
Brachytherapy seed reconstruction with joint-encoded C-arm single-axis rotation and motion compensation
Med. Image Anal.
(2011) - et al.
Needle and seed segmentation in intra-operative 3D ultrasound-guided prostate brachytherapy
Ultrasonics
(2006) - et al.
Intraoperative dosimetry for prostate brachytherapy from fused ultrasound and fluoroscopy images
Acad. Radiol.
(2005) - et al.
Intra-operative 3D guidance and edema detection in prostate brachytherapy using a non-isocentric C-arm
Med. Image Anal.
(2012) - et al.
Automatic segmentation of radiographic fiducial and seeds from x-ray images in prostate brachytherapy
Med. Eng. Phys.
(2012) - et al.
Measurement of prostate rotation during insertion of needles for brachytherapy
Radiother. Oncol.
(2005) - et al.
Dosimetric comparison of interactive planned and dynamic dose calculated prostate seed brachytherapy
Radiother. Oncol.
(2006)
Population-based study of biochemical and survival outcomes after permanent 125I brachytherapy for low- and intermediate-risk prostate cancer
Urology
Intraoperative planning and evaluation of permanent prostate brachytherapy: report of the American brachytherapy society
Int. J. Radiat. Oncol. Biol. Phys.
Intraoperative ultrasound–fluoroscopy fusion can enhance prostate brachytherapy quality
Int. J. Radiat. Oncol. Biol. Phys.
Review of intraoperative imaging and planning techniques in permanent seed prostate brachytherapy
Radiother. Oncol.
Toward a dynamic real-time intraoperative permanent prostate brachytherapy methodology
Brachytherapy
Dynamic intraoperative dosimetry for prostate brachytherapy using a nonisocentric C-arm
Brachytherapy
Localization of linked 125I seeds in postimplant trus images for prostate brachytherapy dosimetry
Int. J. Radiat. Oncol. Biol. Phys.
A three-film technique for reconstruction of radioactive seed implants
Med. Phys.
Clinical implementation of a digital tomosynthesis-based seed reconstruction algorithm for intraoperative postimplant dose evaluation in low dose rate prostate brachytherapy
Med. Phys.
Prostate implant reconstruction from C-arm images with motion-compensated tomosynthesis
Med. Phys.
Registration between ultrasound and fluoroscopy or CT in prostate brachytherapy
Med. Phys.
Prostate brachytherapy seed identification on post-implant TRUS images
Med. Phys.
The CMA evolution strategy: a comparing review
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