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Machine Design

Miniature Motors Help “Paint” Tumors With Radiation

Maxon Precision Motors,

Varian Medical Systems,

A multileaf collimator (MLC) for radiosurgery from Varian Medical Systems, Palo Alto, Calif., contains up to 120 tungsten slats controlled by miniature motors. The slats block the path of X-rays, letting clinicians precisely “paint” radiation doses on tumors close to sensitive tissues such as optic nerves and spinal columns. Motors from Maxon Precision Motors Inc., Fall River, Mass., move the slats in and out, creating an adjustable aperture through which the radiation travels to the tumor.

Because the aperture shape dynamically changes as treatment progresses, clinicians can deliver radiation in doses that closely match the shape of tumors. The computercontrolled movement also lets clinicians deliver different doses to different sections of the tumor to better target aggressive areas while bypassing healthy tissue.

The MLC attaches to the head of a linear accelerator, which generates the beam. Because the motors have limited space in which to move, motor size was a critical factor. Maxon’s RE 8, RE 10, and RE 13 motors are rated between 0.5 and 1.5 W, and measure 17 to 24.6-mm long. Each motor includes an encoder to close the servoloop, allowing radiation delivery with suitably high resolutions and keeping the device’s footprint as small as possible. Maxon also hardened the encoders to withstand radiation because high levels of radiation destroys conventional encoders.

Additional motor elements include rare earth magnets that increase torque and a patented rhombic moving coil that helps provide long life, lower electrical noise, increase accelerations, and improve efficiency. The motor’s ironless rotor exhibits zero cogging.

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