September 27, 2013 7:24 pm

The Doctor: Why X-rays are a grey area

‘Misinterpretation can be disastrous. You need to train your eye to take exactly the same route each time’

Because I knew Mrs H didn’t think much of doctors, I spent a little extra time examining her, hoping to impress her with my thoroughness. After measuring her oxygen saturations and listening to her lungs, I put my spread hands on her back and watched to see how far my thumbs moved apart when she inhaled – the test for lung expansion. You don’t often see doctors doing this test outside of clinical exams (nor hand-washing either, a cynical nurse friend once observed). Even though everything was normal, I went on to percuss her all over, front and back, tapping away like a woodpecker. Mrs H looked at me with pity. “Why don’t you just do an X-ray?” she said.

I felt the same when I started medical school. The consultants’ obsession with history-taking and examination seemed courtly and old-fashioned; what need for chatting and tapping when a CT scanner could reveal a patient’s innards in wafer-thin segments faster than the ham slicer at the deli counter? Who cares about the nature of a cough when we can look through the chest and see the real-life bronchi instead?

Then we discovered what the pictures looked like. The cartoon depiction of an X-ray is broadly true – bones look white (ish), air looks black. But there are endless shades of grey in between. Muscles are one shade, fluid another, pus a third, blood a fourth. There are no bright little arrows indicating a tumour or infection. Because of the smudgy grey-on-grey effect, it is difficult to make out the edges of things, unless they are foreign bodies, in which case they stand out as brightly as a contraband bottle on a baggage-handler’s X-ray. Look! You can identify a safety pin, a crucifix, and a nipple ring!

Sadly, this is not of much clinical use. Everyone spends their four or five or six years of medical school learning to read chest X-rays, and on the ward at midnight there will still be a group of junior doctors staring at the image on the computer, trying to determine what they can see: is it heart failure (the grey is fluid) or pneumonia (the grey is pus)? Does that other grey area look “fluffy” (suggesting infection) or “lacy” (suggesting fibrosis)?

There is, of course, a whole profession devoted to deciphering radiological images – and much of your time in hospital will be spent begging radiologists to help you – but plain chest films are one of the few types of medical image that every doctor is expected to be able to read. However, they can still be tricky. Misinterpretation can be disastrous. When you learn to read an X-ray, you learn to do it in a certain way. Everyone’s certain way is different (some people look at the bones first, some the lungs; some divide the image into eighths and study each quadrant in turn as though they’re working across an actual chest with a stethoscope). Whichever pattern you follow, you need to train your eye to take exactly the same route each time, so you never miss anything out. And you mustn’t get too lost in detail, or too distracted by ghost safety pins, or give up once you’ve spotted the first abnormality.

Some things only appear when you’re not looking too hard, like the shapes that emerge in Magic Eye paintings. Little tumours hide in the increased whiteness at the lung apices. Free air from a leaking gut rises to form a fine black line under the diaphragm. The shadow of the heart draws your eye; it is easy to miss the unexpected bulge of an aneurysm behind it. The owner of one chest X-ray had a pain in his chest: he was given antiplatelet drugs to treat a diagnosis of heart attack. They made his aneurysm bleed, and he died.

Sophie Harrison is a hospital doctor in South Yorkshire

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