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 (i?? ). The coefficient i?? 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 90i?? 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 360i?? 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 360i?? in one direction, take a slice and then spin 360i?? 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.