maxon Story

A new radiotherapy technology.

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A medical engineering company in the USA has developed a new radiotherapy technology designed for the treatment of cancer. Several devices are needed to adjust the gamma rays for this radiotherapy, which are directed with extreme precision by a single integrated motion control. maxon motors, gearheads, and controllers ensure the precision of the movements.

A high-precision radiotherapy technology

The company ViewRay, based in Oakwood Village (Ohio, USA), took on the challenge to design a multileaf collimator (MLC). The goal of the engineers was to develop an end product that was more accurate than any other product on the market at the time. ViewRay collaborated with maxon to produce critical components: the motors, encoders, gearheads, and individual motor control modules. ViewRay is a privately held medical engineering company developing innovative radiation therapy devices for use in oncology. The ViewRay system provides continuous soft-tissue imaging during treatment, using MRI-guided radiotherapy, so that physicians are able to see where the actual radiation dose is being delivered and can adapt to changes in the patient’s anatomy. The ViewRay radiation system includes five sub-systems, which are seamlessly integrated to provide optimal patient care. These are: real time MR imaging, treatment planning, dose prediction and optimization, real time soft-tissue targeting, and remote review and approval. The treatment delivery is performed in a split-magnet MRI system with rotating gantry assembly to position three Cobalt-60 gamma ray sources via the three multileaf collimators.

The imaging in other radiotherapy technologies takes place before or after treatment, not while the beam is on. This is problematic for therapy, as targeting cannot be adjusted during the operation.

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Soft-tissue motion often allows a tumor’s position to shift during radiation treatment, causing soft tissue damage. ViewRay solved this problem by using a combination of MR imaging and radiation therapy delivery technologies. With continuous soft-tissue imaging, during treatment and with the beam on, ViewRay tools can refine the target and re-optimize the dose while the patient is undergoing the treatment.

According to the engineering team, “The MLC motor control system is an important portion of the system. We knew it would also be one of the more challenging to design because of proximity to the MRI magnet, and volume constraints due to the gantry configuration.” The system uses three gamma ray sources, mounted in separate shielded heads. According to ViewRay, the double-focused MLC was designed to sharpen contours in the image to produce penumbra comparable to conventional accelerators, so that clinicians can treat patients with greater confidence.

EPOS2 digital positioning control

The team chose maxon’s EPOS2 Module 36-2 compact digital positioning controllers for their small form factor, which allowed them to package 60 channels of motion control for each collimator. Thirty EPOS modules can fit on a single custom motherboard, and there are two motherboards per collimator. Each ViewRay system uses three of these collimators, one for each of the shielded heads. Each EPOS module implements a flexible and highly efficient power stage, and drives a DC motor with digital encoder. The maxon motors are specially designed to operate as slave nodes in a CANopen network. They feature a maximum output power of 2 amps, and their supply voltage comes from an 11-36 V power supply unit. The ViewRay team communicates with the controllers using the EPOS2 firmware, which has easy-to-implement operating modes and monitoring functions, as required by the control.

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The EPOS2 DC positioning controllers are easy to use and implement, thanks to the presence of several important interfaces, while remaining compact enough to work well in a limited space. © 2012 maxon

180 assemblies comprising a motor, a gearhead, and an encoder

Each of the collimators contains sixty leaves that are arranged in two opposing banks of thirty. Since there are three collimators, the device uses 180 EPOS controllers as well as 180 motor/encoder/gearhead assemblies.

The MLCs are mounted on the gantry system to provide collimation of the three gamma radiation sources with respect to the target. While the gantry is moving into position, each collimator leaf is positioned according to the set treatment plan. To position each leaf, CANopen bus commands are directed to the associated node consisting of a motor, encoder, gearhead, and controller. The result is a precisely collimated shape that matches the patient’s treatment plan.

Author: maxon France

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