Even when e-cigarettes are nicotine-free, they have an immediate effect on blood vessels, a new imaging study shows.

"E-cigarettes have long been marketed as a safer alternative to regular tobacco smoking," said Dr. Marianne Nabbout, a resident at University of Arkansas for Medical Sciences who led new study to identify the effects of vaping, with and without nicotine.

The research, conducted at the University of Pennsylvania, included 31 healthy vapers and smokers ranging from 21 to 49 years of age.

Each underwent an MRI twice. They were imaged both before and once after using three types of smokes: a tobacco cigarette, e-cigarette aerosol with nicotine and e-cigarette without nicotine.

A cuff was placed on their upper thigh to restrict blood flow. Once it was deflated, researchers measured the speed of blood flow in the large, femoral artery and how much oxygen in the blood returned to the heart after supplying it to tissues.

A special type of MRI called phase-contrast was used to measure blood flow in the brain.

Then, researchers compared their results to scans of 10 people ranging from 21 to 33 years of age who don't smoke or vape.

The findings were telling: After inhaling each type of vape or smoke, blood flow in the femoral artery slowed significantly. This artery, which runs along the thigh, supplies oxygenated blood to all of the lower body.

The decline in vein function was greatest with nicotine-containing e-cigarettes. E-cigarettes without nicotine were close behind.

Researchers said vapers also had less oxygen in their veins – suggesting an immediate drop in oxygen uptake after vaping.

She and her team plan to present the findings on Dec. 2 in Chicago at a meeting of the Radiological Society of North America.

"If the acute consumption of an e-cigarette can have an effect that is immediately manifested at the level of the vessels, it is conceivable that the chronic use can cause vascular disease," Nabbout said in a meeting news release.

Her advice for users? "Refraining from smoking and vaping is always recommended," she said.

Research presented at meetings is typically considered preliminary until published in a peer-reviewed journal.