Radiography:
The process or technique of producing images of an opaque object on photographic film or on a fluorescent screen by means of radiation.
The process or technique of producing images of an opaque object on photographic film or on a fluorescent screen by means of radiation.
Tomography:
A technique of X-ray photography by which a single plane is photographed, with the outline of structures in other planes eliminated.
A technique of X-ray photography by which a single plane is photographed, with the outline of structures in other planes eliminated.
Computed Tomography
Computed Tomography (CT) is a powerful nondestructive evaluation (NDE) technique for producing 2-D and 3-D cross-sectional images of an object from flat X-ray images. Characteristics of the internal structure of an object such as dimensions, shape, internal defects, and density are readily available from CT images. Shown below is a schematic of a CT system.
The test component is placed on a turntable stage that is between a radiation source and an imaging system. The turntable and the imaging system are connected to a computer so that x-ray images collected can be correlated to the position of the test component. The imaging system produces a 2-dimensional shadowgraph image of the specimen just like a film radiograph. Specialized computer software makes it possible to produce cross-sectional images of the test component as if it was being sliced.
Brief history of CT:
In 1895 Roentgen discover x- ray.
In 1917 Rodan develops reconstruction mathematics.
In 1963 Cormack formulated X-Rays absortion in tissues.
In 1972 Hounsfield demonstrate CT.
In 1975 1st whole body CT.
In 1983 EBCT demonstrated .
In 1989 spiral CT demonstrated.
In 1991 multislice CT introduce.
Principles of operation:
1) Hounsfield original approach was translate an object through collimated X-ray beam and detect an image projection .
2) The projection is digitally rendering of X-ray attenuation through the object.
3) Multiple projection are obtained by rotating the object between translation.
4) A computer generated matrix of section of object is produce by back projection reconstruction . a special application of simultaneous equation.
How a CT System Works;
The imaging system provides a shadowgraph of an object, with the 3-D structure compressed onto a 2-D plane. The density data along one horizontal line of the image is uncompressed and stretched out over an area. This information by itself is not very useful, but when the test component is rotated and similar data for the same linear slice is collected and overlaid, an image of the cross-sectional density of the component begins to develop. To help comprehend how this works, look at the animation below.
The imaging system provides a shadowgraph of an object, with the 3-D structure compressed onto a 2-D plane. The density data along one horizontal line of the image is uncompressed and stretched out over an area. This information by itself is not very useful, but when the test component is rotated and similar data for the same linear slice is collected and overlaid, an image of the cross-sectional density of the component begins to develop. To help comprehend how this works, look at the animation below.
In the animation, a single line of density data was collected when a component was at the starting position and then when it was rotated 90 degrees. Use the pull-ring to stretch out the density data in the vertical direction. It can be seen that the lighter area is stretched across the whole region. This lighter area would indicate an area of less density in the component because imaging systems typically glow brighter when they are struck with an increased amount of radiation. When the information from the second line of data is stretched across and averaged with the first set of stretched data, it becomes apparent that there is a less dense area in the upper right quadrant of the component's cross-section. Data collected at more angles of rotation and merged together will further define this feature. In the movie below, a CT image of a casting is produced. It can be seen that the cross-section of the casting becomes more defined as the casting is rotated, X-rayed and the stretched density information is added to the image.
In the image below left is a set of cast aluminum tensile specimens. A radiographic image of several of these specimens is shown below right.
CT slices through several locations of a specimen are shown in the set of images below.
A number of slices through the object can be reconstructed to provide a 3-D view of internal and external structural details. As shown below, the 3-D image can then be manipulated and sliced in various ways to provide thorough understanding of the structure
Advantages of CT
1) Better contrast resolution
2) No superimposition of tissue
3) 3D imaging
4) less scatter radiation
5) bone mineral assay
6) CT has proved successful for the visualization of pituitary tumours.
7) It would accurately differentiate between symmetrical bilateral adrenocortical hyperplasia and a unilateral adrenal mass.
8) Anatomical structures and pathological processes in the central nervous system has been examined usefully in small animal.
9) The method is useful in diagnosis of tumors malformations, inflammation, degenerative and vascular diseases and trauma
10) Multiplayer quantities CT has been used for bone mineral analysis in dog.
Limitation of CT:
1) Spatial resolution
2) Relatively high patient doses.
3) X- axis reformation
4) Distant artifacts
Reference:
Veterinary radiology basic principles and radiographic positioning by A.P.Singh/ Jit singh Editio006E 2009.
- ^ "computed tomography—Definition from the Merriam-Webster Online Dictionary". http://www.merriam-webster.com/dictionary/computed+tomography. Retrieved 2009-08-18.
- ^ a b Herman, G. T., Fundamentals of computerized tomography: Image reconstruction from projection, 2nd edition, Springer, 2009
- ^ Smith-Bindman R, Lipson J, Marcus R, et al. (December 2009). "Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer". Arch. Intern. Med. 169 (22): 2078–86. doi:10.1001/archinternmed.2009.427. PMID 20008690.
