X-Ray Micro-Tomography

SkyScan-1072 High-resolution desk-top micro-CT system

RECOMMENDED SAMPLE SIZE : 1.5cm diameter x maximum height of 3cm. RESOLUTION: 4 microns (this can be reached if sample has diameter of < 5mm). MAGNIFICATION: max 120x

SkyScan-1076 in vivo micro-CT system

MAXIMUM SAMPLE SIZE : 6.8cm diameter x maximum length of 15cm of which 2.5cm can be imaged in any one run. RESOLUTION: 9, 18 or 35 microns.

Any conventional optical or electron microscope provides visualisation of 2-dimensional images of a specimen surface or thin section. In most cases a final conclusion about the original 3-dimensional object structures cannot be made on the basis of 2-dimensional information.

3-dimensional information of an object's structures can be obtained by cutting them into thin slices, which can then be visualised in the microscope (light or electron). After storing the images a 3-dimensional structure model can be reconstructed from the 2-dimensional information. This method is not only cumbersome but it is unreliable since the object structure can be altered by the preparation technique and the distance between the slices is usually too coarse to avoid loss of 3D information.

An x-ray (radiography) system produces 2-dimensional shadow images of the complete internal 3-dimensional structures, but in a single 2-dimensional shadow projection the depth information is completely mixed. Only an x-ray tomography system allows us to visualise and measure complete 3-dimensional object structures without sample preparation or chemical fixation.
Existing Medical X-ray Tomography systems have resolutions of 1 to 2mm. To observe the microstructure of materials we need an instrument with several orders of improvement in resolution.

Skyscan, a Belgian based company have spent the past 15 years developing such an instrument. The rapid improvements in electronics and mechanics have been incorporated into a desktop instrument. The Skyscan 1702, X-ray Micro-tomography System was installed in February 2002. Since its installation the system has been applied to a diverse range of samples including bone, teeth, plant materials, diamond bearing rocks, rock core drill samples, dental materials, metal alloys, whole fish, foodstuffs and soft tissue samples.

The Skyscan 1076 is designed for image small, live animals such as mice and rats. The standard scan time for live animals is approximately 30 minutes during which the animals must be anaesthetised. The Imaging chamber is environment controlled ensuring that body temperature is maintained during imaging. The system has the facility for monitoring heart rate, breathing rate and motion. These monitoring systems can be used to synchronise x-ray exposures to specific times in the animal’s breathing/heart beat cycles so that the effects of movement on the tomograms is minimised.

How does it Work
A Micro-focused X-ray source illuminates the object of interest, which is positioned precisely within the x-ray beam. The x-ray Shadow Images are acquired by a sensitive X-ray Camera. During the Image Acquisition the sample is rotated step at a time through 180 degrees. Images are recorded at each rotation. Using complex software 2 dimensional images (or slices) based on x-ray density can be recalculated from the x-ray Shadow Images.

One or more of these 2D images can be stored in the computer and then using another software package a stack of these images can be added together to form a 3D model of the sample. From this model it is possible to make virtual cuts or slices of the object in any direction. The software allows for numerical data to be calculated from the morphology of the samples structure.

Basis principles of micro-tomography
Any x-ray shadow image corresponds to a two-dimensional projection from the three-dimensional object. In the simplest case we can describe it as a parallel x-ray illumination. In this approximation each point on the shadow image contains the integration of absorption information inside the three-dimensional object in the corresponding partial x-ray beam.

For parallel geometry one can divide the problem of a 3-dimensional reconstruction from 2-dimensional projections into the serial reconstruction of 2 dimensional object slices from one dimensional shadow lines.

Consider an example where we have an object with only one significant absorption in an unknown place. In the one dimensional shadow line we will have a decreasing intensity in the shadow of absorption in the object area.

Now we can initialise in the computer memory an empty array of pixels (picture elements) corresponding to possible object displacement. We must be sure that all parts of the reconstructed object will be inside the field of view.

Because we have the position of the shadow from the absorption points of the object, we can mark on the area of reconstruction in the computer memory all possible positions of absorption points inside the object as lines.

If we now rotate our object and repeat this operation, each new rotation of the position of the object will add to the area of reconstruction the lines of possible object positions corresponding to the position of the shadow. This operation is called "back projection". After several rotations we can localise the position of the absorption point inside the area of reconstruction. By increasing the number of shadow projections from different angles of rotation the localisation becomes more and more defined.

In the case of reconstruction from an infinite number of projections one can get an image with a good definition of the absorption area position inside the initial object. At the same time a blur area will accompany the pointer image because it is produced as a superposition of lines with all inclinations. Now we know what image will be produced from the pointer object and we can "pre-correct" the initial information in absorption lines to make the resulting image more corresponding to the real object. This correction adds some "negative absorption" outside the point of the object shadows to eliminate the positive blur in the back projection process (convolution).

From the reconstructed 2D "cross-Sections" a 3D object can be created by the addition of 2 or more of the serial 2D images.

The example to the left is a 3-D reconstruction of a Calcium depleted human bone recorded with the X-ray Micro-tomography System.

For more information on instrument capabilities and examples of applications go to the manufacturers website at http://www.skyscan.be