As George Bisacca, a conservator of paintings at the Metropolitan Museum of Art, drove to New York University Medical Center for a CT scan, he was feeling just fine. The “patient” being transported was a 15th-century painting by the Sienese master Sassetta.

The painting was at the center of a long-standing art historical conundrum, which Mr. Bisacca hoped the CT scan could solve. Scholars have suspected that the Met’s painting, which is in the Robert Lehman Collection, was originally part of Sassetta’s famous, but long ago fragmented, altarpiece from the Franciscan church of Borgo San Sepolcro in Arezzo. But there were problems with this theory, most importantly that the Met’s panel appeared to be on the wrong kind of wood.

The altarpiece, double-sided and about 10 feet wide by 16 feet tall, was commissioned in 1437 by Franciscan monks; it was the costliest of its kind in the 15th century. Sometime in the late 16th century, it was removed from the church, dismantled, and the panels sliced in half so that paintings on the front and back sides could be used separately. Today, those that survive — 27 out of an original 60 — are dispersed among two private collections and ten museums around the world, including the Met, the Louvre Museum, the National Gallery in London, the Pushkin State Museum of Art in Moscow, the Detroit Institute of Art, and the Cleveland Museum of Art.

For the last two years, a researcher at the University of Amsterdam, Machtelt Israëls, has led an international effort to reconstruct the altarpiece. She contacted Mr. Bisacca and asked him to do a fresh examination of the Met’s panel.

Because of a description of the altarpiece from the period, art historians have long believed that the Met’s panel, which shows an Annunciation, was originally the reverse side of a panel now at the Cleveland Museum of Art, which shows a Crucifixion. There were two problems. First, the two panels are different shapes. Second and more important, they appeared to be made of different kinds of wood. The Cleveland panel, like the rest of the panels from the altarpiece, and most Italian paintings of the period, is made of poplar. The Met’s is made of spruce. Because of this, some scholars doubted that the Met’s panel was part of the Borgo San Sepolcro altarpiece at all, rather than an orphan from another altarpiece, of which the rest has disappeared.

The curator of the Robert Lehman Collection, Laurence Kanter, explained that art historians have a tendency, right or not, to try to connect fragments of an artist’s work that survive. “When we have bits and pieces floating around, we tend to think it’s possible to link them together in some way,” Mr. Kanter said.

On examining the painting more closely, Mr. Bisacca developed a hunch that might explain the discrepancy of the Met panel being on different wood from the others.

“I noticed that in a strong, raking light I could see some distortions in the surface,” Mr. Bisacca said. “It made me think what happened was, after the Cleveland panel and our panel were sawn apart, ours was then further thinned down, to about three or four millimeters, and attached onto a new panel. Then they added modern gilt pilasters at the sides and a base at the bottom, so that you couldn’t see any lamination line where our picture was glued to a new backing.”

The only way to prove his hypothesis, Mr. Bisacca realized was to do a cross-sectional view of the wood with a CT scan. A colleague in the museum’s department of scientific research, Tony Frantz, referred Mr. Bisacca to Emilio Vega, who supervises a very sophisticated multi-detector CT scan at NYU Medical Center.

A CT scan works by sending a stream of X-rays from an X-ray tube on one side of the machine through the body (or painting) to a detector on the other side. Through the level of attenuation of the X-rays, the machine can create images of what is inside the body (or painting). The individual images are cross-sections, like slices of salami.

In the case of the Sassetta, Mr. Bisacca got exactly what he wanted from a scan conducted in March. “This machine is capable of taking an astronomical amount of data: It takes 1,800 images in a matter of seconds,” he said. “What we got is a kind of film, as if you made 1,800 cuts and were able to look at the wood on the endgrain as you went in — a little film seeing a cross-section as it moves through the panel.”

The film showed the lamination line where the poplar was attached to the spruce. “It also showed us the cut of the wood — a tangential section cut, which is a specific cut in the tree,” Mr. Bisacca said. “We were able to match that up with the Cleveland piece to show that they were in fact once the same piece of wood.” The CT scan of the painting “pretty much proved that it was the other half of the Cleveland panel.”

Mr. Frantz said in an e-mail that CT scanning has proven so useful in answering art historical questions — not only about the remounting of Renaissance panel paintings, but also about “the coherency of texture between rejoined parts of Southeast Asian stone sculpture [and] the fabrication methods of just about everything from early Chinese bronzes to modern European furniture” — that his department is hoping to acquire its own CT scan.

The dispersal of the pieces of the Borgo San Sepolcro altarpiece is not unusual. “Very few altarpieces from the 14th- and 15th-century are still in place today,” Mr. Kanter said.

When a town or family could afford to commission a new altarpiece for its church, it would either destroy the old one or dismantle it, putting the pieces to different uses. A panel depicting the name saint of a child in the family might be put in that child’s bedroom, for example. “By and large these things weren’t saved,” Mr. Kanter said. “They weren’t valuable as art, because they were old-fashioned, but they were valuable as gold or wood. So they were reused as table tops or doors, or burned to recover the gold.”

The researcher at the University of Amsterdam, Ms. Israëls, has invited all the people who have studied the panels at their respective institutions to a conference in September, at Villa I Tatti on the outskirts of Florence — Bernard Berenson’s villa, now run by Harvard University as a center for Italian Renaissance studies. A book on the reconstruction of the altarpiece will also be published next year. Ms. Israëls said that while many new tests have been done in the course of this project, the CT scan on the Annunciation was the most dramatic — including its satisfying result, she noted, that “the Virgin is as immaculate as we thought.”