Measurements of diagnostic x-ray backscatter by a novel ion chamber method.

There is a major gap in backscatter information for diagnostic x-ray beams. Such information is increasingly needed for dose measurements and calculations, as well as for designing devices and techniques. We have therefore carried out measurements on both low Z materials and metals, using an ion chamber method designed specifically for the purpose. Lucite and two D.R. White tissue substitutes were studied extensively (BR 12 "average breast" and MS 11 "water"). Measured percent backscatter (BS) was greatest for Lucite and least for MS 11, with BR 12 in between. Backscatter buildup is rapid: 50% of full backscatter is achieved with 6 mm thickness for all three materials using mammographic beams and with about 12 mm using general diagnostic beams. A simple relationship between BS and field area permits close estimates of BS values for fields for which measured data is not available. Among metals tested, copper exhibited greatest backscatter (39% BS maximum), aluminum least, and lead in between--information of potential importance in cassette design and similar applications.