X-ray attenuation

This leads naturally to the concept of X-ray attenuation, defined as the natural logarithm of the transmitted intensity fraction ... 

X-ray attenuation occurs via four basic modes of interaction, shown schematically in Figure 4. Coherent X-ray scatter (CXRS) is an energy preserving interaction between an X-ray photon and an entire atom or crystalline matrix. Incoherent or Compton scatter represents a direct interaction between an X-ray and an... 

In the mid-1970s, Alvarez and Macovski [27, 28] made a significant finding that had been overlooked for many years. For purposes of computing X-ray attenuations, any target can be closely represented as a unique mixture of two basis... 

Figure 20 Ambiguity in locating point masses from two X-ray views. The observed X-ray attenuations may equally well be attributed to the two masses shown as the circles or alternatively to the two masses shown as the triangles. A third X-ray view would resolve this ambiguity.

E.B. Saloman,J.H. HubbeU andJ.H. Scofield, X-Ray Attenuation Cross Sections for Energies lOOeV to lOOkeVand Elements Z 1 to —92, At. Data Nucl. Data Tables 38 (1988) 1—197. 

In locating contraband the CT approach has two major advantages over conventional X-ray imaging. First, and most importantly, it can measure and record, unambiguously, the material property of individual objects packed in a bag. The quantity is the X-ray attenuation coefficient, which is proportional to a material s... 

Dual-energy is a CT imaging approach aimed at generating and extracting additional information beyond material density from image slices. The additional information, a second material property, is known as the effective atomic number, Zeff. Although the X-ray attenuation coefficient is proportional to a material s density p, it is also a function (i) of the atomic number Z of each of the material s... 

Since there is direct proportionality between the iodine concentration within the tissue or vessel and X-ray attenuation, pharmacokinetic parameters can be calculated from time-density curves. In CT, 30 HU correspond to approximately 1 mg iodine g tissue [6]. This is a rather high local concentration of radioopaque material, thus underlining the relatively low sensitivity of CT that counterbalances its high specificity. 

A numerical matrix correction technique is used to linearise fluorescent X-ray intensities from plant material in order to permit quantitation of the measurable trace elements. Percentage accuracies achieved on a standard sample were 13% for sulfur and phosphorus and better than 10% for heavier elements. The calculation employs all of the elemental X-ray intensities from the sample, relative X-ray production probabilities of the elements determined from thin film standards, elemental X-ray attenuation coefficients, and the areal density of the sample cm2. The mathematical treatment accounts for the matrix absorption effects of pure cellulose and deviations in the matrix effect caused by the measured elements. Ten elements are typically calculated simultaneously phosphorus, sulfur, chlorine, potassium, calcium, manganese, iron, copper, zinc and bromine. Detection limits obtained using a rhodium X-ray tube and an energy-dispersive X-ray fluorescence spectrometer are in the low ppm range for the elements manganese to strontium. 

Brain tissue water content is inversely correlated with X-ray attenuation and can thus be measured with CT (Rieth et al., 1980 Unger et al., 1988 Dzialowski et al.,... 

The oil phase, Soltrol 130, a refined kerosene, was doped with iodated oils of similar molecular structure. The dopants are strong photoelectric absorbers and increase the accuracy of saturation determination by increasing the X-ray attenuation. The refined, nearly single-component, oil was also used to insure complete first-contact miscibility. Because this is a single-component oil, multiple contact developed miscibility is not observed below the miscibility pressure. 

The introduction of more stable x-ray sources and detectors resulted in the development of simpler, commercially viable systems. Kalshoven [40,41] described an x-ray instrument which used a special program for scanning the sedimentation tank. As the concentration measurements by means of x-ray attenuation are very rapid, the scanning greatly speeds up the analysis, reducing the measurement time down to a few minutes. In this instrument it was done in such a way that the concentration, and hence the cumulative mass percentage undersize, is recorded as a function of Stokes diameter rather than time. An x-ray tube was used as a source and... 

Many quality assurance procedures in diagnostic radiology require the use of a phantom to simulate the X-ray attenuation of the patient. The phantom should transmit the same quantity and quality (i.e. spectrum) of radiation as that transmitted by the patient. The knowledge of photon spectra at the position of measurement on the surface, inside and behind standard dosimetric and imaging phantoms is helpful for performing dosimetric or calibration measurements and hence helpful in the course of quality control. The experimental approach is limited however, simulation with Monte Carlo methods has been proved to be a very powerful technique (Petoussi et al., 1992). 

The use of gadolinium-based contrast media for radiographic examinations is not recommended to avoid nephrotoxicity in patients with renal impairment, since they are more nephrotoxic than iodinated contrast media in equivalent X-ray attenuating doses... 

Figure 6 X-ray microcomputed tomography. An axial cross-section through a freeze-dried 15-minute swollen high-amylose starch pellet, in which variations in X-ray attenuation provide evidence of an outer surface membrane. The pellet size is 3.0 (height) x 3.3 (width). The membrane thickness is 300 pm on the right and left edges (at mid height) and 450 pm at the top and bottom sides of the pellet. The smooth layer ranges from 130 pm on the right and left edges to 200 pm on the top and bottom sides. Source From Ref. 159.

Absorption coefficients define the X-ray attenuation effect from a given solids. We use both mass absorption coefficients, //m, and linear absorption coefficients, p, with these being related by ju = j.umpx, where p is the density of a given solid and x is the X-ray path length. The mass absorption for any material can be calculated from the elemental mass absorption coefficients by the relation pm = Z/ / where the f and the are the weight fraction and linear absorption coefficients, respectively, for each element in the material. 

Foster AL, Brown GE, Tingle TN, Parks GA (1997) Quantitative arsenic speciation in mine tailings using X-ray absorption spectroscopy. Am Mineral 83 553-568 Gaillard JF, Webb SM, Quintana JPG (2001) Quick X-ray absorption spectroscopy for determining metal speciation in environmental samples. J Synch Rad 8 928-930 Gerward L (1981) Analytical approximations for X-ray attenuation coefficients. Nucl Instr Meth Phys Res 181 11-14... 

Portal to all NIST data systems see B.l Atomic and molecular spectra, cross sections, X-ray attenuation, and dosimetry data Portal to all National Library of Medicine databases NMR data submitted by users Cr) tal structures of nucleic acids See B.ll... 

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