Three Multileaf Collimators
When it came to designing a multi-leaf collimator (MLC) device, ViewRay (Oakwood Village, OH) took on the challenge to produce an end product that would be more accurate than others on the market. ViewRay collaborated with maxon Precision Motors for important components, including a custom motherboard, DC Servo motors, encoders, gearheads, and individual motor control modules,” according to an engineering team member.
ViewRay Incorporated of Cleveland, Ohio, is a privately held medical device company developing advanced radiation therapy technology for the treatment of cancer. The ViewRay system provides continuous soft-tissue imaging during treatment, using MRI-guided radiotherapy, so that clinicians are able to see where the actual radiation dose is being delivered and adapt to changes in the patient’s anatomy. Overall, the ViewRay system includes five sub-systems, which are seamlessly integrated to provide optimal patient care. The major subsystems are: real time MR imaging, treatment planning, dose prediction and optimisation, 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 shielded Cobalt-60 sources with the three multileaf collimators.
The imaging in other radiotherapy technologies took place before or after treatment, not while the beam was on. This was a limitation in providing therapy because targeting could not be adjusted dynamically. Soft-tissue motion often allowed a tumor’s position to shift during treatment, causing soft tissue damage.
The ViewRay system 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-optimise the dose while the patient is on the treatment table. This is the first MRI-guided Radiotherapy System produced. 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. For ViewRay, the double-focused MLC is designed to sharpen field edges to produce penumbra comparable to conventional accelerators, so that clinicians can treat patients with greater confidence.
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 modules can fit on a single custom motherboard, and there are two motherboards per collimator. Each ViewRay system required three of these collimators, one for each of the three heads used in the system. EPOS controllers are compact, fully digital, smart motion controllers designed for use as plug-in modules in customer-specific motherboards, and can be used for single axis or multi axis motion control systems. Each EPOS module implements a flexible and highly efficient power stage, and drives a brushed DC servo motor with digital encoder.
180 motor, encoder, gearhead assemblies
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 DC servo 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 treatment plan. To position each leaf, CANopen bus commands are directed to the associated node consisting of a DC servo motor, encoder, gearhead, and controller. The result is a precisely collimated shape that matches the treatment plan.
The 180 motors used in the system are RE16 brushed DC, 4.5 Watt motors. Each motor operates at speeds up to 11,000 rpm. Along with the DC servo motors are MEnc13 Hall-effect encoders for precise DC motor control and GP16A gearheads with 19:1 reduction providing adequate torque for positioning speeds of 1cm per second. The EPOS Studio software package provides an easy-to-learn, easy-to-use development environment for proof of concept and end-product development activities, according to engineering. Due to MRI magnetic field effects, the MLC controllers had to be remote from the motors, and housed in a convection cooled chassis.
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