Motion Control Meets Medical Imaging

Medical imaging original equipment manufacturers (OEMs) are turning to motion control platforms that help control costs, streamline production globally, and provide systems capable of delivering the most sophisticated 2D and 3D clinical images possible. Motion control requirements are driven by these goals, and the need to satisfy specific kinematics requirements.

By Kevin Steele, Bosch Rexroth Corp. August 1, 2007

Medical imaging original equipment manufacturers (OEMs) are turning to motion control platforms that help control costs, streamline production globally, and provide systems capable of delivering the most sophisticated 2D and 3D clinical images possible. Motion control requirements are driven by these goals, and the need to satisfy specific kinematics requirements.

Imaging systems designers usually begin by defining the machine’s kinematics and the specific algorithms needed to achieve that motion. In this industry motion control systems fit into two categories:

1) Basic– single plane, horizontal xy movement, such as a patient table moving in and out of an imaging cylinder, or raising and lowering for patient access; and

2) Complex– multi-axis coordinated motion, with extremely demanding kinematics.

Complex systems typically consist of a patient table and an imaging arc or C-arc gantry, which is a large, C-shaped apparatus with the X-ray source at one end, and the detector on the opposing end. The patient table moves horizontally, vertically, lifts the head or feet, and tilts side to side. The C-arc rotates 180

Some of the most complex kinematics are required when a test or procedure calls for isocentric motion: keeping the imaging beam on a point in the patient’s body, while the imaging apparatus and patient table move independently through multiple passes to create a 360

Isocentric motions require very demanding kinematics loop computations with up to nine axes of motion, moving through x , y , and z planes, while retaining extremely clear image resolution that may only be a few millimeters in diameter.

In the past, motion control subsystems were custom-made by the OEMs, which required diverting valuable engineering and programming resources to motion control from core image processing functions. A new generation of motion controls offers “off-the-shelf” platforms. These support complex kinematics with:

  • 32 KHz servo loop update rates;

  • Configuration tools that support rapid creation of complex control loops with interfaces to calculation engines, such as Matlab, and optimized functionality for features like safety routines, accurate axis synchronization, gearing and spline functionality;

  • Controller-level processing to ensure tighter integration with drives, I/O and imaging functions; and

  • Open architecture C/C++ API.

Most imaging systems use servo drives in the 500 W to 2 kW range to handle gantry loads and patient-weight loads that can range widely and reach 400 lbs.

Most medical imaging systems also use a range of human-machine interfaces (HMIs), such as touchscreens, switch and dial panels, and handheld joysticks. Joystick control is similar to a teaching pendant or handheld controller used in traditional robotics or industrial automation applications.

Because complex medical imaging systems can have multiple elements with multiple axes of motion with demanding kinematics, their motion control subsystems were often custom-made by OEMs.

Open motion architecture

With the wider use of off-the-shelf motion control platforms, there are advantages to selecting systems with open software architectures that support adaptive re-use of existing algorithms and intellectual property (IP). Unlike motion control in automotive and machine tool industries, which typically use IEC 61131-3 programming, medical imaging systems need platforms with control loops optimized for complex applications such as isocentric motion. Plus, they need tight integration of I/O functions, safety systems, and operator controls. This advanced functionality is often implemented via a C/C++ API by the system programmer.

For example, the Bosch Rexroth NYCe 4000 platform has full-featured C/C++ libraries that let programmers efficiently import existing motion control code, easing integration with the imaging system, and reducing time to market for the OEM.

Motion control platforms that use open communications protocols, such as the FireWire/IEEE 1349B protocol optimized for complex industrial applications, can also help speed motion control/imaging machine integration, and deliver added operational advantages.

For example, FireWire lets imaging system builders create a ring network topology that supports loop healing; if there is a cable or interface failure at any point, motion control resets while maintaining communications among all nodes, without risk of sudden interruption.

Modular control adds value

Modularity is a key design consideration for imaging motion control platforms. As design, engineering and manufacturing teams are frequently distributed worldwide, they seek modular systems that simplify engineering, reduce development costs, and make final assembly on-site more efficient.

Modular design also enhances engineering flexibility. Control systems with flexible plug-in slots for drive and CPU interfaces, and flat panel backplanes with multiple drive, controller and I/O interfaces mounted for convenient access are preferable.

A modular backplane lets engineers specify their choice of connectors that serve a specific application best. There’s no need to design extra breakout boards or special interfaces to connect controllers to other components such as encoders, sensing devices or drives.

Modular motion control systems can have a major ripple effect on machine costs, too, helping drive down total cost of ownership (TCO).

A modular control system standardized for use across multiple imaging platforms (CT, X-ray, MRI, etc.) reduces costs in several ways: design time, motion programming, fewer components and cabling, and simplified purchasing and inventory.

As with many other industries, imaging OEMs are constantly striving to make their machines more compact. The machines are inherently large, and hospital and lab floor space can be limited and expensive, especially in urban medical centers and university hospitals with limited room to expand. Off-the-shelf motion control systems designed with form factors specifically to fit into tight machine spaces can help keep the imaging system footprint smaller.

Compact motion control elements can also help improve imaging machine design aesthetics. This has important therapeutic value: Imaging machines are deliberately designed to have smooth flowing, sculptured lines which provide a more comfortable environment for patients.

Safety matters most

The safety and well-being of the patient is of paramount importance. Machine designers have implemented stringent safety technologies to protect patients, physicians, and operators—anyone who may be present within the arc of motion for any imaging component.

Safety features on most imaging machines include proximity sensors and emergency stop controls to instantly halt any motion that could come in contact with a person. Motion control plays a critical role here. The control loop must deliver microsecond response in a smooth-flowing and controlled fashion. For example, for the detection of an obstacle such as a human hand, the actual motor currents are monitored, which generates a real-time event and local response to bring the axes to a swift controlled stop.

Given these requirements, controller-level complex-loop processing provides the safest, surest architecture. Since safety control loops are typically machine-specific, platforms that support fast, custom algorithm development and local real-time C sequences on the motion node can save valuable programming time.

Off-the-shelf motion control platforms must comply with healthcare and environmental regulations (in the U.S., FDA and FCC.) At the electronics level, this typically includes Class B certification for low noise and electrical grounding. Components should also be hardened and shielded from potential effects of long-term use with high-energy and exotic electromagnetic sources such as X-rays, MR magnets, gamma ray and positron emissions.

Costs of high-end imaging systems can run into the millions. To stay competitive and sustain R&D investments in the next generation of systems, OEMs must integrate multiple product lines, technology platforms and engineering groups located across the globe. Increasing standardization, especially in motion control, and improving global supply chain management are crucial to improving their return on investment.

For example, one major device manufacturer has 20 patient table designs. Most OEMs now depend on multiple cross-border design, engineering and manufacturing mechatronics teams to build one machine: For an MRI machine, The Netherlands team may handle the patient table, a team in the UK engineers the imaging apparatus, and the finished system must be integrated into a suite in Houston by a third team.

An efficient integrated global supply chain must deliver and assemble the finished imaging system, assuring that all elements have been tested and verified ready for use, before they’re shipped to Houston. To help achieve this goal, OEMs are trying to implement, as much as possible, standard motion control platforms across multiple product lines and markets.

Partnering advantages

Many medical OEMs have found it more efficient to partner with single-source motion control suppliers with global reach and experience. Benefits include:

  • Engineering and support resources present in all major system design, development, and manufacturing locations.

  • Proven expertise for successfully working with cross-border and cross-functional teams.

  • In-depth expertise implementing, installing, and supporting motion control platforms in markets on every continent.

Global partnering with suppliers generates additional supply chain optimization. Standardizing machine components across multiple product lines gives design teams more opportunities to simplify machine architectures, reduce component count, and drive further design standardization. This approach frees medical OEMs to concentrate engineering resources on the imaging technologies that give them clear market differentiation.

Author Information
Kevin Steele is semiconductor and medical branch manager at Bosch Rexroth Corporation.