Novel MRI Technique Provides Clear Images Of Blood Flow
DURHAM, N.C. -- Duke University Medical Center researchers have created for the first time moving images of blood traveling through vessels, non-invasively and without the use of contrast agents or radiation. They used a novel application of magnetic resonance imaging (MRI) technology.
Just as importantly, the researchers said, this technology can easily be applied to existing MRI machines, since the advances reported by the Duke team do not involve new hardware, but are rather the result of new conceptualization of the technology.
MRI uses harmless magnetic fields and radio-frequency signals to image tissues in the body. Basically, the magnetic fields cause hydrogen nuclei, or protons, that are part of water molecules in tissue to align. Pulses of radio frequency waves perturb this alignment, and the molecules give off telltale signals as they lose energy. The signature of such water molecules differs according to the tissue, providing the contrast that is a key to MRI's ability to sensitively image tissues.
The new approach, called global coherent free precession (GCFP), allows researchers to selectively "tag" protons within the water of blood cells with radio frequency waves as they pass through the plane of the MRI scan. Since all other tissues surrounding the blood do not pass through the scanner's plane, they are not tagged, leaving images solely of the blood as it moves downstream through the vessel.
The results of the Duke experiments, which will appear in the May issue of the journal Nature Medicine, were posted early on-line April 4, 2004.
Lead researcher Robert Judd, Ph.D., co-director of the Duke Cardiovascular Magnetic Resonance Center, recently received a grant from the National Institutes of Health to further study the MRI physics of the new phenomenon. Judd collaborated with Wolfgang Rehwald, Ph.D., a physicist with Siemens Medical Systems, manufacturer of the MRI equipment, as well as Duke researchers Raymond Kim, M.D. and Enn-Ling Chen, Ph.D.
"While further work is necessary to refine this new approach, GCFP already represents the only diagnostic technique capable of examining the functional effects of cardiovascular disease with real-time physician-scanner interaction, without an invasive procedure, without a contrast agent, and without radiation," Judd said.
The problems presented by contrast agents can be potentially significant, Judd explained, since these agents can cause kidney damage, and many patients with cardiovascular disease already have weakened kidneys due to their disease.
Currently, physicians wanting to see images or potentially blocked vessels typically use X-ray angiography. In this approach, a contrast agent is injected into the blood stream, and a series of X-rays are taken at the site of interest. These images are then assembled by a computer and the result is a short movie known as a cine (pronounced sin-ee).
"In our new approach, the act of MRI scanning itself excites protons in blood cells as they pass through the plane of the scan," Judd explained. "They are still excited as they flow downstream and the scanner can detect that signal. So the scanner is simultaneously tagging protons and collecting data."
During an MRI examination, a patient is guided through the cavity of a large doughnut-shaped magnet. The magnet causes hydrogen nuclei in cells to align, and when perturbed by radio waves, they give off characteristic signals, which create thousands of "cross-sectional "slices." These slices are then converted by computers into three-dimensional images.
While MRI technology itself is 20 years old, only in the past few years has technology improved to the point where accurate images of moving tissues can be taken. It was while studying these images of the heart and surrounding tissues that Judd continued to notice strange anomalies, or artifacts, appearing in the periphery of the scans. These artifacts often looked like pulsating spots of light.
"When we looked into it further, we found that these artifacts occurred in 5 to 10 percent of the scans," Judd explained. "After more research into the phenomenon, we decided to try to harness the effect rather than get rid of it."
As it turned out, the strange images appeared whenever a blood vessel ran perpendicular to the plane of the scan. So the researchers rotated the scanner, adjusted the radiofrequency of the scanner and developed a complex new way of capturing the images.
"As the blood cell flows through the plane, it is excited by the MRI," Judd explained. "While the next cell is also excited when it flows through, the first cell still gives off a radio-frequency signal that we can detect, on so on. We receive a continuous image of the blood flow."
The blood remains "excited" for about 13 centimeters from the point of excitation, Judd said.
For Judd, the new approach gives clear three-dimensional information on two important aspects of cardiovascular disease -- the actual anatomy of the vessel, as well as the speed and direction of blood flow.
"Information on flow is important because anatomy and function are not always related," he explained. "In one patient, a 70 percent blockage of an artery may hinder blood flow enough to cause cell death, while in the next patient, due to subtle differences in the three-dimensional shape and length of the blockage, there may not be the same problem."
The current studies were performed on the aortas of human volunteers. The aorta was chosen for the initial studies because it is one of the largest vessels in the body and it is easily accessible.
Judd sees one of the immediate uses of the new technique on the renal arteries. Constrictions in the renal arteries are common causes of hypertension, and since the new technique does not use contrast agents, patients should be able to better tolerate the procedure, Judd said.
"Since GCFP can image both the magnitude and direction of blood flow, we can also use this new approach to assess the patency of grafts after bypass surgery, shunt procedures and points of attachment of blood vessels," Judd said.
"There is also reason to believe that GCFP might ultimately play an important role in imaging the coronary arteries that supply the heart with blood," Judd continued. "Because of the small size of these arteries and the complex topography of the heart's left ventricle, more research is needed to get accurate and useful images."