Photon attenuation

Figure 19. Influence of UV radiation as polymerization agent on the subsequent total photon attenuation in silica fibers, as a function of wavelength in nanometers. When no absorption of the UV is arranged lowest losses are assured. (The loss expressed in decibels per kilometer is a modern figure of merit for light guide...

You would like to analyze the atomic structure of Pd by grazing incidence X-ray diffraction. After penetrating a distance x, the intensity of the beam is decreased by I = J0 e x. Your diffractometer uses photons of 10 keV energy. Is the wavelength sufficiently small to analyze atomic structures At this energy a handbook tells you that the photon attenuation coefficient, nip, is 691 cm2/g. The density is 12.0 g/cm3. Assume you want 20% of the incident X-rays to be scattered within the topmost layer of 1 nm thickness. Which angle do you have to choose ... 

The projection data acquired in the form of sinograms are affected by a number of factors, namely variations in detector efficiencies between detector pairs, random coincidences, scattered coincidences, photon attenuation, dead time, and radial elongation. Each of these factors contributes to the sinogram to a varying degree depending on the 2D or 3D acquisition and needs to be corrected for prior to image reconstruction. These factors and their correction methods are described below. 

Although PET/CT modality is widely employed for diagnostic purpose, CT data acquired either for fusion of PET and CT images or for photon attenuation correction are affected by several factors that cause artifacts in PET/CT images. The factors include patient positioning, metal objects, CT contrast agents, truncation artifacts, and respiratory movement, which are discussed below. 

Figure 12.31 Photon attenuation coefficients for germanium. The dashed line is the approximate total linear attenuation coefficient.

Although gamma rays are much less subject to attenuation than alpha and beta particles, a density correction is needed if the density of the sample deviates significantly from the density of the calibration standards. The effect of density on self-absorption for both the standard and the sample is estimated by Eq. (7.2) [x for this purpose is the photon attenuation coefficient in cm /g and x is the sample area density in g/cm. Values for ix in some common materials are listed in Table 2.2 and in its cited reference. If a large set of samples with consistent density is analyzed, it may be possible to prepare radioactivity standards at the same density to avoid the need for correction. Interpolating efficiency values as a function of density is feasible at energies above 0.1 MeV because the effect of minor density difference on counting efficiency is small. 

PHOTON ATTENUATION COEFFICIENTS Martin J. Berger and John H. Hnbbell... 

Table 8.1. Comparison of the uptake within liver lesions in pre-therapeutic MAA SPECT ims es and in post-therapeutic bremsstrahlung SPECT. The analysis is confined to metabolically active liver lesions as determined from F-18-FDG PET/CT scanning. Note the high number of lesions showing high uptake on MAA scintigraphy, but low uptake on post-therapeutic SPECT, most probably explained by photon attenuation of the bremsstrahlung images...

The energy loss of photons in matter can be characterized by the cross section of the process and also by the mean free path of photons between interactions. The latter can be expressed in units of surface density, and then it is called mass attenuation length. Photon attenuation lengths for various elements are shown inO Fig. 8.13 as taken from Bichsel et al. (2002). 

Nowotny R (1998) Photon attenuation date on PQ program XMuDat (version 1.0.1, August 1998), report IAEA-NDS-195. International Atomic Energy Agency, Vienna... 

Calculations according to O Eq. (30.43) require the knowledge of the photon attenuation coefficients /r of the sample matrix (see Sect. 11.5.1 in the Appendix of Vol. 1). Experimental calibrations are readily conducted by placing an unirradiated sample between the activated sample and the detector, thus determining the attenuation coefficients for an unknown or unlisted matrix. By using an irradiated standard identical to the sample, as is possible in the comparator method, the need for geometry, self-shielding, and self-absorption corrections is eliminated, and thus the potential errors are reduced. 

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