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 120° apart, the oscillations of each of these are 120° 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. I believe the information quoted from this website is accurate as the University of Herefordshire recommends it, to its undergraduates in radiography.
Figure 1.7 summarises what we have learned from the previous sections. The generator and the X-ray tube produce the X-ray beams which are then filtered to remove the long wavelength radiation. Finally, the collimator reduces scattered rays and focuses the beams. Figure 1.7 is a diagrammatic representation of CT slides of the brain being taken as the gantry rotates in a circular motion.
Detectors:
From Figure 1.7 (above) we can see that the attenuated X-ray beams that pass through a patient’s head are distinguished by the detectors on the other side of a gantry. In conventional radiography we utilise a film screen to detect the data; in CT the image receptors are the detectors. According to Roberts P.D (1990), the detectors must exhibit several characteristics such as the ability to capture, absorb and convert X-rays into electrical signals. They must also have a good response time - the speed at which they can detect the X-rays. Finally, they must have a high resolution or dynamic range - the ability to measure the largest to the smallest signals. Before continuing any further we must note one very important fact, the number of detectors is the same as the number of slice scanners.
At the present time there are two types of detectors in use, the scintillation detectors (figure 1.8) and the gas ionisation detectors. The scintillation detectors convert the X-rays into light and then the light is then converted into electrical energy, whereas the gas ionisation converts X-rays directly into electrical energy. Both types have their advantages and disadvantages; however, scintillation material detectors are more effective.
Image formation:
Ball J (2006) states that, the construction of the image is based on the degree of attenuated beam which hits the detector, otherwise known as the attenuation coefficient. In the case of CT this is the average linear attenuation coefficient (μ). The coefficient μ reflects the degree to which the x-ray intensity is reduced by the material. CT numbers are given to the degree of beam attenuation; these are measured in Hounsfield units and play a vital part in image formation.
The formation of a CT image is a three-phase process as shown in figure 1.9 below:
As explained under the principles section, an x-ray beam is scanned around the body.
Figure 2.0 shows that the amount of x-ray that penetrates the body is measured by the detectors. From one specific x-ray focal point, only one slice is produced. Many slices from around the patient’s body are required in order to reconstruct the image.
As the x-ray beam is scanned around the body, resulting in many slices, the data is recorded by the detectors and stored in a computer for image reconstruction. The data recorded by the detectors is stored in terms of the total x-ray attenuation (or penetration) along its path.
The CT image:
As we have already learned, the aim of CT imaging is to produce a digital image for a particular slice of tissue. During the process of image reconstruction, the slice of tissue is divided into a matrix of voxels or volume elements. This is analogous to a pixel which is 2D; voxel refer to 3D image. Figure 2.1 shows a voxel.
As mentioned before, the degree of beam attenuation is given a CT number which corresponds to a certain shade of grey. Therefore each pixel in the image has a CT number in it which decides its colour. . Before we carry on we should note that the CT numbers are designated to a greyscale. Diagram 1.0 shows a greyscale unit with the corresponding CT numbers.
Graham T.D (1996) states that, the CT numbers are calculated relative to the attenuation of water. Objects with beam attenuation less than that of water have an associated negative number. Substances with an attenuation greater than water have a positive Hounsfield number. The CT number assigned in each pixel decides the colour of the pixel. From the many pixels an image is built up.
Image reconstruction:
Image reconstruction is the phase in which data obtained from scanning is processed to produce an image. This digital image consists of an array of pixels. Sprawls.org/resources states that, back projection is the process used in CT imaging and it refers to the use of an algorithm (set of instructions). The data collected (x-ray attenuation) by the detectors in the gantry is used to construct an image. This process is complicated. However, figures 2.2 and 2.3 make the understanding easier.
We know that the data collected by the detectors is not a complete image, but a profile of the x-ray attenuation. This profile is then used to draw an image by back projection. The data collected is given a CT number which decides it particular shade of grey on the back projection. From one view or slice of x-ray beams there is only enough information to allow us to draw streaks (Figure2.2). However, many views or slices produce an image.
In figure 2.3 the x-ray beam has been rotated by 90° and another view is obtained. By back-projecting this data onto the image, things become clearer. Two views do not provide a high resolution image; however, hundreds will do.
This process provides the image which is then displayed on the operating console, figure 1.2.
Advances:
Some of the main advances in CT scanners are briefly mentioned under the history section. The powerful advances since its development are its ability to collect and manipulate data quickly, as well as its capability to produce images of the whole body. Figure 2.4 shows a timeline of the advances.
However, the most important progression in the field of medical imaging is the “power slip ring”. Between the period of 1974 to 1987 all original scanners were fixed with high voltage cables which provided the x-ray power needed for the x-ray tube.
The problem with this method is that the cables are wrapped around an elaborate set of rotating drums and pulley, otherwise known as the gantry. As a result, the rotating gantry can only spin 360° in one particular direction at a time and makes a slice. Note, the gantry cannot rotate any further otherwise the high voltage cables would get intertwined and damaged; causing a hazard. Pre-1987 the gantry would spin 360° in one direction, take a slice and then spin 360° in the other direction and take a second slice. The disadvantage of this process is the time it consumes; in between the slice the gantry has to halt and then reverse direction whereas the patient table moves forward an equal amount to the slice thickness.
Bushong C.S (2004) suggests that, during the mid 1980’s, a modernization called the power slip ring was developed so the x-ray cable was no longer needed. The advantage of the slip ring is that it makes it possible for electric power to be transferred from a stationary power source onto a continuously rotating gantry. Some modern CT scanners with slip ring can now rotate continuously and do not have to stop. The development of the power slip ring has created a beginning in CT called spiral or helical scanning.
There is basic physics behind the slip ring which can explain the process and its advantages. Figure 2.5 shows a slip ring inside a CT scanner. Similarly, diagram 1.1 shows the principle of a slip ring. A slip ring is an electric connector designed to carry current form a stationary wire into a rotating device. According to Allday J. (2000), a split ring is used in electric motor to keep them rotating in the same directions by swapping the contacts every half term. Slip rings work on the same basic rule as commutators, with only a slight variation as commutators are specialized for use in DC and generator. CT scanners use the same law.
Characteristically, a slip ring is comprised of a carbon or metal contact brush (as shown in diagram 1.1), which rubs on the outside diameter of a rotating metal ring or the x-ray tube in our case. As the metal ring turns, the electrical current is able to conduct through the carbon brushes and thus maintain its connection to the x-ray tube which as a result can carry on firing x-ray beams. This has lead to what is called helical scanning.
Helical Scanning:
According to Synergy Magazine, which is the largest radiography title in the UK and arguably the most reliable, the use of slip rings has improved the CT vastly. It has eliminated the need for high tension cables, allowing continuous gantry rotation, as well as making helical scanning possible (explained in detail below). In addition, it has meant faster scan times for patients and continuous data acquisition which provides more detail images. During an interview with the radiographer Jonida Cama at Basildon and Thurrock University hospital, I found out that the biggest advantage of split ring is the that it reduces the waiting times for patients as well as benefiting those that are claustrophobic.
What is helical scanning, and how does a slip ring make it possible? Helical scanning combines continuous gantry rotation with table motion. This means the path of the x-ray been around a patient follows a helical path. This is different from pre- 1987 as there in no waiting between slices to move table. Figure 1.3 shows helical scanning.
Applications, Advantages and Risks:
Applications:
CT is different to other medical imaging such as conventional x-rays. CT has the ability to differentiate between soft tissue structures, such as liver, lung or fat. However, its main use is in searching for lesions, tumours and metastasis. In addition to revealing their presence CT can also be used to find their approximate size and spatial location.
Recently CT has also been used for interventional procedures such as CT guided biopsy and minimally invasive therapy. Another important use of CT is the following of a tumour and whether a particular cancer treatment is successful.
In UK hospitals CT are used daily as they are invaluable in diagnosing and treating spinal problems and to the skeletal structures, as it can distinguish even very small bones as well as surrounding tissues.
According to the radiographer Jonida Cama at Basildon and Thurrock University hospital, CT scanner save millions of lives each year by providing early diagnosis to life threatening disease such as tumours, thoracic and abdomen defects.
In this chapter we have only mentioned the applications; there are also limitations which should be noted. Soft-tissue details in areas such as the brain, knee or shoulder can be more readily and clearly seen with magnetic resonance imaging (MRI). The examination is not generally advised for pregnant women. 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
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Allday J, Adams S (2000) Advanced Physics. Oxford University Press
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Ball J, More D.A (2006) Essential Physics for Radiographers. Blackwell Publishing
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Bushong C.S (2004) Radiologic Science for Technologist. Mosby Inc
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Duncan T, (1987) Physics; A Textbook for Advanced Level Students. John Murray
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Elliott A, McCormick A (2004) Health Physics. Cambridge University Press
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Foster E (1993) Equipment for Diagnostic Radiographer. MTP Press Limited
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Graham T.D (1996) Principles of Radiological Physics. Churchill Livingstone
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Ogborn et al (2000) Advancing Physics A2. Institute of Physics
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Roberts P.D, Smith L.N (1990) Radiographic Imaging. Churchill Livingstone
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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
Acknowledgements
I would like to thank Basildon and Thurrock University hospital and the University of Hertfordshire for their support and information.