Contrast For X-rays

The effect of different cations on the SAXS scan is shown in Figure 13. Very little change in the position of the reflection is seen with changing cation. The intensity of the reflection decreases as the cation weight increases. This is a result of both the change in water absorption and a change in contrast for x-ray scattering between the ionic clusters and fluorocarbon matrix, since the electron density of the cluster will increase... 

Polymers and PMC are of relatively low density and hence do not absorb X-rays or y-rays efficiently. Low energy radiation (typically 10-40 keV) yields enhanced contrast for X-ray radioscopy or radiography of polymers and PMC due to the nonlinear shape of the absorption versus energy curve (72). Higher energy y- and X-ray sources are hence less suitable. 

The sensitivity of the luminescence IP s in the systems employed here decreases with increasing x-ray energy more strongly than in the case of x-ray film. Therefore, this phenomenon must be compensated by using thicker lead front and back screens. The specific contrast c,p [1,3] is an appropriate parameter for a comparison between IP s and film, since it may be measured independently of the spatial resolution. Since the absorption coefficient p remains roughly constant for constant tube voltage and the same material, it suffices to measure and compare the scatter ratio k. Fig. 2 shows k as a function of the front and back screen thickness for the IP s for 400 keV and different wall thicknesses. The corresponding measured scatter ratios for x-ray films with 0,1 mm front and back screens of lead are likewise shown. The equivalent value for the front and back screen thicknesses is found from the intersection of the curves for the IP s and the film value. 

Considering existing microscopical techniques, one can find that non-destmctive information from the internal stmcture of an object in natural conditions can be obtained by transmission X-ray microscopy. Combination of X-ray transmission technique with tomographical reconstmction allows getting three-dimensional information about the internal microstmcture [1-3]. In this case any internal area can be reconstmcted as a set of flat cross sections which can be used to analyze the two- and three-dimensional morphological parameters [4]. For X-ray methods the contrast in the images is a mixed combination of density and compositional information. In some cases the compositional information can be separated from the density information [5]. Recently there has been a... 

Water-insoluble barium salts are poorly absorbed. In fact, barium sulfate is used as a contrast material for x-ray examination of the gastrointestinal tract based on its limited solubility andlow toxicity (52). Barium sulfate fed to mice at various levels up to 8 ppm dietary Ba ( 1.14 mg/kg-d as Ba " ) for three generations had no significant effects on growth, mortality, morbidity, or reproductive performance (53). 

Another application is for X-ray security machines. Here, the contrast between different materials is large, so the X-ray intensity is less high, but a high acceleration voltage is needed, as the penetration depth should be large. Thus a number of pulsed X-ray devices have been made, following the early work of Zhou et al. [59 62]. 

NMR is another powerful tool for determining the 3D structures of compounds. In contrast to X-ray crystallography, NMR requires that the compound be in solution, rather than crystalline. It provides information about the number and types of atoms in the molecule and the electronic environment around these atoms. 

A possible way to improve differentiation between normal and diseased tissues is the use of non-specific contrast agents which achieve contrast enhancement based on morphological and physiological properties of tumour tissue. Contrast agents for X-ray, CT and MRI are preferentially taken up by tumour tissue shortly after intravenous injection due to increased tumour vasculature, leaky endothelial structures and enlarged extracellular volumes [34], a phenomenon that is in principle exploitable for optical contrast agents. 

These different approaches in the search for MRI liver contrast agents have also been tried for X-ray imaging. In the following chapters, the literature will be reviewed for compounds with potential as X-ray liver contrast agents. 

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