Medical Device Link.

IMAGING

Drivers who have GPS systems in their cars know how easy it is to get used to the confidence of knowing where to go. They also realize that it would be difficult, if not impossible, for them to go back to using a paper map. Likewise, surgeons who have used highly accurate image-guidance or surgical navigation systems are likely to have a similar reaction to this technology. The primary reason for the popularity of such systems is their ability to facilitate seamless transitions between preoperative imaging, surgical planning, and actual surgical procedures. They can increase surgeons’ productivity and improve the outcome of diagnostic or surgical procedures.

To position themselves well in this growing field, medical device OEMs must make their image-guided systems both operating room– and surgeon-friendly. Surgeon confidence will be key to healthcare industry acceptance of image-guided surgery. Therefore, equipment should facilitate seamless surgical planning and execution without being cumbersome to install or to use. Developing systems with demonstrable accuracies and high-quality image models will also be critical. This article highlights ongoing trends and technology advances within the medical guided imaging field. It discusses approaches that OEMs can take to address some of today’s challenges and help set the direction for the future.

Background

Image-guided surgery has been shown to make surgeries minimally invasive, thereby reducing procedure duration, complexity, and patient recovery time.^1,2 Angioplasty provides the best example of reducing invasiveness among image-guided surgical procedures. It has provided a minimally invasive alternative to complex open-heart surgeries. It has significantly changed the landscape for cardiac treatment. Similarly, endoscopy has revolutionized the diagnosis and treatment of inner gastrointestinal (GI) ailments such as ulcers or tumors, eliminating the need for exploratory surgery. Image-guided navigation has also simplified traditionally complicated procedures such as spinal and hip-replacement surgeries. For those procedures, sensors strategically positioned over a patient’s bones enable surgeons to see a patient’s joints during surgery and more-accurately determine the best placement of implants.

The healthcare systems of the United States and many other developed countries face severe financial constraints and staff shortages amid increasing pressure to improve quality of service. The ability of image-guided surgery to address some of these challenges could fuel the image-guided surgical market’s growth over the next decade. Some industry experts believe that this market in the United States alone will likely double to more than $300 million in the next five years, with an even higher growth rate should surgeon adoption increase.³ The market will likely draw additional growth from the increasing demand for medical imaging, which is expected to exceed $20 billion in the United States by 2010.⁴

FDA classifies image-guided surgical systems as Class III medical devices because they have a direct and immediate effect on patient safety. The added scrutiny imposed by the agency during an already tough approval process means manufacturers also need to be vigilant about their processes and quality measures.

Current Surgical Navigation

The performance of image-guided intervention relies on high-resolution imaging and fast 3-D visualization. However, it also depends on localization technologies that enable surgeons to navigate beneath the surface of the tissue.

Over the last decade, there have been tremendous improvements in image quality as well as a rapid rise in acquisition speeds of imaging systems. These advances make such systems increasingly suitable for intraoperative use.

Figure 1. Image-guided surgical devices, such as this percutaneous spine navigation system, offer surgeons accurate and efficient position and orientation information.

In addition, smarter algorithms implemented on silicon and faster computers that can register and render large volumes of data in a few seconds have led to near-real-time 3-D visualization. However, localization remains a pivotal research subject because of the inherent complexity associated with real-time tracking of instruments or anatomy during surgery. Localization directly affects the accuracy and efficacy of image-guided surgery as well. Position and orientation information about an instrument or about anatomy, when fused with an appropriate image of the surgical volume, provides surgeons with the critical navigational road map (see Figure 1).

Most localization systems commercially available today are based on some variation of electromagnetic field measurements. They track a small sensor, or receiver, that is attached to the tip of an instrument within a magnetic field generated by one of several means. Position and orientation information is then computed using data obtained from the sensor. Most commercial systems offer up to six degrees of freedom in their measurements and claim accuracies of up to 1 mm. The position information is then overlaid, ideally in real time, onto images taken during or before surgery, providing the surgeon with a so-called live context of the instrument being used.

Although the products available in the market today have been reasonably successful, the acceptance of navigation systems among the surgical community is not yet as wide as it could be. There are a number of reasons for this, including the following:

• The presence of large ferromagnetic objects, such as a c-arm, in the vicinity affects the accuracy of electromagnetic localization methods.
• Anatomical dynamics, as in breathing lungs or a beating heart, result in temporal misalignment of tracking and imaging information.
• Lack of precise registration of preoperative images with intraoperative images and navigation data results in imprecise tracking information.
• Possibility of problems with interchangeability of surgical instruments with various tracking systems.
• Cables and other infrastructure necessary for image-guided surgery lead to added bulk and clutter in the operating room (OR) and in the surgical volume.

Innovations in Tracking Methods

Today’s surgical instrument tracking systems distinguish themselves by their adaptation of dc, switched dc, or ac sources to generate a magnetic field. The configuration of transmitters and receivers also provides differentiation and affects performance. These electromagnetic navigation systems often require extensive on-site calibration to ensure that the system can process the signal received by a sensor in its specific environment. Calibration is time-consuming, requires support from field service teams, and may restrict a system’s use to the environment within which it is calibrated.

Under ideal conditions, these systems can achieve submillimeter positional accuracy and subdegree orientation accuracy in static measurements. However, the problems occur in practice. In use, a system’s accuracy depends on many other factors, including the number of sensors, interference from other ferromagnetic objects in the vicinity, and the dynamics of the surgical instrument. Applying multiphysics models can lead to adaptive tracking that can compensate for the presence of large ferromagnetic objects in the OR. Mathematical modeling of the magnetic field using software tools such as HFSS, by Ansoft Corp., can yield important information regarding system behavior in the presence of interference. It can also reduce the burden of on-site calibration requirements. In addition, novel configurations of sensor placement (e.g, static points at the vertices of a tetrahedron or an array of reference points) can improve the accuracy of localization. The static frame of reference being determined by multiple sensors can provide tracking accuracies approaching theoretical predictions.

All navigation platforms on the market impose stringent requirements on surgical instruments, and thus tend to be restrictive. Currently, instruments must be designed and manufactured for use with specific tracking systems. For example, drivers and drills used with one company’s surgical navigation workstation require tracking sensors that are compatible with its proprietary tracking technology. Those sensors would be incompatible with another company’s navigation system. These restrictions relate to the particular sensors and tracking technology implemented within the system. Smarter surgical instruments that can interoperate with different navigation systems could drive industry acceptance of image guidance. For example, an autonomous tracking mechanism via a gyroscope could be embedded into surgical instruments and the tracking data could be broadcast wirelessly, leading to an open platform. However, certain considerations must be addressed when designing wireless systems. Such systems must be able to cope with intentional and unintentional interference from other radio systems. They also need to be protected against electromagnetic interference (EMI) from other devices, just as wired devices need to be protected against data loss and potential breaks.

Although wireless links in electronic devices are inherently intermittent, it is possible to transfer high-quality data across them. The key requirements of data quality and integrity can be met by applying extra protection mechanisms to the wireless connection. Such protection mechanisms are often overlooked in simple off-the-shelf radio designs, but system design specific to the medical environment could mitigate this issue.

Figure 2. The Da Vinci surgical system has enabled traditionally complex surgeries to be minimally invasive through image guidance and localization techniques.

Image guidance and localization have also enabled robotic or stereotactic surgery. A camera placed within or inserted into the surgical volume provides a live image that guides the surgeon to drive a robot, which in turn controls precise surgical instruments. Development of systems such as the Da Vinci surgical system by Intuitive Surgical (shown in Figure 2) has allowed complex and traditionally open surgeries to become minimally invasive. Where normally a surgeon would need to make a large incision, the robot can control and guide miniature instruments through small openings in the body.

Robotic surgery is still in its infancy, but device manufacturers can drive its uptake by designing instruments and user interfaces that enhance surgeons’ comfort level with remote manipulation. A surgeon will feel comfortable and confident in performing robotic surgery if, for example, the ergonomics and mechanism of interacting with the robotic arms and controls are similar to the surgeon’s normal use of scalpels and other surgical tools. Improving the accuracy of the instrument movement and minimizing response time would go a long way toward increased adoption. This could also open the door for telesurgery, wherein a doctor could perform a procedure miles away from a patient. A major roadblock for telesurgery has been the latency between the doctor moving the controls and the remote robotic device responding to those movements. OEMs will have to address this time delay, probably by developing reliable high-speed means of data communication.

Ultrasound Image Guidance

Intraoperative imaging has been dominated by fluoroscopy for many years, followed more recently by mobile computed tomography (CT) platforms. However, both modalities make radiation exposure a concern for patients and surgeons alike. In the last five years, interventional magnetic resonance imaging (MRI) machines have been finding their way into the OR, making high-contrast soft-tissue details available during surgery. But the size of these MRI machines makes them bulky for use in an already crowded OR. Moreover, MRI still remains prohibitively costly, severely restricting its use.

Although ultrasound has established itself as an important imaging modality for applications like obstetrics and breast imaging, its use as a guidance tool has been fairly limited. Compared with other modalities, ultrasound yields images that are too low in quality for surgical guidance. Also problematic is ultrasound’s lack of registration coordinates, owing to the freehand nature of ultrasound image acquisition. However, digital beam formers and harmonic imaging have greatly improved ultrasound image quality. Extensive research is being conducted to tackle the speckle and echoes that plague ultrasound images. Motorized transducers for volume data acquisition and advanced rendering algorithms have made 3-D ultrasound a reality. These advances, combined with the inherent safety and real-time nature of ultrasound, indicate that ultrasound could be a viable modality for surgical navigation.

Ultrasound guidance has been successfully used for breast biopsies and other tissue aspiration for almost a decade. More recently, ultrasound-guided injections have proved effective for treating musculoskeletal conditions like injured tendons, ligaments, and joints. Intravascular ultrasound is also becoming invaluable in stent placements. By visualizing stents in real time, surgeons can position them precisely, thereby reducing occurrence of restenosis. With the advent of portable handheld equipment that lends itself to the space limitations within the OR, ultrasound is now even more attractive for intraoperative use.

Ultrasound can also be used in conjunction with other imaging modalities to provide real-time updates on soft-tissue movement during surgery. For example, comparing intraoperative ultrasound images with preoperative MR images can provide neurosurgeons with vital information on brain deformation that occurs after the cranial case has been opened. Combining ultrasound with cardiac CT data can help generate combination soft-tissue and bone models, giving surgeons a complete understanding of the cardiac volume.

Figure 3. Fusing images such as MRI and PET can provide real-time updates during surgeries.

Various techniques using these types of multimodal image fusion have been explored and have been shown to work with reasonable success (see Figure 3).^5-7 Linear registration techniques work well with static data but suffer when capturing anatomical movement. Nonlinear registration methods use complex mathematical transforms for data characterization and can be computationally intensive. For improving the quality of the fused image and 3-D visualization thereafter, other methods such as image segmentation, finite element modeling, or Bayesian algorithms need to be applied. Using sensors and markers to define fixed positions on the anatomy during image capture can also help with accurate alignment of pre- and intraoperative data.

Reduce the Clutter: Go Wireless

Complex cabling requirements of current navigation systems add to the already crowded space in the OR and create clutter in the surgical area. If smart sensors could untether surgical instruments, and if EMI concerns could be allayed, the next generation of surgical navigation systems could go wireless and greatly increase freedom of movement for the surgeon.

A variety of wireless standards are already setting precedents in the medical industry. Wi-Fi is being deployed widely in hospitals, providing improvements in workflow and convenience for staffs. It can facilitate everything from tablet PCs that provide access to patient records by the bedside to ambulatory patient monitors. Other technologies, such as Bluetooth and ZigBee, offer advantages when low-data-rate and short-range connections are sufficient.

However, before designing a wireless system, it is important to understand and quantify the data-loss policy that will be required for the particular medical device. In conducting risk analyses, questions relating to quality of service need to be raised and answered. These considerations include the response time required, amount of acceptable sensor data loss, detection of gaps in data transmission, etc. Quality-of-service issues can be appropriately tackled by introducing layered protection mechanisms during data transmission. Some techniques that can be used include forward error protection, which can offer near-perfect data correction using high-performance coding schemes, or block error detection, which uses cyclic redundancy check (CRC) coding on data blocks. Another option is automatic message repeat request, which ensures data integrity by using a high-level handshaking scheme. When balanced with a supporting radio protocol, these mechanisms can yield a wireless system design with adequate reliability and response time for medical devices.

Table I. (click to enlarge) FCC has allocated frequency bands specifically for the use of medical devices.

Selecting an appropriate radio spectrum is also critical to reducing the likelihood of in-band interference. FDA has released a guidance that instructs manufacturers to use frequency bands that have been allocated by FCC specifically for the use of medical devices.⁸ These frequency bands and their associated regulations provide a framework for products to transmit information wirelessly in the sensitive and data-critical hospital environment. Table I shows the assigned frequencies for the various allocated bands.

Manufacturers that design their hospital telemetry products for the wireless medical telemetry service (WMTS) band are likely to encounter a simpler regulatory route compared with those that design products in general unlicensed bands. By following the rules of the WMTS band, devices will automatically minimize the risk of interference from other in-band sources.

A WMTS-based system can allow the transmission of sensor data from surgical tools to remote systems that can process the data. This could allow surgeons to access patients without the obstruction of trailing cables and image-processing equipment. The compiled image and sensor data would then be presented to surgeons on a single visual interface.

For implantable applications, FCC has provided the medical implant communications service (MICS) band, wherein data can be transmitted from within the body to a receiver outside. As simple as the concept may sound, transmitting sensor data reliably and continuously from within the body presents a challenge for radio system design. The hostile environment within the human body and the attenuation of signals by human tissue require special attention. Design is further complicated by the need for the transmitter to be compact while achieving a battery life that meets the requirements of the surgical procedure. Low power consumption must be complemented by high-data-rate transmission to maximize battery life. Even so, implantable radio technology is already providing greater access to anatomy. Pill-based sensors that can be swallowed have already been designed. They gather various measurements (pH, temperature, pressure, etc.) from inside the body. The same technology could be used for wireless cameras that can offer image mapping of the entire GI tract, allowing remote visualization and diagnosis.

Other wireless technologies such as ultrawideband (UWB), which is available for unlicensed operation in the 3.1–10.6-GHz band and is capable of high data rates, present opportunities for video transmission over a short range of a few meters. UWB-based systems can generate composite images of a surgery and then transmit them to screens close to surgeons without the need for cables. Such products would minimize the need for image-processing equipment to be kept near the patient and improve surgeons’ working environment. Specific applications in microscopic surgery (e.g., retinal procedures) can also be envisioned, wherein the wireless video-based systems would eliminate the need for surgeons to continually look through a microscope while performing the procedure.

Conclusion

Technological advances continue to revolutionize medical care by enabling earlier diagnosis and safer, more-efficient treatments. Advances in image-guided surgery have the potential to bring in a new era for surgical procedures and may well become the standard of care in the digital OR of tomorrow. Future developments should focus on speed, accuracy, and simplicity. Key areas for improvement include obtaining real-time localization data in combination with high-resolution imaging. Special attention should be paid to technology that could be leveraged to make wireless surgical suites a reality. There is potential for breakthrough innovation that could enable a giant step forward in several areas, such as stereoscopic displays for accurate presentation of 3-D volumes.

However, the road ahead will not be easy for manufacturers. The consequences that a single malfunction can bring are enough to warrant strict due diligence when developing products for image-guided surgery. High quality standards need to be maintained during design, implementation, and verification and validation. Proving the accuracy and reliability of an image-guided surgical system can be a significant burden. However, it can be tackled by adopting a disciplined, risk-based approach throughout the product life cycle.

With so many challenges on the horizon, the image-guided surgical market may seem fraught with hurdles. However, companies that make the leap and conquer the market will reap large rewards.

Vaishali Kamat leads the sensing and imaging group at Cambridge Consultants (Cambridge, MA). She can be contacted at vaishali.kamat@cambridgeconsultants.com.