OSAKA - MR-guided interventional procedures are becoming common in Japan, particularly in the prefecture of Shiga, thanks to scanner innovation and the concurrent development of MR-compatible tools and software to facilitate the procedures.
At Wednesday's opening sessions of the 2006 Computer Assisted Radiology and Surgery (CARS) meeting, Dr. Shigehiro Morikawa from the Shiga University of Medical Science in Japan talked about the tools -- including several developed at his institution -- that have facilitated the growing use of MR-guided procedures in the heart, abdominal organs, liver, brain, and bones.
To be sure, MR guidance has its drawbacks, notably in its limited access to patients due to the proximity of the magnets, and the lack of MR-compatible surgical equipment and tools, Morikawa said. On the other hand, it has more than enough advantages to make it worth the effort.
MRI has good soft-tissue image contrast, is free from ionizing radiation, has multiplanar and multislice capabilities, is useful even with bone and air space, and temperature changes can be monitored during treatments, according to Morikawa.
As a result, the use of MR interventional procedures is spreading, both with traditional closed-bore systems and newer open systems, he said.
The University of Minnesota in Minneapolis is using a 1.5-tesla MR scanner (Gyroscan ACS-NT, Philips Medical Systems, Andover, MA) for its interventional procedures. The surgery and imaging don't occur simultaneously, but the setup does have the advantage of a full-strength magnet.
The MRI suite is comprised of several adjoining rooms that make up the work space. During interventional procedures, the scanner table slides back and forth between the surgical area and the magnet, Morikawa said.
Tokyo Women's Medical University in Japan uses an "off-the-wall" MRI system for neurosurgery based on a 0.3-tesla Hitachi Medical Systems (Tokyo) scanner called the "hamburger" for its horizontal double-bun design, Morikawa said. The patient is accessed from a 43-cm gap between the magnets.
Morikawa's facility in Shiga stands the hamburger concept on its head with its use of a 0.5-tesla GE Healthcare (Chalfont St. Giles, U.K.) Signa SP open MRI scanner, dubbed the "double doughnut." The gap between the two magnets is 58 cm.
"Because of the configuration of our magnet, we can easily access the patient," Morikawa said, noting that continuous image guidance is possible due to MR's lack of ionizing radiation. "The surgeons can perform procedures in a natural position while monitoring MR images on the monitors."
There are two ways of placing the patient. With the front-load option, the table is placed through the two parallel doughnut holes, and can be accessed by two surgeons, one on each side. With the side-load option, used to access the parietal region of the brain or the pelvic region for prosthesis placement, the table is placed through the gap between the two magnets, and can be accessed by only one surgeon.
An MR-compatible microscope can be deployed between the magnets, and MR images can be acquired as needed during the procedure without moving the patient.
"With a custom-made boot, the pelvic region can be treated in the lithotomy position," Morikawa said.
A system comprised of LED panels paired with 3D detectors is used to track the path of a needle or other interventional tool, with MR images confirming the trajectory every two seconds. "Surgeons can interactively control the plane," he said.
The group has developed a number of handpiece adaptors to facilitate control and targeting of the needle in tight angles. Three varieties of the light, MR-compatible tools are the spherical, offset, and torch-shaped adaptors. They allow the interventionalist to hold the handpiece steadily to enable punctures or guidance without having the handpiece block the line of sight of the LED, or have the magnet get in the way.
"When the surgeons rotate the handpiece, the MR images also rotate, and images are updated every two seconds," Morikawa said. The torch-shaped adapter allows puncture access from a side angle, thereby creatively overcoming the limitations of the scanner configuration. In one case "a brain tumor in the parietal region of the vein was accessed by a puncture from the side using the torch adapter," he said.
Finally, Morikawa's group has developed MR-compatible endoscopic tools, including a nasal scope, thoracoscope, and laparoscope, all of which can be used interventionally under MR guidance.
The facility has used MR guidance, and its homegrown tools, in hundreds of procedures involving the breast, ear, nose and throat, orthopedic applications, and so far 212 microwave liver tumor ablations, a mainstay of the facility.
Innovative tumor ablation
Theromocoagulation of liver tumors has been under development for the past 25 years, and has now become the standard of care for liver lesions, which grow more common each year in the Japanese population, Morikawa said.
The group uses a microwave tissue coagulator (Microtaze, Azwell, Osaka) capable of irradiating microwaves at a frequency of 2.45 GHz, along with the open MRI system and an MR-compatible ablation electrode. A second "firefly" electrode determines the position of the ablative electrode.
Because the frequency of MR is far removed from that of microwaves, "we can observe noise-free real-time images, even during microwave irradiation," Morikawa said.
Software, also developed at Morikawa's facility, color-codes and tracks in 3D where the electrode has been placed and held, and which regions have not yet been ablated, ensuring consistency of results. A footprinting function notes the regions where the electrode has been placed, and biplanar temperature maps are used for temperature monitoring.
However, MR temperature monitoring is highly susceptible to motion, so respiratory image triggering is also used to eliminate the effects of breathing on image guidance with the patient under general anesthesia, Morikawa said.
One patient had a liver tumor just below the diaphragm, and "puncture of the abdominal wall was not easy," he said. "In this case thoracoscopic assistance was very useful. The thoracoscopic image and the MRI image were combined with the picture function of a video mixer."
In another case, an 8-year-old boy presented with a cystic lesion in the thoracic spinal cord. With the assistance of the thoracoscope, the venous plexus was dissected and the thoracic spine punctured under MRI guidance. A new set of multislice spin-echo images acquired the following day showed that the cyst size had decresed substantially, producing an excellent outcome for the patient with minimal invasiveness, Morikawa said.
"I believe this therapy can only be done under MR guidance," he said.
"MRI guidance is quite useful in many fields," Morikawa concluded. "Further development of MRI-compatible instruments and software is required."
An audience member suggested the use of MR spectroscopy to bring guidance down to the molecular level, particularly in brain lesions. Morikawa said he was working on a spectroscopy guidance study, but the 0.5-tesla field strengh of the scanner is too low for spectroscopy. "We need a fusion technique," he said.
By Eric Barnes
Auntminnie.com staff writer
June 29, 2006
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