Looking inside rat lungs

A magnetic resonance imaging technique developed by Duke University Medical Center researchers has enabled them to visualise the lungs of live laboratory rats.

A magnetic resonance imaging (MRI) technique developed by Duke University Medical Center researchers in the US has enabled them to visualise the lungs of live laboratory rats as the animals receive a drug, called methacholine, used to test for asthma.

The non-invasive imaging method has enabled them to observe the airways in the lung as they respond to medication, providing the researchers with more detailed information about lung function than traditional diagnostic methods.

The ‘magnetic resonance microscopy’ technique enables scientists to zoom in on airways as small as 100 microns, about the size of a human hair. ‘We’re visualising the behaviour of the smallest airways, a critical element in understanding such pulmonary diseases as asthma,’ said G. Allan Johnson, PhD, director of Duke’s Center for In Vivo Microscopy.

The study results were prepared for presentation at the American Thoracic Society meeting in Seattle on May 21, 2003. The research was sponsored by the NIH National Center for Research Resources and the National Heart Lung and Blood Institute.

‘Methacholine challenge’ tests are a standard means of diagnosing asthma. The drug causes the airways of lungs in both humans and rats to constrict. Asthmatic patients’ lungs tend to overreact to the medication, an indication of the disease. Traditionally, a patient’s response to the drug must be determined indirectly by measuring the amount of air exhaled.

‘Using the traditional techniques, it’s hard to pinpoint the site of constriction in the lung to identify the triggers for an asthma attack,’ said Ben Chen, PhD, a research associate at the Duke center and lead investigator of the study. ‘The lung is a complex structure, yet air only enters and exits through the mouth and nose making lung measurement difficult. This imaging technique opens up a whole new potential for measuring local lung function. With this method, we can see regional changes in flow and determine visually which part of the lung is impaired.’

In combination with the latest molecular genetics techniques, the imaging method can aid in identifying the root causes of lung disease, Chen said. By homing in on the precise location of lung abnormalities, it might ultimately lead to more targeted treatments for asthma and other lung disorders, he added.

The researchers’ basic discoveries made in animal models of disease can be applied toward understanding human disease, Johnson emphasised.

To visualise air-filled lung structures, the new diagnostic method, called hyperpolarised helium magnetic resonance dynamic lung imaging, uses helium gas that is excited, or ‘hyperpolarised,’ with high power laser beams. Conventional MRI scanners, in contrast, work by measuring the behaviour of protons in water in tissues. Proton imaging alone isn’t an effective means of visualising internal lung structure because the organ’s volume is primarily made up of air.

The supercharged helium is inert and can be safely inhaled into the lungs, said the researchers. The helium-filled lungs can then be scanned to create dynamic images as air is inhaled and exhaled – highlighting abnormalities in particular lung regions. Earlier work with the technique revealed early stage emphysema in rat lungs before symptoms became apparent.

In the current study, the Duke researchers placed rats on a ventilator designed to deliver a constant flow of oxygen and hyperpolarised helium gas mixture. Each rat breathed the helium mixture while being imaged on one of the Duke Center’s MR microscope systems.

The MR microscope delivers images at a spatial resolution more than 500 times that of clinical MRI scanners, said Johnson. The researchers scanned rats both before and after a dose of methacholine was administered.

On a computer screen, the researchers observed directly as the major airways in the rat lungs bulged and peripheral airways constricted in response to the methacholine.

What’s more, by overlaying proton and helium MRI scans, the team could identify air trapping in the lung, a common reaction when airways narrow and stiffen, Chen said. Standard MRI highlighted the structure of the lung in broad outline, while the new method revealed only lung portions filled with fresh air spiked with helium. The difference between the images indicated that air had become trapped.

The next step, the researchers say, will be to apply the method to study mice genetically altered to have asthma.