Introduction : Phase Contrast Imaging
XRT Limited is the pioneer of Phase Contrast X-ray Imaging using technologies originally developed by CSIRO (Commonwealth Scientific Industrial Research Organization), Australia.
When X-rays penetrate a sample, two major events happen.
- Absorption - X-rays are attenuated by the sample through mechanisms such as scattering and fluorescence.
- Phase Change - X-rays are "bent" or deflected by refraction as they pass through the sample.
In X-Ray Imaging Systems these two effects lead to Absorption Contrast and Phase Contrast respectively. There are three important things to understand about these contrast mechanisms:
- Phase Contrast effects are significantly larger than absorption contrast in the normal X-ray energy range.
- Absorption Contrast is inversely proportional to (kV)3 and it drops off very rapidly with increasing X-ray energy. Phase Contrast on the other hand is only inversely proportional to kV, so it drops off much less rapidly with increasing X-ray energies.
- Absorption Contrast is proportional to Z (atomic number)4 whereas Phase Contrast is approximately proportional to Electron Density.
Other commercially available X-ray Micro CT and imaging systems are only able to image the absorption contrast component of a sample. This has generally limited the application of X-ray imaging to materials of relatively high atomic number, but now, using the latest advances in source and detector technology (in conjunction with patented propagation methodology) XRT can deliver practical phase contrast X-ray imaging systems with the following advantages:
- Systems enabled for phase contrast can generally achieve much higher image contrast (quality) on all types of samples, compared to those using absorption contrast alone. Simply put, features become visible or more visible with phase contrast.
- Phase contrast systems are able to use higher X-ray energy and still achieve good contrast. This important fact enables the imaging of thicker/more dense samples thereby increasing the types of samples that may be imaged while easing the requirements for sample preparation.
- Low atomic number structures can be imaged by phase contrast which would be impossible by absorption contrast alone, phase contrast is also particularly sensitive to voids, cracks, delamination etc.
Examples of samples that benefit from Phase Contrast X-Ray Imaging include low atomic number materials - biological samples, structural composites, and microelectronic samples such as 3D Chip Scale Packages where delamination and voiding is particularly difficult to see by traditional means.
Introduction : Phase Retrieval
Phase Contrast imaging is particularly useful for X-ray imaging of structures made from weakly absorbing materials, either alone or in the company of more dense structures. Good examples are biological samples like small animal parts, or samples composed of low atomic number materials such as structural composites and some microelectronic samples such as 3D CSPs. Often, only phase contrast can give a sensible degree of contrast in X-ray imaging of samples in their native state.
The observed natural edge enhancement in a phase contrast image is produced by refraction/propagation effects and although it gives excellent image contrast, it is not linear and so does not in itself always give an accurate indication of the thickness or density of the sample. Phase retrieval however transforms this observed intensity distribution into a retrieved phase distribution (see Fig.1) which can more accurately reflect sample structure. Application of phase retrieval to an in-line X-ray image thus allows one to obtain important qualitative information directly from the phase retrieved image (such as the shape of the object), as well as quantitative data about the internal composition of the sample (i.e. projected electron-density distribution).
The following series of images is an example of phase retrieval at work. Fig. 2 is an image of a latex sphere showing excellent phase contrast with very strong edge enhancement and good resolution of internal bubbles. From this image alone however it would be impossible to determine the cross sectional shape of the sample.
Fig. 3 is the intensity distribution AFTER phase retrieval. The observed intensities in the image have been transformed into retrieved phase signatures and here it is quite apparent that the sample is spherical.
Fig. 4 is an X, Y plot of the intensity distribution, again the spherical shape is readily apparent.
Introduction : Computed Tomography (CT) Imaging
Tomography is imaging by sections or sectioning. A device used in tomography is called a tomograph, while the image produced is a tomogram. The word is derived from the Greek word tomos which means "a section", "a slice" or "a cutting". X-ray micro tomography progressively rotates the sample through 180 degrees or 360 degrees taking an image at preset intervals, typically every half or quarter degree. The images are then reconstructed using a back-projection algorithm to allow a 3D model of the structure to be derived which can then be visualized using specialized software.
Advantages of tomography include:
- CT completely eliminates the superimposition on the image of structures outside the area of interest.
- Because of the inherent high-contrast resolution of CT, differences between structures that differ very little in physical density can be more easily distinguished from one another. Phase tomography further increases the contrast in the images thus extending the technique to more difficult structures.
- Data from a single CT scan can be viewed as cross section images in different planes without any mechanical cutting or sectioning. This is sometimes referred to as multi-planar imaging.
Tomograms reconstructed from phase images have extra contrast because of the edge enhancement. This can be a very good thing when structural contrast is naturally poor, such as may be the case with weakly absorbing materials or with voids and cracks. These structures can be extremely difficult to see with absorption contrast alone in which case phase can be the only way forward.
However, in some cases the reconstructions from the unretrieved phase projections can lead to excessive noise in the slices and significant artifacts in the cross sections. This can make thresholding of structures if interest extremely difficult and quantitative 3D measurement impossible.
Phase retrieval however transforms this non-linear observed intensity image into a retrieved phase distribution which will more accurately reflect sample structure and also has the effect of smoothing the background of the reconstructed slices.
The result is a high contrast, metrologically accurate reconstruction of weakly absorbing structures that cannot be obtained any other way.
A comprehensive discussion of Micro CT systems is now available for download.