Novel x-ray attenuation mechanism: role of interatomic distance.

Little progress has been made over the last 30 years for improving attenuation by x-ray contrast agents, in part because the mechanisms of x-ray attenuation are thought to be well understood. We hypothesized that x-ray absorbance can be modulated by altering the interatomic spacing between K-edge attenuating atoms. Iodomethane, diiodomethane, 2,6-diiodo-4-nitroanaline, and diiodobenzene isomers were dissolved in DMSO and imaged with an OEC Compact 7600 fluoroscope. At a tube voltage of 42 kVp, absorbance of equimolar diiodomethane (150 mM) was significantly (p<0.01) greater than iodomethane (150 mM) by 45%. Interestingly, 150 mM diiodomethane absorbance was significantly greater than 300 mM iodomethane (by 5%, p<0.01) despite equal amounts of iodine in both solutions. 1,3-diiodobenzene absorbance was significantly greater than 1,2- and 1,4-diiodobenzene (p<0.01). However, 2,6-diiodo-4-nitroanaline absorbance was similar to 1,3-diiodobenzene. When a linear model was fit for absorbance as a function of density and harmonic error (the fractional remainder of the inter-iodine distance and the K-shell ionizing wavelength) at different beam energies, a significant overall fit was obtained for both unfiltered and hardened beams (p<0.01). While the slope of absorbance as a function of harmonic error was significant for all conditions (p<0.01), the slope with respect to density was significant only when the beam was unfiltered (p<0.05). Also harmonic error, but not density, displayed significant energy-dependent effects on absorbance (p <0.01). These data suggest that harmonic error is a strong determinant of absorbance, particularly when the beam energy is concentrated near the iodine K-edge and is likely a descriptor of K-characteristic photon interactions. Therefore, x-ray absorbance may be modulated by the distance between covalently linked x-ray K-edge attenuating atoms. This finding has important implications for increasing contrast agent absorbance as well as for designing molecular transducers capable of modulating K-edge attenuating atomic distances and thereby x-ray absorbance.