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Correspondence

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									1971, British Journal of Radiology, 44, 75

JANUARY

1971

Correspondence
(The Editors do not hold themselves responsible for opinions expressed by Correspondents)
THE EDITOR—SIR, DEPENDENCE OF EXPOSURE RATE UPON FIELD SIZE AND SHAPE THE EDITOR—SIR, BLINDS IN RADIOGRAPHIC ROOMS

It has been previously reported that the exposure rate of teletherapy machines for a given SSD is a linear function of the variable loge (A/P) where A is the area and P is the perimeter of the field (Patomaki, 1968). At this hospital it has been noted that a straight line regression of the exposure rate of a Theratron 80 on the variable logio(^4/P) has a multiple correlation coefficient of 0-998. The regression of absorbed dose-rate at a depth of 0-5 cm on logio(^4/P) has a similar value of the multiple correlation coefficient. It has also been noted here that the exposure rate for an orthovoltage machine is a quadratic function of this variable, \ogw{AjP). More specifically, it has been found that the second-order regressions of exposure rate for a given quality of radiation on the logarithmic function of field size and shape at 50 cm SSD for diaphragm-limited fields having areas between 4 cm2 and 484 cm2 will have multiple correlation coefficients between 0-961 and 0-990 depending upon the quality of the radiation. The overall regression is significant at 1 per cent. The second-order term is significant at 1 per cent. The several regressions for different qualities of radiation can be subsumed under the two more convenient forms below with only a slight loss in accuracy. For 100 kVp 0-25 mm Cu HVL 100 kVp l-00mm AIHVL 140 kVp 0-50 mm Cu HVL F=0-951 +0-433*[X*(l-X)-0-103] For 300 kVp 2-00 mm Cu HVL 300 kVp 3-00 mm Cu HVL ] Here X=logio(^4/P) and Y is 2 the exposure rate in R/minute normalised to a 10 X 10 cm field. Yours, etc.,
DONALD HERBERT.

Is it not time that we discarded the practice of installing black light-tight blinds in radiographic rooms and photographic darkrooms? These usually have a rough or matt surface on each side. Many years ago when I was taught physics I learned that the ideal absorber and radiator of heat is a small depression or hole in a black surface. This is just what the surfaces of such dark blinds contain in large numbers. It is not surprising, therefore, that in summer they convey a tremendous amount of heat into an X-ray room or darkroom making such places most unpleasant to work in. If these blinds, particularly on their outer surfaces, were light in colour and smooth they could be made just as opaque without being diathermic. The colour and texture of the inside of the blinds is not so important, as heat transfer on this side is mostly by conduction and convection of the air, but there is no advantage in a black blind. Yours, etc.,
F. R. BERRIDGE.

Addenbrooke's Hospital, Trumpington Street, Cambridge.

Physics Department, Penrose Cancer Hospital, Colorado Springs, Colorado. REFERENCE
PATOMAKI, L. K., 1968, British Journal of Radiology, 41, 381.

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