The resulting image has nearly nothing in common with a CT image of a human head. The horizontal dark line in the upper part of figure 8 a) is again resulting from the absorption by the bones above the eye-sockets. In figure 8 a) the back projection from the 90° and from the 0° signal have been overlayed. To do so, the intensity of the detected signals are projected back from the detector to the source and overlayed in the area of the X-rayed object. The CT-image can be calculated from the absorption signals detected with the CCD-line array while the source and the detector circle around the head by a so called back projection. 7: Absorption signal detected when a slice of a head is X-rayed ( in this example the source only passes through a quarter circle). In the actual horizontal direction of the X-rays the optical path through the bones above the eye-sockets is very long and so most of the X-rays are absorbed in these bones, resulting in the dark line on the detector!įig. Observe the dark line in the upper part of the detected CCD-signal. How is a CT image extracted from the detected X-ray absorption? Figure 7 shows the absorption detected on the CCD-detector array when a slice of the head from figure 6 is X-rayed. 6: Compter tomographic 3D-image of a human head Dark areas mark bone, grey areas tissue and white areas antras or air.įig. The image below shows a computer tomography 3D-reconstruction of 17 slices of a human head. 5: Clinical computer tomograph (left) and CT-image (right ) ©02 In micro-CT, the spacial resolution is (in 2013) in the range of 1/1000 to 1/2000 of the sample diametre and it reaches a few hundred nanometres.įig. Figure 5 shows a picture of a real CT system and the quality of images taken with such systems. 75 dpi or about 2000 million voxel (=volumetric pixel) for a 75 kg person). The disadvantage is the necessity of a large vacuum tube and electron beam steering systems.Ī large number of CTs is used for medical applications with the capability to scan and image volumes as large as the human body with a maximum resolution of about three points per millimeter (i.e. The advantage of this system is the absence of moving parts. The detector is a stationary ring shaped array. 4) directed to the target points, where X-rays are generated. In the fifth generation systems the X-ray source consists of a ring shaped target anode and several electron beams (marked green in fig. In the fourth generation systems the X-ray source rotates around the patient. These systems can resolve different tissues more precisely. Since 2005 systems using two source-detector units with different photon energies are available. 2007 these systems provided up to 320 image slices per second or a 12 frames per second live-video of an entire heart. A modern variant is the Helix-CT, where the patient is shifted with constant speed. The patient is shifted along the rotation axis to get the whole information about the investigated volume. In more recent systems the detector is a 2D-array taking up to hundreds of image slices in one rotation ("multislice", fig. In the third generation systems one X-ray source and a ring segment detector array at the opposite side of the patient rotate synchronously around the patient to acquire the image data. In the second generation systems up to ten sources and detectors increased data acquisition speed. ![]() For the next image, the patient is shifted along the rotation axis. Then source and detector are rotated a few degrees before shifting them for the data acquisition of the next projection direction. ![]() In the first generation systems one X-ray source and one detector are shifted synchronously perpendicular to the axis of rotation to take the data of one projection direction. Some steps in the technical development of CTs are illustrated below. ![]() Cormack and Godfrey Hounsfield and still is subjected to continuous improvement. A 3D-image can then be calculated from a stack of many slice images.Ĭomputer tomography has been developed since 1957 by the groups of Allan M. From the entity of all these projections in different directions, each slice image can be calculated by Fourier-transformation. The X-ray projection in one direction is the so called Radon-transformed of the sample. Either the sample is rotated between the X-ray source and the detector or (mostly in medical applications) the source-detector unit rotates around the sample. The principle is, to X-ray the object in different directions to get more information about the distribution of the absorption contrast than with radiography. X-ray computer tomography (CT) allows to create 3D-images of the inner structure of objects without opening them.
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