S in( E) is the energy spectral density of the incident photons. (1) does not hold and must be replaced by I t =∫S in (E) exp −∫μ(x,y,E) d s d E. If the radiation used to probe the specimen is monochromatic as in γ-tomography with 137Cs source, the reconstruction yields a good representation of the actual distribution of μ values. In case of transmission tomographic imaging, it is the reconstruction of this approximate distribution in the CT plane. The function f( x, y) is the distribution of μ( x, y, E eff), the linear absorption coefficients at the effective energy E eff of the penetrating radiation. CT obtains the object function f( x, y) from the set of projection measurements P φ( t). The projection data recorded for tomographic reconstruction of a single slice in case of parallel beam configuration is given by P φ (t)= ln I 0 (φ) I t (φ) = ∫ (φ,t) f(x,y) d swhere φ is the projection angle, t is the distance in the detector plane from the projected rotation axis and I t and I 0 are detected beam intensities with and without the object in its path. For an object surrounded by air, this is accomplished by taking the logarithm of the ratio of the incident intensity to the transmitted intensity. The initial data, which are acquired in the form of intensity measurements, must be converted to projection data, which approximate the line integral of the linear attenuation coefficients characterising the material within the object. The goal of tomographic imaging in its simplest terms, is to reconstruct a two-dimensional array of linear attenuation coefficients from a series of one-dimensional transmission profiles. The huge cost involved in customised detectors and other sub-systems have been a bottleneck in their generalised use for NDT&E. Recent developments in the field of detector technology, X-ray equipment, mechanical system designs and desktop computers based on fast processors have a cumulative effect on industrial CT systems. Wide range of industrial applications, which include not only object dimensions but also material characteristics, ask for a specific implementation of a given configuration. As a result, the technology is continuously evolving and different configurations of industrial tomographic imaging systems designed for visualisation of defects at the micron level at one end, to systems for scanning large diameter rocket motors can be found. Though the basic principle of image reconstruction in case of industrial tomography is similar to that developed for medical systems, there exists a wide difference in application areas. The industrial application of CT is important in areas where reliability, safety and quality control are of fundamental importance and where material faults may have implications. ![]() An industrial CT system must be tailored to the object under inspection and to the type of information required from the inspection. ![]() The great power of CT lies in its ability to unfold X-ray absorption data taken from many different angles and produce a map of local X-ray absorption at all points inside an object. This new technique of CT with X-rays or γ-rays inspired other workers to experiment with these methods in non-destructive evaluation of non-medical objects where some of the constraints like patient movements and dose restrictions encountered in medical tomography are absent. These advances revolutionised much of medical radiography and this was recognised with the award of the Nobel Prize for Medicine to Cormack and Hounsfield in 1979. Cormack and Hounsfield led the foundations of CT imaging using radioisotope sources and X-rays. Computed Tomography (CT) was originally developed as a medical diagnostic tool for providing unambiguous quantitative information about interior details of human body.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |