11/27/95
STANFORD -- Stanford researchers have shown that magnetic resonance imaging (MRI) may provide a non-invasive alternative to current methods for diagnosing blockages and other serious problems in the coronary arteries.
Standard MRI techniques cannot produce sharp images of the coronary arteries, which are located in the chest. The techniques require several minutes of exposure time, so do not work well in the chest, which continually moves as a patient breathes. In addition, the arteries are difficult to distinguish from surrounding structures in normal images.
But postdoctoral student Todd S. Sachs, electrical engineering Professors Dwight G. Nishimura and Albert Macovski, and Bob S. Hu, assistant professor of cardiovascular medicine, have overcome the obstacles involved by combining two advanced imaging techniques. Their work was reported by Sachs at a meeting of the Radiological Society of North America in Chicago on Monday, Nov. 27
According to the scientists, their research shows that MRI can resolve features of the coronary arteries in sufficient detail to reveal problems. But the researchers have not directly compared their MRI images with current diagnostic methods to determine MRI's clinical usefulness. If such tests do demonstrate its value, the combined techniques could be implemented relatively swiftly. They do not require changes in MRI hardware, only in the way that the images are taken and processed.
If proven effective, MRI would have substantial benefits over the current method for diagnosing coronary artery disease. The standard approach involves inserting a catheter into the femoral artery at the groin, then snaking it up through the aorta and into each of the coronary arteries. Once the catheter is in place, a material, such as iodine, that is opaque to
X-rays is squeezed into the arteries and X-ray images are taken.
MRI is a non-invasive way to study what is going on in the body. It relies on the fact that when a body is placed in an external magnetic field, its atomic nuclei absorb radio waves in certain precise wavelengths. This effect is used to create images of living tissue. But it has a number of limitations, including distortion due to a patient's movement.
A common approach used to overcome motion distortion has been to reduce the scan time to the length of time that patients can hold their breath. This reduces the resolution of the images, however, and requires the patient's cooperation.
The Stanford researchers have developed a different approach to the motion problem that they call a "diminished variance algorithm." This method reduces motion distortion during scan times that are equivalent to those used in normal MRI imaging. MRI images are built up through a series of data lines. Groups of data lines are called frames. The researchers developed a test to determine the degree of motion distortion in each frame. After acquiring an initial image, all the frames are tested for distortion and the worst frame is rescanned. This process continues until all the frames are distortion-free or the scan time runs out. The method was reported in the September 1995 issue of the journal Magnetic Resonance in Medicine.
In addition to this "motion artifact reduction," the researchers used a second technique called selective inversion recovery (SIR) to make the coronary arteries stand out clearly. SIR was developed by Dwight Nishimura and Samuel Wang, a former student of Nishimura's with a doctoral degree in electrical engineering who is currently finishing his M.D. at the Stanford Medical School.
SIR employs radio waves to "tag" the blood in the aortic root before it flows in the coronary arteries. Tagging involves changing the spin of the atomic nuclei in the blood so that it is opposite that of the other tissues in the body. That allows the researchers to image just the tagged blood as it flows through the coronary arteries, making them stand out clearly.
Two images demonstrate the ability of MRI to image the coronary arteries. The first image shows the heart on the left. The coronary arteries are the "y-shaped" feature extending horizontally from the heart on the right. The second image shows the coronary arteries. They stand out brightly because the blood in them has been specially "tagged" to show up in the MRI scan.
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