NETWORKING

This cardiovascular CT system by Medtronic is networked via CANopen.

CAN stands for controller area network, and CANopen is a real-time-capable communication network. CANopen, which is a standard, open architecture, can be used in many medical applications to simplify system development and device integration. CANopen can extend a medical device’s capabilities and accuracy while reducing the manufacturer’s development time and cost. This article describes several applications in which CANopen is used.

The communication network enables medical devices to interact. The CANopen network is the means of communication between, for example, a CT and a contrast media injector. It is also the communication channel between the CT and the patient bed, without which the patient would not be in the correct position to be x-rayed. It also controls the CT to ensure that x-rays are taken only after the contrast fluid injector has injected the fluid into the patient’s veins. It does so within a predefined time span to ensure the best possible scan of the head. This could be a matter of milliseconds, much faster and more accurate than any human could time this process manually. The CANopen network even sends a signal to the CT that there is a patient on the bed and that his head is in the right position, and so on.

Medical device manufacturers such as GE Healthcare, Philips Medical Systems, Siemens Medical Solutions, and Toshiba Medical Systems have developed several CANopen application profiles, meaning they have specified the data content that the networks send to and from the connected machines to the CT, angiograph, magnetic resonators, etc. Connected machines can be subsystems such as collimators, dosimeters, and contrast media injectors.

Medical system manufacturers that decide to use the CANopen protocol to connect their medical devices are often driven by the need to reduce development costs. By using CANopen, device manufacturers can concentrate on device development rather than communication. Using a nonproprietary protocol such as CANopen enables device manufacturers to buy components and CANopen devices from different suppliers instead of relying on a single supplier or developing a proprietary system.

Better Interfaces Improve Exam Consistency

The medical device industry has developed the CANopen specification for connecting CT and contrast injector devices. The specification supports a physical connection and software for triggering the scan and injection with a single button push on the CT console. CAN in Automation (CiA) 425 harmonizes the protocol for the injection and scan. It also supports the transfer of information (such as the time of injection, rate, and dose) gathered by the injector to the scanner, so that these data are added to the patient record. This promotes consistency among exams.

The Medrad Stellant 5000 CT suite uses an injector networked via a CANopen CiA 425 device.

For example, Medrad provides a contrast media injector that uses a CiA 425–compliant interface. It is linked to a CT scanner made by Siemens Medical Solutions. Such a digital link between the power injector and the CT scanner for integrated scanning and contrast injection simplifies the technician’s job and enhances the accuracy of the x-rays. The system enables the initiation of the power injector and scanner using a single button. It also improves synchronization of the bolus timing and helps improve accuracy of contrast timing in CT angiography.

The technology behind one-button scanning and contrast injection is more important than it might seem. The system represents a workable future platform for all kinds of data transfer between scanner and injector. The synchronized scanning and contrast injection system is built on the CiA 425 CANopen profile specification that allows the Siemens and Medrad products to share detailed procedural information. The next generation of injectors is slated to use the technology interface to provide these communication tools and more, enabling the transfer of information regarding actual and programmed flow rates, scan rates, dual flow rates, and peak pressures. And if the scanner is connected to a remote installation service (RIS), it will also be possible to distribute information to individual patient records. An RIS capability would eliminate the need to dictate scanning and injection information into radiology reports.

Proton Beam Facility Uses CANopen

Networks exist even within subsystems. For example, within patient beds there can be one or several networks that connect devices such as controllers, drives, and sensors to each other. A patient bed that moves the patient into the imaging device would need a network to connect the following elements:

• A sensor (to determine whether there is a patient on the bed).
• A drive (to move the bed into the tube).
• A controller that coordinates the movements to a signal from the caretaker (e.g., the push of a button).

Compared with an x-ray beam, a proton beam that is delivered with sufficient energy (or modulated) has a low entrance dose (the dose in front of the tumor), a high-dose Bragg peak region, which is designed to cover the entire tumor, and no exit dose beyond the tumor, whereas x-ray beams may deposit most of their dose in tissues in front of the tumor.

The DSCT system by Siemens uses two x-ray sources and two detectors at the same time at different power levels. The device is networked to other medical devices via CANopen.

The proton beam machine consists of a large steel cylinder weighing 100 t, housing a gantry with large magnets that guide the accelerated proton beam. The patient is driven into the cylinder and positioned in the path of the beam to within a half millimeter, by means of a table with control of x, y, z, rotation, pitch, and roll axes based on closed-loop servomotors.

The table itself weighs 4.5 t, to provide the stiffness required for accurate and repeatable positioning of the long load. At the heart of the control system is a PC. A Linux-based application issues motion commands to the system’s actuator.

Because the proton beam machine has the potential to injure patients who are positioned incorrectly, the company designed safeguards into the motion system. Each axis has at least two absolute position sensors, one of which is connected to a security monitoring application program running on a separate PC. The other is used for normal motion control applications.

The movement commands issued by the control system are monitored by the security PC. At the end of a movement, for fail-safe operation, the two sensors must agree that the system is in the right place before the proton beam can be switched on. The motion subsystem consists of a PCI bus card plugged into the PC, connected via CANopen to six drives and servomotors. Two further axes on the card are also used by the integrator to control the linear extension of the proton beam nozzle as it is set up for a treatment session and a heavy-duty industrial motor that adjusts the magnet gantry.

For manufacturers of motion control systems, the CANopen protocol is a cost-effective network on a per-node basis. More than 580 CANopen vendors purchase the widely available and reasonably priced silicon. A variety of software libraries and integration tools help device manufacturers reduce development time. CANopen has well-documented and supported profile definitions and uses a simple, low-cost physical layer.

Embedded CANopen in Hospital Beds

Regular patient beds, such as those designed for use in intensive care units, can use an embedded CANopen network. The hospital environment has a diverse set of requirements such as long-term-stay facilities, medium acuity (med-surge) general short-term stay, and critical care. Depending on the environment, as many as 20 independent microprocessors can be used to communicate together. Raising the patient’s head uses one of six motors with position sensing and battery backup. Multiple types of sensors are employed. User controls are located in many places on the product.

CANopen technology reduces development costs and time as well as per-node cost. It is a robust technology (with collision detection and arbitration and cycle redundancy check) and provides bandwidth, expandability, and design flexibility. A typical bed can be configured with at least one or more of the following modules, depending on the area in the hospital where the bed would be used. Some features, such as the following, are optional and require plug-and-play scalability:

• Scale.
• User interface.
• Motor control.
• Nurse station.
• Patient detection.
• Air mattress.
• TV controls.

The core unit of this complete operating room endoscopy system is a central systems and device management unit that ensures the control of all integrated medical products. It uses a central touch panel in a sterile, direct-access area.

User controls are typically event-driven updates. The user expects the product to start moving when an articulation button is pressed. Periodic updates happen on a fixed timer or schedule. The patient weight or pressure in an air mattress is monitored over time. Remote nurse stations may request information from the bed as needed. CANopen can handle even complex systems because of its flexible configuration options. Even large systems are possible with transparent gateways to connect several CANopen networks.

There are several benefits to using a standard open architecture. For example, software stacks can be purchased readily for almost any microcontroller and code language. In addition, many third-party Windows-based development tools are user-friendly and provide more than enough power. The software and tools are mature, flexible, fast, capable, and economical.

Monica Mack is technical editor for CAN in Automation (Erlangen, Germany). She can be contacted at mack@can-cia.org.