Tuesday, February 12, 2008

Darwin and Radiology

If radiologists imaged promiscuously up and down the animal kingdom, evolution would be a bit more obviously germane to my specialty. However, most of us don't actually image around a lot outside our own species. Instead, we spend most of our professional lives looking at one particular group of apes, which suppose themselves to already be the pinnacle of creation. How, then, can Darwin and evolution be relevant in the medical imaging of these uppity primates?

The short answer is that we visually confront the consequences of evolution daily in radiographs and other images of our patients.

When a birth defect is first discovered in a child, radiology is often one of the next steps in the diagnostic workup. Defects in one body part are often accompanied by others, and X-rays, CT, MRI and other imaging methods are used to help spot these. We've learned a lot about this clustering of anomalies by studying the development of gill arches in embryos from humans, sharks and other species. A defect in the first gill arch, for example, may result in a child with a tiny jaw and non-functioning ears.

Some body parts develop via a convoluted patchwork process that seems more worthy of the Three Stooges than any "intelligent" designer. The inferior vena cava is a great example of this. In several mammals, this large vein is formed between the sixth and eighth weeks of gestation by sequential formation, anastomoses, and regression of three paired veins. Variations in the development of these veins can lead to all sorts of wacky anomalies. Most of these venous variations are well-visualized with CT.

Another problematic legacy from the past is our spine. Our vertebral bodies are separated and cushioned by a series of discs, which are the remnant of our embryonic notochord -- a structure we share with creatures all the way back to Amphioxus. A tear in the fibrous lining of a disc can allow the gelatinous contents to extrude out of the disc, where it may cause painful pressure on spinal nerves. Neil Shubin summarized it thusly:
When we injure a disk, a very ancient part of our body plan is rupturing. Thanks a lot, Amphioxus.
On the topic of Amphioxus, I can't resist adding the following lines performed by folk musician and marine biologist Sam Hinton, who sang these words to the tune of "It's a Long Way to Tipperary":

It's a long way from Amphioxus,
It's a long way to us,
It's a long way from
To the meanest human cuss.
It's good-bye, fins and gill-slits,
Welcome, lungs and hair!
It's a long, long way from
But we all came from there.
Traces of an inner fish are not only present within our patients, but within us radiologists as well. The eyes we use to view an X-ray have built-in image-processing that goes way, way back in the evolutionary tree to early sea dwellers. As I posted earlier, the human retina performs this imaging-processing before the image even reaches the brain. The edge-enhancement that occurs in the retina can help a radiologist to spot an abnormality. Unfortunately, this phenomenon can occasionally make us perceive abnormalities which actually aren't there.

So, Happy Darwin Day! I'll be spending mine ostensibly looking at human images. However, I'll also be seeing bits of a lot of other creatures today, even though I don't mention any of them on my official reports.

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