CT Scans Aid Lung Injury Diagnosis, Management

October is Healthy Lung Month. . . . .

The increased use of computed tomography (CT) scanning has led to better understanding, diagnosis and management of acute lung injury (ALI), known in its more severe forms as acute respiratory distress syndrome (ARDS). And faster, more advanced CT equipment is now providing an even better computerized view of ALI/ARDS.

Lawrence R. Goodman, MD, FACR, Medical College of Wisconsin Chief of Thoracic Imaging and Professor of Radiology, collaborated with experts at an institute in Milan, Italy, on a paper about CT and ARDS published in the American Journal of Respiratory Critical Care Medicine in 2001. Dr. Goodman discussed lung injuries gave us a recent update on the current role of CT in ARDS.

“ARDS is reaction of the lung to many different serious injuries,” said Dr. Goodman. “Some of the serious injuries are directly to the lung, such as pneumonia or inhalation of toxic substances like smoke or ammonia. Sometimes ARDS is due to things that occur outside the lung and affect the lungs indirectly, such as sepsis, which is a blood-borne infection, or prolonged low blood pressure from blood loss or trauma.”

“Regardless of the cause, the capillaries of the lung, where the blood meets the lung to be oxygenated, become leaky. Instead of separating the blood from the lung they allow fluid to leak out into the lung, so the gas exchange is more difficult.”

Going Beyond Chest X-Rays in the ICU
“For years the only way to look at ARDS to see what was going on had been the chest X-ray,” said Dr. Goodman. “That gave us a moderate amount of information, but there were many gaps. The CT, which allows us to look at the lung in cross-section rather than from the outside, has helped us to understand physiologic abnormalities in the lungs and make a more accurate diagnosis.”

For example, before the emergence of CT scanning in these conditions, it was believed that ARDS affected all parts of the lung equally. “With that belief, artificial ventilation (breathing with the help of a machine) was set up in a way that assumed the entire lung was involved, or uniformly involved,” said Dr. Goodman. “The CT has shown us that the chest X-ray was not giving us the whole picture – the lung was not uniformly involved.”

“There are areas of the lung that are severely affected by an injury, areas that are mildly involved, and areas that are not involved at all. So the type of artificial ventilation that was used to keep the patients alive in the ICU (intensive care unit) was not optimal. The CT has helped us to modify the way ventilation is given.”

“CT also showed us that when patients are lying on their backs, as they tend to do in the ICU, most of the disease occurs along the back, partially because of gravity,” said Dr. Goodman. “It helped tell us that if you turn the patient face down, some of that disease in the back – which is really in part a collapse of the lung, opens up and it disease may shift to the front of the lung.

“That’s important for two reasons. One, it shows us that some of the disease is related to lung collapse rather than inflammation or edema in the lung. Second, some people are now using this discovery therapeutically. If patients are not responding well to being on a ventilator while lying on their backs, some facilities are turning the patients onto their bellies for a few hours a day to try and shift the disease to different places and open up other parts of the lung. This has proven to be variably effective, certainly not uniformly effective. It’s not universally accepted and not universally done, but it seems to me that there will be a subgroup of ARDS patients who may benefit from this kind of variation in positioning.”

How Many “Slices,” How Fast?
The role of CT scans in diagnosis and management of ARDS patients is still growing, in some ways moving in step with improvements in the technology and the now-widespread availability of modern CT equipment.

“Patients who are on the ventilator and in the ICU get secondary complications of their ARDS,” said Dr. Goodman. “They get infections, they get air leaks in their lungs, they get abscesses, they get fluid around the lung. Very often these are more devastating than ARDS itself. CT is much, much better in illustrating these abnormalities than was the chest x-ray. So, CT is very, very helpful and used on a daily basis now. For patients who are on a ventilator and are not responding the way one would expect, CT is used to look for some of these complications.”

CT scans have been used with ARDS patients for more than ten years. They offer a more dimensional view than “one shot” x-rays by applying radiation to take a picture in a different way. CT scans use more radiation than simple x-rays and can provide a varying number of “slices” that add up to an image that can be interpreted with a high degree of clarity.

“CT scanners are now faster,” said Dr. Goodman. “Patients with ARDS, of course, can’t hold their breath well. With the older scanners it would take eight seconds, ten seconds, fifteen seconds to scan the patient, per slice, so there was a lot of blurring and respiratory motion. Now you can scan the patient, per slice, in five-tenths of a second. You get rid of a lot of the blurring artifacts and get a better understanding of what’s going on.

“It’s like taking a photograph of a moving car with a fifteen-second exposure versus a tenth of a second exposure. With the newer scanners you can now scan up the entire lung in about four seconds rather than twenty-five to thirty seconds. Researchers can do better explorations of how the lung responds to ventilation and the different maneuvers in real time. Also, computers are so much more powerful that we can now turn to computer analysis of what’s going on in the lung rather than just looking at images and saying ‘this is what I think is going on in the lung.”

Most large hospitals now have state-of-the-art CT scanners or will within the next year or two, said Dr. Goodman. “There have been several generations of scanners. Around 1992 the first helical, continuous scanners came out, so you didn’t have to scan, stop, move the patient, scan, stop, and so on. Toward the end of the 1990s, instead of having one CT detector, they went from four, and then to eight, and then to sixteen detectors. Now you can scan a patient sixteen times as fast as you could in the early 1990s – and they’re talking about going to thirty-two. Every few years the technology gets better.”

Dan Ullrich
HealthLink Contributing Writer

MCW Health News presents up-to-date information on patient care and medical research by the physicians of the Medical College of Wisconsin.