Re: HEALTH: Diagnostic imaging details urgently desired

From: Geraint Rees (
Date: Fri Jan 18 2002 - 03:28:10 MST

On 18/1/02 4:06 am, "Technotranscendence" <>
> You should worry seriously about radiation dosage. Many of these
> devices are really not meant for diagnosing asymptomatic people. You
> might be putting yourself in for unhealthy doses of radition just to
> satisfy your curiosity. Is the benefit worth the risk? Has anyone done
> studies?

There is no scientific evidence (that I am aware of) to justify recommending
whole body CT screening for patients with no symptoms or a family history
suggesting disease. There is no evidence that whole body CT scanning is
effective in terms of reducing total health care costs; nor that it is
useful in prolonging life.

In addition to their specific risks, screening procedures with low
specificity (like CT) can also expose patients to the risks of further
diagnostic investigations after a false-positive finding.

The risks of radiation are well researched but (as you might expect) highly
controversial. Sticking to dose limits, here's the current UK guidelines:

1. for employees aged 18 years or over, 20 millisieverts in a calendar year
2. for trainees, 6 millisieverts in a calendar year; and
3. for any other person, including members of the public and employees under
18 who cannot be classed as trainees, 1 millisievert in a calendar year.

These are the limits for healthy employees and the general public (obviously
not patients, who may require a diagnostic investigation due to disease).
Environmental exposure to radiation is around 3mSv/annum but can rise (e.g.
in Cornwall, where there is a lot of granite, it can be around 10mSv/annum).
Finally, some occupational groups (e.g. aircrew) get significantly higher

To put this in the context of CT, a typical CT scan of the chest gives a
radiation dose of 8mSv which is equivalent to 400 chest radiographs (chest
radiography = 0.02 mSv) and a high resolution CT scan of chest by
conventional scanner (thin slices, high mA) gives a dose of more than 600
chest radiographs. Whole body CT will obviously be a lot more.

What this means in terms of risk is less clear, but it will be greater than

> (Granted, if these were low risk and low cost, I would not mind having
> them done. I wonder about ultrasound in this area. You might not be
> able to spot the same things as other types of scan, but it seems less
> risky. If the risk is only 1% of other scans and the benefit is only
> 10% as much as ditto, that would still be 10 times more benefit over
> risk.)

MRI is another non-invasive technique with low risk. I suspect that the
problem is not the risk of the procedure, but the sensitivity and
specificity of the test for diagnosing potentially life threatening
problems. If the test has low sensitivity, it will not be useful as a
screening tool (as lots of stuff will be missed). If it has low specificity,
it will expose large numbers of people to unncessary and potentially harmful
further investigation for false positives.

Best wishes,


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