A series of scans taken last autumn of my aortic artery concluded that I faced either grave danger -- or none at all. Aneurysm! Warning! A whopper! concluded the first test I took, a CT scan. Chill out -- there's no aneurysm, concluded the second scan, an echocardiogram.

You're both wrong, said the third, an MRI with contrast: There IS an aneurysm -- but it's not a whopper.

This debate would have been fascinating to witness if the artery in question hadn't belonged to me. Or if the artery had been a minor vessel -- say, the one that carries blood to my little toe. But this was my aorta -- carrier of blood to every organ in the body -- meaning that this battle of the scans bore life-and-death implications. Watching the scans bounce my prognosis from grim to great and back, I was bewildered.

This experience wasn't unusual. The great news is that aortic aneurysms -- an often-fatal condition that never showed up well on X-ray -- are captured easily on new scans such as CT, MRI and echocardiogram. The not-so-great news is that these scans aren't perfect at determining size -- and size is paramount when it comes to aortic aneurysms. Worse yet, human error can render any of these scans useless.

"What this underscores is that as a consumer, you're building a case," says Michael Dake, chief of interventional radiology at Stanford University School of Medicine. Building a solid case may require more than one scan and more than one radiologist.

The first scan I received was just a snapshot of my heart, taken to search for calcium in the coronary arteries. It found none of that, but caught a glimpse of the nearby aortic artery -- and what a whopper it appeared to be: 4.6 centimeters, or nearly two centimeters too large. Meaning: I had an aneurysm nearly large enough to be in danger of bursting. Yikes!

Except my internist wasn't convinced. To make sure, he ordered up a second scan, this one an echocardiogram. When this test came back negative -- No aneurysm at all! -- I did a little dance. Prematurely, as it turned out. Echocardiogram, it seems, is highly operator dependent. In the right hands, it can be more accurate than CT or MRI, and it has the added advantage of emitting no radiation. In the wrong hands, however, it's worthless.

Fortunately, my internist knew this, and he ordered up a third test -- an MRI. This differed from the original CT in that it involved 90 minutes of image-taking, instead of just a snapshot. In addition, I had contrast, or dye, inserted into my veins -- a factor that bolsters reliability.

The MRI confirmed the presence of an aneurysm but at 4.1 centimeters. A subsequent echocardiogram (this time administered by a pro) validated this finding, as did a second MRI.

So I'm proof that newfangled scans can detect a condition that not so long ago was undetectable, and that is more common than most doctors realize, killing more Americans each year than does AIDS and most kinds of cancer. For me, the drill now is to keep an eye on the aneurysm through twice-annual scans, looking in particular for growth. If the bulge reaches a certain size -- say, five centimeters -- then the risk of rupture will be greater than the risk of open-chest surgery to repair the aneurysm.

But can I rest assured that it isn't five centimeters now? Often, people who undergo aneurysm-repair surgery are told afterward that the bulge was much larger than any scan had shown. But Stanford's Dr. Dake is skeptical about some of these tales. Quoting a former radiological mentor, he says, "There's no such crude diagnostic instrument as the surgeon's hand and eye."