Tracking Of Organ Motion Research Articles

MR SIGnature MAtching (MRSIGMA) with retrospective self-evaluation for real-time volumetric motion imaging

Objective. MR SIGnature MAtching (MRSIGMA) is a real-time volumetric MRI technique to image tumor and organs at risk motion in real-time for radiotherapy applications, where a dictionary of high-resolution 3D motion states and associated motion signatures are computed first during offline training and real-time 3D imaging is performed afterwards using fast signature-only acquisition and signature matching. However, the lack of a reference image with similar spatial resolution and temporal resolution introduces significant challenges for in vivo validation. Approach. This work proposes a retrospective self-validation for MRSIGMA, where the same data used for real-time imaging are used to create a non-real-time reference for comparison. MRSIGMA with self-validation is tested in patients with liver tumors using quantitative metrics defined on the tumor and nearby organs-at-risk structures. The dice coefficient between contours defined on the real-time MRSIGMA and non-real-time reference was used to assess motion imaging performance. Main Results. Total latency (including signature acquisition and signature matching) was between 250 and 314 ms, which is sufficient for organs affected by respiratory motion. Mean ± standard deviation dice coefficient over time was 0.74 ± 0.03 for patients imaged without contrast agent and 0.87 ± 0.03 for patients imaged with contrast agent, which demonstrated high-performance real-time motion imaging. Signficance. MRSIGMA with self-evaluation provides a means to perform real-time volumetric MRI for organ motion tracking with quantitative performance measures.

Research on Intraoperative Organ Motion Tracking Method Based on Fusion of Inertial and Electromagnetic Navigation

The intraoperative movement of target organs is currently a key and difficult problem that restricts clinical surgery diagnosis and treatment, especially radiotherapy. Unexpected posture changes can cause the deviation of the target area during the operation, thereby affecting the treatment effect and even causing severe complications. Therefore, it is necessary to track the movement of target organs in real-time and accurately to improve the effect and safety of the surgical treatment. This paper proposes an intraoperative organ motion tracking method based on the fusion of inertial navigation and electromagnetic navigation., which can directly measure the target organ's movement under non-invasive or minimally invasive conditions using the human natural cavity. The proposed method for identifying the magnetic field's interference state uses the extended Kalman filter to fuse the inertial measurement unit and the electromagnetic positioning information to achieve target organ movement tracking under different interference conditions. It effectively improved the real-time and robustness of intraoperative organ tracking. Simultaneously, based on the 9-axis inertial measurement unit and a 6-DOF electromagnetic positioning system, the catheter-type target organ tracker was developed, and then under the simulated electromagnetic interference environment, dynamic and static verification experiments were carried out, respectively. The maximum tracking error of displacement and attitude is 2.75 mm and 0.127 rad, respectively, under severe electromagnetic disturbance through experiments. Under non-electromagnetic interference conditions, its displacement and attitude's maximum tracking errors are 0.94 mm and 0.011 rad, respectively. The results prove that the target organ tracking method based on the fusion of inertial navigation and electromagnetic navigation is feasible. It can realize the tracking and measurement of the target organ's movement within the clinical permission error range.

Feasibility of real-time motion tracking using cine MRI during MR-guided radiation therapy for abdominal targets.

Real-time high soft-tissue contrast magnetic resonance imaging (MRI) from the MR-Linac offers the best opportunity for accurate motion tracking during radiation therapy delivery via high-frequency two-dimensional (2D) cine imaging. This work investigates the efficacy of real-time organ motion tracking based on the registration of MRI acquired on MR-Linac. Algorithms based on image intensity were developed to determine the three-dimensional (3D) translation of abdominal targets. 2D and 3D abdominal MRIs were acquired for 10 healthy volunteers using a high-field MR-Linac. For each volunteer, 3D respiration-gated T2 and 2D T2/T1-weighted cine in sagittal, coronal, and axial planes with a planar temporal resolution of 0.6 for 60s was captured. Datasets were also collected on MR-compatible physical and virtual four-dimensional (4D) motion phantoms. Target contours for the liver and pancreas from the 3D T2 were populated to the cine and assumed as the ground-truth motion. We performed image registration using a research software to track the target 3D motion. Standard deviations of the error (SDE) between the ground-truth and tracking were analyzed. Algorithms using a research software were demonstrated to be capable of tracking arbitrary targets in the abdomen at 5Hz with an overall accuracy of 0.6mm in phantom studies and 2.1mm in volunteers. However, this value is subject to patient-specific considerations, namely motion amplitude. Calculation times of<50ms provide a pathway of real-time motion tracking integration. A major challenge in using 2D cine MRI to track the target is handling the full 3D motion of the target. Feasibility to track organ motion using intensity-based registration of MRIs was demonstrated for abdominal targets. Tracking accuracy of about 2mm was achieved for the motion of the liver and pancreatic head for typical patient motion. Further development is ongoing to improve the tracking algorithm for large and complex motions.

Optimizing robot motion for robotic ultrasound-guided radiation therapy

An important aspect of robotic radiation therapy is active compensation of target motion. Recently, ultrasound has been proposed to obtain real-time volumetric images of abdominal organ motion. One approach to realize flexible probe placement throughout the treatment fraction is based on a robotic arm holding the ultrasound probe. However, the probe and the robot holding it may obstruct some of the beams with a potentially adverse effect on the plan quality. This can be mitigated by using a kinematically redundant robot, which allows maintaining a steady pose of the ultrasound probe while moving its elbow in order to minimize beam blocking. Ultimately, the motion of both the beam source carrying and the ultrasound probe holding robot contributes to the overall treatment time, i.e. beam delivery and robot motion. We propose an approach to optimize the motion and coordination of both robots based on a generalized traveling salesman problem. Furthermore, we study an application of the model to a prostate treatment scenario. Because the underlying optimization problem is hard, we compare results from a state-of-the-art heuristic solver and an approximation scheme with low computational effort. Our results show that integration of the robot holding the ultrasound probe is feasible with acceptable overhead in overall treatment time. For clinically realistic velocities of the robots, the overhead is less than 4% which is a small cost for the added benefit of continuous, volumetric, and non-ionizing tracking of organ motion over periodic x-ray-based tracking.

Optimizing Configurations for 7-DoF Robotic Ultrasound Guidance in Radiotherapy of the Prostate.

Robotic ultrasound guidance is promising for tracking of organ motion during radiotherapy treatments, but the radio-opaque robot and probe interfere with beam delivery. The effect on treatment plan quality can be mitigated by the use of a robot arm with kinematic redundancy, such that the robot is able to elude delivered beams during treatment by changing its configuration. However, these changes require robot motion close to the patient, lead to an increased treatment time, and require coordination with the beam delivery. We propose an optimization workflow which integrates the problem of selecting suitable robot configurations into a linear-programming-based workflow for treatment plan optimization. Starting with a large set of candidate configurations, a minimal subset is determined which provides equivalent plan quality. Our results show that, typically, six configurations are sufficient for this purpose. Furthermore, we show that optimal configurations can be reused for dose planning of subsequent patients.

Analysis and optimization of the robot setup for robotic-ultrasound-guided radiation therapy.

Robotic ultrasound promises continuous, volumetric, and non-ionizing tracking of organ motion during radiation therapy. However, placement of the robot is critical because it is radio-opaque and might severely influence the achievable dose distribution. We propose two heuristic optimization strategies for automatic placement of an ultrasound robot around a patient. Considering a kinematically redundant robot arm, we compare a generic approach based on stochastic search and a more problem-specific segmentwise construction approach. The former allows for multiple elbow configurations while the latter is deterministic. Additionally, we study different objective functions guiding the search. Our evaluation is based on data for ten actual prostate cancer cases and we compare the resulting plan quality for both methods to manually chosen robot configurations previously proposed. The mean improvements in the treatment planning objective value with respect to the best manually selected robot position and a single elbow configuration range from 8.2 to 32.8% and 8.5 to 15.5% for segmentwise construction and stochastic search, respectively. Considering three different elbow configurations, the stochastic search results in better objective values in 80% of the cases, with 30% being significantly better. The optimization strategies are robust with respect to beam sampling and transducer orientation and using previous optimization results as starting point for stochastic search typically results in better solutions compared to random starting points. We propose a robust and generic optimization scheme, which can be used to optimize the robot placement for robotic ultrasound guidance in radiation therapy. The automatic optimization further mitigates the impact of robotic ultrasound on the treatment plan quality.

Reducing the tracking drift of an uncontoured tumor for a portal-image-based dynamically adapted conformal radiotherapy treatment.

Accurate tracking of organ motion during treatment is needed to improve the efficacy of radiation therapy. This work investigates the feasibility of tracking an uncontoured target using the motion detected within a moving treatment aperture. Tracking was achieved with a weighted optical flow algorithm, and three different techniques for updating the reference image were evaluated. The accuracy and susceptibility of each approach to the accumulation of position errors were verified using a 3D-printed tumor (mounted on an actuator) and a virtual treatment aperture. Tumor motion up to 15.8mm (peak-to-peak) taken from the breathing patterns of seven lung cancer patients was acquired using an amorphous silicon portal imager at ~ 7.5frames/s. The first approach (INI) used the initial image detected, as a fixed reference, to determine the target motion for each new incoming image, and performed the best with the smallest errors. This method was also the most robust against the accumulation of position errors. Mean absolute errors of 0.16, 0.32, and 0.38mm were obtained for the three methods, respectively. Although the errors are comparable to other tracking methods, the proposed method does not require prior knowledge of the tumor shape and does not need a tumor template or contour for tracking. Graphical abstract.

Surrogate-driven deformable motion model for organ motion tracking in particle radiation therapy

The aim of this study is the development and experimental testing of a tumor tracking method for particle radiation therapy, providing the daily respiratory dynamics of the patient’s thoraco-abdominal anatomy as a function of an external surface surrogate combined with an a priori motion model. The proposed tracking approach is based on a patient-specific breathing motion model, estimated from the four-dimensional (4D) planning computed tomography (CT) through deformable image registration. The model is adapted to the interfraction baseline variations in the patient’s anatomical configuration. The driving amplitude and phase parameters are obtained intrafractionally from a respiratory surrogate signal derived from the external surface displacement. The developed technique was assessed on a dataset of seven lung cancer patients, who underwent two repeated 4D CT scans. The first 4D CT was used to build the respiratory motion model, which was tested on the second scan. The geometric accuracy in localizing lung lesions, mediated over all breathing phases, ranged between 0.6 and 1.7 mm across all patients. Errors in tracking the surrounding organs at risk, such as lungs, trachea and esophagus, were lower than 1.3 mm on average. The median absolute variation in water equivalent path length (WEL) within the target volume did not exceed 1.9 mm-WEL for simulated particle beams. A significant improvement was achieved compared with error compensation based on standard rigid alignment. The present work can be regarded as a feasibility study for the potential extension of tumor tracking techniques in particle treatments. Differently from current tracking methods applied in conventional radiotherapy, the proposed approach allows for the dynamic localization of all anatomical structures scanned in the planning CT, thus providing complete information on density and WEL variations required for particle beam range adaptation.

A new method for tracking organ motion on diagnostic ultrasound images.

Respiratory-gated irradiation is effective in reducing the margins of a target in the case of abdominal organs, such as the liver, that change their position as a result of respiratory motion. However, existing technologies are incapable of directly measuring organ motion in real-time during radiation beam delivery. Hence, the authors proposed a novel quantitative organ motion tracking method involving the use of diagnostic ultrasound images; it is noninvasive and does not entail radiation exposure. In the present study, the authors have prospectively evaluated this proposed method. The method involved real-time processing of clinical ultrasound imaging data rather than organ monitoring; it comprised a three-dimensional ultrasound device, a respiratory sensing system, and two PCs for data storage and analysis. The study was designed to evaluate the effectiveness of the proposed method by tracking the gallbladder in one subject and a liver vein in another subject. To track a moving target organ, the method involved the control of a region of interest (ROI) that delineated the target. A tracking algorithm was used to control the ROI, and a large number of feature points and an error correction algorithm were used to achieve long-term tracking of the target. Tracking accuracy was assessed in terms of how well the ROI matched the center of the target. The effectiveness of using a large number of feature points and the error correction algorithm in the proposed method was verified by comparing it with two simple tracking methods. The ROI could capture the center of the target for about 5 min in a cross-sectional image with changing position. Indeed, using the proposed method, it was possible to accurately track a target with a center deviation of 1.54±0.9 mm. The computing time for one frame image using our proposed method was 8 ms. It is expected that it would be possible to track any soft-tissue organ or tumor with large deformations and changing cross-sectional position using this method. The proposed method achieved real-time processing and continuous tracking of the target organ for about 5 min. It is expected that our method will enable more accurate radiation treatment than is the case using indirect observational methods, such as the respiratory sensor method, because of direct visualization of the tumor. Results show that this tracking system facilitates safe treatment in clinical practice.

Simultaneous acquisition of image and navigator slices using CAIPIRINHA for 4D MRI

Respiratory organ motion is still the major challenge of various image-guided treatments in the abdomen. Dynamic organ motion tracking, necessary for the treatment control, can be performed with volumetric time-resolved MRI that sequentially acquires one image and one navigator slice. Here, a novel imaging method is proposed for truly simultaneous high temporal resolution acquisition. A standard balanced steady state free precession sequence was modified to simultaneously acquire two superimposed slices with different phase cycles, namely an image and a navigator slice. Instead of multiband RF pulses, two separate RF pulses were used for the excitation. Images were reconstructed using offline CAIPIRINHA reconstruction. Phantom and in vivo measurements of healthy volunteers were performed and evaluated. Phantom and in vivo measurements showed good image quality with high signal-to-noise ratio (SNR) and no reconstruction issues. We present a novel imaging method for truly simultaneous acquisition of image and navigator slices for four-dimensional (4D) MRI of organ motion. In this method, the time lag between the sequential acquisitions is eliminated, leading to an improved accuracy of organ motion models, while CAIPIRINHA reconstruction results in an improved SNR compared with an existing 4D MRI approach.

Current progress on augmented reality visualization in endoscopic surgery

Advancements in surgery are progressing at a rapid rate; however, there are still limitations, including the ability to accurately visualize the target organ, in particular during laparoscopic surgery. Augmented reality visualization is a novel technique that has been developed to allow the fusion of three-dimensional medical images, such as those from transrectal ultrasound or computed tomography/MRI, with live camera images in real-time. In this review, we describe the current advancements and future directions of augmented reality and its application to laparoscopic surgery. Geometrically-correct superimposed images can be generated by tracking of the laparoscope and registration of the target organ. The fused image between the live laparoscopic images and the reconstructed three-dimensional organ model aides the surgeon in his or her understanding of anatomical structures. Laparoscopic and robot-assisted surgeries in both general surgery and urology have been performed with technical success to date. The primary limitation of the current augmented reality systems is its infancy in dynamic tracking of organ motion or deformation. Recently, augmented reality systems with organ tracking based on real-time image analysis were developed. Further improvement and/or development of such new technologies would resolve these issues. Augmented reality visualization is a significant advancement, improving the precision of laparoscopic/endoscopic surgery. New technologies to improve the dynamic tracking of organ motion or deformation are currently under investigation.

TU‐G‐BRC‐09: A Comparison of Step‐And‐Shoot and DMLC IMRT‐Delivery for Real‐Time Tumor Tracking

Purpose: The dose distributions of static and dynamically delivered IMRT fields for real‐time target tracking were evaluated to determine which methods work best for the compensation of breathing‐induced organ motion.Methods: A single IMRT beam of a clinical plan was applied with the step‐and‐shoot delivery technique and the sliding‐window technique with the research Siemens 160MLC in order to account for a two‐dimensional, sinusoidal target motion and for a real patient breathing‐curve. For the sliding‐window dose delivery, two different maximum leaf speeds, as well as synchronized and non‐synchronized leaf trajectories were employed. All fields were delivered without and with tracking of the target using three electromagnetic transponders of the Calypso‐System. The dose distributions were measured with radio‐chromic films in a water equivalent phantom in 5cm depth and were compared to the step‐and‐shoot delivery for a non‐moving phantom. For the dose comparison a gamma criterion of 2%/2mm was utilized.Results: For both motions tracking improved the gamma‐index by 13%–32%. For the dynamic delivery no significant advantage was observed between unsynchronized and synchronized leaf trajectories. Moreover no remarkable differences were found by comparing results obtained with different maximum leaf speeds. For both target motions the step‐and‐shoot delivery shows a better dose distribution using the tracking approach: For the 2D‐motion a gamma‐passing rate of 86.3% can be achieved using step‐and‐shoot compared to 78.2% on average for the dynamic dose delivery. For the patient breathing‐motion 83.3% compared to an average of 77.1% were obtained. When comparing the results of both motion patterns, slightly higher passing rates were found with the periodic 2D‐sinusoidal motion. Conclusions: Only small differences were observed for different IMRT dose delivery modes with organ motion tracking, except for the tracked step‐and‐shoot delivery. However, more experiments have to be performed to investigate other IMRT plans and explicit dependencies on the field modulation.Supported by Siemens Healthcare/CR

SU‐HH‐BRB‐09: Real‐Time Soft‐Tissue Imaging Concurrent with External Beam Radiation Therapy Delivery

Purpose: The challenge of real‐time visualization, localization, and tracking of organ motion and deformation concurrently with external beam radiation therapy (EBRT) delivery remains unmet and unattainable by existing image guidance technologies. We propose to address this challenge in the development of a novel, minimally interfering, robotic ultrasound image guidance system that can be integrated with existing medical linear accelerators. Method and Materials: The span of ultrasound transducer motion required for trans‐abdominal prostate imaging was quantified via optical tracking of a probe during free‐hand acquisitions. This information was incorporated into the design of a customized human‐safe robotic manipulator to remotely control the transducer position and pressure while avoiding LINAC gantry collisions. The effect of accelerator‐induced electromagnetic interference on the robot was investigated. In addition, a treatment plan with beam directions restricted to sectors that do not interfere with the ultrasound transducer was evaluated. Results: Prostate image integrity was sensitive to probe pitch, thus the manipulator was designed to actively control abdominal pressure (0–10N) and transducer pitch (−40°−90°). Probe placement in other directions remains adjustable to accommodate anatomy variations. Human‐safety requirements translated into the design via a torque limiting friction clutch, lightweight backdriveable links, and remote center of motion pitch mechanism. Collision avoidance was confirmed by rotating the gantry 360° around the robot mounted on the LINAC couch in a typical treatment setup. The control system of the robot behaved similarly with the LINAC beam off and on, with mean servo cycle intervals of 1000.03 μs and 1000.06μs, respectively. For a representative prostate setup, remotely‐controlled robotic ultrasound image acquisition was successfully demonstrated on a volunteer. Dose volume histogram (DVH) for an IMRT plan with restricted sectors remained virtually identical to a clinically employed DVH. Conclusion: Remotely‐controlled robotic ultrasound imaging is feasible and may offer real‐time intra‐fractional soft‐tissue guidance concurrent with EBRT delivery.

Image-guided surgery in minimally invasive urology

Image-guided surgery (IGS) is a new type of surgery in which indirect visualization of the surgical anatomy helps the surgeon to enable minimally invasive percutaneous, laparoscopic, or robotic-aided treatment more precisely and safely, to achieve complete tumor removal and sparing of the function of critical organs. Although 2D ultrasonography has played a main role as conventional intraoperative image guidance, image-fusion system of ultrasonography with computed tomography (CT) or MRI, 3D ultrasonography imaging, MRI-compatible navigation system, and image-overlay technology of augmented reality navigation system have emerged with computer aid. Development of augmented reality in soft tissue navigation is challenging, especially in tracking of organ motion and deformation. In IGS, the surgeon may see any angled tomogram of the patient's body, or 3D anatomies beyond the direct vision. IGS would not only provide the delivery of energy or medicines to the therapeutic target, but also monitor the precision and effectiveness of the treatment. Emerging imaging technology gives surgeons a new powerful opportunity to realize where surgical pathological targets and vital healthy anatomies are located beyond the surgeon's direct vision.

Tracking of Organ Motion during Arc Therapy with MV Treatment Beam Imaging and As-needed kV Imaging

Tracking of Organ Motion during Arc Therapy with MV Treatment Beam Imaging and As-needed kV Imaging

Performance of ultrasound based measurement of 3D displacement using a curvilinear probe for organ motion tracking

Three-dimensional (3D) soft tissue tracking is of interest for monitoring organ motion during therapy. Our goal is to assess the tracking performance of a curvilinear 3D ultrasound probe in terms of the accuracy and precision of measured displacements. The first aim was to examine the depth dependence of the tracking performance. This is of interest because the spatial resolution varies with distance from the elevational focus and because the curvilinear geometry of the transducer causes the spatial sampling frequency to decrease with depth. Our second aim was to assess tracking performance as a function of the spatial sampling setting (low, medium or high sampling). These settings are incorporated onto 3D ultrasound machines to allow the user to control the trade-off between spatial sampling and temporal resolution. Volume images of a speckle-producing phantom were acquired before and after the probe had been moved by a known displacement (1, 2 or 8 mm). This allowed us to assess the optimum performance of the tracking algorithm, in the absence of motion. 3D speckle tracking was performed using 3D cross-correlation and sub-voxel displacements were estimated. The tracking performance was found to be best for axial displacements and poorest for elevational displacements. In general, the performance decreased with depth, although the nature of the depth dependence was complex. Under certain conditions, the tracking performance was sufficient to be useful for monitoring organ motion. For example, at the highest sampling setting, for a 2 mm displacement, good accuracy and precision (an error and standard deviation of <0.4 mm) were observed at all depths and for all directions of displacement. The trade-off between spatial sampling, temporal resolution and size of the field of view (FOV) is discussed.

SU‐FF‐J‐63: Comparison of Daily Setup Corrections Using On‐Board Cone‐Beam Computed Tomography, and Two Planar Kilovoltage Imaging Systems

Purpose: The purpose of this study is to compare bony landmark‐based setup corrections derived from a linac gantry‐mounted kilovoltage (kV) imaging system that can either be used in orthogonal radiography (OBI) or kV circular orbit computed tomography (CBCT) imaging mode. A third commercial image‐guided radiotherapy system, consisting of orthogonal room‐mounted planar x‐ray imaging system, Exac Trac (ET), was also tested. In addition to daily setup corrections, imaging doses were compared. Methods and Materials: Both patient and phantom data were used to perform this study. Five patients were selected and couch shifts ware determined using both the OBI and ET systems. Using a pelvic phantom, intentionally introduced shifts of 0, 5, and 20 mm in the anterior/posterior (AP/PA), superior/inferior (S/I), and lateral directions, respectively, were applied. The estimated couch shifts were compared using all three Image Guided Radiotheraphy (IGRT) systems. Two cranial cases, one liver case, one lung case, and one prostate case were selected and the estimated couch shifts using the OBI and the ET systems were determined while the patient is in place. Couch shifts are not applied until all imaging modalities are performed. Results: Comparison of simulated shifts using all three imaging modalities showed a standard deviation of setup uncertainties is within 1 mm in all directions. Patients' data showed that the biggest difference is in the order 4 mm in the superior/inferior direction for liver patient and 2 mm in the superior/inferior direction for cranial patient. Conclusions: This study showed that while using 3D/3D image fusion is an excellent choice for online daily patient setup, it is possible to determine couch shift to high degree of accuracy using either 2D/2D or 2D/3D image fusion. For lung and liver patient, tracking of organ motion must be used to any imaging modality.

Management Strategies for Locally Advanced Prostate Cancer

Locally advanced prostate cancer represents a subpopulation of prostate cancer diagnosed in patients who are either untouched by screening efforts or whose disease has an unusually rapidly progressive natural history. The diagnostic work-up for the locally advanced patient is distinct from that of early stage disease in several respects in that it is related principally to ruling out metastases. The typical metastatic work-up consists of a serum alkaline phosphatase, bone scan, CT of the abdomen/pelvis, and chest x-ray. Once metastatic disease has been ruled out, individual components of the management of locally advanced prostate cancer patients may include surgery (palliative or curative), external beam radiation therapy (with photons or particles) or brachytherapy (with low-dose rate/permanent or high-dose rate/temporary radiation sources), and hormone therapy. Unlike in early stage disease, observation/watchful waiting is typically not a treatment option in locally advanced prostate cancer. Of the curative local control modalities, the one most commonly used, and the one which has emerged as the clinical standard, is photon external beam radiotherapy (EBRT). Numerous randomised studies have shown that androgen ablation has an established role in conjunction with radiotherapy for locally advanced disease--the current standard of care is thus photon EBRT plus neoadjuvant and adjuvant androgen ablation. Long-term androgen ablation appears to be better than short-term ablation, even when hormone complications are considered. EBRT is typically delivered to the prostate, seminal vesicles and pelvic lymph nodes, although in some circumstances local fields to the prostate and seminal vesicles may be adequate. New treatment planning and delivery techniques, such as intensity-modulated radiotherapy and organ motion tracking, are being developed to reduce the morbidity of radiotherapy while permitting a higher delivered dose. Further work is necessary to determine the precise sequencing and duration of hormone therapy in conjunction with radiotherapy and the optimum radiotherapy treatment volume. Additional work is also needed to determine the precise groups benefiting from other local control modalities such as surgery and brachytherapy. Finally, novel investigational strategies such as chemotherapy and gene therapy are being applied in an attempt to improve outcomes of locally advanced prostate cancer patients.

Real‐time organ motion tracking and fast image registration system for MRI‐guided surgery

AbstractThe advantage of magnetic resonance imaging (MRI)‐guided surgery, in which MR images taken during surgery are used to guide the surgery, has been recognized recently. However, there is a problem, due to long imaging time of MRI, that it is difficult to capture the respiratory organ motion. It is often a constraint that the patient must hold his or her breath, especially in the treatment of the abdominal region. Consequently, this paper proposes a method of eliminating this constraint in MRI‐guided surgery by real‐time monitoring of the movement of the organ involved in the surgery by means of the signal derived from MRI. Specifically, the navigator echo is extracted in imaging during surgery. The movement of the organ involved in surgery is detected on the basis of the change in the calculated projection profile, and the results are presented to the surgeon. In this study, the procedure is built into a 1.5‐T clinical MRI system. The feasibility of the proposed system was examined under clinical conditions by evaluating its accuracy and delay in a phantom experiment, and also in an in vivo experiment on the liver of a volunteer. The results are reported in this paper. © 2005 Wiley Periodicals, Inc. Syst Comp Jpn, 37(1): 83–92, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/scj.20186

Clinical Experience with the Calypso® 4D Localization System in Prostate Cancer Patients: Implantation, Tolerance, Migration, Localization and Real Time Tracking

Purpose/Objective: To study the implantation and long-term tolerance of Beacon® transponders, and localization and tracking in patients with localized prostate cancer. Materials/Methods: Calypso 4D Localization System (“Calypso System”, Calypso Medical, Seattle, WA) is an investigational patient positioning device including markers (transponders: 8.5 × 1.85 mm, glass-encapsulated electrical circuits used for permanent implantation), and external electronics that continuously localize the transponders using electromagnetic signals at a 10 Hz rate. Three transponders were implanted with 14G needles at the apex, left and right base under transrectal ultrasound guidance in 20 patients. Adverse event checks were conducted at Day 0 (day of implant), 4, 7, 10, 14 (simulation); Fraction 4, 10, 15, 20; and 1, 6, and 12 months post- RT. CTs or pair of kV X-rays were acquired at Day 0, 4, 7, 10, 14; at Fractions 1, 2, 3, 4, 10, 15, 20; and 1 month post-RT to document transponder geometry and compute inter-transponder distance, a migration indicator. Results: The implantation procedures completed without any major complications. Of 60 transponders, one was presumably implanted in the urethra and subsequently voided (patient was asymptomatic) and one migrated cephalad approx. 3 cm from the apex prior to Day 4 presumably within the venous plexus (the transponder subsequently remained stable). In 2 cases, there was a minimal change in urinary status (IPSS scores) prior to RT. All acute bleeding (hematuria, hematospermia, and/or hematochezia) resolved by Day 10 (n=7). To date, 5/20 subjects have completed the 12-month post-RT follow-up. Dysuria was reported in 11 patients after RT. One case of prostatitis was reported at 6 months, and one patient with a very large prostate gland had urinary obstruction requiring surgical intervention at 8 months post-RT that was considered unrelated to the transponders. Analysis of inter-transponder distances indicated stable geometry in 15/20 patients by Day 4, and in all patients by Day 14. Distances at 4-week post-RT again showed stable geometry. In 11/20 patients, localization and tracking were performed (1 patient not localizable and 8 not evaluated). Comparison of the Calypso System coordinates with transponder locations on X-rays showed average (±+SD) 3D difference between the two systems was 1.5 ± 0.9 mm. Finally, during tracking, 2/11 patients showed significant organ motion (>1 cm) over 8 minutes. Longitudinal and vertical motion followed the same pattern of change, with no significant motion in the lateral direction. The excursions persisted for over one minute. Conclusions: Transrectal implantation of transponders for use with the Calypso System has an acceptable adverse event profile that appears consistent with gold marker implantation. Care should be taken to implant transponders within the prostate gland, off mid-line, to avoid migration. Real-time target position monitoring is possible for the first time without requiring X-rays for imaging. Tracking the prostate showed significant motion over time periods of interest in radiation therapy in some patients. Real-time tracking of organ motion will play an important role in modern external beam radiotherapy.