Biomedical subjects

Paul S Cho

Publications and source records attributed to Paul S Cho.

15 recordsLinked to original sources

PubMed2006

Semiautomatic 3-D prostate segmentation from TRUS images using spherical harmonics.

Prostate brachytherapy quality assessment procedure should be performed while the patient is still on the operating table since this would enable physicians to implant additional seeds immediately into the prostate if necessary thus reducing the costs and increasing patient outcome. Seed placement procedure is readily performed under fluoroscopy and ultrasound guidance. Therefore, it has been proposed that seed locations be reconstructed from fluoroscopic images and prostate boundaries be identified in ultrasound images to perform dosimetry in the operating room. However, there is a key hurdle that needs to be overcome to perform the ultrasound and fluoroscopy-based dosimetry: it is highly time-consuming for physicians to outline prostate boundaries in ultrasound images manually, and there is no method that enables physicians to identify three-dimensional (3-D) prostate boundaries in postimplant ultrasound images in a fast and robust fashion. In this paper, we propose a new method where the segmentation is defined in an optimization framework as fitting the best surface to the underlying images under shape constraints. To derive these constraints, we modeled the shape of the prostate using spherical harmonics of degree eight and performed statistical analysis on the shape parameters. After user initialization, our algorithm identifies the prostate boundaries on the average in 2 min. For algorithm validation, we collected 30 postimplant prostate volume sets, each consisting of axial transrectal ultrasound images acquired at 1-mm increments. For each volume set, three experts outlined the prostate boundaries first manually and then using our algorithm. By treating the average of manual boundaries as the ground truth, we computed the segmentation error. The overall mean absolute distance error was 1.26 +/- 0.41 mm while the percent volume overlap was 83.5 +/- 4.2. We found the segmentation error to be slightly less than the clinically-observed interobserver variability.

Algorithms↗

PubMed2005

Intraoperative fluoroscopic dose assessment in prostate brachytherapy patients.

PURPOSE: To evaluate a fluoroscopy-based intraoperative dosimetry system to guide placement of additional sources to underdosed areas, and perform computed tomography (CT) verification. METHODS AND MATERIALS: Twenty-six patients with prostate carcinoma treated with either I-125 or Pd-103 brachytherapy at the Puget Sound VA using intraoperative postimplant dosimetry were analyzed. Implants were performed by standard techniques. After completion of the initial planned brachytherapy procedure, the initial fluoroscopic intraoperative dose reconstruction analysis (I-FL) was performed with three fluoroscopic images acquired at 0 (AP), +15, and -15 degrees. Automatic seed identification was performed and the three-dimensional (3D) seed coordinates were computed and imported into VariSeed for dose visualization. Based on a 3D assessment of the isodose patterns additional seeds were implanted, and the final fluoroscopic intraoperative dose reconstruction was performed (FL). A postimplant computed tomography (CT) scan was obtained after the procedure and dosimetric parameters and isodose patterns were analyzed and compared. RESULTS: An average of 4.7 additional seeds were implanted after intraoperative analysis of the dose coverage (I-FL), and a median of 5 seeds. After implantation of additional seeds the mean V100 increased from 89% (I-FL) to 92% (FL) (p < 0.001). In I-125 patients an improvement from 91% to 94% (p = 0.01), and 87% to 93% (p = 0.001) was seen for Pd-103. The D90 increased from 105% (I-FL) to 122% (FL) (p < 0.001) for I-125, and 92% (I-FL) to 102% (FL) (p = 0.008) for Pd-103. A minimal change occurred in the R100 from a mean of 0.32 mL (I-FL) to 0.6 mL (FL) (p = 0.19). No statistical difference was noted in the R100 (rectal volume receiving 100% of the prescribed dose) between the two techniques. The rate of adverse isodose patterns decreased between I-FL and FL from 42% to 8%, respectively. The I-125 patients demonstrated a complete resolution of adverse isodose patterns after the initial isodose reconstruction (I-FL). The Pd-103 patients demonstrated a final rate of 8% gaps, 0% islands, and 0% holes on corrected isodose reconstruction. CONCLUSION: The use of intraoperative fluoroscopy-based dose assessment can accurately guide in the implantation of additional sources to supplement inadequately dosed areas within the prostate gland. Additionally, guided implantation of additional source, can significantly improve V100s and D90s, without significantly increasing rectal doses.

Brachytherapy↗

PubMed2005

Detection and correction of patient movement in prostate brachytherapy seed reconstruction.

Intraoperative dosimetry of prostate brachytherapy can help optimize the dose distribution and potentially improve clinical outcome. Evaluation of dose distribution during the seed implant procedure requires the knowledge of 3D seed coordinates. Fluoroscopy-based seed localization is a viable option. From three x-ray projections obtained at different gantry angles, 3D seed positions can be determined. However, when local anaesthesia is used for prostate brachytherapy, the patient movement during fluoroscopy image capture becomes a practical problem. If uncorrected, the errors introduced by patient motion between image captures would cause seed mismatches. Subsequently, the seed reconstruction algorithm would either fail to reconstruct or yield erroneous results. We have developed an algorithm that permits detection and correction of patient movement that may occur between fluoroscopy image captures. The patient movement is decomposed into translational shifts along the tabletop and rotation about an axis perpendicular to the tabletop. The property of spatial invariance of the co-planar imaging geometry is used for lateral movement correction. Cranio-caudal movement is corrected by analysing the perspective invariance along the x-ray axis. Rotation is estimated by an iterative method. The method can detect and correct for the range of patient movement commonly seen in the clinical environment. The algorithm has been implemented for routine clinical use as the preprocessing step for seed reconstruction.

Algorithms↗

PubMed2005

Syntactic parsing of medical reports using evolutionary optimization.

We report on a syntactic parser for medical reports which uses a genetic algorithm to efficiently identify the highest ranking parse configuration based on a scoring scheme developed as part of our prior work. The approach was tested on a set of 250 sentences from the domain of radiology. Performance time and comparison to exhaustive methods are given.

Algorithms↗

PubMed2004

Three-dimensional seed reconstruction from an incomplete data set for prostate brachytherapy.

Intra-operative dosimetry in prostate brachytherapy requires 3D coordinates of the implanted, radioactive seeds. Since CT is not readily available during the implant operation, projection x-rays are commonly used for intra-operative seed localization. Three x-ray projections are usually used. The requirement of the current seed reconstruction algorithms is that the seeds must be identified on all three projections. However, in practice this is often difficult to accomplish due to the problem of heavily clustered and overlapping seeds. We have developed an algorithm that permits seed reconstruction from an incomplete data set. Instead of all three projections, the new algorithm requires only one of the three projections to be complete. Furthermore, even if all three projections are incomplete, it can reconstruct 100% of the implanted seeds depending on how the undetected seeds are distributed among the projections. The method utilizes the principles of epipolar imaging geometry and pseudo-matching of the undetected seeds. The algorithm was successfully applied to a large number of clinical cases where seeds imperceptibly overlap in some projections.

Algorithms↗

PubMed2004

Application of influence diagrams to prostate intensity-modulated radiation therapy plan selection.

The purpose is to incorporate clinically relevant factors such as patient-specific and dosimetric information as well as data from clinical trials in the decision-making process for the selection of prostate intensity-modulated radiation therapy (IMRT) plans. The approach is to incorporate the decision theoretic concept of an influence diagram into the solution of the multiobjective optimization inverse planning problem. A set of candidate IMRT plans was obtained by varying the importance factors for the planning target volume (PTV) and the organ-at-risk (OAR) in combination with simulated annealing to explore a large part of the solution space. The Pareto set for the PTV and OAR was analysed to demonstrate how the selection of the weighting factors influenced which part of the solution space was explored. An influence diagram based on a Bayesian network with 18 nodes was designed to model the decision process for plan selection. The model possessed nodes for clinical laboratory results, tumour grading, staging information, patient-specific information, dosimetric information, complications and survival statistics from clinical studies. A utility node was utilized for the decision-making process. The influence diagram successfully ranked the plans based on the available information. Sensitivity analyses were used to judge the reasonableness of the diagram and the results. In conclusion, influence diagrams lend themselves well to modelling the decision processes for IMRT plan selection. They provide an excellent means to incorporate the probabilistic nature of data and beliefs into one model. They also provide a means for introducing evidence-based medicine, in the form of results of clinical trials, into the decision-making process.

Aged↗

PubMed2004

Three-dimensional seed reconstruction for prostate brachytherapy using Hough trajectories.

In order to perform intra-operative or post-implant dosimetry in prostate brachytherapy, the 3D coordinates of the implanted radioactive seeds must be determined. Film or fluoroscopy based seed reconstruction techniques use back projection of x-ray data obtained at two or three x-ray positions. These methods, however, do not perform well when some of the seed images are undetected. To overcome this problem we have developed an alternate technique for 3D seed localization using the principle of Hough transform. The Hough method utilizes the fact that, for each seed coordinate in three dimensions, there exists a unique trajectory in Hough feature space. In this paper we present the Hough transform parametric equations to describe the path of the seed projections from one view to the next and a method to reconstruct the 3D seed coordinates. The results of simulation and phantom studies indicate that the Hough trajectory method can accurately determine the 3D seed positions even from an incomplete dataset.

Algorithms↗

PubMed2004

Parametric shape modeling using deformable superellipses for prostate segmentation.

Automatic prostate segmentation in ultrasound images is a challenging task due to speckle noise, missing boundary segments, and complex prostate anatomy. One popular approach has been the use of deformable models. For such techniques, prior knowledge of the prostate shape plays an important role in automating model initialization and constraining model evolution. In this paper, we have modeled the prostate shape using deformable superellipses. This model was fitted to 594 manual prostate contours outlined by five experts. We found that the superellipse with simple parametric deformations can efficiently model the prostate shape with the Hausdorff distance error (model versus manual outline) of 1.32 +/- 0.62 mm and mean absolute distance error of 0.54 +/- 0.20 mm. The variability between the manual outlinings and their corresponding fitted deformable superellipses was significantly less than the variability between human experts with p-value being less than 0.0001. Based on this deformable superellipse model, we have developed an efficient and robust Bayesian segmentation algorithm. This algorithm was applied to 125 prostate ultrasound images collected from 16 patients. The mean error between the computer-generated boundaries and the manual outlinings was 1.36 +/- 0.58 mm, which is significantly less than the manual interobserver distances. The algorithm was also shown to be fairly insensitive to the choice of the initial curve.

Algorithms↗

PubMed2003

Isodose patterns in patients with inadequate prostate brachytherapy coverage.

PURPOSE: The development of a practical, real-time dosimetry system should result in improved implant dose distributions and higher prostate cancer control rates. Our purpose here is to demonstrate that intraoperative isodose reconstruction in relation to the seed distribution, even without accurate registration with the prostatic volume, can likely identify an inadequate implant intraoperatively and guide corrective seed placement. METHODS AND MATERIALS: A total of 102 Pd-103 implants performed by standard techniques, using a modified peripheral loading pattern, were studied. A postimplant computed tomography (CT) scan was obtained 2-4 h after the implant. The contoured images and sources were entered into a Varian Variseed 7.0 treatment planning system. Dosimetric parameters analyzed included the percent of the postimplant prostate or rectal volume covered by the prescription dose (V100), and the dose that covers 90% of the postimplant prostate volume (D90). Isodose patterns were analyzed at midprostate, and for the entire prostate. Adverse isodose patterns were defined as gaps, holes, islands. Isodose gaps are subprescription intervals between the prostatic margin and the prescription isodose. Isodose holes are regions of subprescription dose within the prostate. Isodose islands are isolated regions > or =prescription dose inside the prostatic margins. RESULTS: Characteristic isodose patterns were predictive for V100 values. Midprostatic isodose holes were seen in 55% of patients with a V100 < 80%, 5% of patients with a V100 of 80-90%, and only 1% of patients with a V100 > 90%. When analyzing the entire prostate, isodose holes were seen in 55% of patients with a V100 < 80%, 18% of patients with a V100 of 80-90%, and 9% of patients with a V100 > 90%. Midprostatic isodose islands were seen in 55% of patients with a V100 < 80%, 5% of patients with a V100 of 80-90%, and no patient with a V100 > 90%. When analyzing the entire prostate, isodose islands were seen in all patients with V100 < 80%, 36% of patients with a V100 of 80-90%, and only 1% of patients with a V100 > 90%. The likelihood of a V100 less than 80% was best predicted by the presence of isodose holes or islands at midprostate. Patients with either finding had an 86% chance of having a V100 < 80%. CONCLUSIONS: These semiquantitative findings can provide practical guidelines for intraoperative dosimetry, to provide a more rational guide to intraoperative postimplant assessment and modification. If isodose holes or islands are seen within the implanted volume, additional seeds are added to the affected region to obtain a V100 > 80%.

Brachytherapy↗

PubMed2003

Random and systematic beam modulator errors in dynamic intensity modulated radiotherapy.

This paper reports on the dosimetric effects of random and systematic modulator errors in delivery of dynamic intensity modulated beams. A sliding-widow type delivery that utilizes a combination of multileaf collimators (MLCs) and backup diaphragms was examined. Gaussian functions with standard deviations ranging from 0.5 to 1.5 mm were used to simulate random positioning errors. A clinical example involving a clival meningioma was chosen with optic chiasm and brain stem as limiting critical structures in the vicinity of the tumour. Dose calculations for different modulator fluctuations were performed, and a quantitative analysis was carried out based on cumulative and differential dose volume histograms for the gross target volume and surrounding critical structures. The study indicated that random modulator errors have a strong tendency to reduce minimum target dose and homogeneity. Furthermore, it was shown that random perturbation of both MLCs and backup diaphragms in the order of sigma = 1 mm can lead to 5% errors in prescribed dose. In comparison, when MLCs or backup diaphragms alone was perturbed, the system was more robust and modulator errors of at least sigma = 1.5 mm were required to cause dose discrepancies greater than 5%. For systematic perturbation, even errors in the order of +/- 0.5 mm were shown to result in significant dosimetric deviations.

Brain Neoplasms↗

PubMed2003

Tomosynthesis-based localization of radioactive seeds in prostate brachytherapy.

Accurately assessing the quality of prostate brachytherapy intraoperatively would be valuable for improved clinical outcome by ensuring the delivery of a prescribed tumoricidal radiation dose to the entire prostate gland. One necessary step towards this goal is the robust and rapid localization of implanted seeds. Several methods have been developed to locate seeds from x-ray projection images, but they fail to detect completely-overlapping seeds, thus necessitating manual intervention. To overcome this limitation, we have developed a new method where (1) a three-dimensional volume is reconstructed from x-ray projection images using a brachytherapy-specific tomosynthesis reconstruction algorithm with built-in blur compensation and (2) the seeds are located in this reconstructed volume. In contrast to other projection-based methods, our method can detect completely overlapping seeds. Our simulation results indicate that we can locate all implanted seeds in the prostate using a tomosynthesis angle of 30 degrees and seven projection images. The mean localization error is 1.27 mm for a case with 100 seeds. We have also tested our method using a prostate phantom with 61 implanted seeds and succeeded in locating all seeds automatically. We believe this new method can be useful for the intraoperative quality assessment of prostate brachytherapy in the future.

Brachytherapy↗

PubMed2003

Automatic section segmentation of medical reports.

Automated segmentation of medical reports can significantly enhance the productivity of the healthcare departments. While many algorithms have been developed for document summarization, passage retrieval, and story segmentation of news feeds, much less effort has been devoted to parsing of medical documents. We present an algorithm specifically developed for medical applications. The algorithm consists of two components. First, a rule-based algorithm is used to detect the sections that contain labels. It utilizes a knowledge base of commonly employed heading labels and linguistic cues seen within training examples. The second part of the algorithm handles the detection of unlabeled sections. It uses a combination of lexical pattern recognition and a classifier based on an expectation model for a particular class of medical reports. The proposed method was evaluated on three test corpora containing a total of 129,303 report sections. The detection rates for labeled and unlabeled sections for individual corpus ranged from 97.4% to 99.4% and from 96.5% to 99.0%, respectively. The rule-based approach is particularly effective for medical reports due to inherently structured nature of these documents.

Algorithms↗

PubMed2002

Ultrasonography and fluoroscopic fusion for prostate brachytherapy dosimetry.

PURPOSE: To investigate the feasibility of performing postimplant and intraoperative dosimetry for prostate brachytherapy by fusing transrectal ultrasound (TRUS) and fluoroscopic data. METHODS AND MATERIALS: Registration of ultrasound (prostate boundary) and fluoroscopic (seed) data requires spatial markers that are detectable by both imaging modalities. In this study, the needle tips were considered as such fiducials. Prostate phantoms were implanted with the seeds, and four localization needles were inserted. In the TRUS frame of reference, the longitudinal coordinate of the needle tip was determined by advancing the needle until the echo from its tip just registered at a known probe depth. The tip's transverse coordinates were determined from the associated TRUS slice. The three-dimensional needle tip positions were also calculated in the fluoroscopic coordinate system using a seed reconstruction method. The transformation between the TRUS and fluoroscopy coordinate systems was established by the least-squares solution using the singular value decomposition. RESULTS: With three of four needle tips as fiducials and the one remaining needle as a test target, the mean fiducial registration error was 0.8 mm and the test target registration error was 2.5 mm. When all four points were used for registration, the errors decreased to 1.1 mm. A comparison between the proposed method and CT-based dosimetry yielded a percentage of prostate volume receiving 100% and 150% of the prescribed minimal peripheral dose and minimal dose received by 90% of the prostate gland that agreed within 0.4%, 2.7%, and 4.2%, respectively. CONCLUSION: The combination of TRUS and fluoroscopy is a feasible alternative to the currently used CT-based postimplant dosimetry. Furthermore, because of online imaging capability, the method lends itself to real-time intraoperative applications.

Brachytherapy↗

PubMed2002

Fast cross-projection algorithm for reconstruction of seeds in prostate brachytherapy.

A fast method of seed matching and reconstruction in prostate brachytherapy is proposed. Previous approaches have required all seeds to be matched with all other seeds in other projections. The fast cross-projection algorithm for the reconstruction of seeds (Fast-CARS) allows for matching of a given seed with a subset of seeds in other projections. This subset lies in a proximal region centered about the projection of a line, connecting the seed to its source, onto other projection planes. The proposed technique permits a significant reduction in computational overhead, as measured by the required number of matching tests. The number of multiplications and additions is also vastly reduced at no trade-off in accuracy. Because of its speed, Fast-CARS can be used in applications requiring real-time performance such as intraoperative dosimetry of prostate brachytherapy. Furthermore, the proposed method makes practical the use of a larger number of views as opposed to previous techniques limited to a maximum use of three views.

Algorithms↗

PubMedDate not supplied

Spatial accuracy of fractionated IMRT delivery studies in canine paraspinal irradiation.

Intensity modulated radiation therapy (IMRT) theoretically allows detailed tailoring of the dose distribution in tissue. The goal of this study was to determine if a method of dynamic IMRT could be used to deliver a high dose of radiation to a concave shaped target around the cervical spinal cord. Fifteen young adult dogs from our laboratory population were randomly divided into two groups. A radiation dose of 84 Gy in 4 Gy fractions was delivered with a conventional 4 field technique for Group A dogs, and with dynamic IMRT for Group B dogs to a "C-shaped" target close to the cervical spinal cord. Neurologic status, magnetic resonance imaging results and histopathologic changes were compared among dogs in the two groups. Group A dogs developed myelomalacia with a latency period of 65 +/- 9 days. Group B dogs did not have any histologic changes to the cervical spinal cord when euthanasia was performed 12 months after irradiation. The results demonstrate that this IMRT technique can be safely and precisely delivered to a patient in a clinical situation.

Animals↗