Dual-energy CT method with 192-Iridium

This is a newly developed CT method in determining local density and effective atomic number using dual-energy gamma radiation from 192-Ir radioisotope (C. Rizescu et al./ Determination of local density and effective atomic number by the dual-energy computerized tomography method with 192-Ir radioisotope, Nuclear Instruments and Methods in Physics Research , A 465 (2001), 584 - 599).
The applied method is based on: 1) modeling total effective cross-section of gamma radiation in the natural materials in function of the atomic number (Z from 1 to 92), in terms of the photoelectric and Compton cross-sections, and 2) using a new parameter (composition factor k) given by the ratio of the linear attenuation coefficients from the two tomograms.
The authors investigated 16 materials with effective atomic number Zeff between 6 and 72.2 and bulk density ranging from 2.15 g/cm³ to 10.51 g/cm³, using 310.5 keV and 469.1 keV gamma radiation. The results show very good accuracy of the dual-energy CT method for most materials. The error in determining the effective atomic number is less than 5% for Zeff >25 and less than 10% for light materials (Zeff <15), while the error in density estimation is always smaller than 3%. go to top

Tomograms for the set of investigated probes. a) 310.5 keV gamma-ray tomogram. b) 469.1 keV gamma-ray tomogram. In the left upper corner of each tomogram, the distribution of the linear attenuation coefficient along a line that passes through materials Sn, Ag and Zn is shown. Scanning parameters: 120 angles, 0.4 mm ray-spacing, scan time: 14 hours, reconstruction: 400 x 400 grid, 0.177 by 0.177 mm pixel. Materials: graphite, Al, sintered ceramics (99.4%Al2O3), Ti, Zn, Fe, Sn, Co, Cd, Mo and Ag.	Histograms of the linear attenuation coefficients µ CT for the set of investigated probes. a) Histogram of µ CT in the 310.5 keV gamma-ray tomogram. The typical value of FWHM for this distribution is 0.0175 cm-1. b) Histogram of µ CT in the 469.1 keV gamma-ray tomogram. The typical value of FWHM is 0.0186 cm-1. Values of µ CT are given in cm-1. go to top
Results of the constrained k-factor procedure in dual-energy method for the set of investigated probes. a) Map of effective atomic numbers b) Map of densities. In the left upper corner of each representation, the distribution of the effective atomic number and the density along a line that passes through materials Sn, Ag and Zn is shown. go to top	Histograms of ZCT and densityCT obtained by the constrained k-factor procedure for the set of investigated probes. a) Histogram of the effective atomic numbers in the map of ZCT. The typical value of FWHM for ZCT distribution is 2.1 for molybdenum (the same value for aluminum). b) Histogram of the densities in the map of densityCT. The typical value of FWHM is 0.21 g/cm3.

Original collimator for micro-tomography

Sketch of micro-collimator for X and Gamma-rays computer tomographs

2-D tomogram with Hg micro-collimator. Material: cooper wires; Source: 192-iridium, Energy: 310 keV Collimator: 0.08 mm aperture. go to top

Identifying of minerals' abundance by Gamma- ray tomodensitometry

A fragment of a polisulfide rock containing quartz and high-density mineral inclusions has been investigated by using a 192 Ir gamma-ray dual computer tomograph (C. Rizescu et al./ High density mineral inclusions in rocks evidenced by Gamma-ray tomodensitometry, Journal of Trace and Microprobe Techniques, Vol. 19, No. 1 (2001), 119 -129). The tomographic image reveled the existence of three different mineral fraction, whose densities and effective atomic numbers were equal to 2.57 ± 0.24 g/cm³ (Zeff = 11.6 ± 2.1), 4.01 ± 0.22 g/cm³ (Zeff = 25.1 ± 2.1) and 6.01 ± 0.90 g/cm³ (Zeff = 68.5 ± 6.0) respectively.

Dual-energy CT method has been found to be a suitable technique for the investigation of rock samples because this method can represents simultaneous two parameters: density and average atomic number over a section the entire object. For this complex polymetallic rock we have identified the existence of three different mineral fractions: light (quartz), medium (mainly pyrite, blenda, chalkopyrite, and sphalerite), and heavy (galena). More, we have been calculated the abundance of each fraction for the entire sample: 70.6% for quartz, 26.8% for medium fraction and 2.6% for galena. A detailed mineralogical analysis confirmed these data. (Left) 2-D images and histograms of density and Zeff distributions of the section S. go to top

INVESTIGATION OF GEOLOGICAL SAMPLES

Three specimens of metamorphic and sedimentary rocks were investigated using 192-Iridium dual energy gamma-ray computer tomography. (C. Rizescu et al. / Dual energy gamma-ray axial computer tomography investigation of some metamorphic and sedimentary rocks (2002) Neues jahrbuch fur geologie und paleontologie, Stutgart, Germany, in print)). Using this methodology, tomographs, depicting density as well as effective atomic number distribution with a 0.3 mm resolution, were obtained. Various details were evidenced including the granular structure of augen gneiss, the well-developed foliation pattern of mica rich gneiss, the banded configuration of nummulitic limestone or the concentric disposition of calcite microcrystals in a speleothem. In addition, by using the corresponding image histograms, it was possible to identify both major and minor components of investigated rocks. go to top

Nummulitic limestone CT images (density and Zeff) indicate a layered disposition of individual nummulites. The investigated rock is composed by a great number of nummulites, disposed more or less parallel, and bound together by a cementing materiel which contains heavy fractions as iron or manganese minerals.	Quartz-feldspar shale The investigated section consists of 13 layers with an average thickness of 1.54 mm. A lighter material, depicted in more pale hues separates each lamina, which appears in darker tones. The investigated structure consists of an alternation of quartz and feldspar layers separated by muscovite rich strata.	Hydrothermal quartz The lighter components are quartz with a porous structure, i.e. micro-crystals aggregation with linear sizes smaller than 0.3 mm. On the left side of density CT image appears a thin layer of a heavier compact material, formed by most probably by limonite. go to top

3-D investigation of polymetallic nodules

Polymetallic nodules collected from the north-east Pacific Clarion-Clipperton region were investigated with 192-iridium dual-energy gamma-ray computer tomograph (C. Rizescu et al. / 3-D dual gamma-ray computer axial tomography investigation of polymetallic nodules, Deep Sea Research I, 48 (2001), 2529 - 2540).
Sixty-five successive digital sections, each with a thickness of 0.8 mm, reveal the 3-D density as well as the effective atomic number distribution function over the entire nodule volume, allowing one to obtain virtual sections across any desired plane. The tomograms show many details of internal structure of the investigated nodule such as and old nucleus fissure networks or layer structures. The total concentration of iron and manganese that determined by this method was equal to 19.4 ± 1.7. go to top

Photo of the investigated nodule

3-D map of density

Equatorial section (map of Zeff)

Two vertical cross-sections

		The argillaceous material of the nodule has an average density of 1.47 ± 0.37 g/cm3 and an average Zeff equal to 12.8 ± 2.1 and the mineralized part has an average density of 1.83 g/cm3 ans an average Zeff equal to 20.9. The global mineralization of the nodule is equal to 19.4 ± 1.7 %, according with the literature.	3-D image of nodule material whose density is greater than 1.91 g/cm3 go to top
		The sections show the evolution of the nodule configuration from an initial asymmetric pattern towards a more regular, elliptic shape. In the center, darker shades define the presence of a nucleus that consists of a fragment of an older nodule with alternating layers. The average density of the nucleus (1.85 g/cm3) is significantly higher than of the rest of nodule (1.31 g/cm3).

Sedimentary cores

Sedimentary core collected from the south-east Black Sea (near Bosphorus region at 1.400 m depth) were investigated with 192-iridium dual-energy gamma-ray computer tomograph.

Complex Analysis of the "Cannon of Giurgiu" by Computerized Tomography with ¹⁹²Ir

The historical piece of artillery 'Cannon of Giurgiu' is investigated by computed tomography and X-ray diffraction methods (C. Rizescu et al. / Complex analysis of the "Cannon of Giurgiu" by Computerized Tomography with 192-Ir, Journal of Archaeological Science, Vol. 29, No.3 (2002), 267 - 275). 30 cross sections transversal to the cannon axis, spaced at 5 to 20 mm, are analyzed by dual-energy CT method with 192Ir radioisotope. Samples of corrosion material from the cannon pipe are tested by X-ray diffraction method. Numerous casting defects, large internal defects grown during cannon use, as well as severe corrosion regions have been detected. Structural analysis provides estimations of local material density and effective atomic number, leading to the identification of three domains in the cannon internal structure (bronze basic material, intermediate material and corrosion material). The degrading process is deep, with thickness of the corrosion layer ranging between 3.5 mm and 10 mm. These results determine the choice of appropriate restoration and conservation procedures.

Analysis of material parameters determined by dual-energy CT and X-ray diffraction methods provides the following important information:

• The effective atomic number (Zeff = 33.6) and density (density= 8.78 g/cm³) of bronze correspond to the following material composition: 91.4%Cu7.8%Sn0.8%Pb. This formula is typical for bronze materials cast in the Middle Age metallurgic workshops.
• The intermediate material with Zeff = 26.2 and density= 5.04 g/cm³ corresponds to a structure of the following type: 45% basic material (bronze) and 55% corrosion material.
• The effective atomic number of the corrosion material, determined by X-ray diffraction method (Zeff =12.3), is very close to the one determined by dual-energy CT method (Zeff =12.9). go to top