Robotized 4D Ultrasound for cardiac Image- Guided Radiation Therapy

The subject of this thesis is the development and test of a robotized four-dimensional (4D) ul-trasound system for automatic real time image acquisition in cardiac image-guided radiationtherapy (IGRT).The compensation for motion is a promising research field to minimize the irradiation ofhealthy tissue during radiation therapy sessions. Due to image guidance in radiation ther-apy the patient’s position and the motion of the patient’s body surface can be determinedusing different techniques. These techniques show drawbacks. The imaging techniques usedfor the acquisition of the interior of the patient’s body are based on ionizing radiation, whichincrease the exposed radiation dose. Optical imaging techniques tracking the surface of thepatient’s body lack the ability to capture the motion of the interior.Ultrasound as an imaging technique is capable of the acquisition of three-dimensional im-ages of the interior of the patient’s body in real time. This imaging technique has no knownmedical hazards. Furthermore it has been shown that tracking of organs is possible. Addi-tionally, the feasibility of motion compensation based on the tracking has been discussed.Therefore ultrasound imaging can be used to compensate for the drawbacks of the currentimaging technique used in IGRT. This motivates the development of a robotized system ca-pable of acquiring three-dimensional ultrasound images over a certain time period withouthuman interaction during the acquisition.The developed system is based on a standard industrial robot carrying the ultrasound trans-ducer with a force-torque sensor attached. This setup enables the robot to position the trans-ducer without limitations within the working space of the robot. Furthermore, the force-torque sensor measures the resulting forces to control the contact pressures between thepatient and the transducer. The force-torque sensor used is a specially designed embeddedsystem functioning as a safety guard and compensating for dynamically occurring forces andtorques. The developed system is extended by a patient positioning setup for testing pur-poses.Additionally, a control and visualization software has been implemented. The software fea-tures computer tomography image visualization for the selection of the point of interest, thecontrol of the ultrasound transducer’s pose with respect to the selected point of interest, aprepositioning of the patient and the visualization of the system’s state. Furthermore a con-trol algorithm for contact pressure control of the ultrasound transducer during the treatmenthas been implemented.
In the evaluation of the developed system the key characteristics of the robotized 4D ultra-sound system are analyzed. Since the key feature is to provide continuous ultrasound imageswhile holding the transducer on the patient’s skin with a defined contact pressure. The fea-tures evaluated are the visibility of the point of interest or the image quality respectively, thecontinuity of the image-flow and the actual applied contact pressure. All evaluated charac-teristics turn out to be in acceptable ranges.Thus, it has been shown that the developed robotized 4D ultrasound system for cardiac IGRTserves its purpose in the automatic continuous acquisition of ultrasound images and thusmay serve as the basis for further research in the field of motion compensation using ultra-sound imaging in cardiac IGRT.