A cooperatively controlled robot for ultrasound monitoring of radiation therapy

Abstract

Image-guided radiation therapy (IGRT) involves two main procedures, performed in different rooms on different days: (1) treatment planning in the simulator room on the first day, and (2) radiotherapy in the linear accelerator room over multiple subsequent days. Both the simulator and the linear accelerator include CT imaging capabilities, which enables both treatment planning and reproducible patient setup, but does not provide good soft tissue contrast or allow monitoring of the target during treatment. We propose a cooperatively-controlled robot to reproducibly position an ultrasound (US) probe on the patient during simulation and treatment, thereby improving soft tissue visualization and allowing real-time monitoring of the target. A key goal of the robotic system is to produce consistent tissue deformations for both CT and US imaging, which simplifies registration of these two modalities. This paper presents the robotic system design and describes a novel control algorithm that employs virtual springs to implement guidance virtual fixtures during “hands on” cooperative control.

Cooperative Control with Ultrasound Guidance for Radiation Therapy

“…In this mode, the operator holds the US probe and applies a force toward an intended direction of motion. Static VFs impose motion constraints; in our implementation, they are in the form of linear and torsional springs, which provide haptic feedback toward a reference US probe position and orientation, as described in Sen et al (2013). The fixtures are called static because their parameters do not change over time.…”

“…6 are the elements of the diagonal admittance gain matrix, and − → f F and − → τ F are the linear and torsional force measurements in the force sensor frame. We use nonlinear admittance gains to enable fine control for smaller applied forces, while also allowing faster motions for higher applied forces (Kazanzides et al, 1992;Sen et al, 2013). Next, we convert the velocity inputs into the robot frame through the series of transformations shown in Figure 3, in which F R represents the robot frame, F Pr represents the US probe tip frame (the frame origin is near the center of the convex probe surface but does not need to be precisely located, as discussed in Section 4), and F F represents the force sensor frame.…”

“…But, radiation therapists typically are not experienced ultrasonographers, so we developed a co-manipulation strategy where the robot shares control of the ultrasound probe with the radiation therapist and provides haptic cues (via virtual fixtures) to help the therapist to correctly place the US probe. This paper presents a co-manipulation strategy that incorporates real-time US imaging in the control loop to update the virtual fixtures (VFs), which represent an advance over our prior work (Sen et al, 2013), where the VFs were based on the reference position and orientation and were not updated. A phantom study is performed to demonstrate that this strategy enables an operator to quickly and accurately reproduce a reference US image.…”

In vivo reproducibility of robotic probe placement for a novel ultrasound-guided radiation therapy system

“…The cooperatively controlled robot 23 could then be used to place the US probe to visualize the tumor being treated. Once a suitable position is found, the robot will record the probe pose in room coordinates for repositioning on the treatment day.…”

“…22,23 The model probe would be placed during the CT image acquisition for treatment planning, while the true US probe would be used for real-time monitoring of organ motion on each day of treatment delivery. Our previous publications introduce the cooperative control algorithm for the robotic system 23 and investigate ex vivo reproducibility of markers located at different depths from the probe. 22 This work is the first to investigate the in vivo reproducibility of tissue deformations with robotic probe placement for guiding radiation therapy.…”