Originally Published MPMN October 2009

PRODUCT UPDATE

Motorizing Medical Imaging Applications

Motors and motion control systems for medical imaging must be able to operate in environments with extreme electromagnetic fields

Bob Michaels

It’s hard to imagine modern medical devices without motors and motion control systems. From centrifuges and oxygen-concentration equipment to medical pumps and an array of surgical instruments such as saws and drills, motion control technology moves medicine. But not all motors are created equal. When it comes to medical imaging systems, including magnetic resonance imaging (MRI) machines, special motors are required that are made with nonmetallic materials. A host of motion control systems fit that bill.

Motors Lacking the Powers
of Attraction
Because traditional electromagnetic motors contain ferrous metals, they represent a safety hazard in applications with strong magnetic fields, such as MRI technology. Electromagnetic motors also generate their own magnetic and radio-frequency (RF) fields, which can result in RF arcing and cause hardware damage and image artifacts. In addition, conventional motor operation can be influenced by the static and gradient magnetic fields used during MRI data acquisition, causing unpredictable motor function or damage.
As a supplier of motors for a range of medical imaging technologies, Nanomotion Inc. (Ronkonkoma, NY; www.nanomotion.com) manufactures models based on ultrasonic standing-wave principles with a piezocrystal, a technology that yields unlimited linear or rotary motion in a small footprint. While all of the vendor’s motors are suitable for various types of imaging applications, a completely nonmagnetic version is suitable for use in the MRI field, according to company president Alan Feinstein. “These motors have no magnetic materials and no intrinsic magnetic fields and can function in 1.5- to 3-T MRI machines while they are running,” he explains. “The key feature is the ability to operate the motor inside the magnetic field and not generate artifacts on the screen. Any magnetic field or material from the motor would distort the MRI image, rendering it useless.” Using a piezo-based technology enables the technology to eliminate the use of all magnetic materials.
Suppliers of motors for medical imaging equipment face several electrical and mechanical challenges, according to Feinstein. Electrically, noise can create unacceptable artifacts on a screen. Even if a nonmagnetic motor is used, the proximity of the drive electronics is important. This complication requires electronics that are filtered and tuned to operate outside the range of MRI frequencies to eliminate interference. Depending on whether the MRI is a 1.5- or 3-T system, different motor amplifier platforms may be required.
Mechanically, the motion structure, whether it is linear or rotary, requires nonmagnetic materials, in addition to precision and stiffness. However, while it is easy to find commercially available bearings and other materials for industrial-grade motors, motors used in MRIs must employ special bearings made from ceramics and plastics to overcome the magnetism challenge. “Completely nonmagnetic bearings are hardly commercialized, and they are often hard to come by as well as expensive,” Feinstein remarks. “Since Nanomotion motors transmit motion through frictional contact of ceramic-on-ceramic, it is necessary that we have relatively high stiffness in the mating structure that we are moving, which means 30 N•µm or higher. This eliminates the use of plastic bearings and requires some creative solutions for managing this in the design.”
MRI is becoming the tool of choice among doctors because it provides high image quality and causes minimal disruption to the patient. “The objective is to use the technology as much as possible to guide surgical tools,” Feinstein comments. But to use surgical tools and other medical devices in conjunction with MRI, devices with nonmagnetic motors are necessary. “The aim today is to find ways to couple treatment with the MRI to achieve real-time guidance,” Feinstein says. “This is the foundation for efforts to have completely nonmagnetic systems that can provide motion inside a high magnetic field.”
Perfecting Piezoelectric Technology
“Because standard motion technologies are based on electromagnetic motors, they generate an electromagnetic field, which can interfere with imaging while endangering people within the MRI field itself,” explains Dan Viggiano III, director of the custom products division of New Scale Technologies Inc. (Victor, NY; www.newscaletech.com). To avoid the problems associated with conventional motors, the company offers a piezoelectric ultrasonic motor that is constructed from nonferrous materials. “Our technology, because it is piezo-based, is inherently nonmagnetic and offers a suitable replacement for devices that need to be manipulated, moved, and positioned within the MRI field.”
The company’s Squiggle motor consists of four piezoelectric ceramic plates bonded to a nonmagnetic metal nut. Two-phase drive signals cause the piezoelectric plates to vibrate at an ultrasonic frequency of 40 to 200 kHz, matching the first bending resonant frequency of the nut. The motion of the plates is synchronized to make the nut vibrate in an orbital motion, causing the mating screw to rotate and translate.
“The Squiggle motor is a unique ultrasonic actuator that uses a screw and nut to generate linear motion in a very small volume,” Viggiano comments. “However, like all other ultrasonic motors, the Squiggle uses contact friction. Because contact friction affects the threads, careful material engineering is required. The material used must be hard and durable but also capable of forming precise thread dimensions.”
Standard Squiggle motors are made from ferrous steel, which is not suitable for MRI applications. While titanium is a well-accepted material for MRI equipment, tests have shown that it exhibits insufficient force, speed, and lifetime characteristics. The galling of titanium is a particular problem. “We’ve performed significant material research, development, and testing to find better material combinations for motors used in MRI environments,” Viggiano says. “We have discovered that medical-grade steels, which are used for implantable devices, achieve sufficient lifetime in MRI applications and are as compatible as titanium.”
Providing Feedback

In motorized systems, the motor’s position and speed information must be fed back to the drive—a function commonly performed by encoders. “An encoder gives you continuous information,” notes Dennis Horwitz, vice president of sales and marketing at Micronor Inc. (Newbury Park, CA; www.micronor.com). “However, encoders based on metallic or ferrous materials will disrupt the electromagnetic fields of MRI machines, corrupting the whole point of the imaging system.”

Because of its passive optical design and nonmetallic construction, the company’s MR318 fiber-optic rotary encoder is designed for use in MRI equipment, providing feedback in motion control systems operating in electromagnetic fields without affecting system operation or measurements. Offering 360-ppr (1°) resolution and measuring both angular position and shaft speed, the encoder provides feedback for actuator or motor control, patient-position monitoring, and correlating foot pedal position for functional MRI diagnostics.
“Most encoders are either magnetic or optical rotary systems,” Horwitz remarks. “While magnetic encoders, by definition, use magnetic sensors, optical rotary encoders use optoelectronics.” As in standard rotary encoders, the MR318’s optoelectronics are remote—a function of the system’s fiber-optic technology. Based on fiber optics, the encoder is passive and immune to electromagnetic interference. The key principle is the fiber optics, Horwitz says. The light signal in and out is based on an all-optical design and is passive, whereas conventional optical rotary encoders are active. “When you combine fiber-optic technology with a nonmetallic housing, that’s what the MR318 is all about.”
For more articles on motors and motion control technologies, go to www.devicelink.com/mpmn/motors
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