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Imaging guides surgery in real time
Operating-room imaging and new material applications take surgeons on a 'fantastic voyage' within the human body
By Rick DeMeis -- Design News, June 8, 1998
Raquel Welch. Other than that screen icon, what the '60s science-fiction flick "Fantastic Voyage" brings to the minds of most engineers is the futuristic medical technology it envisioned. In it, a miniaturized, radar-guided submarine and crew performed microsurgery within the body of a defecting Cold War scientist.
Today, technology has caught up with imagination--on the verge of widespread use are systems that virtually allow surgeons to "look around" and explore, in real time, deep internal sites of interest. Minimally invasive devices then "extend" the doctor's skills into these areas for remediation.
To advance real-time image-guided surgical procedures, General Electric Medical Systems (Milwaukee, WI) now has Signa SP, its IntraOperative MR (magnetic resonance) Imaging(TM) (IO/MRI) system, in tests at 10 clinical investigative sites around the world (see sidebar). Unlike previous MR imagers, this one allows surgeons to see detailed internal views during procedures conducted directly within the imaging apparatus. And while endoscope, radiological imaging, and micro video camera technology have advanced, they only allow viewing surfaces in front of the instrument. MR's ability to see what lies beneath, from any angle, opens up opportunities for a whole range of minimally invasive procedures.
Genesis. A decade ago, Ferenc Jolesz, a physician in neuroradiology at the Harvard Medical School (Boston, MA) visited the company to talk about MRI technology (DN 6/10/96 p. 78). While MR was monitoring therapy, the discussions "got us to thinking about using it for actually guiding therapy," says GE Medical Systems' Mark Augusti, manager of global intraoperative medical programs. "But we had to find a way to design a magnet to give surgeons patient access" (see sidebar). Up until then, MRI magnets were huge affairs that required patients to lie claustrophobically enclosed within them for long periods.
Subsequent developments over the next half decade resulted in cryogenically cooled superconductive materials. Their use for MR systems gave rise to less-confining "open gap" units--posing possibilities for physician access to patients. The real breakthrough, according to Augusti, came from the company's R&D center in Schenectady, NY. Researchers there produced a cryoless superconductor, a unique ceramic tape, functioning at 10K rather than 4K. "This may not seem like much difference," he says, "but it allows using just water chillers" at a significant cost savings.
In bringing about an interventional MRI system, "the surgeons ideally wanted no magnet," notes Augusti. That would give them total access, but was not realistic. "We settled on the most gap [to allow operating] at the most acceptable price and performance. A key factor was the detailed mockup sessions for ergonomics." Such efforts paid off in the twin vertical-coil Signa SP, which has not only imaging features, but also integrates other systems, such as anesthesia gas supplies.
Feedback. The clinical investigation sites provide the Signa design team with feedback in different areas of expertise. "We get lots of great stuff" says Augusti. While the developers saw the system mainly as a tool for surgery, especially to guide minimally invasive procedures, the test facilities are using them in other ways.
The London site is looking into pediatric orthopedics and the problem of infants born with hip displacement. These babies should have their legs properly set in a cast to correct the condition. Just applying a cast may not result in proper growth, but using x-rays to verify positioning would be harmful. With the Signa SP, doctors can verify proper leg positioning quickly, before and after casting. In Zurich, research into lower digestive tract dynamics is taking place. For this, an MR compatible commode had to be designed. And in Boston, doctors are verifying the position of cancer-treating radioactive "seeds" in the complex area of the prostate.
Signa images are displayed to physicians on Sun workstations driven by GE-developed software. Refresh rate is about once a second. Augusti says this is fast enough for most real-time uses. Some users want frequent updates, while others only will occasionally glance at the display. The Stanford, CA, investigative site is working on updating at 12 frames per second. "Resolution depends on the patient and pixel size," Augusti adds, as well as contrast between elements in the image. "And we can take 1/2-mm slices."
The workstations also show 3D models of the patient's anatomy from previous MRI data, facilitating surgical planning. And the system can fuse surface anatomy images with MRI internal views.
On track. An array of light-emitting diodes (LEDs) mounted on rigid devices, such as scalpels, allows their position in the body to be tracked. A set of cameras views the LEDs. Software calculates the 3D position and orientation of the device and its tip. This location is indicated on the real-time MR images. For flexible catheter tips, used in intravascular procedures, for example, the MR system tracks a miniature, passive RF coil at the end of the catheter (DN 2/2/98, p. 29).
Biocompatibility is always an issue in medical procedures (see sidebar). Additionally, ensuring MR compatibility in using the Signa system requires safe operating practices. These are given in an extensive document for users and equipment suppliers. Compatibility goes beyond ferromagnetic materials attracted to the magnet, into EMI (electromagnetic interference), and the production of false imaging artifacts, notes Augusti. The MR operating room is divided into four zones. These range from the most restrictive, the imaging area, to the least, near the room walls. One example he cites is a non-magnetic material not necessarily being image compatible. "A titanium needle may produce small artifacts, that, if used in certain procedures, could obscure millimeter-size tumors.
"Real-time MR will be an integral part of more effective patient care in the future," Augusti concludes. "There is much we are learning. It will be a necessity for certain classes of surgery. Obviously it won't be used to take out an appendix, but will enable site-specific drug delivery, cancer treatment, and super-small endoscope use." Such minimally invasive surgery enters the body through cuts under 4 mm in length and allows natural healing. These procedures are safer, keep internal organs cleaner with less potential damage to healthy tissue, speed patient recovery, cut hospital stays, and also reduce medical costs.
But because the surgeon does not "open up" the patient to directly view the region of concern, accurate imaging becomes critical. "While super software and 3D models will be tools in aiding surgical planning, the comprehensiveness of real-time MR imaging will be vital," says Augusti. Future developments may include combinations of imaging modalities, such as MR with traditionally cheaper, more mobile ultrasound imaging that reflects off internal body surfaces. When asked about surgeons using head-mounted displays to overlay MR images within their field of view, Augusti replies, "These are here today, with the question being how to make them MR compatible." And, he adds, "There are always lots of changes coming from the software standpoint." Effective real-time medical imaging should give future surgeons the means to effectively plan and execute procedures that just a few years ago would have been impossible, subject to uncertainty, and slow and elaborate to implement.
Real-time MRI engineering challenges
- RF shielding within the operating room
- MRI compatibility of tools and equipment
- System ergonomics and access to patient
- Timely image updates for each application
How MRI works
A magnetic-resonance imaging (MRI) unit must be in a room shielded "from the world's RF" (radio frequencies), according to GE Medical Systems' Mark Augusti. Thus, some of the system's own equipment is placed in a separate "RF room" linked by fiberoptics and shielded cables. Inside the shielded room, the familiar circular, high-flux coils generate a homogeneous magnetic field within the open volume at their center. This field aligns the patient's hydrogen atoms. Uniform gradient "slices" within the field then cause different tissues to resonate at different frequencies.
Augusti adds, "An RF signal rotates the atoms 90 degrees, then 180 degrees. We then listen for the signals the atoms give off when realigning with the magnetic field. The signals distinguish between tissue types, such as fat, water, cancer, etc." As concern about exposure to electromagnetic fields, he notes, "the magnets only align 1 out of every 100,000 hydrogen atoms" for minimal temporary physiological change.
Enabling technologies for OR MRI
- Cryoless superconducting materials
- Workstation displays and image fusion software
- Vision systems and RF tracking
- Integration of OR systems into MRI platform
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