Inside out: CT scans and your body

Why would you gratuitously expose your brain or your thorax to a big dose of radiation?

I was reading a book on the train when I began to feel watched. The man in the next seat was reading my book too. I was hoping that Human Cross-Sectional Anatomy would improve my understanding of medical imaging. Each page showed a computerised tomography (CT) image with a photograph of the corresponding segment of a cadaver positioned alongside it. Arrowheads identified various key structures such as “ascending aorta”, “pancreatic duct”. I was looking at a coronal view of a human face: to visualise this plane, imagine an Alice band becoming a cheese wire and sinking through the head to the chin. The cadaveric section had more holes in it than a slice of Emmenthal: the matching CT image looked like a doily. It’s easy to forget how much empty space there is behind one’s face: air passages, sinuses. “It looks like The Scream,” said my neighbour.

CT allows us to see more of what’s inside than plain X-rays. A CT scanner is a white ring like a giant polo mint. The patient lies down on a couch which the radiographer slides into the ring, positioning it over whichever body part she wants to scan. The polo contains a rotating X-ray source; spinning round the body, it takes pictures from every angle, which a computer then composits into detailed cross-sectional images. A CT scan gives a clearer picture of tissues that are only just visible or invisible on an ordinary X-ray: the lungs, the guts, the gallbladder, the brain. CT’s ability to show brain tissue makes it a first-choice investigation when you suspect your patient has had a stroke, although imaging in such perilous situations is why the CT scanner sometimes used to be called “The Doughnut of Death”. The old nickname highlights the need to balance diagnosis with care: your patient must be well enough to cope with the investigation. A CT involves an excursion to the X-ray department and then a period of relative solitude in the scanning room (although this is very much briefer than it used to be). This is why the first question the radiographer asks is “Is your patient stable?” (meaning, are you going to kill them in your well-intentioned effort to find out what’s wrong?).

CT images come in “slices” which are produced as the scanner inches along the body (doctors all seem to describe this as “a bacon slicer”, which communicates the idea even while it conjures a slightly unsettling picture). The pictures are sophisticated but they’re still X-rays; and they’re composed of 50 shades of grey. Even if you’ve read the whole of Human Cross-Sectional Anatomy you need a highly trained eye to decipher them, which is why it is usual to wait for the radiologist’s report before acting on anything.

A while back there were a number of companies selling the opportunity to have your whole body imaged in this way – an odyssey through the polo mint. This was marketed as a useful screening test, despite breaking every rule of useful screening tests. Full body CT scans are not specific: they’re not targeted at any particular condition. And they’re not sensitive, that is, they’re imprecise (a “nodule” seen on a CT scan could be cancer, or something entirely different). And the benefits do not outweigh the risks. A standard chest X-ray exposes you to 0.02 millisieverts of radiation; a CT head, by comparison, gives you two millisieverts, a hundred times the dose. You’re quite well at the moment, with no or few symptoms: why would you gratuitously expose your brain or your thorax to a big dose of radiation?

Sophie Harrison is a hospital doctor in South Yorkshire. This column appears fortnightly

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