New medical imaging modality arises from hard work, inspiration

This month's cover story ("MR elastography inspires new wave of hepatic imaging," page 20) is devoted to a rare event: the birth of a new imaging modality. MR elastography is a wonder of human ingenuity that employs MRI, a modality that itself still seems like a miracle. Few physicians and scientists could have imagined in the era of Drs. Jonas Salk and Michael DeBakey what it would be like to noninvasively examine the human body with the detail that MRI brings.

The basic concept of elastography is easy to fathom. The idea that different substances have different elastic properties is understandable to any child who has shaken a bowl of fruit suspended in gelatin. But exploring this concept was more than an intellectual exercise for Dr. Richard Ehman. From the start in the early 1990s, Ehman, the inventor of at least 30 imaging-related devices, aimed at developing an MR technique that could quantify and image the mechanical properties of tissues.

The process was not easy. Ehman and colleagues at the Mayo Clinic in Rochester, MN, experimented with the basic concept for two years just to demonstrate it could be done. His group described the proof of principle in a Science cover story (1995;269:1854-1857).

Often, a hypothesis for a given experiment to develop MR elastography would be right, but the methods for testing them would be wrong. Ehman and colleagues were persuaded to move in unproductive directions, consuming months of time before they found a way back on course.

They nearly rejected an early plan to focus on liver imaging after mechanical waves would not propagate through liver tissue specimens. It was only after Ehman tested the liver in vivo that he discovered, five years later, that live liver tissue is well suited for elastography.

"You really need to be persistent," he said in a conversation with Diagnostic Imaging. "You have to have a degree of faith that the original idea is a workable idea."

About 40 physicians, scientists, postdoctoral fellows, and medical students-mainly working in the Ehman MRI lab at Mayo-contributed to MR elastography's development. The group included radiologists, oncologists, mammographers, nephrologists, gastroenterologists, hepatologists, musculoskeletal biomechanics specialists, sonographers, and acoustic physicists.

They faced three main technical challenges. One was the design of nonferrous devices that could generate a reproducible pattern of mechanical waves in targeted organs during MRI. The second involved a strategy to acquire imaging data from the waves, and the third pertained to processing the data to produce clinically relevant elastograms.

Wave generation was the hardest problem to crack, Ehman said. After failed experiments with electromechanical devices, the group adopted a passive/active driver technique. Sound waves were transmitted from an acoustic speaker outside the imaging suite through a tube to a passive device that oscillated on the patient's abdomen.

A new branch of imaging algorithm development grew out of the data processing effort. The research group knew it was theoretically possible to process acoustic data to produce an elastogram, but the mathematical calculations did not exist until Armando Manduca, Ph.D., head of the Biomathematics Resource at Mayo, created them. At the Naval Research Lab in Washington, DC, Anthony Romano, Ph.D., contributed knowledge first developed for defense applications on the high seas to help Ehman improve the acuity of his acoustic wave measures.

The process of MR elastography's development involved a few parts inspiration and many parts perspiration for Ehman and his colleagues. But now Ehman's original vision appears finally to have been realized.