In order to minimize radiation directed to a region outside of a target volume of a patient during a radiation therapy and minimize the cost of an image-guided particle therapy system, the particle therapy system includes an ultrasound device having at least one ultrasound transducer positioned abutting or adjacent to an external surface of the patient. The ultrasound device and thus the ultrasound transducer are positioned outside of a beam path of a particle beam between a radiotherapy device generating the particle beam, and the target volume. The ultrasound device is operable to generate data representing the target volume and/or a region outside of the target region while the radiotherapy device directs the particle beam to the target volume. The particle therapy system includes a processor operable to control the radiotherapy device based on a comparison between the generated data representing the target volume and a predetermined treatment plan.