Re: a dose query

[JGinniver@AOL.COM]

Mark,

       I can't answer your question as it relates to a radiograph where the 

entrance skin dose is 3 mGy absorbed dose in air.  However if you are 

interested in this subject you might wish to try and obtain a copy of a document 

published by the National Radiological Protection Board in the UK.  The publication 

is "Guidelines on Radiology Standards for Primary Dental Care, NRPB Vol. 5 No. 

3 Published in 1994.  Table 3 in this document provides Typical effective 

doses for a range of dental and conventional medical diagnostic examinations.  

It's difficult to reproduce the table in plain e-mail text but each type of 

examination is listed below:



1.  Two dental bitewings, 70 kV set, 200 mm Focus to skin distance (FSD), 

rectangular collimation, E Speed Film,  effective dose 0.002 (mSv)

2.  Two dental bitewings, 70 kV set, 200 mm fsd, round collimation, E speed 

film, effective dose 0.004 (mSv)

3.  Two dental bitewings, 50 - 60 kV set, 100 mm fsd, round collimation, E 

speed film, effective dose 0.008 (mSv)

4.  Two dental bitewings, 50- 60 kV set, 100 mm fsd, round collimation, D 

speed film, effective dose 0.016 (mSv)

5.  Dental panoramic, rare earth intensifying screens, effective dose 0.007 

(mSv)

6.  Dental panoramic, calcium-tungstate intensifying screens, 0.014 (mSv)

7.  Skull, effective dose 0.1 (mSv)

8.  CT : Head, effective dose 2 (mSv)

9.  Chest, effective dose 0.004 (mSv)

10.  CT: Chest, effective dose 8 (mSv)

11. Barium meal, effective dose 5 (mSv)



The report includes details of how, or from which source, these values were 

derived.



I'm pleased to say that using this report, and with the support of the NRPB, 

we achieved a good example of ALARA (or ALARP in the UK) dose reduction.  We 

had a dental facility on site that required a new X-ray set.  We replaced it 

with a new 70 kV set which cost little more than a direct replacement for the 

faulty unit.  The new set came with a rectangular collimator at the same cost as 

a round collimator which was suitable for use at a fsd of 200 mm.  We changed 

to a higher speed film which cost the same to purchase and didn't require 

modification to our existing developing equipment and processes we were able to 

reduce the patient doses for a standard dental bitewing radiograph by a factor 

of 8.  IMHO this is the true meaning of ALARA, sensible dose reductions where 

possible with disproportionate cost for the dose saved.



For those that are interested there are a further 2 NRPB reports that may be 

of interest.

Protection of the patient in X-ray Computed Tomography NRPB Vol. 3 No. 4 

published 1992

Guidelines on Patient dose to promote the optimisation of protection for 

diagnostic medical exposures NRPB Vol. 10, No. 1 published 1999



Regards,

       Julian