Fiducial Systems Research Articles

Fiducial reference systems for time and coordinates in satellite altimetry

Time is the fundamental measurement in satellite altimetry and the key parameter in building and keeping up a long-term, consistent, and reliable record for monitoring changes in sea level. Over the years, different time scales, although interconnected, have been used in altimetry and also in satellite positioning. This sometimes leads to inexplicit descriptions and ambiguous use of time and orbit coordinates as well as of their transformations between various reference and measuring systems. Altimetry satellites, like Sentinel-3, CryoSat-2, Jason-3, HY-2A/-2B, IceSat-2, etc., observe and practically realize ranges by measuring time differences between the transmission and reception of an electromagnetic wave (either radar or laser at present). Similar principles apply for global navigation satellite systems and for their terrestrial reference systems upon which altimetry is linked and tied together. Yet, the “meter” of any terrestrial reference systems is also defined by time. This work seeks to establish a standard reference system for “time” and “coordinates” in an effort to reach uniform and absolute standardization for satellite altimetry. It describes various errors generated from differences, discontinuities and interactions in time, frequency, range, time tagging, and reference coordinate frames. A new approach, called “fiducial reference measurements for altimetry”, is here given to examine ways to connect errors with metrology standards in order to improve the estimation of uncertainty budgets in ocean and water level observation by altimetry. Finally, the geocentric inertial reference system and the international atomic time are proposed in this paper for satellite altimetry observations and products.

Laser geodetic satellites: a high-accuracy scientific tool

Satellite Laser Ranging (SLR) began in the mid-1960s on satellites of opportunity with retro-reflectors intended as a part of intercomparison tests of satellite tracking techniques. Shortly thereafter, data from these satellites began to work their way into geodetic solutions and dedicated geodesy experiments. By early 1970s when future requirements for centimeter accuracy were envisioned, planning began for dedicated, spherical retro-reflector geodetic satellites. Built with high mass-to-area ratios, these satellites would have important applications in gravity field modeling, station geolocation and fiducial reference systems, Earth rotation, and fundamental physics. Early geodetic satellites were Starlette, launched in 1975 by Centre National d’Etudes Spatiales (CNES), and LAGEOS in 1976 by the National Aeronautics and Space Administration (NASA). Recent geodetic satellites include LARES, launched in 2012, and LARES-2 under development, both by the Italian Space Agency (ASI). Today, a complex of these ‘geodetic satellites’ from low to high altitude Earth orbits supports many space geodesy requirements. This paper will discuss the evolution of the geodetic satellites from the early days, through current programs and out to future needs as we approach our goal for millimeter accuracy.

Technical advances in endoscopic ultrasound-guided fiducial placement for the treatment of pancreatic cancer.

Radiation therapy has an important role in the treatment of locally advanced or metastatic pancreatic cancer and can be used alone or in conjunction with surgery and/or systemic chemotherapy. Because of the challenge of delivering an accurate and optimal radiation dose, image-guided radiation therapy can be used to improve targeting. Fiducial markers can be placed in the tumor and used for localization in patients undergoing image-guided radiation therapy. The safety and feasibility of endoscopic ultrasound (EUS)-guided placement of fiducials has been assessed and reported for the management of pancreatic cancer. We herein review the technique, efficacy, and safety profile of EUS-guided fiducial placement. In addition, we highlight recent advances and technological upgrades in EUS-guided fiducial delivery systems for pancreatic cancer most relevant to practicing gastroenterologists and interventional endoscopists.

Frameless, Real-Time Surface Imaging-Guided Radiosurgery System

Stereotactic radiosurgery (SRS) refers to the delivery of a single, high dose of radiation to a precisely defined target.1 This is achieved by using multiple, noncoplanar radiation beams that converge on the target lesion.2,3 The goal of SRS is to deliver highly conformal radiation therapy in an attempt to ablate the lesion, while minimizing damage to the surrounding normal tissue. SRS has proven to be a safe and efficacious treatment option for many lesions that are surgically inaccessible or are associated with unacceptable surgical morbidity.4–6 Due to the precision required for the planning and execution of SRS, implementation historically required the surgical attachment of a frame to the patient's skull.7 The frame is, in turn, rigidly mounted to the treatment table to minimize patient movement during treatment. Subsequent technological advances included the development of frameless SRS, where patient positioning is monitored through serial X-rays or externally attached fiducials.8,9 Whereas these systems have been proven accurate,10,11 they result in additional radiation to the patient and an increase of time and complexity in treatment delivery.12,13 To overcome these issues, we here provide a 9-minute 51-second video illustration of what can be described as a third generation SRS technology where patient immobilization is monitored by video surface imaging cameras designed to detect and monitor the surface contour of the patient's facial features. The system, AlignRT (VisionRT Ltd.), utilizes three ceiling-mounted nonionizing camera pods that capture facial landmarks of the patient and, in turn, builds a three-dimensional surface image of the patient. Each pod consists of three cameras (two for stereovision, one for texture), a speckle flash unit for static imaging, and a speckle projector for dynamic imaging. Once the surface rendering is complete, the system monitors any cranial displacement from the initial position. Radiation delivery is halted, if significant and sustained displacements occur, and resumed only after the patient returned to the initial position. We termed this system as Surface Imaging-Guided Stereotactic Radiosurgery (SIG SRS). We previously demonstrated that the physical accuracy and the clinical efficacy of SIG SRS are comparable to the frame-based X-ray or external fiducial-based systems.14,15 We have treated more than 350 patients with the AlignRT system. In terms of clinical efficacy, we have analyzed the outcome of our first 44 patients with 115 intracranial metastases treated with SIG SRS.14 The median follow-up for all patients was 6.0 months, with a range of 0.3–21.6 months. Actuarial 6- and 12-month local control was 90% and 76%, respectively. Regional failure was noted in 16 patients (46%). The median actuarial overall survival was 7.7 months.14 These results are comparable to those achieved using conventional frame-based systems,16,17 frameless systems with serial X-ray monitoring,10,18 as well as external fiducial systems.8,9 In this study, we provide a video documentation of the SIG SRS procedure based on minimal patient immobilization and patient monitoring with a video surface imaging system. This treatment constitutes a step forward in patient comfort and treatment speed, while maintaining accuracy and clinical results seen with previous stereotactic immobilization systems. Acknowledgments: This work was supported, in part, by National Institutes of Mental Health R25 grant MH71544 (P.J.). The authors would like to thank Tara Henson for her participation in demonstrating the procedure. K.T.M. and C.C.C. receive speakership honorarium from Varian Medical Systems. Runtime of video: 9 mins and 51 secs

ON THE DESIGN AND EVALUATION OF A PRECISE, ROBUST AND SCALABLE FIDUCIAL MARKER FRAMEWORK

In this paper we present an improved color-based planar fiducial marker system. Our framework provides precise and robust full 3D pose estimation of markers with superior accuracy when compared with many fiducial systems in the literature, while color information encoding enables using over 65 000 distinct markers. Unlike most color-based fiducial frameworks, which requires prior classification training and color calibration, ours can perform reliably under illumination changes, requiring but a rough white balance adjustment. Our methodology provides good detection performance even under poor illumination conditions which typically compromise other marker identification techniques, thus avoiding the evaluation of otherwise falsely identified markers. Several experiments are presented and carefully analyzed, in order to validate our system and demonstrate the significant improvement in estimation accuracy of both position and orientation over traditional techniques.

Spinal Radiosurgery for Metastatic Disease of the Spine

Metastatic tumor in the spinal column is common, causing symptomatic spinal cord compression in approximately 25,000 patients annually. Although surgical treatment of spinal metastases has become safer, less invasive, and more efficacious in recent years, there remains a subset of patients for whom other treatment modalities are needed. Stereotactic radiosurgery, which has long been used in the treatment of intracranial lesions, has recently been applied to the spine and enables the effective treatment of metastatic lesions. We review the evolution of stereotactic radiosurgery and its applications in the spine, including a description of two commercially available systems. Although a relatively new technique, the use of stereotactic radiosurgery in the spine has advanced rapidly in the past decade. Spinal stereotactic radiosurgery is an effective and safe modality for the treatment of spinal metastatic disease. Future challenges involve the refinement of noninvasive fiducial tracking systems and the discernment of optimal doses needed to treat various lesions. Additionally, dose-tolerance limits of normal structures need to be further developed. Increased experience will likely make stereotactic radiosurgery of the spine an important treatment modality for a variety of metastatic lesions.

SU‐FF‐I‐86: MRI and MRS Assessment of Bone Marrow Cellularity

Purpose: Bone marrow cellularity is an important parameter in the diagnosis and treatment of bone marrow disease, as disease affects marrow cellularity and marrow adipocyte concentration significantly alters beta‐ or alpha‐particle energy deposition from radiopharmaceuticals to radiosensitive hematopoietic cells. The current “gold standard” for the assessment of cellularity is the histological analysis of painful bone marrow biopsies of the iliac crest. There have been a few limited attempts to quantify cellularity non‐invasively by MR Chemical Shift Imaging (CSI), but the accuracy of this technique has never been validated. Here we demonstrate a method to correlate ex‐vivo CSI cellularity values to the results obtained by standard histological methods, using stereotactic image co‐registration. Method and Materials: Cellularity phantoms were created by preparing a series of lipid/water emulsions in which the water fractions (WFs) ranged from 0% to 100%, and were precisely known a priori. WFs were then measured in each phantom by CSI using a Phillips 3T horizontal bore scanner. Following this, tubular Silicon fiducials were inserted into the spongiosa of an excised canine limb using a Teflon template guide. Ex‐vivo cellularity measurements were obtained by 3T‐CSI within the fiducial region, and then bone cores were extracted with the fiducial systems intact for subsequent histological analysis and digital microscopy imaging using standard protocols. The accuracy of the CSI cellularity was then evaluated by co‐registration of the MR spectroscopy to the histological ‘gold‐standard’. Results and Conclusions: Here we demonstrate a novel technique to co‐register NMR Spectroscopy with standard digital imaging methods, in phantoms and histological specimens in which the lipid to water fraction is precisely quantifiable. Further studies are suggested, but these results imply that it may be possible to accurately sample bone marrow cellularity in both healthy and diseased individuals, for improved prognostic outcome in the treatment of metastatic bone disease.

Submillimetric target-registration error using a novel, non-invasive fiducial system for image-guided otologic surgery

Objective: Otologic surgery is undertaken to treat ailments of the ear, including persistent infections, hearing loss, vertigo, and cancer. Typically performed on otherwise-healthy patients in outpatient facilities, the application of image-guided surgery (IGS) has been limited because accurate (<1 mm), non-invasive fiducial systems for otologic surgery have not been available. We now present such a fiducial system.Methods: A dental bite-block was fitted with a custom-designed rigid frame with 7 fiducial markers surrounding each external ear. The bones containing the ear (i.e., the temporal bones) of 3 cadaveric skulls were removed and replaced with discs containing 13 surgical targets arranged in a cross-hair pattern about the centroid of each ear. The surgical targets (26/skull) and fiducial markers (14/skull) were identified both within CT scans using a published algorithm and in physical space using an infrared optical tracking system. Fiducial registration error (FRE), fiducial localization error (FLE), and target registration error (TRE) were calculated.Results: For all trials, root mean square FRE=0.66, FLE=0.72, and TRE=0.77mm. The mean TRE for n=234 independent targets was 0.73 with a standard deviation of 0.25mm.Conclusions: Using a novel, non-invasive fiducial system (the EarMark™), submillimetric accuracy was repeatably achieved. This system will facilitate image-guided otologic surgery.

Multiring Fiducial Systems for Scalable Fiducial-Tracking Augmented Reality

In augmented reality (AR), a user can see a virtual world as well as the real world. To avoid registration problems between the virtual world and the real world, the user's viewing pose in both worlds should be kept the same. Fiducial-tracking AR is an attractive approach to the registration problem. However, most of the developed fiducialtracking AR systems have restricted workspaces. To provide a wide range of workspaces (from a small-scale desktop space to a large-scale space) and a wide range of views (from far views to detailed views), an AR system should have scalability. In this paper, we present multiring color fiducial systems and a real-time fiducial detection method for scalable fiducial-tracking AR. We analyze the optimal ring width and develop formulas to obtain the optimal fiducial set with applicationspecific inputs. We develop a real-time ring-detection method that converts the five-DOF ellipse-detection problem to a series of simple steps with a 1-D segmentfilter and multithreshold segmentation. The results lead to a simple and inexpensive means of achieving scalable-area tracking for AR and an approach that is suitable as an optical tracking method for VR as well.

Target area and diagnostic interface issues on the National Ignition Facility (invited)

The National Ignition Facility (NIF) is under construction at Lawrence Livermore National Laboratory for the DOE Stockpile Stewardship Program. It will be used for experiments for inertial confinement fusion ignition, high energy density science, and basic science. Many interface issues confront the experimentalist who wishes to design, fabricate, and install diagnostics, and to help this process, a set of standards and guideline documents is being prepared. Compliance with these will be part of a formal diagnostic design review process. In this article we provide a short description of each, with reference to more complete documentation. The complete documentation will also be available through the NIF Diagnostics web page. Target area interface issues are grouped into three categories. First are the layout and utility interface issues which include the safety analysis report, target area facility layout; target chamber port locations; diagnostic interferences and envelopes; utilities and cable tray distribution; and timing and fiducial systems. Second are the environment interface issues which include radiation electromagnetic interference/electromagnetic pulse effects and mitigation; electrical grounding, shielding, and isolation; and cleanliness and vacuum guidelines. Third are the operational interface issues which include manipulator based target diagnostics, diagnostic alignment, shot life cycle and setup, diagnostic controllers; integrated computer control system; shot data archival; classified operations; and remote operations.