Encapsulated Contrast Agent Markers for MRI-based Post-implant Dosimetry

Low-dose-rate prostate brachytherapy involves the implantation of tiny radioactive seeds into the prostate to treat prostate cancer. The current standard post-implant imaging modality is computed tomography (CT). On CT images, the radioactive seeds can be distinctively localized but delineation of the prostate and surrounding soft tissue is poor. Magnetic resonance imaging (MRI) provides better prostate and soft tissue delineation, but seed localization is difficult. To aid with seed localization, MRI markers with encapsulated contrast agent that provide positive-contrast on MRI images (Sirius MRI markers; C4 Imaging, Houston, TX) have been proposed to be placed adjacent to the negative-contrast seeds. This dissertation describes the development of the Sirius MRI markers for prostate post-implant dosimetry. First, I compared the dose-volume histogram and other dosimetry parameters generated by MIM Symphony (a brachytherapy treatment planning system that allow the use of MRI images for treatment planning; MIM Software Inc., Cleveland, OH) and VariSeed (a widely used brachytherapy treatment planning system; Varian Medical Systems, Inc., Palo Alto, CA), and found the dosimetry between both brachytherapy treatment planning systems to be comparable. To gain more insight into the MRI contrast characteristics of the Sirius MRI markers, I measured the Sirius MRI marker contrast agent\u27s spin-lattice and spin-spin relaxivities, and studied the relaxation characteristics\u27 dependence on MRI field strength, temperature, and orientation. From the Sirius MRI marker\u27s contrast agent relaxation characteristics, I systematically studied the effect of varying MRI scan parameters such as flip angle, number of excitations, bandwidth, field of view, slice thickness, and encoding steps, on the Sirius MRI markers\u27 signal and contrast, as well as image noise, artifact and scan time. On patients implanted with Sirius MRI markers, I evaluated the visibility of the Sirius MRI markers and image artifacts. Lastly, I semi-automated the localization of markers and seeds to more enable the efficient incorporation of Sirius MRI markers as part of the clinical post-implant workflow. Ultimately, the Sirius MRI markers may change the paradigm from CT-based to MRI-based post-implant dosimetry, for a more accurate understanding of dose-response relationships in patients undergoing low dose rate prostate brachytherapy

Medical image computing and computer-aided medical interventions applied to soft tissues. Work in progress in urology

Until recently, Computer-Aided Medical Interventions (CAMI) and Medical Robotics have focused on rigid and non deformable anatomical structures. Nowadays, special attention is paid to soft tissues, raising complex issues due to their mobility and deformation. Mini-invasive digestive surgery was probably one of the first fields where soft tissues were handled through the development of simulators, tracking of anatomical structures and specific assistance robots. However, other clinical domains, for instance urology, are concerned. Indeed, laparoscopic surgery, new tumour destruction techniques (e.g. HIFU, radiofrequency, or cryoablation), increasingly early detection of cancer, and use of interventional and diagnostic imaging modalities, recently opened new challenges to the urologist and scientists involved in CAMI. This resulted in the last five years in a very significant increase of research and developments of computer-aided urology systems. In this paper, we propose a description of the main problems related to computer-aided diagnostic and therapy of soft tissues and give a survey of the different types of assistance offered to the urologist: robotization, image fusion, surgical navigation. Both research projects and operational industrial systems are discussed

Deformable registration of X-ray and MRI for post-implant dosimetry in low-dose-rate prostate brachytherapy

Purpose Dosimetric assessment following permanent prostate brachytherapy (PPB) commonly involves seed localization using CT and prostate delineation using coregistered MRI. However, pelvic CT leads to additional imaging dose and requires significant resources to acquire and process both CT and MRI. In this study, we propose an automatic postimplant dosimetry approach that retains MRI for soft‐tissue contouring, but eliminates the need for CT and reduces imaging dose while overcoming the inconsistent appearance of seeds on MRI with three projection x rays acquired using a mobile C‐arm. Methods Implanted seeds are reconstructed using x rays by solving a combinatorial optimization problem and deformably registered to MRI. Candidate seeds are located in MR images using local hypointensity identification. X ray‐based seeds are registered to these candidate seeds in three steps: (a) rigid registration using a stochastic evolutionary optimizer, (b) affine registration using an iterative closest point optimizer, and (c) deformable registration using a local feature point search and nonrigid coherent point drift. The algorithm was evaluated using 20 PPB patients with x rays acquired immediately postimplant and T2‐weighted MR images acquired the next day at 1.5 T with mean 0.8 × 0.8 × 3.0 mmurn:x-wiley:00942405:media:mp13667:mp13667-math-0001 voxel dimensions. Target registration error (TRE) was computed based on the distance from algorithm results to manually identified seed locations using coregistered CT acquired the same day as the MRI. Dosimetric accuracy was determined by comparing prostate D90 determined using the algorithm and the ground truth CT‐based seed locations. Results The mean ± standard deviation TREs across 20 patients including 1774 seeds were 2.23 ± 0.52 mm (rigid), 1.99 ± 0.49 mm (rigid + affine), and 1.76 ± 0.43 mm (rigid + affine + deformable). The corresponding mean ± standard deviation D90 errors were 5.8 ± 4.8%, 3.4 ± 3.4%, and 2.3 ± 1.9%, respectively. The mean computation time of the registration algorithm was 6.1 s. Conclusion The registration algorithm accuracy and computation time are sufficient for clinical PPB postimplant dosimetry

A Feasibility Study of Ultra-Short Echo Time MRI for Positive Contrast Visualization of Prostate Brachytherapy Permanent Seed Implants for Post-Implant Dosimetry

Purpose: Ultra-short echo time (UTE) imaging is a magnetic resonance imaging (MRI) technique that uses very short echo times (on the order of microseconds) to measure rapid T2 relaxation. An application of UTE is the visualization of magnetic susceptibility-induced shortening of T2 in tissues adjacent to metal, such as prostate tissue with implanted brachytherapy seeds. This study assessed UTE imaging of prostate brachytherapy seeds on a clinical 3T MRI scanner to provide images for post-implant dosimetry. Methods: A prostate tissue phantom was made of gelatin mixed with Gd and other materials to mimic the prostate peripheral zone’s T1 and T2 relaxation times; this phantom was used to investigate the effect of UTE acquisition parameters on brachytherapy seed visibility. A second phantom was made to model prostate tissue surrounded by muscle tissue; this pelvic phantom was implanted with 85 titanium brachytherapy seeds (STM1251, Bard Medical). Both phantoms were scanned on a 3T GE scanner with a 3D UTE pulse sequence and a fast spin echo (FSE) pulse sequence. The average seed SNR, the CNR between seed and prostate material, and visual characteristics of the seeds were assessed. A seed counting procedure was developed based on the visual seed characteristics, and subsequently used by two physicists to locate seeds in UTE images of the pelvic phantom. Results: On 3D UTE images, the metal seeds caused a bright ring-link artifact in adjacent prostate tissue due to susceptibility-induced T2 shortening. The average seed SNR was 15.99±1.52 for UTE compared to 32.32±22.43 for FSE; CNR between seed and prostate was 6.73±1.85 for UTE vs. 23.76±12.87 for FSE. The ring was larger in diameter than a seed itself; apparent seed diameters were 4.65±0.363 mm for UTE compared to 1.46±0.38 mm for FSE. The 3D spatial ring pattern facilitated differentiation of seeds from needle tracks and seed spacers. The two physicists counted 83 and 86 seeds respectively in the UTE images. Prostate boundaries were less well visualized with UTE compared to FSE. Conclusion: With its ability to visualize brachytherapy seeds, UTE imaging appears to provide an alternative approach to CT for seed identification. Compared to fusion of separately-acquired CT images and T2-weighted MR images (for delineation of prostate boundaries), UTE and T2-weighted MR can be acquired in a single imaging session – a convenience to patients while potentially minimizing inter-modality image registration issues. A study in prostate brachytherapy patients of the quality of post-implant dosimetry with UTE imaging compared to CT imaging is recommended

Defining a magnetic resonance scan sequence for permanent seed prostate brachytherapy postimplant assessment

AbstractPurposeWe describe a magnetic resonance (MR) scan sequence for prostate brachytherapy postimplant assessment.Methods and MaterialsOne brachytherapy team at the British Columbia Cancer Agency has incorporated MR–CT fusion into their permanent seed prostate brachytherapy quality assurance procedure. Several attempts were required to ensure that the diagnostic MR scanner at the adjoining general hospital performed the desired sequence, providing many examples of suboptimal scans and underlining the pitfalls for a center trying to incorporate the use of MR scanning into their brachytherapy program.ResultsThe recommended sequence (Fast Spin Echo T2-weighted, repetition time [TR]/echo time [TE] 4500/90, echo train length [ETL] 10, 20×20 field of view [FOV], 80 bandwidth [BW]) is associated with superior edge detection when compared with those images in which a typical diagnostic sequence was used. The use of a low bandwidth sequence does not compromise edge detection or seed identification when compared with a higher bandwidth.ConclusionsWe have defined a magnetic resonance imaging sequence, which appears to optimize both prostate delineation and identification of seeds, lending itself to straightforward fusion with CT images and allowing for less uncertainty in permanent seed prostate brachytherapy quality assurance

Prostate volume delineation and seed localization using a 3-T magnetic resonance spectrometer

With approximately one in six men affected by prostate cancer at some point in their lives, effective treatment of the disease remains a focus of oncology research. Effective treatment using radiation requires the delivery of a significant dose to the prostate volume while sparing surrounding sensitive structures. Treatment success can then be determined by localization of the seeds following implantation and the calculation of a dose distribution across the target volume. Magnetic Resonance Imaging (MRI) yields images with soft tissue contrast that is superior to CT or ultrasound, but has been under-appreciated as a dosimetric tool due to the difficulty in localizing the implanted seeds; To optimize scan parameters for seed localization, a phantom was constructed of tissue-equivalent gelatin. Seeds were implanted during construction so various scan protocols could be tested for seed visualization and volume calculation prior to patient studies. Five healthy volunteers and five patients with permanently implanted seeds were then imaged to validate the phantom studies. Images were evaluated based on anatomical clarity and seed visualization rates; Optimization of the scan protocols for use with this equipment yields images with clearly defined anatomical boundaries as well as clearly defined seeds. Phantom volume measurements deviated from known values by less than 2.5% T2-weighted images provide superior anatomical delineation, but suffer from broad susceptibility artifacts that make determination of seed locations difficult. Proton density-weighted images clearly show seed locations and tissue margins. The selection of a 1 mm slice thickness and a 4 mm interstice gap allowed maximum seed visualization rates of 93.3%; Keywords: prostate, brachytherapy, dosimetry, magnetic resonance imaging

Post-implant dosimetry analysis of brachytherapy patients using pre and post-implant Mri

Post-implant dosimetry analysis is a critical step in brachytherapy for identifying the quality of the implant based on seed localization in relation to the volume of the prostate. However, the accuracy of post-implant dosimetry analysis is dependent on accurate delineation of the prostate from surrounding tissue in post-implant CT images. Research has shown that adequately delineate the prostate from surrounding tissue in CT images is difficult, resulting in significant variation between the dose received by the prostate and the dose prescribed by the physician. This research compared prostate volumes delineated from pre-implant US, post-implant CT, and pre and post-implant MR images in order to develop a more reliable methodology to delineate the prostate. The results illustrated the superiority of MR imaging over CT imaging in delineation of the prostate thereby producing more individual and mean dosimetry values, D90 and V100, above their respective cut points of 140 Gy and 85%

Segmentation, separation and pose estimation of prostate brachytherapy seeds in CT images.

International audienceIn this paper, we address the development of an automatic approach for the computation of pose information (position + orientation) of prostate brachytherapy loose seeds from 3D CT images. From an initial detection of a set of seed candidates in CT images using a threshold and connected component method, the orientation of each individual seed is estimated by using the principal components analysis (PCA) method. The main originality of this approach is the ability to classify the detected objects based on a priori intensity and volume information and to separate groups of closely spaced seeds using three competing clustering methods: the standard and a modified k-means method and a Gaussian mixture model with an Expectation-Maximization algorithm. Experiments were carried out on a series of CT images of two phantoms and patients. The fourteen patients correspond to a total of 1063 implanted seeds. Detections are compared to manual segmentation and to related work in terms of detection performance and calculation time. This automatic method has proved to be accurate and fast including the ability to separate groups of seeds in a reliable way and to determine the orientation of each seed. Such a method is mandatory to be able to compute precisely the real dose delivered to the patient post-operatively instead of assuming the alignment of seeds along the theoretical insertion direction of the brachytherapy needles

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