Minimally invasive procedures involve small incisions, tiny cameras, and miniature fiber-optic flashlights. They reduce trauma, blood loss, and recovery time for patients. Their success hinges on novel imaging systems that clearly reveal both soft and hard tissues inside the patient.
X-rays and magnetic resonance imaging (MRI) are two of the most common imaging modalities available today. X-ray imaging reveals hard materials, such as bones and surgical tools. However, it cannot image soft tissues like internal organs and blood vessels, which can be seen only if a special dye is injected prior to imaging.
MRI, on the other hand, reveals intricate details of soft tissues and vessels but doesn’t show tool movement accurately inside these tissues. Because MRI and X-rays complement each other, physicians have wanted to integrate the two types of imaging – but a fundamental law of physics, namely the effects of magnetism on moving electrons, has gotten in the way.
Recently, Dr. Rebecca Fahrig, assistant professor of radiology at Stanford University School of Medicine, and her colleagues discovered how to overcome these effects. Their efforts have resulted in an imaging system that completely integrates MRI and X-ray imaging.
To develop the hybrid system the team had two problems to solve. The first involved designing a system in which the high-energy electrons needed to generate X-rays would remain on course and hit their metal target thereby creating the X-rays. Dr. Fahrig and her colleagues used modified MR equipment that is “particularly well-suited for the close integration of the two systems,” she says. Most MR imagers contain a wire coil wound around a long cylinder that fully encloses a patient’s body. When electricity passes through the cylinder, a magnetic field is generated. The hybrid system uses two coils lying parallel to each other, with an X-ray tube placed between them. This arrangement aligns the electric field of the X-ray tube and the magnetic field of the MR system in parallel, which reduces unwanted interactions between the two, Dr. Fahrig says.
The second problem – that the image produced by X-rays in the image detector gets distorted before the image is recorded – was solved by new digital flat-panel detectors. These X-ray detectors eliminate the long distances electrons travel through a vacuum in a conventional detector. Now, they travel less than a millimeter through a solid medium, which makes them essentially immune to magnetic fields. “It means that you can place the flat-panel detector in the magnetic field,” Dr. Fahrig says. “That opened up a whole new range of possibilities as far as the positioning of the X-ray system relative to the MR system.” No longer do physicians have to shuttle patients tens of feet between separate X-ray and MR systems.
The team performed a series of experiments to determine how electrons would be affected by the magnetic field in the new system. They found that, although the magnetic field does disturb electron paths in the X-ray tube, they’re not affected so severely that they damage the X-ray tube or miss hitting the metal target. The resulting X-ray image is slightly blurred, but it’s still entirely suitable for clinical uses, Dr. Fahrig says. “In general, placing the tubes in the magnetic field was easier than we expected.”
Upgrading the System
Dr. Fahrig and her colleagues first implemented a prototype of their system at a Stanford hospital more than three years ago, and they’ve since installed a completely upgraded system. So far, physicians have used the system to perform about 30 different procedures, Fahrig says.
Over half the procedures involved placing shunts in patients with cirrhosis of the liver. In this condition, accumulated scar tissue prevents blood from flowing through the liver. Doctors bypass damaged tissue by placing shunts between the portal vein that feeds blood to the liver and blood vessels inside the organ. The hybrid imager allows surgeons to hit the right vessel almost every time rather than puncturing liver tissue repeatedly to place the shunt.
The researchers are now working on a next-generation machine, Dr. Fahrig says, aiming for an improved X-ray tube, higher magnetic field strength, and imagers that can rotate around the patient, which will allow hybrid imaging at any angle in the body. With these enhancements, the system will image faster and gather more physiological information, Dr. Fahrig says, which will make it useful for other clinical procedures, such as real-time cardiac imaging.
Research for the hybrid imaging system is funded by the National Institute of Biomedical Imaging and Bioengineering (NIBIB), the Lucas Foundation, and was aided by a collaboration with GE Healthcare.
Fahrig R, Wen Z, Ganguly A, DeCrescenzo G, Rowlands JA, Stevens GM, Saunders RF, Pelc NJ. Performance of a static-anode/flat-panel x-ray fluoroscopy system in a diagnostic strength magnetic field: A truly hybrid x-ray/MR imaging system, Medical Physics 32, 1775-1784, 2005.