Washington (CNN) -- Full-body imaging machines that see through clothes have significantly improved security in airports where they are deployed, and have revealed more than 60 "artfully concealed" illegal or prohibited items in the past year, the Transportation Security Administration says.

To date, no explosives have been detected by the machines, but their ability to spot even small concealed objects demonstrates their effectiveness as a security tool, officials said.

"It is absolutely a tremendous improvement of what we can detect at the checkpoints," TSA Acting Administrator Gale Rossides said this week. "It is an excellent piece of technology that will significantly improve our detection capabilities."

As evidence of the machines' capabilities, the security agency released five photos of drugs or suspected drugs that airport screeners found after scans revealed anomalies on the ghost-like images of people's bodies. The agency said metal detectors would not have revealed the items.

Screeners using the technology also found a knife hidden in the small of a person's back at the Richmond, Virginia, airport, a concealed razor blade on a passenger in Phoenix, Arizona, and other concealed items such as large bottles of lotion, which are prohibited as carry-on items.

In addition, the machines have revealed numerous prohibited items that passengers evidently inadvertently left in pockets. Those items are confiscated but are not counted in the tally, a TSA spokesman said.

U.S. to implement new airport security measures

The agency field-tested the full-body imagers for more than a year before announcing last month the deployment of machines to 11 airports nationwide. Today, 46 machines are in place in 23 airports, and the agency is stepping up deployments and plans to have about 1,000 set up by the end of 2011.

Interest in the machines has heightened since the Christmas Day incident in which a man allegedly attempted to detonate an explosive concealed in his underwear. In an appearance before Congress last month, Rossides declined to say whether the machines could have detected the underwear bomb.

But to illustrate the machines' effectiveness, Rossides showed a packet of white powder smaller than a tea bag, saying it was identical to a concealed bag detected by an imager.

"The amazing thing is that our officers, as they get more and more familiar with this technology, are actually finding very, very small things that are being secreted on the body," she said.

But some passengers say the machine's capabilities are presenting new Fourth Amendment questions about the government's searches, saying the machines -- in detecting very small objects -- are subjecting passengers to scrutiny beyond what is needed to safeguard the plane.

"I can't imaging an explosive that is powerful enough in that [tea-bag size] quantity to endanger an aircraft," said John Perry Barlow, a former Grateful Dead lyricist who once took the TSA to court after a search of his checked luggage revealed a small amount of drugs.

"Every time technology makes another leap forward, we have to reclaim the Fourth Amendment, and often we have to reclaim the entire Bill of Rights, because technology gives us powers that were not envisioned by the Founding Fathers," Barlow said.

The security agency said that it searches only for prohibited items -- not illegal items such as drugs. When it finds illegal items during a search, it refers the item to local law enforcement officers, it says.

"What we're trying to resolve is the anomaly that we're seeing on the body," said Rossides. "If it's drugs, then we call in local law enforcement and they handle it from there."

Barlow predicted that the body scanner will lead to another court case to clarify the extent it can be used to search the body.

"Eventually they're going to bust somebody for something that was clearly and obviously not a threat to the aircraft, and any reasonable person would have known that [while looking at the] body scan. And at that point somebody is going to make it an issue," he said.

Rossides said the body imagers are especially useful because they can expose contraband on parts of the body that aren't fully explored in pat-downs, such as the groin.

"I think what was so telling about the Christmas Day attack was that it exploited our cultural norms, that we don't frequently pat down persons in that part of the body. This technology will give us the image of the entire body," she said.

But Rossides said the imagers are not a "silver bullet" because "those who intend to do harm are constantly adapting."

"We still have to have multiple layers of security," she said.

For all his reservations about scanners, Barlow said he does not hesitate going through one.

"I've got nothing to hide," he said. "I go through the scanner. If anybody wants to see me naked, they're welcome to the sight."