Jeffrey Schlosser

Telerobotic system concept for real‐time soft‐tissue imaging during radiotherapy beam delivery

PURPOSE The curative potential of external beam radiation therapy is critically dependent on having the ability to accurately aim radiation beams at intended targets while avoiding surrounding healthy tissues. However, existing technologies are incapable of real-time, volumetric, soft-tissue imaging during radiation beam delivery, when accurate target tracking is most critical. The authors address this challenge in the development and evaluation of a novel, minimally interfering, telerobotic ultrasound (U.S.) imaging system that can be integrated with existing medical linear accelerators (LINACs) for therapy guidance. METHODS A customized human-safe robotic manipulator was designed and built to control the pressure and pitch of an abdominal U.S. transducer while avoiding LINAC gantry collisions. A haptic device was integrated to remotely control the robotic manipulator motion and U.S. image acquisition outside the LINAC room. The ability of the system to continuously maintain high quality prostate images was evaluated in volunteers over extended time periods. Treatment feasibility was assessed by comparing a clinically deployed prostate treatment plan to an alternative plan in which beam directions were restricted to sectors that did not interfere with the transabdominal U.S. transducer. To demonstrate imaging capability concurrent with delivery, robot performance and U.S. target tracking in a phantom were tested with a 15 MV radiation beam active. RESULTS Remote image acquisition and maintenance of image quality with the haptic interface was successfully demonstrated over 10 min periods in representative treatment setups of volunteers. Furthermore, the robots ability to maintain a constant probe force and desired pitch angle was unaffected by the LINAC beam. For a representative prostate patient, the dose-volume histogram (DVH) for a plan with restricted sectors remained virtually identical to the DVH of a clinically deployed plan. With reduced margins, as would be enabled by real-time imaging, gross tumor volume coverage was identical while notable reductions of bladder and rectal volumes exposed to large doses were possible. The quality of U.S. images obtained during beam operation was not appreciably degraded by radiofrequency interference and 2D tracking of a phantom object in U.S. images obtained with the beam on/off yielded no significant differences. CONCLUSIONS Remotely controlled robotic U.S. imaging is feasible in the radiotherapy environment and for the first time may offer real-time volumetric soft-tissue guidance concurrent with radiotherapy delivery.

Online Image-based Monitoring of Soft-tissue Displacements for Radiation Therapy of the Prostate

PURPOSE Emerging prolonged, hypofractionated radiotherapy regimens rely on high-dose conformality to minimize toxicity and thus can benefit from image guidance systems that continuously monitor target position during beam delivery. To address this need we previously developed, as a potential add-on device for existing linear accelerators, a novel telerobotic ultrasound system capable of real-time, soft-tissue imaging. Expanding on this capability, the aim of this work was to develop and characterize an image-based technique for real-time detection of prostate displacements. METHODS AND MATERIALS Image processing techniques were implemented on spatially localized ultrasound images to generate two parameters representing prostate displacements in real time. In a phantom and five volunteers, soft-tissue targets were continuously imaged with a customized robotic manipulator while recording the two tissue displacement parameters (TDPs). Variations of the TDPs in the absence of tissue displacements were evaluated, as was the sensitivity of the TDPs to prostate translations and rotations. Robustness of the approach to probe force was also investigated. RESULTS With 95% confidence, the proposed method detected in vivo prostate displacements before they exceeded 2.3, 2.5, and 2.8 mm in anteroposterior, superoinferior, and mediolateral directions. Prostate pitch was detected before exceeding 4.7° at 95% confidence. Total system time lag averaged 173 ms, mostly limited by ultrasound acquisition rate. False positives (FPs) (FP) in the absence of displacements did not exceed 1.5 FP events per 10 min of continuous in vivo imaging time. CONCLUSIONS The feasibility of using telerobotic ultrasound for real-time, soft-tissue-based monitoring of target displacements was confirmed in vivo. Such monitoring has the potential to detect small clinically relevant intrafractional variations of the prostate position during beam delivery.

Monte Carlo modeling of ultrasound probes for image guided radiotherapy.

PURPOSE To build Monte Carlo (MC) models of two ultrasound (US) probes and to quantify the effect of beam attenuation due to the US probes for radiation therapy delivered under real-time US image guidance. METHODS MC models of two Philips US probes, an X6-1 matrix-array transducer and a C5-2 curved-array transducer, were built based on their megavoltage (MV) CT images acquired in a Tomotherapy machine with a 3.5 MV beam in the EGSnrc, BEAMnrc, and DOSXYZnrc codes. Mass densities in the probes were assigned based on an electron density calibration phantom consisting of cylinders with mass densities between 0.2 and 8.0 g/cm(3). Beam attenuation due to the US probes in horizontal (for both probes) and vertical (for the X6-1 probe) orientation was measured in a solid water phantom for 6 and 15 MV (15 × 15) cm(2) beams with a 2D ionization chamber array and radiographic films at 5 cm depth. The MC models of the US probes were validated by comparison of the measured dose distributions and dose distributions predicted by MC. Attenuation of depth dose in the (15 × 15) cm(2) beams and small circular beams due to the presence of the probes was assessed by means of MC simulations. RESULTS The 3.5 MV CT number to mass density calibration curve was found to be linear with R(2) > 0.99. The maximum mass densities in the X6-1 and C5-2 probes were found to be 4.8 and 5.2 g/cm(3), respectively. Dose profile differences between MC simulations and measurements of less than 3% for US probes in horizontal orientation were found, with the exception of the penumbra region. The largest 6% dose difference was observed in dose profiles of the X6-1 probe placed in vertical orientation, which was attributed to inadequate modeling of the probe cable. Gamma analysis of the simulated and measured doses showed that over 96% of measurement points passed the 3%/3 mm criteria for both probes placed in horizontal orientation and for the X6-1 probe in vertical orientation. The X6-1 probe in vertical orientation caused the highest attenuation of the 6 and 15 MV beams, which at 10 cm depth accounted for 33% and 43% decrease compared to the respective (15 × 15) cm(2) open fields. The C5-2 probe in horizontal orientation, on the other hand, caused a dose increase of 10% and 53% for the 6 and 15 MV beams, respectively, in the buildup region at 0.5 cm depth. For the X6-1 probe in vertical orientation, the dose at 5 cm depth for the 3-cm diameter 6 MV and 5-cm diameter 15 MV beams was attenuated compared to the corresponding open fields to a greater degree by 65% and 43%, respectively. CONCLUSIONS MC models of two US probes used for real-time image guidance during radiotherapy have been built. Due to the high beam attenuation of the US probes, the authors generally recommend avoiding delivery of treatment beams that intersect the probe. However, the presented MC models can be effectively integrated into US-guided radiotherapy treatment planning in cases for which beam avoidance is not practical due to anatomy geometry.

Ultrasound Imaging in Radiation Therapy: From Interfractional to Intrafractional Guidance.

External beam radiation therapy (EBRT) is included in the treatment regimen of the majority of cancer patients. With the proliferation of hypofractionated radiotherapy treatment regimens, such as stereotactic body radiation therapy (SBRT), interfractional and intrafractional imaging technologies are becoming increasingly critical to ensure safe and effective treatment delivery. Ultrasound (US)-based image guidance systems offer real-time, markerless, volumetric imaging with excellent soft tissue contrast, overcoming the limitations of traditional X-ray or computed tomography (CT)-based guidance for abdominal and pelvic cancer sites, such as the liver and prostate. Interfractional US guidance systems have been commercially adopted for patient positioning but suffer from systematic positioning errors induced by probe pressure. More recently, several research groups have introduced concepts for intrafractional US guidance systems leveraging robotic probe placement technology and real-time soft tissue tracking software. This paper reviews various commercial and research-level US guidance systems used in radiation therapy, with an emphasis on hardware and software technologies that enable the deployment of US imaging within the radiotherapy environment and workflow. Previously unpublished material on tissue tracking systems and robotic probe manipulators under development by our group is also included.

WE‐D‐220‐01: Tissue Displacement Monitoring for Prostate and Liver IGRT Using a Robotically‐Controlled Ultrasound System

Purpose: We previously reported the implementation of a novel telerobotic ultrasound system capable of real‐time soft‐tissue imaging during radiotherapy beam delivery. Expanding on this capability, the aim of this work was to develop and characterize an image‐based technique for real‐time detection of prostate and liver displacements. Methods: Image processing techniques were implemented on spatially localized images to generate two tissue displacement parameters (TDPs) in real‐time. The TDPs were derived from the cross correlation similarity measure between a reference image template and the current image within the incoming stream. In five volunteers, soft‐tissue targets were continuously imaged with a customized robotic manipulator while recording the TDPs. Variations of the TDPs in the absence of tissue displacements were evaluated, as well as the sensitivity of the TDPs to prostate translations and rotations. Robustness of the method to template window selection and total time lag of the system were also investigated. The systems applicability to liver respiratory gating was explored with two volunteers. Results: Prostate translations of 1.8 mm, 2.1 mm, and 2.0 mm were detectable in the M/L, A/P, and S/I directions at the 95% confidence level. Prostate pitch of 3.8° was also detectable. False positives in the absence of displacements were registered at a rate of 1 false positive event per 7 minutes of continuous imaging time. Total system time lag was 173 ms, mostly limited by ultrasound acquisition rate. Respiratory signals based on liver blood vessel monitoring differed significantly from signals based on an external marker. Conclusions: For the first time, quantitative real‐time monitoring of soft‐tissue target displacements was demonstrated in‐vivo using telerobotic ultrasound. Such monitoring has the potential to detect small clinically‐relevant intra‐fractional variations of the prostate position during radiotherapy. An internal ultrasound‐based respiratory signal could be a better predictor of liver target motion than an external surrogate.

Intelligent road sign detection using 3D scene geometry

This paper proposes a new framework for fast and reliable traffic sign detection using images obtained from a single front-facing road vehicle camera. Our focus is on a methodology for reducing the computational requirements and increasing the performance of existing detection methods by refining the image space search using 3D scene geometry. Information concerning physical traffic sign dimensions and vehicle camera parameters is integrated into a model that predicts the image scales and locations at which traffic signs are likely to appear. We apply our framework to a Haar-feature-based detection method trained on a collection of stop signs. Experimental results show that the refined image search space results in much less computation time while retaining the same true positive detection performance as existing methods that search all image scales and locations. In addition, false positives at physically implausible traffic sign locations are eliminated.

Radiolucent 4D Ultrasound Imaging: System Design and Application to Radiotherapy Guidance

Four-dimensional (4D) ultrasound (US) is an attractive modality for image guidance due to its real-time, non-ionizing, volumetric imaging capability with high soft tissue contrast. However, existing 4D US imaging systems contain large volumes of metal which interfere with diagnostic and therapeutic ionizing radiation in procedures such as CT imaging and radiation therapy. This study aimed to design and characterize a novel 4D Radiolucent Remotely-Actuated UltraSound Scanning (RRUSS) device that overcomes this limitation. In a phantom, we evaluated the imaging performance of the RRUSS device including frame rate, resolution, spatial integrity, and motion tracking accuracy. To evaluate compatibility with radiation therapy workflow, we evaluated device-induced CT imaging artifacts, US tracking performance during beam delivery, and device compatibility with commercial radiotherapy planning software. The RRUSS device produced 4D volumes at 0.1-3.0 Hz with 60⁰ lateral field of view (FOV), 50⁰ maximum elevational FOV, and 200 mm maximum depth. Imaging resolution (-3 dB point spread width) was 1.2-7.9 mm at depths up to 100 mm and motion tracking accuracy was ≤0.3±0.5 mm. No significant effect of the RRUSS device on CT image integrity was found, and RRUSS device performance was not affected by radiotherapy beam exposure. Agreement within ±3.0% / 2.0 mm was achieved between computed and measured radiotherapy dose delivered directly through the RRUSS device at 6 MV and 15 MV. In-vivo liver, kidney, and prostate images were successfully acquired. Our investigations suggest that a RRUSS device can offer non-interfering 4D guidance for radiation therapy and other diagnostic and therapeutic procedures.

SU‐D‐220‐01: Hybrid X‐Ray/Ultrasound Imaging Approach for Patient Positioning and Real‐Time Tracking in IGRT

Purpose: Projection x‐ray imaging and telerobotic diagnosticultrasound can complement each other as image guided radiation therapy(IGRT) tools. The purpose of this study was to develop and evaluate an IGRTsystem and workflow that integrates these two modalities for patient positioning and real‐time tracking. Methods: In the designed workflow, pre‐treatment positioning is performed by stereoscopic x‐ray imaging of target surrogates, such as implanted fiducial markers. When the target is in position, a reference soft‐tissue ultrasoundimage of the target is acquired using a novel telerobotic ultrasoundsystem. During treatment beam delivery, the ultrasoundsystem continuously acquires spatially localized soft tissueimages concurrently with the acquisition of megavoltage (MV) x‐ray cine images from the treatment beam. Target position is monitored in real‐time using both modalities. Ultrasound monitoring signals result from comparison of the reference image template to the current ultrasoundimage using cross correlation based techniques. X‐ray monitoring signals rely on tracking fiducial markers visible in the MV images. To evaluate the approach experimentally, an intensity modulated radiation therapy(IMRT) plan for a tissue‐mimicking multi‐modality phantom with implanted gold markers was simulated. The phantom was placed on a motion stage, and target shifts before and during treatment were simulated. Results: Target displacements were monitored in real‐time by the ultrasoundsystem and detected below 3 mm. The displacements were subsequently confirmed with kV x‐ray localization, and the target was repositioned. During IMRT delivery, view of fiducial markers was blocked by multi‐leaf collimator leaves in some MV cine images.Ultrasound complemented MV image tracking by successfully monitoring the target during periods of marker blockage. Conclusions: Strengths of x‐ray and ultrasound modalities may be combined to achieve an IGRTsystem capable of (1) absolute pre‐treatment patient positioning, and (2) robust real‐time displacement monitoring during beam delivery without additional dose to the patient.

Robotic intrafractional US guidance for liver SABR: System design, beam avoidance, and clinical imaging

PURPOSE To present a system for robotic 4D ultrasound (US) imaging concurrent with radiotherapy beam delivery and estimate the proportion of liver stereotactic ablative body radiotherapy (SABR) cases in which robotic US image guidance can be deployed without interfering with clinically used VMAT beam configurations. METHODS The image guidance hardware comprises a 4D US machine, an optical tracking system for measuring US probe pose, and a custom-designed robot for acquiring hands-free US volumes. In software, a simulation environment incorporating the LINAC, couch, planning CT, and robotic US guidance hardware was developed. Placement of the robotic US hardware was guided by a target visibility map rendered on the CT surface by using the planning CT to simulate US propagation. The visibility map was validated in a prostate phantom and evaluated in patients by capturing live US from imaging positions suggested by the visibility map. In 20 liver SABR patients treated with VMAT, the simulation environment was used to virtually place the robotic hardware and US probe. Imaging targets were either planning target volumes (PTVs, range 5.9-679.5 ml) or gross tumor volumes (GTVs, range 0.9-343.4 ml). Presence or absence of mechanical interference with LINAC, couch, and patient body as well as interferences with treated beams was recorded. RESULTS For PTV targets, robotic US guidance without mechanical interference was possible in 80% of the cases and guidance without beam interference was possible in 60% of the cases. For the smaller GTV targets, these proportions were 95% and 85%, respectively. GTV size (1/20), elongated shape (1/20), and depth (1/20) were the main factors limiting the availability of noninterfering imaging positions. The robotic US imaging system was deployed in two liver SABR patients during CT simulation with successful acquisition of 4D US sequences in different imaging positions. CONCLUSIONS This study indicates that for VMAT liver SABR, robotic US imaging of a relevant internal target may be possible in 85% of the cases while using treatment plans currently deployed in the clinic. With beam replanning to account for the presence of robotic US guidance, intrafractional US may be an option for 95% of the liver SABR cases.

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, roboticultrasoundimage guidance system that can be integrated with existing medicallinear 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 roboticultrasoundimage 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 roboticultrasound imaging is feasible and may offer real‐time intra‐fractional soft‐tissue guidance concurrent with EBRTdelivery.