Correspondence revised "0-3" scoring system described in the fourth paragraph. It is hoped that this phantom will become more readily available so that wider use and experience will accrue and interchange of information on image quality will be facilitated. Yours etc., A. ROBINSON A. C. UNDERWOOD
Department of Medical Physics & Clinical Engineering, Sheffield University and Health Authority, Weston Park Hospital, Whitham Road, Sheffield S10 2SJ {Received March 1991)
Author's reply THE EDITOR—SIR,
I would like to comment briefly on the interesting letter from Mr A. Robinson and Dr A. C. Underwood. The "0-3 scale of visiblity" referred to in the fourth paragraph of the letter from Robinson and Underwood was the basis of the "common scoring system" referred to in my reply to Dr Moores. Since learning of it from them, I have made some use of it with the Du Pont phantom and elsewhere, and found it helpful. I understand it is used by colleagues in the FAXIL laboratory in Leeds with their new TOR (MAM) phantom and it may prove to be of value for phantom film scoring in general. The Leeds TOR (MAM) phantom is referred to by Robinson and Underwood in their penultimate paragraph. Preliminary results from a few film-screen combinations for this phantom, the Du Pont and the Bart's phantom are well correlated. Yours etc., J. LAW {Received May 1991)
Body aprons in radiation protection THE EDITOR—SIR,
The Guidance Notes associated with the Ionising Radiation Regulations (1985) state that protective aprons for the body shall comply with British Standard 3783 and that they should be examined visually at frequent intervals. This latter clause further states that thorough examination should be carried out at least once a year to ensure that no cracks have developed, and that radiographic examinations are only likely to be justified when the protective material is covered. Uncovered body aprons would seem rarely to be used and it is therefore required of those involved to carry out radiographic, or possibly fluorographic examinations of body aprons at least once a year. It is the carrying out and assessment of such tests which is the subject of this communication. Firstly, British Standard 3783 states that aprons "shall be free from defects such as patches, blisters, porosity, embedded foreign matter or other defects in the material". Even a cursory examination of available aprons shows that this is far from the actual case. Most aprons would seem to suffer from these defects although the relevant British Standrad was published 26 years ago. It is probably
Vol. 64, No. 763
correct to say, however, that many of these defects are quite minor and do not represent any significant increased hazard to the wearer. Indeed one manufacturer states that in fact the overall thickness of the material may be slightly increased from the nominal value in order to compensate for any such deficiencies. Consideration of how to assess the significance of these defects naturally leads on to assessment of the significance of cracks developing in the protective material. The Guidance Notes state that "any defects .... should be reported immediately and the defective items replaced as soon as possible". But what constitutes a defect? The extreme cases are of course obvious — where the material is cracked through the entire thickness — but what of fine and superficial cracking where the increase in density on a radiograph may be no greater than that of some of the manufacturing defects already mentioned? Turning to the British Standard again we can use the test specified by which the lead equivalent of an apron is determined. In the foreward to the Standard it is recommended that we measure the lead equivalent of a crack or defect to discover whether the required nominal standard of protection is being achieved. Some simple practical tests were carried out to this end. The prescribed procedure is to compare the density of the radiograph exposed through the apron with that of a lead stepwedge exposed simultaneously to a primary beam at 150 kVp. Two problems immediately arise. Firstly, any cracks visible on the film are often too narrow to read the density satisfactorily with the ordinary type of densitometer and, secondly, cracks may be present which are not visible on the radiograph. There are two reasons why the cracks may not be seen. The cracks may close up tightly (or open up) when the apron is laid on a flat surface and associated with this the angle of the crack may be acute with respect to the surface of the apron. However, it should be noted that this test is rather unrealistic from another point of view — aprons are never to be used as protection against an unattenuated primary beam. The occasions when aprons are worn to protect the wearer from the primary beam after attenuation by the patient are few and, in the large majority of cases, aprons are worn for protection against scattered radiation only. Some further simple experiments were then performed using water as the scattering medium — thus testing the apron by exposing it to scattered radiation only. As might be expected frank cracks in the material were no longer sharply defined but the increased density that represented them was diffused over a wider area. This made possible not only the accurate reading of the photographic density with an ordinary densitometer but also represents, far more realistically, the actual leakage of radiation through the garment under typical conditions of use. As in all areas of radiation protection it is usual to take the most cautionary approach so it may be presumed that staff are not at any particular or significant risk from this uncertainty. However, it is reasonable to conclude that in this matter some highly subjective judgements are continually being made. Is there not a case for a review of this entire subject? Yours etc., L. J. RAMSEY
45 Harvest House, Cobbold Road, Felixstowe IP 11 7SP {Received January 1991)
641
Department of Medical Physics & Clinical Engineering, Sheffield University and Health Authority, Weston Park Hospital, Whitham Road, Sheffield S10 2SJ {Received March 1991)
Author's reply THE EDITOR—SIR,
I would like to comment briefly on the interesting letter from Mr A. Robinson and Dr A. C. Underwood. The "0-3 scale of visiblity" referred to in the fourth paragraph of the letter from Robinson and Underwood was the basis of the "common scoring system" referred to in my reply to Dr Moores. Since learning of it from them, I have made some use of it with the Du Pont phantom and elsewhere, and found it helpful. I understand it is used by colleagues in the FAXIL laboratory in Leeds with their new TOR (MAM) phantom and it may prove to be of value for phantom film scoring in general. The Leeds TOR (MAM) phantom is referred to by Robinson and Underwood in their penultimate paragraph. Preliminary results from a few film-screen combinations for this phantom, the Du Pont and the Bart's phantom are well correlated. Yours etc., J. LAW {Received May 1991)
Body aprons in radiation protection THE EDITOR—SIR,
The Guidance Notes associated with the Ionising Radiation Regulations (1985) state that protective aprons for the body shall comply with British Standard 3783 and that they should be examined visually at frequent intervals. This latter clause further states that thorough examination should be carried out at least once a year to ensure that no cracks have developed, and that radiographic examinations are only likely to be justified when the protective material is covered. Uncovered body aprons would seem rarely to be used and it is therefore required of those involved to carry out radiographic, or possibly fluorographic examinations of body aprons at least once a year. It is the carrying out and assessment of such tests which is the subject of this communication. Firstly, British Standard 3783 states that aprons "shall be free from defects such as patches, blisters, porosity, embedded foreign matter or other defects in the material". Even a cursory examination of available aprons shows that this is far from the actual case. Most aprons would seem to suffer from these defects although the relevant British Standrad was published 26 years ago. It is probably
Vol. 64, No. 763
correct to say, however, that many of these defects are quite minor and do not represent any significant increased hazard to the wearer. Indeed one manufacturer states that in fact the overall thickness of the material may be slightly increased from the nominal value in order to compensate for any such deficiencies. Consideration of how to assess the significance of these defects naturally leads on to assessment of the significance of cracks developing in the protective material. The Guidance Notes state that "any defects .... should be reported immediately and the defective items replaced as soon as possible". But what constitutes a defect? The extreme cases are of course obvious — where the material is cracked through the entire thickness — but what of fine and superficial cracking where the increase in density on a radiograph may be no greater than that of some of the manufacturing defects already mentioned? Turning to the British Standard again we can use the test specified by which the lead equivalent of an apron is determined. In the foreward to the Standard it is recommended that we measure the lead equivalent of a crack or defect to discover whether the required nominal standard of protection is being achieved. Some simple practical tests were carried out to this end. The prescribed procedure is to compare the density of the radiograph exposed through the apron with that of a lead stepwedge exposed simultaneously to a primary beam at 150 kVp. Two problems immediately arise. Firstly, any cracks visible on the film are often too narrow to read the density satisfactorily with the ordinary type of densitometer and, secondly, cracks may be present which are not visible on the radiograph. There are two reasons why the cracks may not be seen. The cracks may close up tightly (or open up) when the apron is laid on a flat surface and associated with this the angle of the crack may be acute with respect to the surface of the apron. However, it should be noted that this test is rather unrealistic from another point of view — aprons are never to be used as protection against an unattenuated primary beam. The occasions when aprons are worn to protect the wearer from the primary beam after attenuation by the patient are few and, in the large majority of cases, aprons are worn for protection against scattered radiation only. Some further simple experiments were then performed using water as the scattering medium — thus testing the apron by exposing it to scattered radiation only. As might be expected frank cracks in the material were no longer sharply defined but the increased density that represented them was diffused over a wider area. This made possible not only the accurate reading of the photographic density with an ordinary densitometer but also represents, far more realistically, the actual leakage of radiation through the garment under typical conditions of use. As in all areas of radiation protection it is usual to take the most cautionary approach so it may be presumed that staff are not at any particular or significant risk from this uncertainty. However, it is reasonable to conclude that in this matter some highly subjective judgements are continually being made. Is there not a case for a review of this entire subject? Yours etc., L. J. RAMSEY
45 Harvest House, Cobbold Road, Felixstowe IP 11 7SP {Received January 1991)
641
