The Fundamentals

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Projections acquired during scanning of a bird skull.

The following summary is to provide the basic principles of how X-ray computer tomography (CT) works. All CT imaging systems are based on the same basic set up. A sample is placed between an X-ray source and a detector and rotated while images, or radiographs, are collected. The x-rays leave the source, pass through the sample and then hits the detector. Each radiograph image is a 2D projection of the 3D object and the rotation between images is between 0.5 and 0.01 degrees, with over 1000 images collected per scan.

As the x-rays pass through the sample they interact with it, a process known as attenuation. The amount to which any material attenuates x-rays is controlled by the density and atomic number of the material, and the energy of the x-rays. For complex samples containing several different materials, each material will attenuate x-rays differently. In most scanning systems the energy of the x-rays can also be controlled. 

X-rays only travel in straight lines (referred to as beam paths), so the x-rays hitting each point on the detector pass through a different part of the object and the total attenuation and intensity at each point will vary depending on the structure of the sample. A small amount of the energy of each x-ray is absorbed as it interacts with each atom in the beam path. The higher the energy of the x-rays, the more material the x-ray will penetrate before it is fully absorbed. High energy x-rays can therefore pass through greater thicknesses of denser material before they are absorbed. The energy of the x-ray beam should therefore be chosen carefully depending on the size and composition of the sample being investigated. 

When the X-ray beam is turned on, an image of the object is projected onto the detector, a familiar example is of a doctor taking a radiograph of a broken bone. The bone absorbs more x-rays than the surrounding muscle, and so shows up in the image. The greyscale value of each pixel in the image represents the number of x-rays that passed through the sample and reached the detector. As the density of the material controls the amount of x-rays absorbed, the thicker the material, the fewer x-rays reach the detector.  

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