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British Journal of Radiology (2006) 79, 266-267
© 2006 British Institute of Radiology
doi: 10.1259/bjr/71660401
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Correspondence

Authors' reply

The Editor—Sir,

In response to the comments by Brennan and O'Leary on our paper in the September 2005 issue of this journal [1], we are pleased that they agree that collimation to a fixed field size at the entrance surface, as the source to image distance (SID) is changed, is clinically irrelevant. They further claim that collimation to the image receptor as the SID is changed is also clinically irrelevant. Whilst we agree that this should be the case we would like to point out, however, that collimation to the image receptor is the default requirement in many codes of practice. Furthermore, where autocollimation systems or digital image systems that permit collimation of the image post-exposure are used, there is at least anecdotal evidence that collimation to the image receptor only, becomes all too common.

As for our claim that the dose reduction to be obtained by an increase in SID is modest, we stand by this. The case quoted by Brennan and O'Leary from our results is obtained by changing from 75 cm to 200 cm SID, an extreme case not likely to occur in practice. The effective dose reduction to be obtained by increasing from the commonly used 100 cm to 125 cm is typically 10% or so according to our results. This is not to say that such dose reductions are not worth obtaining.

Finally, we accept that patient variability is an issue. This leads to obvious difficulties in experimental work with current theoretical approaches also deficient. We look forward to developments in voxel patient modelling for Monte Carlo simulations that can more realistically give insight into changes in organ doses and hence effective doses that occur due to patient variability, and consequently the impact of this on changes to the SID.

Yours etc.,

J Poletti1 and D McLean2

1 Health and Community Studies, Unitec, Private Bag 92025, Carrington Road, Auckland, 1003 New Zealand, 2 Adjunct A/Prof University of Sydney, School of Medical Radiation Sciences, Faculty of Health Sciences, University of Sydney, PO Box 170 Lidcombe NSW Australia 2141

Received for publication November 14, 2005. Accepted for publication November 18, 2005.

References

  1. Poletti JL, McLean D. The effect of source to image-receptor distance on effective dose for some common X-ray projections. Br J Radiol 2005;78:810–5.[Abstract/Free Full Text][Abstract/Free Full Text]

Related articles in BJR:

Source to image receptor distance
P C Brennan and D O'leary
BJR 2006 79: 266. [Full Text]  




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