imum-intensity, ray-tracing algorithm to provide the three-dimensional images of blood vessels" used in the MRI angiogram referred to above.

Contrast Agents

As David Stark observed, "Roentgen himself introduced the first contrast agents," and thus the principle of using contrast to enhance diagnostic images is as old as the x ray itself. "The general term" for the class of materials used, said Stark in his presentation to the symposium's scientists, is "diagnostic contrast media." They are pharmaceuticals that alter tissue characteristics [so as] to increase the information on diagnostic images. Each of the diagnostic imaging modalities—nuclear medicine, ultrasound, x-ray-based techniques—has its own family of dedicated contrast media."

The contrast agents used in MRI in certain situations can improve accuracy, reduce the number of possible diagnostic procedures, or shorten the course (and therefore the cost) of an MRI sequence. One significant distinction between MRI and CT contrast media is that with MRI the chemical agent is not itself being imaged. Rather, it produces an enhancing effect on one of the other MRI parameters already discussed, that is, relaxation. Stark pointed out that the key is to give a drug that goes to normal, but not abnormal, tissue and increases the signal difference between the two, thus giving a primary diagnostic test, which is to identify the abnormality as a result of the distribution of the drug. The fundamental point, he emphasized, is that these drugs can help us only if they target—or physically localize in—either the normal or the abnormal tissue but not both.

The only paramagnetic agent fully approved for clinical use by the Food and Drug Administration at this writing is gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA), although oral iron compounds and molecular oxygen also are being used in clinical studies. "In fact, some 30 percent of the patients who undergo NMR exams worldwide receive the drug," Stark said. Calling it, therefore, "a big ticket item," Stark reported that a number of corporations would like to break into this market, which "leads to some interesting issues in academia with sponsored research."

Stark explained that paramagnetic agents like Gd-DTPA serve to increase the parallel magnification of materials placed in the main field. "Paramagnetism is characterized by independent action of individual atomic or molecular magnetic moments due to unpaired electron spins" and "can occur in individual atoms or ions, as well as in collections of atoms or ions in solids such as ferritin and hemosid-

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