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In this report I will start by exploring Essay

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In this report I will start by exploring the history of the Computerised Tomography (CT) scanner and the technological advances which have made this type of medical imaging one of the most successful in its field. In addition, I will give a detailed explanation of the physics used to generate and manipulate a three-dimensional image. These images are used by physicians to diagnose cancers and vascular diseases or identify other injuries within the skeletal system, which can cause millions of deaths each year.

This area of research has been chosen because I plan to enter the world of medicine in the next academic year.

Medicine is constantly changing and developing. Cost containment and limitations reimbursed for high-tech studies such as CT and Magnetic Resonance imagining (MRI) are part of the future for the health care system. For CT to grow, or at least survive, it must provide more information than other imaging modalities in a cost-effective, time-efficient manner and at this present time it is able to achieve its aim.

History: Computed Tomography (CT) imaging is also known as “CAT scanning” (Computed Axial Tomography). Tomography is from the Greek words “tomos” meaning “slice” and “graphia” meaning “describing”.

The first CT scanner was invented in Britain by the EMI Medical Laboratories in 1973 and was designed by the engineer Godfrey N Hounsfield. Hounsfield was later awarded the Nobel Peace Prize for his contributions to medicine and science. Figure 1. 0 (below left) show the first ever CT scanner produced, with its designer Hounsfield: Foster E. (1993) and Imaginis. com state that: “the first clinical CT scanners were installed between 1974 and 1976. ” The original systems were dedicated to head imaging only, but “whole body” systems with larger patient openings became available in 1976.

CT became widely available by about 1980. According to Imaginis. com, at this present time there are approximately 6,000 CT scanners in the United States and about 30,000 worldwide. However, it should be noted that many third-world counties do not have the financial capability to purchase CT scanners and as a result do not posses them. The first consignment of CT scanners developed by the EMI took several hours to acquire the data for a single scan. In addition, it would take days to reconstruct a single image from this raw data.

Bell J.(2006), suggest that modern CT scanners can collect up to 4 slices of data in about 350ms and reconstruct a 512 x 512 matrix from millions of data in less than a second. Since its development 36 years ago CT has made advances in speed, patient comfort and resolution . A bigger volume can be scanned in less time and artefacts can be reduced as faster scans can eliminate faults caused from patient motion. Another advance took place in 1987. Bushong C. S (2004) suggests that, in the original CT scanners the x-ray power was transferred to the x-ray tube by high voltage cable; however modern CT scanners use the principle of slip ring.

This is explained in more detail under ‘advances’. Figure 1. 1 (below right) shows what a modern CT scanner looks like. CT examinations are now quicker as well as being more patient-friendly. Much research has been undertaken in this field, which as a result has led to the development of high-resolution imaging for diagnostic purposes. In addition, the research has also reduced the risk of radiation by being able to provide good images at the lowest possible x-ray dose. Principles and Components of CT: CT scanners are based on the x-ray principle; x-rays are high-energy electromagnetic waves which are able to pass through the body.

Roberts P. D (1990) states, that as they are absorbed or attenuated at different levels, they are able to create a matrix of differing strength. In x-ray machines this matrix is registered on film, whereas in the case of CT the film is replaced by detectors which measure the strength of x-ray. To understand how a CT scanner works in more detail, I shall start by looking at the equipment used. Firstly, we must analyse the basic components which make a CT scanner work. These are the gantry, operating console and a computer. Figure 1. 2 shows the order in which the information passes.

Figure 1.2 shows only basic components; other components will be explained later in the course of this report. Arguably, the most important part of a CT scanner is the gantry. Gantry: According to Foster E (1993) and Impactscan. org, the gantry consists of an x-ray source. Opposite the x-ray source, on the other side of the gantry, is an x-ray detector. During a scan a patient will lie on a table which slides into the centre of the gantry until the part of the body to be scanned is between the x-ray source and detector. The x-ray machine and x-ray detector both rotate around the patient’s body, remaining opposite each other.

As they rotate around, the x-ray machine emits thin beams of x-rays through the patient’s body and into the x-ray detector. Figure 1. 3 shows the inside of a gantry. The detectors detect the strength of the x-ray beam that has passed through the body. The denser the tissues, the less x-rays pass through. The x-ray detectors feed this information into a computer as shown is Figure 1. 3. Different types of tissue with different densities show up in a picture on the computer monitor as different colours or shades of grey. Therefore, an image is created by the computer of a ‘slice’ (cross- section) of a thin section of a body.

Before advancing any further we must understand the physics behind this process. X-ray tube: The X-ray tube inside the gantry (figure 1. 4) produces the X-ray beams by converting electrical energy into an electromagnetic wave. Graham T. D (1996) and Bbc. co. uk/dna/h2g2 suggest that, this is achieved by accelerating electrons from an electrically negative cathode towards a positive anode. As the electrons hit the target they are decelerated quickly, causing them to lose energy which is converted into heat energy and X-rays. The anode and cathode form a circuit which is completed by the flow of electrons through the vacuum of the tube.

The basic layout of an X-ray tube is shown below (figure 1. 4). Figure 1. 4 shows that a high voltage is applied between the anode and the cathode. This very high potential is supplied by a high-voltage generator. The high voltage is the provider of the electrical energy needed for conversion and thus production of X-ray beams. A generator is a device that converts mechanical energy into electrical energy. The process is based on the relationship between magnetism and electricity. In 1831, Faraday discovered that when a magnet is moved inside a coil of wire, electrical current flows in the wire.

Three-phase Generator: Three-phase generators are typical of CT scanners. Ogborn J. (2001) and koehler. me. uk, state that this process can be thought of as three phase AC generators combined into one. The poles of the permanent rotating armature magnet swing past each of the non-permanent stator magnets. This induces an oscillating voltage across each of the three coils. Figure 1. 5 shows a three phase generator. As we can see from figure 1. 5, each of the three coils has a wire leading from it. These three wires join together to form the purple wire that leads to the purple terminal see from figure 1.

5 As the three separate coils are arranged 120i?? apart, the oscillations of each of these are 120i?? out phase. This means the purple (or neutral) wire can be quite thin since the different phases add up to approximately zero. The potential difference generated needs to be high; high potential difference has a number of advantages in CT scanners. High potential difference reduces bone attenuation (greater penetration) allowing wider range of image (larger grey scale as bone is not merely white as on normal x-ray- (this will be explained later).

In addition, the higher the radiation intensity at the detectors in the gantry, the better the information acquired. Gantry: The Collimator: In this section we shall look at the gantry (figure 1. 3) in more detail. Figure 1. 6 shows a diagrammatic representation of the inside of a gantry. According to Foster E (1993), inside the gantry is a beam restrictor called, collimator. Beam restrictors are lead obstacles placed near to the anode of the X-ray tube (figure 1. 4) and are used to control the width of the X-ray beam allowed to pass through the patient.

Beam restrictors are needed as they keep patient exposure to a minimum and also reduce scattered rays. This is very important as X-rays are produced by a centre spot on the anode; they are not all produced at the same point. In addition, restrictors also maintain beam width travelling through the patient, which as a result affects the image quality (stronger beam means better image). The most effective form of a beam restrictor is a collimator. This is situated in front of the X-ray tube and consists of two sets of four sliding lead shutters which move independently to restrict the beam.

The Filters: By looking at figure 1. 6 we can see another apparatus positioned between the collimator and the X-ray tube. This is the filter and its job is to remove the long wavelength X-rays produced from the X-ray tube. Impactscan. org suggests that, the X-ray tube produces radiation which consists of long and short wavelengths. However, the filter removes long wavelength radiation as this does not play a role in CT image formation, but increases patient dose. We know that long wavelength radiation is less energetic, and as a result passes through the body and cannot be detected.

 

Furthermore, a person who is very large may not fit into the opening of a conventional CT scanner or may be over the weight limit for the moving table. This could possibly be the next technological advancement in CT scanners. Advantages: The main advantage of CTs is that a short scan time of 600 milliseconds to a few seconds can be used for all anatomic part of the body. This is a big advantage especially for people who are claustrophobic. In addition, it is painless, non-invasive and accurate. As CT scans are fast and simple, in emergency cases they can reveal internal injuries and bleeding quickly enough to help save lives.

Also, in this period of economic recession the CT has shown to be cost-effective imaging tool for a wide range of clinical problems. Comparing CT to its competitors the MRI scan, CT is less sensitive to patient movement and can be performed even if the patient has an implanted medical device, unlike MRI. At the present time the CT scanner is superior to the MRI scanner. MRIs are bigger machines, with much more sensitive electronics in addition to requiring bigger support structures to operate them. To sum that all up- MRI machines cost more and this could be the underlying reason that CT are used more than MRI scans.

Finally, a diagnosis determined by CT scanning may eliminate the need for exploratory surgery. Risks: The main risk of CT is the chance of cancer from exposure to radiation. The radiation ionises the body cells which mutate when they replicate and form a tumour. However, the benefits of an accurate diagnosis outweigh the risks. In our recent study of ionisation radiation we have learned about the unit of Sievert. Radiologyinfo. org states that a radiation dose from this procedure ranges from 2 to 5 mSV, which is approximately the same as the background radiation received in 4 years.

The main risk of CT scanner is cancer; however this is only if they are used excessively. Research for the New Scientist suggests that the risk is very small and the benefits greatly weight it. Summary: In this report I started by looking at the history behind the CT scan and how this medical imaging has taken the science world by storm. I then explained the basic principles behind the scanner. As understanding of these principles grew, we were then led into the physics and a more in depth explanation. The different components of the CT were explained in detail such as the three-phase generator and how an x-ray tube works.

This links in with our recent study of physics. During the report we were also able to understand how slip ring and thus helical scanning has proven to be a major advance is this field. Once again, the physics behind this was explained in some detail. The report concluded by looking at the various applications, advantages and risks. The medical imaging world is constantly changing and improving like any field of medicine. Companies are always trying to produce imaging machines which are faster, more accurate, more economical and present less risk to the patient.

Therefore, the life span of the CT scanner could be limited with its competitors waiting to emerge in the background. The information in this report is very factual and accurate. I used a variety of sources to obtain the information. Most of the information in this coursework is attained from universities and radiology books. In addition, well-known articles were used from the monthly radiology magazine, ‘Synergy’ as well as information from the ‘New Scientist’ and ‘Nature’. Synergy is the biggest radiography magazine in the UK, which makes me believe that the information obtained it accurate.

In addition, ‘New Scientist’ and ‘Nature’ are well established titles which more often than not provide accurate information. The websites I used are all recommended by The University of Hertfordshire to its undergraduates in radiography. This means they are also reliable sources of information. In addition, I also used a number of well recognised radiology books. By using different sources of information, I was able to eliminate any bias or inaccurate information provided in some sources. To sum up, I believe the information provided is accurate and reliable.

Bibliography: Book References > Allday J, Adams S (2000) Advanced Physics. Oxford University Press > Ball J, More D. A (2006) Essential Physics for Radiographers. Blackwell Publishing > Bushong C. S (2004) Radiologic Science for Technologist. Mosby Inc > Duncan T, (1987) Physics; A Textbook for Advanced Level Students. John Murray > Elliott A, McCormick A (2004) Health Physics. Cambridge University Press > Foster E (1993) Equipment for Diagnostic Radiographer. MTP Press Limited > Graham T. D (1996) Principles of Radiological Physics. Churchill Livingstone.

> Ogborn et al (2000) Advancing Physics A2. Institute of Physics > Roberts P. D, Smith L. N (1990) Radiographic Imaging. Churchill Livingstone > Thompson C, Wakeling J (2003) AS Level Physics. Coordinate Group Publication. On Line References > Figure 1. 0 obtained from, www. catscanman. net > Figure 1. 1 obtained from, www. mh. org. au > Figure 1. 3 and Figure 1. 4 obtained from, www. impactscan. org/slides > Figure 1. 5 obtained from, www. koehler. me. uk > Figure 1. 6 and Figure 1. 7 obtained from www. impactscan. org/slides > Figure 1. 8 obtained from, www. itnonline. net.

> Figure 1. 9 and Figure 2. 0 obtained from www. sprawls. org/resources > Figure 2. 1 obtained from, www. csmc. edu > Figure 2. 2 and Figure 2. 3 obtained from, www. sprawls. org/resources > Figure 2. 4, Figure 2. 5 and Figure 2. 6 obtained from www. impactscan. org/slides > www. radiologyinfo. org (25 February 2009) > www. imaginis. com/ct-scan/ (12 March 2009) > www. bbc. co. uk/dna/h2g2 (15 February 2009) > www. impactscan. org/slides (12 March 2009) > www. sprawls. org/resources (14 March 2009) Other References > Synergy Magazine > New Scientist Magazine > Nature Magazine.

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