EM-enhanced US-based seed detection for prostate brachytherapy.

PURPOSE: Intraoperative dosimetry in low-dose-rate (LDR) permanent prostate brachytherapy requires accurate localization of the implanted seeds with respect to the prostate anatomy. Transrectal Ultrasound (TRUS) imaging, which is the main imaging modality used during the procedure, is not sufficiently robust for accurate seed localization. We present a method for integration of electromagnetic (EM) tracking into LDR prostate brachytherapy procedure by fusing it with TRUS imaging for seed localization. METHOD: Experiments were conducted on five tissue mimicking phantoms in a controlled environment. The seeds were implanted into each phantom using an EM-tracked needle, which allowed recording of seed drop locations. After each needle, we reconstructed a 3D ultrasound (US) volume by compounding a series of 2D US images acquired during retraction of an EM-tracked TRUS probe. Then, a difference image was generated by nonrigid registration and subtraction of two consecutive US volumes. A US-only seed detection method was used to detect seed candidates in the difference volume, based on the signature of the seeds. Finally, the EM-based positions of the seeds were used to detect the false positives of the US-based seed detection method and also to estimate the positions of the missing seeds. After the conclusion of the seed implant process, we acquired a CT image. The ground truth for seed locations was obtained by localizing the seeds in the CT image and registering them to the US coordinate system. RESULTS: Compared to the ground truth, the US-only detection algorithm achieved a localization error mean of 1.7 mm with a detection rate of 85%. By contrast, the EM-only seed localization method achieved a localization error mean of 3.7 mm with a detection rate of 100%. By fusing EM-tracking information with US imaging, we achieved a localization error mean of 1.8 mm while maintaining a 100% detection rate without any false positives. CONCLUSIONS: Fusion of EM-tracking and US imaging for prostate brachytherapy can combine high localization accuracy of US-based seed detection with the robustness and high detection rate of EM-based seed localization. Our phantom experiments serve as a proof of concept to demonstrate the potential value of integrating EM-tracking into LDR prostate brachytherapy.

CT AND MRI FUSION FOR POSTIMPLANT PROSTATE BRACHYTHERAPY EVALUATION.

Postoperative evaluation of prostate brachytherapy is typically performed using CT, which does not have sufficient soft tissue contrast for accurate anatomy delineation. MR-CT fusion enables more accurate localization of both anatomy and implanted radioactive seeds, and hence, improves the accuracy of postoperative dosimetry. We propose a method for automatic registration of MR and CT images without a need for manual initialization. Our registration method employs a point-to-volume registration scheme during which localized seeds in the CT images, produced by commercial treatment planning systems as part of the standard of care, are rigidly registered to preprocessed MRI images. We tested our algorithm on ten patient data sets and achieved an overall registration error of 1.6 ± 0.8 mm with a running time of less than 20s. With high registration accuracy and computational speed, and no need for manual intervention, our method has the potential to be employed in clinical applications.

An image-guidance system for dynamic dose calculation in prostate brachytherapy using ultrasound and fluoroscopy.

PURPOSE: Brachytherapy is a standard option of care for prostate cancer patients but may be improved by dynamic dose calculation based on localized seed positions. The American Brachytherapy Society states that the major current limitation of intraoperative treatment planning is the inability to localize the seeds in relation to the prostate. An image-guidance system was therefore developed to localize seeds for dynamic dose calculation. METHODS: The proposed system is based on transrectal ultrasound (TRUS) and mobile C-arm fluoroscopy, while using a simple fiducial with seed-like markers to compute pose from the nonencoded C-arm. Three or more fluoroscopic images and an ultrasound volume are acquired and processed by a pipeline of algorithms: (1) seed segmentation, (2) fiducial detection with pose estimation, (3) seed matching with reconstruction, and (4) fluoroscopy-to-TRUS registration. RESULTS: The system was evaluated on ten phantom cases, resulting in an overall mean error of 1.3 mm. The system was also tested on 37 patients and each algorithm was evaluated. Seed segmentation resulted in a 1% false negative rate and 2% false positive rate. Fiducial detection with pose estimation resulted in a 98% detection rate. Seed matching with reconstruction had a mean error of 0.4 mm. Fluoroscopy-to-TRUS registration had a mean error of 1.3 mm. Moreover, a comparison of dose calculations between the authors' intraoperative method and an independent postoperative method shows a small difference of 7% and 2% forD90 and V100, respectively. Finally, the system demonstrated the ability to detect cold spots and required a total processing time of approximately 1 min. CONCLUSIONS: The proposed image-guidance system is the first practical approach to dynamic dose calculation, outperforming earlier solutions in terms of robustness, ease of use, and functional completeness.

Electromagnetic tracking for catheter reconstruction in ultrasound-guided high-dose-rate brachytherapy of the prostate.

PURPOSE: The accurate delivery of high-dose-rate brachytherapy is dependent on the correct identification of the position and shape of the treatment catheters. In many brachytherapy clinics, transrectal ultrasound (TRUS) imaging is used to identify the catheters. However, manual catheter identification on TRUS images can be time consuming, subjective, and operator dependent because of calcifications and distal shadowing artifacts. We report the use of electromagnetic (EM) tracking technology to map the position and shape of catheters inserted in a tissue-mimicking phantom. METHODS AND MATERIALS: The accuracy of the EM system was comprehensively quantified using a three-axis robotic system. In addition, EM tracks acquired from catheters in a phantom were compared with catheter positions determined from TRUS and CT images to compare EM system performance to standard clinical imaging modalities. The tracking experiments were performed in a controlled laboratory environment and also in a typical brachytherapy operating room to test for potential EM distortions. RESULTS: The robotic validation of the EM system yielded a mean accuracy of <0.5 mm for a clinically acceptable field of view in a nondistorting environment. The EM-tracked catheter representations were found to have an accuracy of <1 mm when compared with TRUS- and CT-identified positions, both in the laboratory environment and in the brachytherapy operating room. The achievable accuracy depends to a large extent on the calibration of the TRUS probe, geometry of the tracked devices relative to the EM field generator, and locations of surrounding clinical equipment. To address the issue of variable accuracy, a robust calibration algorithm has been developed and integrated into the workflow. The proposed mapping technique was also found to improve the workflow efficiency of catheter identification. CONCLUSIONS: The high baseline accuracy of the EM system, the consistent agreement between EM-tracked, TRUS- and CT-identified catheters, and the improved workflow efficiency illustrate the potential value of using EM tracking for catheter mapping in high-dose-rate brachytherapy.

Intraoperative segmentation of iodine and palladium radioactive sources in C-arm images.

PURPOSE: Dynamic dosimetry is becoming the standard to evaluate the quality of radioactive implants during brachytherapy. For this, it is essential to obtain a 3D visualization of the implanted seeds and their relative position to the prostate. A method was developed to obtain a robust and precise segmentation of seeds in C-arm images, and this approach was tested using clinical datasets. METHOD: A region-based implicit active contour approach was used to delineate implanted seeds. Then, a template-based matching was employed to segment iodine implants whereas a K-means algorithm is implemented to resolve palladium seed clusters. To validate the method, 55 C-arm images from 10 patients were used for the segmentation of iodine sources, whereas 225 C-arm images from 16 patients were used for the palladium case. RESULTS: Compared to manual ground truth segmentation of 6,002 iodine seeds and 15,354 palladium seeds, 98.7 % of iodine sources were automatically detected and declustered showing a false-positive rate of only 1.7 %. A total of 98.7 % of palladium sources were automatically detected and declustered with a false-positive rate of only 2.0 %. CONCLUSION: An automated segmentation method was developed that is able to perform the identification and annotation processes of seeds on par with a human expert. This method was shown to be robust and suitable for integration in the dynamic dosimetry workflow of prostate brachytherapy interventions.

A Dynamic Dosimetry System for Prostate Brachytherapy.

The lack of dynamic dosimetry tools for permanent prostate brachytherapy causes otherwise avoidable problems in prostate cancer patient care. The goal of this work is to satisfy this need in a readily adoptable manner. Using the ubiquitous ultrasound scanner and mobile non-isocentric C-arm, we show that dynamic dosimetry is now possible with only the addition of an arbitrarily configured marker-based fiducial. Not only is the system easily configured from accessible hardware, but it is also simple and convenient, requiring little training from technicians. Furthermore, the proposed system is built upon robust algorithms of seed segmentation, fiducial detection, seed reconstruction, and image registration. All individual steps of the pipeline have been thoroughly tested, and the system as a whole has been validated on a study of 25 patients. The system has shown excellent results of accurately computing dose, and does so with minimal manual intervention, therefore showing promise for widespread adoption of dynamic dosimetry.

Seed localization in ultrasound and registration to C-arm fluoroscopy using matched needle tracks for prostate brachytherapy.

We propose a novel fiducial-free approach for the registration of C-arm fluoroscopy to 3-D ultrasound images of prostate brachytherapy implants to enable dosimetry. The approach involves the reliable detection of a subset of radioactive seeds from 3-D ultrasound, and the use of needle tracks in both ultrasound and fluoroscopy for registration. Seed detection in ultrasound is achieved through template matching in 3-D radio frequency ultrasound signals, followed by thresholding and spatial filtering. The resulting subset of seeds is registered to the complete reconstruction of the brachytherapy implant from multiple C-arm fluoroscopy views. To compensate for the deformation caused by the ultrasound probe, simulated warping is applied to the seed cloud from fluoroscopy. The magnitude of the applied warping is optimized within the registration process. The registration is performed in two stages. First, the needle track projections from fluoroscopy and ultrasound are matched. Only the seeds in the matched needles are then used as fiducials for point-based registration. We report results from a physical phantom with a realistic implant (average postregistration seed distance of 1.6 ± 1.2 mm) and from five clinical patient datasets (average error: 2.8 ± 1.5 mm over 128 detected seeds). We conclude that it is feasible to use RF ultrasound data, template matching, and spatial filtering to detect a reliable subset of brachytherapy seeds from ultrasound to enable registration to fluoroscopy for dosimetry.

Ultrasound-fluoroscopy registration for prostate brachytherapy dosimetry.

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.

SLC34A3 intronic deletion in a new kindred with hereditary hypophosphatemic rickets with hypercalciuria.

OBJECTIVE: Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is an autosomal recessive form of hypophosphatemia with hyperphosphaturia, hypercalciuria, and hypercalcemia. In two reports on six affected kindreds with HHRH, the disease was mapped to chromosome 9q34, which contains the SLC34A3 gene that encodes the renal type 2c sodium-phosphate cotransporter. Our objective was to define the clinical course of these cases in a family with HHRH and to screen for SLC34A3 gene in order to determine whether these mutations are responsible for HHRH. METHODS: After clinical and biochemical evaluations, the entire SLC34A3 gene was screened using PCR amplification followed by direct sequencing technique. In this paper, we describe a new kindred with HHRH and a case of progressive and complicated HHRH presenting at age 27 years. RESULTS: We found 101-bp deletion in intron 9 of the SLC34A3 gene. The index patient was homozygous for this mutation which has been previously reported in a Caucasian population. This is the first report for presence of SLC34A3 intron 9 deletion in an Iranian population. CONCLUSIONS: These data showed that HHRH can be easily missed or underdiagnosed. Genetic evaluation of patients with familial hypercalciuria, hypophosphatemia and nephrolithiasis is needed for further information on the prevalence and management of this rare disorder.

Segmentation of iodine brachytherapy implants in fluoroscopy.

PURPOSE: In prostate brachytherapy, intraoperative dosimetry would allow for evaluation of the implant quality while the patient is still in treatment position. Such a mechanism, however, requires 3-D visualization of the deposited seeds relative to the prostate. It follows that accurate and robust seed segmentation is of critical importance in achieving intraoperative dosimetry. METHODS: Implanted iodine brachytherapy seeds are segmented via a region-based implicit active contour model. Overlapping seed groups are then resolved using a template-based declustering technique. RESULTS: Ground truth seed coordinates were obtained through manual segmentation. A total of 57 clinical C-arm images from 10 patients were used to validate the proposed algorithm. This resulted in two failed images and a 96.0% automatic detection rate with a corresponding 2.2% false-positive rate in the remaining 55 images. The mean centroid error between the manual and automatic segmentations was 1.2 pixels. CONCLUSIONS: Robust and accurate iodine seed segmentation can be achieved through the proposed segmentation workflow.

Use of needle track detection to quantify the displacement of stranded seeds following prostate brachytherapy.

We aim to compute the movement of permanent stranded implant brachytherapy radioactive sources (seeds) in the prostate from the planned seed distribution to the intraoperative fluoroscopic distribution, and then to the postimplant computed tomography (CT) distribution. We present a novel approach to matching the seeds in these distributions to the plan by grouping the seeds into needle tracks. First, we identify the implantation axis using a sample consensus algorithm. Then, we use a network flow algorithm to group seeds into their needle tracks. Finally, we match the needles from the three stages using both their transverse plane location and the number of seeds per needle. We validated our approach on eight clinical prostate brachytherapy cases, having a total of 871 brachytherapy seeds distributed in 193 needles. For the intraoperative and postimplant data, 99.31% and 99.41% of the seeds were correctly assigned, respectively. For both the preplan to fluoroscopic and fluoroscopic to CT registrations, 100% of the needles were correctly matched. We show that there is an average intraoperative seed displacement of 4.94±2.42 mm and a further 2.97±1.81 mm of postimplant movement. This information reveals several directional trends and can be used for quality control, treatment planning, and intraoperative dosimetry that fuses ultrasound and fluoroscopy.

Point-to-volume registration of prostate implants to ultrasound.

Ultrasound-Fluoroscopy fusion is a key step toward intraoperative dosimetry for prostate brachytherapy. We propose a method for intensity-based registration of fluoroscopy to ultrasound that obviates the need for seed segmentation required for seed-based registration. We employ image thresholding and morphological and Gaussian filtering to enhance the image intensity distribution of ultrasound volume. Finally, we find the registration parameters by maximizing a point-to-volume similarity metric. We conducted an experiment on a ground truth phantom and achieved registration error of 0.7 +/- 0.2 mm. Our clinical results on 5 patient data sets show excellent visual agreement between the registered seeds and the ultrasound volume with a seed-to-seed registration error of 1.8 +/- 0.9mm. With low registration error, high computational speed and no need for manual seed segmentation, our method is promising for clinical application.

Intra-operative prostate brachytherapy dosimetry based on partial seed localization in ultrasound and registration to C-arm fluoroscopy.

Intraoperative dosimetry during prostate brachytherapy is a long standing clinical problem. We propose a novel framework to address this problem by reliable detection of a subset of seeds from 3D transrectal ultrasound and registration to fluoroscopy. Seed detection in ultrasound is achieved through template matching in the RF ultrasound domain followed by thresholding and spatial filtering based on the fixed distance between stranded seeds. This subset of seeds is registered to the complete reconstruction of the implant in C-arm fluoroscopy. We report results, validated with a leave-one-needle-out approach, both in a phantom (average post-registration seed distance of 2.5 mm) and in three clinical patient datasets (average error: 3.9 mm over 113 seeds).

Quantifying stranded implant displacement following prostate brachytherapy.

We aim to compute radioactive stranded-implant displacement during and after prostate brachytherapy. We present the methods used to identify corresponding seeds in planned, intra-operative and postimplant patient data that enable us to compute seed displacements. A minimum cost network flow algorithm is used, on 8 patients, for needle track detection to group seeds into needles that can be matched between datasets. An iterative best line detection algorithm is used both to help with needle detection and to register the different datasets. Our results show that there was an average seed misplacement of 5.08 +/- 2.35 mm during the procedure, which then moved another 3.10 +/- 1.91 mm by the time the quality assurance CT was taken. Several directional trends in different regions of the prostate were noted and commented on.

Prostate implant reconstruction from C-arm images with motion-compensated tomosynthesis.

PURPOSE: Accurate localization of prostate implants from several C-arm images is necessary for ultrasound-fluoroscopy fusion and intraoperative dosimetry. The authors propose a computational motion compensation method for tomosynthesis-based reconstruction that enables 3D localization of prostate implants from C-arm images despite C-arm oscillation and sagging. METHODS: Five C-arm images are captured by rotating the C-arm around its primary axis, while measuring its rotation angle using a protractor or the C-arm joint encoder. The C-arm images are processed to obtain binary seed-only images from which a volume of interest is reconstructed. The motion compensation algorithm, iteratively, compensates for 2D translational motion of the C-arm by maximizing the number of voxels that project on a seed projection in all of the images. This obviates the need for C-arm full pose tracking traditionally implemented using radio-opaque fiducials or external trackers. The proposed reconstruction method is tested in simulations, in a phantom study and on ten patient data sets. RESULTS: In a phantom implanted with 136 dummy seeds, the seed detection rate was 100% with a localization error of 0.86 ± 0.44 mm (Mean ± STD) compared to CT. For patient data sets, a detection rate of 99.5% was achieved in approximately 1 min per patient. The reconstruction results for patient data sets were compared against an available matching-based reconstruction method and showed relative localization difference of 0.5 ± 0.4 mm. CONCLUSIONS: The motion compensation method can successfully compensate for large C-arm motion without using radio-opaque fiducial or external trackers. Considering the efficacy of the algorithm, its successful reconstruction rate and low computational burden, the algorithm is feasible for clinical use.

Brachytherapy seed reconstruction with joint-encoded C-arm single-axis rotation and motion compensation.

C-arm fluoroscopy images are frequently used for qualitative assessment of prostate brachytherapy. Three-dimensional seed reconstruction from C-arm images is necessary for intraoperative dosimetry and quantitative assessment. Seed reconstruction requires accurately known C-arm poses. We propose to measure the C-arm rotation angles and computationally compensate for inevitable C-arm motion to compute the pose. We compensate the translational motions of a C-arm, such as oscillation, sagging and wheel motion using a three-level optimization algorithm and obviate the need for full pose tracking using external trackers or fiducials. We validated our approach on simulated and 100 clinical data sets from 10 patients and gained on average, a seed matching rate of 98.5%, projection error of 0.33 mm (STD=0.21 mm) and computation time of 19.8s per patient, which must be considered as clinically excellent results. We also show that without motion compensation the reconstruction is likely to fail.

Intraoperative 3D reconstruction of prostate brachytherapy implants with automatic pose correction.

The success of prostate brachytherapy critically depends on delivering adequate dose to the prostate gland, and the capability of intraoperatively localizing implanted seeds provides potential for dose evaluation and optimization during therapy. REDMAPS is a recently reported algorithm that carries out seed localization by detecting, matching and reconstructing seeds in only a few seconds from three acquired x-ray images (Lee et al 2011 IEEE Trans. Med. Imaging 29 38-51). In this paper, we present an automatic pose correction (APC) process that is combined with REDMAPS to allow for both more accurate seed reconstruction and the use of images with relatively large pose errors. APC uses a set of reconstructed seeds as a fiducial and corrects the image pose by minimizing the overall projection error. The seed matching and APC are iteratively computed until a stopping condition is met. Simulations and clinical studies show that APC significantly improves the reconstructions with an overall average matching rate of ⩾99.4%, reconstruction error of ⩽0.5 mm, and the matching solution optimality of ⩾99.8%.

Prostate brachytherapy seed reconstruction using C-arm rotation measurement and motion compensation.

During prostate brachytherapy, C-arm flouroscopy images are used for a qualitative assessment of the procedure. Three dimensional reconstruction of the implanted seeds can be used for intraoperative dosimetry and quantitative assessment. Accurate C-arm pose estimation is necessary for 3D localization of the seeds. We propose to measure the C-arm rotation angles and computationally compensate the inevitable C-arm translational motion to estimate the pose. We compensate the oscillation, sagging and wheel motion of the C-arm using a three-level optimization algorithm, without which the reconstruction can fail. We validated our approach on simulated and 10 data sets from 5 patients and gained on average 99.1% success rate, 0.33 mm projection error and computation time of less than one minute per patient, which are clinically excellent results.

Modeling and simulation of flexible needles.

Needle insertion is performed in many clinical and therapeutic procedures. Tissue displacement and needle bending which result from needle-tissue interaction make accurate targeting difficult. For performing physicians to gain essential needle targeting skills, needle insertion simulators can be used for training. An accurate needle bending model is essential for such simulators. These bending models are also needed for needle path planning. In this paper, three different models are presented to simulate the deformations of a needle. The first two models use the finite element method and take the geometric nonlinearity into account. The third model is a series of rigid bars connected by angular springs. The models were compared to recorded deformations during experiments of applying lateral tip forces on a brachytherapy needle. The model parameters were identified and the simulation results were compared to the experimental data. The results show that the angular spring model, which is computationally the most efficient model, is also the most accurate in modeling the bending of the brachytherapy needle.

Needle insertion study using ultrasound-based 2D motion tracking.

Needle insertion simulators are in demand to train physicians for surgical procedures such as prostate brachytherapy. In order to design a needle insertion simulator, a needle-tissue interaction model is necessary. Such a model can be identified using force and displacement data measured during the insertion of the needle. In this paper, an experiment is conducted in which a needle is inserted into a two-layered PVC phantom, while the needle position and insertion forces are measured and the tissue is imaged using a trans-rectal probe in the sagittal plane. A 2D block matching algorithm is used to estimate tissue deformation from the envelope of the recorded radio-frequency signals. This algorithm can be used to estimate the rotation and displacement of the prostate during prostate brachytherapy. The block matching method was validated in an independent experiment. With the measured force, needle position and tissue displacements, a finite element simulation was conducted to identify the parameters of a needle-tissue force distribution model and Young's moduli of each part of the tissue phantom.