Computer Tomography

Introduction            Tomographic radiography was developed approximately 60 years ago and is now a major clinical imaging modality. Many years lapsed, however, before Kuhl and Edwards applied similar tomographic principles to the problem of ?-ray image formation in medical radioisotope scanning in 1962.            It was not until Anger invented the tomographic scanner in 1968 that was widespread attention was directed to nuclear tomography. The tomographic scanner is basically a large diameter rectilinear scanner which uses camera-like electronics to provide correction signals so that six images can be obtained simultaneously, each displaying a different plane in focus with all other planes out of focus. A tomographic scanner utilizing this concept was marketed later under the name of PHO/CON.

            Around 1970, gamma cameras equipped with slant-hole collimators began to be used for emission tomography. Tomographic images of the brain and liver were obtained using analog reconstruction techniques. The application of digital techniques to tomography was described in 1973 by Freedman using the slant-hole collimator and incremental rotation of the collimator during data acquisition. Even though the digital tomographic techniques had been worked out mathematically, software had not yet been written (Ahluwalia, 1989).

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History            The history of the development of computerized axial tomography is complex. James Bull, the pioneer neuroradiologist, reviewed the history of computed tomography and says that seldom in the history of medicine has a new discovery swept the world quite as quickly as computed tomography. Godfrey Hounsfield and Alan Cormack received the Nobel Prize jointly in 1979 “for the development of computer-assisted tomography.”            In the 1960s Godfrey was working at EMI Ltd in Middlesex.

He was interested in pattern recognition and also in computers. He had worked on radar with the RAF in the Second World War and had built the first solid-state electronic computer in the UK. Hounsfield was looking at internal structure and considered a closed box with an unknown number of items inside. The box could be looked at from any directions using X-ray source and a radiation detector. The results of the transmission readings could then be analyzed by the computer and then presented three-dimensionally as a series of slices in a single plane. Hounsfield developed a mathematical approach to determine the nature of the objects in the box in a process of reconstruction (Thomas, Banerjee, & Busch, 2005).            The introduction of computer tomography (CT) into medicine by G.

Hounsfield in 1971 (skull CT) and computerized transverse axial scanning (tomography) in 1973 was a revolutionary event comparable with the discovery of X-rays by W.C. Röntgen in 1895. As early as 1975, the first normal CT scan of the upper abdomen was reported and in the same year, the first pathological findings from abdominal diseases (relating to the liver) were also presented (Kuntz & Kuntz, 2006).

Principle            In computer tomography, the attenuations of many finely focused X-rays are measured by detectors and converted to electrical signals. These values are transmitted to a computer. Subsequently, the absorption value of each image point is calculated and displayed in a complex digital image. The transmission of X-rays through the body occurs in the form of fan beams and is recorded by a rotating detector fan (3rd CT generation). The 4th CT generation is characterized by a static detector crown, spanning 360 degrees, around which the X-ray source continually rotates. More advanced spiral CT facilitates spiral scanning, permitting continuous imaging of the analyzed area while the patient holds his/her breath. This provides accurate anatomical data without respiratory artifacts and with optimum exploitation of the CM bolus.

            In contrast to ultrasonography, which is based in the recording and imaging of the reflection of sound waves between tissues with varying acoustic impedance, the radiological signal is produced by differences in absorption. With the radiation doses used (100-140 kv), the absorbed dose of energy corresponds to 0.013 Gy (1.

3 rad) per tomographical slice. By using many finely focused X-rays, the dose is largely restricted to the body layer to be imaged. Therefore, only a relatively low scatter of radiation has to be taken in consideration. The radiation exposure of a CT scan is comparable with that of a plain radiograph of the abdomen (Kuntz & Kuntz, 2006).

Advantages of Computer Tomography            Computed tomography presents significant advantages over conventional radiography. Image interpretation can be greatly simplified, as there is no image overlap, and by stacking a series of 2D CT images, one can reconstruct a 3D picture of the inspected specimen. This 3D reconstruction of the test object allows a complete examination of a structure. It was reported that tomographic images are of much better quality than those obtained with radiographym and density differences up to 0.1% can be observed. Moreover, the contrast resolution of CT has been estimated to be 0.1 to 0.

2% better than that of X-ray films, and structural noise is often absent. An additional feature of CT imaging is its faster speed than conventional radiography, as no film processing is required. While 50-75 µm voxels are too coarse for damage quantification in composites, 25 µm voxels are obtainable from a commercial CT scanner. High resolution CT is also known as x-ray tomographic microscopy and is capable of detecting matrix cracking and fiber fracture in composites. Despite certain advantages, CT remains expensive, and in the case of large specimens, data collection can be time-consuming and costly (Brown, 1999).

Computer Tomography in Cancer Diagnosis            Computer tomography is the most commonly used modern standard method for multislice imaging of the body. This method produces continuous slice images of the body based on and X-ray technique. The X-ray tube rotates continuously around the examined part of the human body. The X-ray are variously absorbed by different tissues and the bones and reach the X-ray detectors of the opposite side. Here the partially absorbed X-ray is measured is measured is measured and transferred into different gray steps by computer calculation. Since, for example, X-rays are more absorbed by bones than by lungs, the bones appear more clearly than the lungs in computer tomography images. The continuous movement of the examined part of the body within the rotating X-ray beam produces a helical set of data.

With this data, the computer calculates two-dimensional images. Using these single images, many diseases can be detected. Thus, computer tomography is also used for the detection and follow-up of malignant tumors. Due to the better soft tissue contrast, magnetic resonance imaging is predominantly used to examine diseases of the central nerve system. In contrast, computer tomography has an essentially shorter examination period and a higher spatial resolution.

Therefore computer tomography is only slightly susceptible to artifacts caused by motion or breathing of the patient. Due to the possibility of generating continuous slice images, computer tomography is also applied in treatment planning of radiation therapy (Krebsforschungszentrum, 2002).Conclusion            Computer Tomography is a potent device to examine the absolute geometric pieces of a broad choice of diverse equipments. With onward software improvement the equipped region of CT will broaden and the recognition in the developed field will nurture more. Moreover, the development of methods combining the visual accuracy of computerized tomography with the innocuousness of ultrasound will probably be available in the near future (Gourtsoyiannis & Ros, 2005).

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Brown, R. (1999). Handbook of Polymer Testing Physical Methods. New York: CRC Press.Gourtsoyiannis, N. C., & Ros, P.

R. (2005). Radiologic-Pathologic Correlations From Head To Toe: Understanding The Manifestations of Disease. Burmingham, UK: Springer.Krebsforschungszentrum, D. (2002).

Current Cancer Research 2002. Germany: Springer.Kuntz, E., & Kuntz, H.

-D. (2006). Hepatology Principles and Practice: History, Morphology, Biochemistry.

Heidelberg: Springer.Thomas, A., Banerjee, A. K., & Busch, U.

(2005). Classic Papers In Modern Diagnostic Radiology. Birmingham UK: Springer.