Electromagnetic Localization System Research Articles

A Wearable Capsule Endoscope Electromagnetic Localization System Based on a Novel WCL Algorithm.

The wearable localization system for wireless capsule endoscopy (WCE) is a potential technology to realize rapid diagnosis and treatment of the gastrointestinal (GI). However, the electromagnetic localization accuracy of WCE still needs to be improved. In this paper, based on RSSI electromagnetic fading model, the accurate fitting parameter values are obtained by Kalman filter and the least square method. A novel weighted centroid localization (WCL) algorithm based on exponential weights is proposed, which can achieve high-accuracy localization by using only sparse reception matrix. The simulation results show that when the standard deviation of the localization data is 7.85, the localization root mean square error (RMSE) is 25.4 mm; when the standard deviation of the localization data is 5.475, the localization RMSE is 2.5 mm. These two localization RMSEs are 38% and 79% less than those of the conventional centroid localization algorithm, respectively. An experimental platform of wearable wireless communication and localization system using 24 array receiving antennas is developed in human phantom environment. The experimental results show that the wearable WCE electromagnetic localization system based on the proposed algorithm achieves a localization RMSE of 36.3 mm, which is 17% lower than that of the conventional centroid localization algorithm and meets the needs of clinical diagnosis.

SU‐G‐JeP1‐01: A Combination of Real Time Electromagnetic Localization and Tracking with Cone Beam Computed Tomography in Stereotactic Radiosurgery for Brain Tumors

Purpose:1. online verification of patient position during treatment using calypso electromagnetic localization and tracking system. 2. Verification and comparison of positional accuracy between cone beam computed tomography and calypso system. 3. Presenting the advantage of continuation localization in Stereotactic radiosurgery treatments.Methods:Ten brain tumor cases were taken for this study. Patients with head mask were under gone Computed Tomography (CT). Before scanning, mask was cut on the fore head area to keep surface beacons on the skin. Slice thickness of 0.65 mm were taken for this study. x, y, z coordinates of these beacons in TPS were entered into tracking station. Varian True Beam accelerator, equipped with On Board Imager was used to take Cone beam Computed Tomography (CBCT) to localize the patient. Simultaneously Surface beacons were used to localize and track the patient throughout the treatment. The localization values were compared in both systems. For localization CBCT considered as reference. Tracking was done throughout the treatment using Calypso tracking system using electromagnetic array. This array was in tracking position during imaging and treatment. Flattening Filter free beams of 6MV photons along with Volumetric Modulated Arc Therapy was used for the treatment. The patient movement was observed throughout the treatment ranging from 2 min to 4 min.Results:The average variation observed between calypso system and CBCT localization was less than 0.5 mm. These variations were due to manual errors while keeping beacon on the patient. Less than 0.05 cm intra‐fraction motion was observed throughout the treatment with the help of continuous tracking.Conclusion:Calypso target localization system is one of the finest tools to perform radiosurgery in combination with CBCT. This non radiographic method of tracking is a real beneficial method to treat patients confidently while observing real‐time motion information of the patient.

SU‐E‐J‐160: Comparing the Setup Accuracy of Non‐Ionizing Patient Localization Systems with CBCT to Reduce Imaging Dose in Prone Breast Treatments

Purpose:CBCT is the current gold standard to verify prone breast patient setup. We investigated in a phantom if non‐ionizing localization systems can replace ionizing localization systems for prone breast treatments.Methods:An anthropomorphic phantom was positioned on a prone breast board. Electromagnetic transponders were attached on the left chest surface. The CT images of the phantom were imported to the treatment planning system. The isocenter was set to the center of the transponders. The positions of the isocenter and transponders transferred to the transponder tracking system. The posterior phantom surface was contoured and exported to the optical surface tracking system. A CBCT was taken for the initial setup alignment on the treatment machine. Using the electromagnetic and optical localization systems, the deviation of the phantom setup from the original CT images was measured. This was compared with the difference between the original CT and kV‐CBCT images.Results:For the electromagnetic localization system, the phantom position deviated from the original CT in 1.5 mm, 0.0 mm and 0.5 mm in the anterior‐posterior (AP), superior‐inferior (SI) and left‐right (LR) directions. For the optical localization system, the phantom position deviated from the original CT in 2.0 mm, −2.0 mm and 0.1 mm in the AP, SI and LR directions. For the CBCT, the phantom position deviated from the original CT in 4.0 mm, 1.0 mm and −1.0 mm in the AP, SI and LR directions. The measured values from the non‐ionizing localization systems differed from those with the CBCT less than 3.0 mm in all directions.Conclusions:This phantom study showed the feasibility of using a combination of non‐ionizing localization systems to achieve a similar setup accuracy as CBCT for prone breast patients. This could potentially eliminate imaging dose. As a next step, we are expanding this study to actual patients.This work has been in part supported by Departmental Research Award RODEPT1‐JS001, Department of Radiation Oncology, UC Davis Medical Center.

809 In Vivo Colonic Insufflation via a Disposable CO2 Insufflation System

Background: One of the major obstacles (~30%) of screening colonoscopy is the hurdle of preparation. The standard technologies demands aggressive bowel cleansing that hamper patient's compliance. Aim: To test the safety and proof of concept of a novel prep-less XRay imaging capsule (Check Cap Ltd, Israel) for colon cancer screening. Material and Methods: The capsule (approximately 34 mm in length and 11.5 mm in diameter, similar in size to optical imaging capsules) utilizes a novel ultra-low dose (0.03 mSv) low energy (56 70 Kev) X-ray-based imaging technology to safely generate high resolution 3-D imagery of the colon without bowel cleansing or other aggressive preparation. 10 capsules were swallowed by volunteers (M=7, F=3) (ages 37 66) and were tracked during the whole passage of the capsule in the body using radio frequency telemetry and an Electromagnetic Localization System (ELS). The capsules were programed to perform scans when movement in the colon is detected and transmit the imaging data to an external recorder unit worn by the patients. The study was approved by the IRB of the Tel Aviv Medical Center. Total transit time and total X-ray exposure to the patients were calculated to assess the safety profile of the x-ray capsule imaging technology in human subjects. Results: All capsules were swallowed and naturally exerted by the volunteers without any minor or major side effect. The average transit time of 68 ±31 hours. The volunteers were exposed to an ultralow total radiation dose (Average total exposure (0.03 ± 0.007 mSv), two orders of magnitude less than CTC (with 1 outlier due to manufacturing fault). Data from scans in the colon combined with ELS positioning information was used to reconstruct 3D colon segments in un-prepped colon. Fig1 and Fig.2 are examples of such 3D scans. Laboratory data form synthetic colon phantoms with and without polyps as well as data from cow colon phantom with induced tissue made polyp Fig.3 and Fig.4 have shown reconstruction accuracy of 2 3 mm in colon diameter measurements. (All figure dimensions in mm) Conclusions: A safety and proof of concept study has been performed using a novel prep-less x-ray imaging capsule for colon cancer screening. Quantitative ultra-low dose x-ray 3D imaging was achieved in the colon of human subjects. The efficacy will be confirmed in a multi-center study.

Hardy's Multi-Quadric method of calibrating a wireless electromagnetic localization system for intestinal micro-devices

A wireless electromagnetic localization method for tracking a medical micro-device in the gastrointestinal tract has been proposed and a prototype for the localization system has been developed. In the electromagnetic localization system, several excitation coils were mounted above the abdomen of human body to generate a varying electromagnetic field one by one. Then a wireless magnetic sensor was used to measure the magnetic field strength at the location of the micro-device. Thus the position and orientation of the micro-device could be obtained by solving the inverse problem of magnetic fields. In order to improve the localization precision, a global error correction technique based on the Hardy's Multi-Quadric scattered data interpolation method was investigated to correct the static errors. In the calibration experiment, dependencies between true locations of the micro-device and the corresponding tracked locations within the operational space were established. Based on these data sets, the Hardy's Multi-Quadric correction function was derived. The experimental results show that the localization error can be greatly reduced by using the Hardy's Multi-Quadric correction method. The advantage of the approach is that it allows for smooth continuous mapping from the tracked space to the true space within the entire tracked space.

An evaluation of interference of inflatable penile prostheses with electromagnetic localization and tracking system

The Calypso system is stated by the manufacturer to be contraindicated for cases where the patient has been implanted with a penile prosthesis. This is due to concern for potential metal interference-related reduction of spatial localization and tracking accuracy. Here we quantify the localization and tracking accuracy of the Calypso system in the presence of inflatable penile prosthesis devices from three most widely used models which account for, essentially, 100% of implants in North America. Phantom studies were first performed to quantify the interference of Calypso localization and tracking accuracy from both varying metal (steel) masses, and from the penile prosthetic devices themselves. The interference of varying steel masses was studied as a function of two factors: (a) the mass and (b) the location of steel material. The Calypso daily quality assurance (QA) phantom with three implanted Beacon(®) transponders was used to measure any aliasing of position that might occur due to metal interference. After confirming the safety of use in phantom, we implanted Calypso Beacon(®) transponders in one patient with a previously implanted AMS Model 700 inflatable penile prosthetic device. For each of the 42 delivered treatment fractions, redundant stereotactic ultrasound (US) image guidance was performed to ensure good agreement between US and Calypso guidance. We observed that a steel mass of less than 18 g did not cause any detectable positional aliasing for the Calypso tracking function. The mass of metal material measured to exist in the three penile prosthetic devices studied here (MP35N alloy) was approximately 1 g for each. No positional aliasing was observed for the three prosthetic devices in phantom, and good agreement between redundant US and Calypso was also observed in patient. Both phantom and patient evaluations with the penile prosthetic devices showed no measurable interference with the Calypso system, thus indicating that accurate Calypso-based alignments can be performed in the presence of current industry standard inflatable penile prosthetic devices.

Dose to the muscles of fecal continence during radiation therapy for prostate cancer.

86 Background: Radiation therapy for prostate cancer can lead to loss of fecal continence; our understanding of the dose-volume relationships of this late toxicity continues to develop. The external anal sphincter (EAS), internal anal sphincter (IAS), and the three parts of the levator ani muscle, the puborectalis (PRM), the pubococcygeus (PCM), and the illiococcygeus (ICM), all contribute to fecal continence. We developed a reproducible method for contouring these muscles and in this preliminary study evaluate whether decreased planning target volume (PTV) margins lead to potentially clinically significant decreases in dose to these muscles during definitive radiation therapy for prostate cancer. Methods: Muscles involved in fecal continence were contoured for 10 consecutive patients on a prospective study of reduced PTV margins for treating low-to-intermediate risk prostate cancer with intensity-modulated radiation therapy (IMRT) using an electromagnetic localization system. IMRT plans to a prescribed dose of 7740 cGy were developed using 10mm PTV margins (5mm posteriorly), and compared with actual treatment IMRT plans using 3mm circumferential PTV margins. Decreases in dose were evaluated for statistical significance using an unpaired t-test. Results: Reducing PTV margins decreased the mean PTV volume from 176.2 ml to 91.9 ml. Mean doses to the EAS, IAS, and rectum (REC) decreased significantly; from 11.0 Gy to 4.1 Gy (p=0.005), from 30.5 Gy to 15.0 Gy (p = 0.004), and from 43.7 Gy to 35.6 Gy (p=0.006) respectively. Decrease in the mean dose to the PRM was nearly statistically significant, 48.7 Gy to 34.6 Gy (p = 0.055). Decreases in mean doses to the PCM and ICM were not statistically significant; from 61.9 Gy to 55.2 Gy (p = 0.107), and from 40.7 Gy to 34.8 Gy (p = 0.176), respectively. Conclusions: Using electromagnetic tracking to reduce PTV margins leads to a significant decrease in dose to the muscles of fecal continence, with mean dose decreases in a range that may be clinically significant.

Geometric accuracy of dynamic MLC tracking with an implantable wired electromagnetic transponder

Background. Tumor motion during radiotherapy delivery can substantially deteriorate the target dose distribution. A promising method to overcome this problem is dynamic multi-leaf collimator (DMLC) tracking. The purpose of this phantom study was to integrate a wired electromagnetic (EM) transponder localization system with DMLC tracking and to investigate the geometric accuracy of the integrated system. Material and methods. DMLC tracking experiments were performed on a Trilogy accelerator with a prototype DMLC tracking system. A wired implantable EM transponder was mounted on a motion stage with a 3 mm tungsten sphere used for target visualization in continuous portal images. The three dimensional (3D) transponder position signal was used for DMLC aperture adaption. The motion stage was programmed to reproduce eight representative patient-measured trajectories for prostate and for lung tumors. The tracking system latency was determined and prediction was used for the lung tumor trajectories to account for the latency. For each trajectory, three conformal fields with a 10 cm circular MLC aperture and 72 s treatment duration were delivered: (1) a 358° arc field; (2) an anterior static field; and (3) a lateral static field. The tracking error was measured as the difference between the marker position and the MLC aperture in the portal images. Results. The tracking system latency was 140 ms. The mean root-mean-square (rms) of the 3D transponder localization error was 0.53/0.54 mm for prostate/lung tumor trajectories. The mean rms of the two dimensional (2D) tracking error was 0.69 mm (prostate) and 0.98 mm (lung tumors) with tracking and 3.4 mm (prostate) and 5.3 mm (lung tumors) without tracking. Conclusions. DMLC tracking was integrated with a wired EM transponder localization system and investigated for arc and static field delivery. The system provides sub-mm geometrical errors for most trajectories.

Impact of Translational and Rotational Target Motion during Hypofractionated Prostate Radiotherapy

Using tracking data acquired with a 4D electromagnetic localization system, we have quantified the dosimetric impact of both translational and rotational intrafraction target motion in the delivery of hypofractionated prostate radiotherapy.

Investigation of Potential Interference of 3 Piece Penile Prostheses with Electromagnetic Localization System

Investigation of Potential Interference of 3 Piece Penile Prostheses with Electromagnetic Localization System

Daily Isocenter Correction With Electromagnetic-Based Localization Improves Target Coverage and Rectal Sparing During Prostate Radiotherapy

To evaluate dosimetric consequences of daily isocenter correction during prostate cancer radiation therapy using the Calypso 4D localization system. Data were analyzed from 28 patients with electromagnetic transponders implanted in their prostates for daily target localization and tracking. Treatment planning isocenters were recorded based on the values of the vertical, longitudinal, and lateral axes. Isocenter location obtained via alignment with skin tattoos was compared with that obtained via the electromagnetic localization system. Daily isocenter shifts, based on the isocenter location differences between the two alignment methods in each spatial axis, were calculated for each patient over their entire course. The mean isocenter shifts were used to determine dosimetric consequences of treatment based on skin tattoo alignments alone. The mean SD of the percentages of treatment days with shifts beyond 0.5 cm for vertical, longitudinal and lateral shifts were 62% 28%, 35% 26%, and 38% 21%, respectively. If daily electromagnetic localization was not used, the excess in prescribed dose delivered to 70% of the rectum was 10 Gy and the deficit in prescribed dose delivered to 95% of the planning target volume was 10 Gy. The mean isocenter shift was not associated with the volumes of the prostate, rectum, or bladder, or with patient body mass index. Daily isocenter localization can reduce the treatment dose to the rectum. Correcting for this variability could lead to improved dose delivery, reduced side effects, and potentially improved treatment outcomes.

Real-time Prostate Motion is Highly Variable among Patients Undergoing Prostate Radiotherapy (RT) with Electromagnetic Localization and Tracking

Real-time Prostate Motion is Highly Variable among Patients Undergoing Prostate Radiotherapy (RT) with Electromagnetic Localization and Tracking

Development of a Non-migrating Electromagnetic Transponder System for Lung Tumor Tracking

Real-time localization of implanted fiducials for lung cancer is limited by high non-therapeutic imaging radiation, expensive fluoroscopic installations, subjective image interpretation requirements and poor gold fiducial implant stability. A real-time, non-ionizing, objective, electromagnetic localization and tracking system that uses wireless implanted Beacon transponder for use in the prostate (Calypso Medical Technologies, Seattle, WA), initially showed low rates of transponder fixation with bronchoscopic implantation in a canine model.

The Effect of Transponder Motion on the Accuracy of the Calypso Electromagnetic Localization System

To determine position and velocity-dependent effects in the overall accuracy of the Calypso Electromagnetic localization system, under conditions that emulate transponder motion during normal free breathing. Three localization transponders were mounted on a remote-controlled turntable that could move the transponders along a circular trajectory at speeds up to 3 cm/s. A stationary calibration established the coordinates of multiple points on each transponder's circular path. Position measurements taken while the transponders were in motion at a constant speed were then compared with the stationary coordinates. No statistically significant changes in the transponder positions in (x,y,z) were detected when the transponders were in motion. The accuracy of the localization system is unaffected by transponder motion.

The Effect of Transponder Motion on the Accuracy of the Calypso Electromagnetic Localization System

The Calypso localization system remotely detects miniature electromagnetic transponders implanted in a patient. The system is capable of continuously measuring the position of a treatment site during radiotherapy. If the site is moving during respiration then the system can be used for dynamic tracking of the target. With this application in mind we have made measurements to see if there is any change in localization accuracy due to transponder position or motion. Three localization transponders were mounted on a remote-controlled turntable that could move the transponders along a circular trajectory at speeds up to 3 cm/sec. A stationary calibration established the coordinates of six points on each transponder's circular path. We then compared position measurements taken while the transponders were in motion at a constant speed to the stationary coordinates. To synchronize the position of the moving transponders to a known point on the circular path we injected an RF noise pulse into the Calypso readout antenna when the turntable passed 0°. This produced outliers on the arc of moving transponder positions that could be used to compare the transponder positions in the direction of motion to the calibrated stationary positions. The stationary localization uncertainty varied from 0.02 to 0.06 cm per axis in the (x,y) plane, which is consistent with the observations of other researchers. We detected a small systematic trend in the stationary uncertainty as a function of transponder position along an arc 11 cm long. Lung tumors do not move more than about 3 cm in the most extreme cases. Therefore the Calypso system will have less than 0.02 cm systematic change in localization accuracy over the full range of tumor position. There was no statistically significant change in the transponder positions in either the radial or tangential direction when the transponders were in motion. If the Calypso system is used for real-time respiratory motion tracking, then tracking accuracy will be unaffected by transponder motion, at least for the velocity and position range that will be encountered in clinical applications. If the transponder is moving, its reported location corresponds precisely to its position at the midpoint of the readout interval, regardless of speed.

Daily Isocenter Correction during Prostate Radiotherapy (RT) Helps to Avoid Significantly Decreased Target Volume Dose and Increased Rectal Dose

Highly conformal external beam RT for prostate cancer can deliver high doses of radiation to the prostate while potentially minimizing dose to normal structures. However, errors in isocenter localization may result in major changes in dosimetry. Implanted devices such as gold fiducials or electromagnetic beacons have been shown to help correct daily setup errors. The Calypso electromagnetic localization system allows for accurate daily measurements of pretreatment isocenter shifts that are automatically recorded prior to each fraction of RT.

Bronchoscopic Implantation of a Novel Wireless Electromagnetic Transponder in the Canine Lung: A Feasibility Study

The success of targeted radiation therapy for lung cancer treatment is limited by tumor motion during breathing. A real-time, objective, nonionizing, electromagnetic localization system using implanted electromagnetic transponders has been developed (Beacon electromagnetic transponder, Calypso Medical Technologies, Inc., Seattle, WA). We evaluated the feasibility and fixation of electromagnetic transponders bronchoscopically implanted in small airways of canine lungs and compared to results using gold markers. After approval of the Animal Studies Committee, five mongrel dogs were anesthetized, intubated, and ventilated. Three transponders were inserted into the tip of a plastic catheter, passed through the working channel of a flexible bronchoscope, and implanted into small airways of a single lobe using fluoroscopic guidance. This procedure was repeated for three spherical gold markers in the opposite lung. One, 7, 14, 28, and 60 days postimplantation imaging was used to assess implant fixation. Successful bronchoscopic implantation was possible for 15 of 15 transponders and 12 of 15 gold markers; 3 markers were deposited in the pleural space. Fixation at 1 day was 15 of 15 for transponders and 12 of 12 for gold markers. Fixation at 60 days was 6 of 15 for transponders and 7 of 12 for gold markers, p value = 0.45. Bronchoscopic implantation of both transponders and gold markers into the canine lung is feasible, but fixation rates are low. If fixation rates can be improved, implantable electromagnetic transponders may allow improved radiation therapy for lung cancer by providing real-time continuous target tracking. Developmental work is under way to improve the fixation rates and to reduce sensitivity to implantation technique.

BRONCHOSCOPIC IMPLANTATION OF ELECTROMAGNETIC TRANSPONDERS IN THE CANINE LUNG

PURPOSE: The success of radiation therapy for lung cancer treatment is limited by tumor motion during breathing and thus the ability to target effectively. Real-time localization and tracking may enable increased tumor radiation and decreased healthy tissue irradiation. Real-time localization of implanted gold fiducials is limited by high doses of non-therapeutic imaging radiation, expensive fluoroscopic equipment, subjective image interpretation and poor implant stability. A real-time, non-ionizing, objective, electromagnetic localization system that uses wireless implanted Beacon® transponders has been developed (Calypso Medical, Seattle, WA). We compared bronchoscopic implant feasibility and stability for both transponders and gold fiducials in peripheral airways of canine lungs.

MO‐D‐ValB‐05: Commissioning An AC Electromagnetic Localization System for Radiation Therapy

Purpose/Objective: To establish a commissioning and acceptance test protocol for an AC electromagnetic tracking system for use in target localization and tracking during radiation therapy. (Calypso® 4D Localization System, Calypso Medical, Seattle, WA). Materials/Methods: Following installation of infrared cameras and electromagnetic system, compatibility tests were made to ensure the system did not interfere with radiation output, MLCs or IMRT field fluence. The collision space between the linac and the Calypso System was evaluated. System verification included calibration using software and calibration tools, one of which has embedded RF transponders and optical reflectors. End‐to‐end testing to assess localization accuracy included CT scan of a radiographic phantom with RF transponders, creation of four Calypso Plans, data entry to the Calypso System and execution of treatment sessions. Tracking accuracy was measured using a precision translation table for orthogonal motions ± 5 mm. Results: Calypso System did not affect normal clinical operations of radiation output, MLC, or IMRT field segmentation. Overall accuracy on ten systems at five institutions and 40 treatment plans was 0.068 ± 0.027 cm, incorporating contributing errors from the CT scans, identification of transponders in the phantom, and effects of radiation dose delivery. With stable environment, no systematic drift was noticed over 30 minute. Translations of precise increments from −0.50 cm to +0.05cm were accurately tracked with error of 0.00cm, −0.02cm and 0.01cm (lat, long, vert). The system maintained accuracy with monthly optical calibrations. Changes in system readout of ±0.05 cm (readout quantization) were noticed at certain gantry angles. Conclusion: Evaluations demonstrate the AC electromagnetic system with wireless transponders can be integrated into the radiotherapy environment with existing instrumentation and operates within the designed accuracy specification. Conflict of Interest: Work supported by Calypso Medical Technologies.

Target localization and real-time tracking using the Calypso 4D localization system in patients with localized prostate cancer

The Calypso 4D Localization System is being developed to provide accurate, precise, objective, and continuous target localization during radiotherapy. This study involves the first human use of the system, to evaluate the localization accuracy of this technique compared with radiographic localization and to assess its ability to obtain real-time prostate-motion information. Three transponders were implanted in each of 20 patients. Eleven eligible patients of the 20 patients participated in a study arm that compared radiographic triangulated transponder locations to electromagnetically recorded transponder locations. Transponders were tracked for 8-min periods. The implantations were all successful, with no major complications. Intertransponder distances were largely stable. Comparison of the patient localization on the basis of transponder locations as per the Calypso system with the radiographic transponder localization showed an average (+/-SD) 3D difference of 1.5 +/- 0.9 mm. Upon tracking during 8 min, 2 of the 11 patients showed significant organ motion (>1 cm), with some motion lasting longer that 1 min. Calypso transponders can be used as magnetic intraprostatic fiducials. Clinical evaluation of this novel 4D nonionizing electromagnetic localization system with transponders indicates a comparable localization accuracy to isocenter, (within 2 mm) compared with X-ray localization.