Tomograms

In this paper, discontinuities in cylindrical specimens were studied by ultrasonic reflection tomography. The aim was threefold. First, to localize discontinuities from circular C-scan images. Second, to reconstruct quantitative cross-sectional images from circular B-scan profiles (i.e., reflection tomograms). Finally, to obtain three-dimensional information (i.e., discontinuity location, dimension and type) by stacking these reflection tomograms in multiple planes, in the third dimension. 

In order to ensure perpendicular beam incidence on the cylindrical specimen, the circular B-scan profiles were acquired by high frequency (narrow beam) transducers in a synthetic circular aperture array. From these profiles two-dimensional reflection tomograms were reconstructed using a filtered backprojection technique. Straight line propagation was assumed. Several artificial discontinuity types in a cylindrical Plexiglas (Perspex) specimen were compared with similar artificial discontinuities in a cylindrical A/Si-alloy [2]. Furthermore, examples of real discontinuities (an inclusion and a feed head) in the cylindrical AlSi-alloy are presented. 

Fig. 4. Schematic details of cylinder (left) and volumetric imaging of cylinder by stacking reflection tomograms (right). Dimensions in [mm].

Fig. 5. shows six ultrasonic reflection tomograms. Three of these are from the Plexiglas specimen (shown left) and three are from the AlSi-alloy (shown right). The tomograms are reconstructed from reflection data measured across the plane (b), (c) and (e), respectively. The dark regions indicate high reflectivity and represent specimen interfaces and discontinuities. 

Fig. 5. Reflection tomograms in Plexiglas and A/Si-alloy cylinders, respectively, reconstructed from data measured across plane (d), (e) and (f). High reflectivity is indicated by dark regions.

The reflection tomograms (c) show the axial hole in the Plexiglas specimen, but also a real discontinuity in the A/5i -alloy. The internal discontinuity is located 6 mm from the edge, 50° from the axial hole and its dimension is about 1-2 mm. This may be an inclusion or a porosity (void). Multiple reflections from the measurement were ignored in the calculation of the Plexiglas tomogram (left). This is seen as a bright circle. 

The reflection tomograms (e) show, as expected, solid material in the Plexiglas specimen. A feed head from the casting process is seen clearly in the AZSi-alloy. The diameter of the feed head is about 6 mm in accordance with Fig. 3. 

The three pairs of reflection tomograms are listed in Table 2, showing the artificial and real discontinuities in the Plexiglas and A/5i-alloy cylinder, respectively. 

Finally, by volumetric imaging Three-dimensional information was obtained by stacking reflection tomograms from multiple planes. Using this stacking technique, cubic voxels were obtained and could be numerically dissected in any plane. Although there are several attractive features related to this technique, there are also several questions which need to be addressed before it can be used for industrial applications. For example, the applied sound field must be further characterized. 

Software base complexes are used for this purpose, which realize analytical restoration algorithms and various procedures of tomograms processing Program complexes ensure the control of a measuring process and dialoguing with the operator. 

U, - meaning of linear relaxation coefficient for i - element tomogram s, Vi - volume of object, appropriate i -clement tomogram s, po - meaning of linear relaxation coefficient of a matrix material, (p/p)mei - mass relaxation coefficient of metal, wo - faaor of a pore filling material... 

The control technique of fuel distribution in uranium - graphite fiael elements seems to be most perform. The technique allows to determine weight of uranium or its connections in a chosen zone of fuel elements. There were used the sources of radiation on a basis radionuclide Am. The weight of uranium in fuel element or its parts is determined by combine processing of a tomograms, set received on several parallel layers of fuel element. The comparative results of tomographic researches and chemical analysis of weight of uranium in quarters of spherical fuel elements are resulted in the table. 

With the use of Cs source tomographic layer-by-layer study of nuclear fuel within a range of 5 to 12 g/sm is conducted. In the specialized tomograph the initial information measurement time is 5-30 min, the tomograms restoration time is 4-10 min. The sensitivity to a various density is about 5% when detecting local areas with a diameter exceeding 0.5mm. 

Fig. 6. a, b - typical tomograms of dry cucumber seed of control and GMF groups c, e, g - cucumber seeds of GMF group after 5, 10 and 15 minutes in water accordingly d, f, h -cucumber seeds of the control group (the same time periods). Arrows indicate lens-shaped water absorbing structures. 

Figure 4.6 PET pictures of the heart of a patient with acute myocardial infarction treated with a thrombolytic agent. Top row shows scans after administration of water containing lsO to trace blood flow. Bottom row shows tomograms obtained after administration of acetate containing nC to trace the heart s metabolism, that is, its rate of oxygen use. The defects tire clearly visible on day 1, both in the impaired blood flow (top left) and the impaired metabolic use of oxygen (bottom left). Recovery of blood circulation has taken place on day 2 and is maintained. (Figure also appears in color figure section.)...

Chen, L. F., Winkler, H., Reedy, M. K., Reedy, M. C., and Taylor, K. A. (2002). Molecular modeling of averaged rigor cross-bridges from tomograms of insect flight muscle. / Struct. Biol. 138, 92-104. 

Dennis MS, Bamford JM, Molyneux AJ et al. (1987). Rapid resolution of signs of primary intracerebral hemorrhage in computed tomograms of the brain. British Medical Journal 295 379-381... 

With consecutive tomograms, the thickness of each section of the body is 5-8-10 (-12) mm. In individual cases, additional thin-section tomograms of 1 mm can be obtained. Resolution is 1 x 2 mm in the hepatic area, with an accuracy in attenuation values of up to 0.5%. In this way, the values for a particular cross-section and their spatial distribution are visualized in a scan. This results in blur-free, anatomically precise imaging of a layer of the body in an axial plane. CT scans provide satisfactory information if an object diameter of 1.5 to 3 mm is resolved with a density gradient of 0.5% to the surrounding area at an integral dose of 10 mGy. 

The experimental temperature was adjusted using a cold gas flow from a cryojet (Oxford Instruments) directed onto the head of the sample holder. The cup is surrounded by a double walled Kapton-foil cage. Dry nitrogen was flushed between the inner and the outer walls in order to avoid icing in the beam path while recording the tomogram. 

SPECT single photon emission computed tomogram... 

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