Anomalous scattering effects in elastic photon-atom scattering from biomedically important elements

In a variety of biomedical applications it has been demonstrated that elast ic photon-atom scattering can be important. These include: solving for the crystal structure of a macromolecule, imaging, and radiation dose calculati ons. To the extent that scattering is significant, it is important to remem ber that there are a number of effects which go beyond the form-factor appr oximation. In this paper, we discuss what is presently known about the vali dity of form-factor approximations for predictions of elastic photon scatte ring by atoms. We quantify the uncertainty in form-factor theories for samp le atoms of biomedical interest, including: carbon, oxygen, aluminum, calci um, iron, iodine and lead. The importance of effects that go beyond form-fa ctor approximation is illustrated by demonstrating that errors can be intro duced in using a simple form-factor-based scaling technique using scatterin g factors measured at one energy to predict scattering at another energy. T hese anomalous dispersion have been used extensively in recent years in stu dying structures of biological macromolecules, either in combination with o r as an alternative to isomorphous replacement techniques. We present accur ate, high precision anomalous scattering factors for a range of elements co mmonly used in macro-molecular structure studies, tabulated on a fixed grid interval in the energy range 1- to 100 keV. (C) 1999 Elsevier Science Ltd. All rights reserved.