SkyScan: micro-CT methods

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This web-site is created & updated by Alexander Sasov		Microtomography (Micro-CT) Nowadays different microscopical methods and equipments, each with different possibilities and limitations, are at the disposal of the scientist. Let's try to describe what may be an "ideal" microscope from the user point of view. Of course technical specifications, such as spatial resolution, accessible object size and so on are still very significant. But several other aspects become more and more important. In the last years one can find a strong reorientation of most microscopical methods to study objects in natural (or adjustable) conditions without preparation. This is the reason for the fast growth of variable pressure SEM's and environmental instruments. Microscopical visualisation without vacuum and coating allows to maintain the natural specimen structure as well as examine its behaviour under external influence (loading, chemical reactions, interaction with other solids, liquids, gases etc.) Another important issue for modern microscopy is the three-dimensional information. Most users try to recognize the three-dimensional structure inside an object from two-dimensional micrographs. Most existing microscopes can visualise either the object surface or a transmission image through a thin section. That means the three- dimensional internal object structure can only be investigated destructively. Conclusions about the three-dimensional structure can be obtained from the image of a surface or from a combination of several thin slices. In both ways, the information cannot be reliable. Even with the most delicate preparation or cutting methods the specimen structure can change dramatically. For living or exceptional objects any cutting is not even possible. Another significant aspect of modern microscopy is the quantitative interpretation of the images in terms of the microstructure of the object. Although most microscopes include or can be combined with powerful image processing systems, the interpretation of the contrast is still the main problem. For instance, a two-dimensional micrograph of the surface of an object does not allow to deduce accurate morphological characteristics since this would require information about the third dimension. Moreover, the image contrast is not only generated by the morphology of the object but by other factors such as composition, etc. The interpretation can be improved by independently detecting several signals from the same object area, for example in SEM by combining the secondary electron image with X-ray microanalysis. But even in that case the interpretation is still very cumbersome. On the other hand, reliable micromorphological information could be easily obtained from a set of thin flat cross sections which reveal only density information, from which case accurate two- and three-dimensional numerical parameters of the internal microstructure could be calculated. In this respect, an "ideal 3D microscope" from the user point of view should meet the following requirements: An object should be examined in natural conditions or with the possibility of selecting the environment ( chemical, mechanical ...), Any area inside the three-dimensional object should be visualised nondestructively in 3D at sufficient magnification, The numerical characteristics of the internal structure (morphology and composition) should be deduced reliably in any two-dimensional slice or from the complete three- dimensional area. Considering existing microscopical techniques, one can find that non-destructive information from the internal structure of an object in natural conditions can be obtained by transmission X-ray microscopy. The combination of X-ray transmission technique with tomographical reconstruction allows to get three-dimensional information about the internal microstructure. In this case any internal area can be reconstructed as a set of flat cross sections which can be used to analyse the two- and three-dimensional morphological parameters. 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. Recently there has been a significant improvement in the development of X-ray microscopes using synchrotron sources. However, these facilities are rather complicated and expensive and are not accessible for most researchers. On the other hand, the last few years have shown also a steady improvement in X-ray source technology so that now inexpensive compact sealed X-ray microfocus tubes can be produced with a very long lifetime. Because these sources emit polychromatic radiation one cannot use X-ray lenses for optical magnification. However, since the source spot size is small one can project the object over a large distance to the detector so as to obtain a geometrical magnification. In that case spatial resolution is limited by the X-ray spot size. At this moment, the attainable spot size in of the order of several microns but with the steady technological improvement one can expect submicron X-ray sources in the coming years. SKYSCAN-1172 DESK-TOP X-RAY MICROTOMOGRAPH The desk-top X-ray microscanner "SkyScan-1172" consists of a microfocus sealed X-ray tube 20-100kV/0-250uA with <5um@4W spot size and expected lifetime >10000hours, a precision object manipulator (8 axis), an X-ray CCD-camera and a fastest external computer with Dual Intel Xeon processors operated under Windows-XP Professional operation system. For microtomographical reconstruction transmission X-ray images are acquired from 200-3600 rotation views over 180 or 360 degrees of rotation. Both the X-ray tube and the camera operate under computer control. The system is adequately shielded against X-ray leakage. In its standard form the X-ray detector consists of a 4000x2300 pixels 12-bit digital cooled CCD-camera with fibre optic 2:1 coupled to an X-ray scintillator and digital frame grabber. In this way an integration time up to 10 sec. is possilble (0.2-2 sec typically). As a cost-efficient option, the instrument can be supplied with a CCD camera 1280x1024 pixels array coupled to scintillation screen by lens. A special software package has been developed for system control and microtomographical reconstruction. The microtomographical reconstruction algorithm is based on Feldkamp cone-beam reconstruction for circular and spiral acquisition with specific noise-reduction corrections. The software and hardware is speed-optimised for multiprocessing as 32 bit Windows-application and tested on different Intel and DEC processors operating under MS Windows. The reconstruction for one cross section of 1024x1024 float-point pixels from 200 projections takes less than 1sec. The software package includes also image processing and analysis procedures, stereo-visualisation, realistic three dimensional visualisation with possibilities in software to rotate and to cut 3D-object on the screen. The Windows-XP environments easily allow for network connections. ..... go to"news"> ..... go to home page >






This web-site is
created & updated
by Alexander Sasov

Microtomography (Micro-CT)

Nowadays different microscopical methods and equipments, each with different possibilities and limitations, are at the disposal of the scientist. Let's try to describe what may be an "ideal" microscope from the user point of view. Of course technical specifications, such as spatial resolution, accessible object size and so on are still very significant. But several other aspects become more and more important.

In the last years one can find a strong reorientation of most microscopical methods to study objects in natural (or adjustable) conditions without preparation. This is the reason for the fast growth of variable pressure SEM's and environmental instruments. Microscopical visualisation without vacuum and coating allows to maintain the natural specimen structure as well as examine its behaviour under external influence (loading, chemical reactions, interaction with other solids, liquids, gases etc.)

Another important issue for modern microscopy is the three-dimensional information. Most users try to recognize the three-dimensional structure inside an object from two-dimensional micrographs. Most existing microscopes can visualise either the object surface or a transmission image through a thin section. That means the three- dimensional internal object structure can only be investigated destructively. Conclusions about the three-dimensional structure can be obtained from the image of a surface or from a combination of several thin slices. In both ways, the information cannot be reliable. Even with the most delicate preparation or cutting methods the specimen structure can change dramatically. For living or exceptional objects any cutting is not even possible.

Another significant aspect of modern microscopy is the quantitative interpretation of the images in terms of the microstructure of the object. Although most microscopes include or can be combined with powerful image processing systems, the interpretation of the contrast is still the main problem. For instance, a two-dimensional micrograph of the surface of an object does not allow to deduce accurate morphological characteristics since this would require information about the third dimension. Moreover, the image contrast is not only generated by the morphology of the object but by other factors such as composition, etc. The interpretation can be improved by independently detecting several signals from the same object area, for example in SEM by combining the secondary electron image with X-ray microanalysis. But even in that case the interpretation is still very cumbersome. On the other hand, reliable micromorphological information could be easily obtained from a set of thin flat cross sections which reveal only density information, from which case accurate two- and three-dimensional numerical parameters of the internal microstructure could be calculated.

In this respect, an "ideal 3D microscope" from the user point of view should meet the following requirements:

An object should be examined in natural conditions or with the possibility of selecting the environment ( chemical, mechanical ...),
Any area inside the three-dimensional object should be visualised nondestructively in 3D at sufficient magnification,
The numerical characteristics of the internal structure (morphology and composition) should be deduced reliably in any two-dimensional slice or from the complete three- dimensional area.

Considering existing microscopical techniques, one can find that non-destructive information from the internal structure of an object in natural conditions can be obtained by transmission X-ray microscopy. The combination of X-ray transmission technique with tomographical reconstruction allows to get three-dimensional information about the internal microstructure. In this case any internal area can be reconstructed as a set of flat cross sections which can be used to analyse the two- and three-dimensional morphological parameters. 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. Recently there has been a significant improvement in the development of X-ray microscopes using synchrotron sources. However, these facilities are rather complicated and expensive and are not accessible for most researchers. On the other hand, the last few years have shown also a steady improvement in X-ray source technology so that now inexpensive compact sealed X-ray microfocus tubes can be produced with a very long lifetime. Because these sources emit polychromatic radiation one cannot use X-ray lenses for optical magnification. However, since the source spot size is small one can project the object over a large distance to the detector so as to obtain a geometrical magnification. In that case spatial resolution is limited by the X-ray spot size. At this moment, the attainable spot size in of the order of several microns but with the steady technological improvement one can expect submicron X-ray sources in the coming years.

SKYSCAN-1172 DESK-TOP X-RAY MICROTOMOGRAPH

The desk-top X-ray microscanner "SkyScan-1172" consists of a microfocus sealed X-ray tube 20-100kV/0-250uA with <5um@4W spot size and expected lifetime >10000hours, a precision object manipulator (8 axis), an X-ray CCD-camera and a fastest external computer with Dual Intel Xeon processors operated under Windows-XP Professional operation system. For microtomographical reconstruction transmission X-ray images are acquired from 200-3600 rotation views over 180 or 360 degrees of rotation. Both the X-ray tube and the camera operate under computer control. The system is adequately shielded against X-ray leakage.

In its standard form the X-ray detector consists of a 4000x2300 pixels 12-bit digital cooled CCD-camera with fibre optic 2:1 coupled to an X-ray scintillator and digital frame grabber. In this way an integration time up to 10 sec. is possilble (0.2-2 sec typically). As a cost-efficient option, the instrument can be supplied with a CCD camera 1280x1024 pixels array coupled to scintillation screen by lens.

A special software package has been developed for system control and microtomographical reconstruction. The microtomographical reconstruction algorithm is based on Feldkamp cone-beam reconstruction for circular and spiral acquisition with specific noise-reduction corrections. The software and hardware is speed-optimised for multiprocessing as 32 bit Windows-application and tested on different Intel and DEC processors operating under MS Windows. The reconstruction for one cross section of 1024x1024 float-point pixels from 200 projections takes less than 1sec. The software package includes also image processing and analysis procedures, stereo-visualisation, realistic three dimensional visualisation with possibilities in software to rotate and to cut 3D-object on the screen. The Windows-XP environments easily allow for network connections.

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