Computed Tomography Essays

Computed Tomography Essays Computed Tomography Essay Computed Tomography Essay 1. Introduction One of the most used techniques in the imagiology field is called Computed Tomography ( CT ) , a method to get pieces of the organic structure based on the fading of X raies. This monograph will seek to roll up the most of import information about CT, viz. its history, physical rules, cardinal instrumentality, informations acquisition and processing techniques, every bit good as its applications. First, a brief circuit through the history of the technique will be taken, while some of the most of import accomplishments will be referred. The get downing point will be the find of the X raies, so go throughing through the creative activity of the first CT scanner and the development of informations analysis and processing algorithms. Then, a concise alteration of the development of the scanners will be done, defining the different coevalss of scanners and the cardinal characteristics of each one. In order to understand how an object can be scanned by this technique, a reappraisal of the physical constructs that constitute the footing of CT will be done. More exactly, we will discourse the fading of radiation while go throughing through objects. A short description of how X-rays interact with affair and the construct of additive fading coefficient will be discussed. The instrumentality needed for CT will shortly be referred, in peculiar the most of import constituents of a CT scanner will be briefly explained. As informations acquired by the scanners are non displayed in the manner they are obtained, we will subsequently explicate the most used methods to treat and analyse the great sum of information acquired by the CT sensors. The procedure of making a graduated table to stand for informations the CT Numberss will later be overviewed, in order to understand how images are created and shown to the physicians. A description of how CT allows to separate different anatomical constructions and how it permits to see merely the constructions we want will besides be done. After that, an numbering of some of the many clinical applications of CT will be done, cognizing at the start that it will be impossible to name all the applications, ground why merely a few will be referred. Besides, it is non the chief end of this monograph, although it is indispensable to understand the important importance of CT in the medicine field. Finally, we will seek to speculate about the hereafter of CT, specifically what it can be improved and what are the existent challenges for this technique and how it can be overcame. This monograph is portion of the Hospital and Medical Instrumentation class and pretends to be an overall position of CT, ground why there is non thorough item in each subdivision ( for more item in the approached subjects, please read the mentions ) . three-dimensional Reconstruction techniques will non be discussed because it is the subject of another group. Acute instrumentality will non be exploited because it non exploited in the class every bit good. 2. Historical Background The history of CT started with the find of X raies in 1895 by Wilhelm Conrad Roentgen, which gave him the Physics Nobel Prize in 1901. During 1917, the Austrian mathematician Johann Radon developed a survey in which he demonstrated that doing several projections in different waies of a stuff and animating its associated form, it was possible to obtain a piece where 1 could qualify different densenesss of the stuff. The thought of utilizing these mathematical methods to make images of pieces of the human organic structure in radiographic movies was proposed by the Italian radiotherapist Alessandro Vallebona in 1930. Between 1956 and 1963, the physicist Allan Cormack developed a method to cipher the distribution of captive radiation in the human organic structure based on transmittal measurings, which allowed to observe smaller fluctuations in soaking up. [ 2 ] , [ 3 ] , [ 4 ] In the twelvemonth of 1972, Sir Godfrey Hounsfield ( who won the Nobel Prize in Medicine or Physiology in 1979, shared with Cormack ) invented the first CT scanner in United Kingdom when he was working at EMI Company, which, at the clip, was really best known for its connexion to the music universe. The original paradigm, called EMI Scanner , recorded 160 points for each projection in 180 different angles ( with stairss of 1 A ; deg ; ) and each piece took 5 proceedingss to be acquired. A 180160 matrix was so constructed with these informations, which took 2 and half hours to be analyzed until the concluding 2D-images could be visualized. The first types of scanners required the patient s caput to be immerged in a water-filled container in order to cut down the difference of X raies fading between the beams that crossed the skull and the 1s that merely crossed the environment, because the sensor had a little scope of strengths that it could mensurate. [ 5 ] , [ 6 ] During the subsequent old ages, CT scanners increased its complexness, and based on that development, we can separate five coevalss of machines that will be discussed in the following subdivision ( Section 3 ) . Subsequently, in 1989, it was developed a new technique in which information acquisition was done continuously the coiling CT scanning utilizing the motion of the platform where the patient was lying. [ 4 ] Presents, CT machines have evidently superior public presentations than the paradigms of the 70 s. In fact, several rows of sensors have been added which now allows enrollment of multiple pieces at the same clip the multislices scanners. These betterments allowed to stand for informations in 10241024 matrixes, which have a 1 megapixel pel declaration. [ 7 ] , [ 8 ] 3. Development of CT Scanners Over the clip, the basicss of informations acquisition and the cardinal features of the machines changed in many ways. This fact, let us to divide the development of the CT scanners in five coevalss. 3.1 First Generation Parallel Beam The first technique implemented in CT commercial machines consisted of the emanation of a parallel X-ray beam that passed through the patient until it reached a sensor located on the opposite side. Both X-ray and sensor were topographic point in the border of a ring with the patient as the centre. The X-ray beginning, every bit good as the sensor, suffered a additive interlingual rendition gesture to get informations from all mater s waies. Then, the X-ray tubing and the sensor, was rotated about 1 A ; deg ; , holding the patient as isocenter, and a new beam was emitted and the motion of interlingual rendition restarted. This procedure was repeated until it reached 180 A ; deg ; and, for each rhythm of emitted beams, 160 projections of the stuff on analysis were recorded. The extremely collimated beam provided first-class rejection of scattered radiation in the patient. At this point, the most used image Reconstruction technique was the backprojection. Subsequently in this work ( S ection 6 ) we will explicate the techniques used in Reconstruction. The clip needed for informations acquisition was highly long ( 5 proceedingss per piece ) , due to technological restrictions. [ 8 ] 3.2 Second Generation Fan beam In the 2nd coevals, the collimated beam was replaced by a fan X-ray beam and the simple sensor was replaced by a additive array of sensors. This progress resulted in a shorter scan clip, although this technique still continued to utilize a conjugate source-detector interlingual rendition gesture. At the same clip, the algorithms used to retrace the piece images became more complicated. Because of the huge sum of clip needed to get informations, both the first and 2nd coevalss of scanners were limited to caput and appendages scans, because those were the parts of the organic structure that could stay immobilized during the long scan clip. [ 9 ] , [ 2 ] , [ 8 ] 3.3 Third coevals Revolving sensors The 3rd coevals of scanners emerged in 1976. In this coevals, the fan beam was big plenty to wholly incorporate the patient, which made the interlingual rendition motion redundant and the scanner commenced to put to death merely the rotational motion. Such as the fan beam, besides the sensors became large plenty to enter all informations of each piece at a clip. The sensor consisted of a line with 100s of independent sensors that, like as in the 2nd coevals, rotated attached to the X-ray beginning, which required up to 5 seconds to get each piece. The power supply was now made by a faux pas ring system placed on the gauntry, which allowed to continually revolve it without the demand to change by reversal the revolving gesture to untwist the power overseas telegrams used earlier, as it was needed after each rotary motion in first and 2nd coevalss. [ 2 ] , [ 8 ] 3.4 Fourth coevals Fixed sensors This coevals was implemented in the late 70 s and its invention was a stationary ring of sensors that surrounded the patient. In this instance, merely the X-ray beam had motion. The ring consisted of a 600 to 4800 independent sensors that consecutive recorded the projections, so detector and beginning were no longer associated. However, sensors were calibrated twice during each rotary motion of the X-ray beginning, supplying a self-calibrating system. Third coevals systems were calibrated merely one time every few hours. In the 4th coevals systems, two sensors geometries were used. The first one consists of a revolving fan beam inside the fixed ring of sensors and the 2nd 1 has the fan beam outside the ring. These technological progresss provided a decrease of the scan times to 5s per image and piece spacing below 1 millimeter. Both 3rd and 4th coevalss are available in market and both have success in medical activities. [ 8 ] , [ 2 ] 3.5 Fifth Generation Scaning negatron beam The invention of the 5th coevals of CT scanners ( early 80 s ) was a new system of X-ray beginning. While the ring of sensors remains stationary, it was added a new semicircular strip of wolfram and one negatron gun which is placed in the patient alliance. By directing this negatron beam to the anode of the tungsten strip, the release of X-ray radiation is induced. This method consequences in a no traveling parts system, i.e. no mechanical gesture is needed to enter information because the sensors wholly surround the patients and the electronic beam is directed electronically. The four mark rings and the two sensor Bankss allow eight pieces to be acquired at the same clip, which cut down the scan clip and, accordingly, the gesture artifacts. This fact led to the decrease of scan clip to between 33 and 100 MS, which is sufficient to capture images of the bosom during its cardiac rhythm, ground why it is the most used in diagnostic of cardiac disease. For that ground, this is besides c alled Ultrafast CT ( UFCT ) or Cardiovascular CT ( CVCT ) Because of the uninterrupted scan, particular accommodations in the algorithm are needed to cut down image artefacts. [ 2 ] , [ 8 ] , [ 9 ] 3.6 Coiling Scanners The thought of making a coiling CT came with the demand for scans of three-dimensional images. This system to get three-dimensional CT images was born in the early 90 s and consists of a continue interlingual rendition motion of the tabular array which supports the patient. This technique is based on the 3rd coevals of machines and allows scan times of the venters to be reduced from 10 proceedingss to 1 minute, which reduces the gesture artifacts. Besides, a three-dimensional theoretical account of the organ under survey can be reconstructed. The most complex invention of this technique consists of the information processing algorithms, because they must see the coiling way of X-ray beam around the patient. Technically, this was possible merely due to the faux pas ring system implemented on the 3rd coevals of scanner. [ 9 ] , [ 8 ] , [ 10 ] 3.7 Cone beam After the development of new techniques, sensors, methods and algorithms, nowadays the inquiry is: How many pieces can we get at same clip? . The reply to this inquiry lies in the arrangement of several rows of sensors and the transmutation of a fan beam X ray to a three-dimensional cone beam. Nowadays, makers have already placed 64 rows of sensors ( multislice systems ) and the image quality reached high degrees. Furthermore, the wholly scan of a construction takes now about 15 seconds or even less. [ 2 ] 4. Physical Principles The basic rule of CT is mensurating the spacial denseness distribution of a human organ or a portion of the organic structure. It is similar to conventional X-ray, in which an X-ray beginning of unvarying strength is directed to the patient and the image is generated by the projection of the X raies against a movie. The X raies are emitted with a certain strength I0 and they emerge on the other side of the patient with a lower strength I. The strength decreases while traversing the patient, because radiation interacts with affair. More exactly, X raies used in CT are of the order of 120kV and, with that energy ( 120 keV ) , they interact with tissues chiefly by photoelectric ( largely at lower energies ) and Compton effects ( at higher energies ) , although they can besides interact by coherent spread, besides called Rayleigh spread ( 5 % to 10 % of the entire interactions ) . Photoelectric consequence consists of the emanation of an negatron ( photoelectron ) from the irradiated affair caused by the soaking up of the X ray s energy by an interior negatron of the medium. In Compton consequence, a X-ray photon interacts with an outer negatron of affair and deviates its flight, reassigning portion of its energy to the negatron, which is so ejected. In consistent spread, the energy of the X ray is absorbed by the tissue doing the negatrons to derive harmonic gesture and is so reradiated in a random way as a secondary X ray. [ 10 ] , [ 11 ] , [ 12 ] , [ 13 ] , [ 14 ] CT X raies are non monoenergetic, but for now, to simplify the apprehension of this construct, we will see them monoenergetic. When an X ray ( every bit good as other radiation ) passes through a stuff, portion of its strength is absorbed in the medium and, as a effect, the concluding strength is lower than the initial 1. More exactly, the Beer s Law states that strength transmitted through the medium depends on the additive fading coefficient of the stuff  µ if we consider that we are in presence of a homogenous medium and the thickness of the stuff ten harmonizing to the undermentioned look: The job with conventional radiogram is that it merely provides an incorporate value for  µ along the way of the X-ray, which means that we have a two-dimensional projection of a three-dimensional anatomy. As it can be easy understood, all the constructions and variety meats at the same degree will look overlapped in the image. As a effect, some inside informations can non be perceived and some variety meats may non be wholly seen. For illustration, it is really difficult to see the kidneys in a conventional skiagraphy because the bowels appear in forepart of them. [ 15 ] , [ 16 ] , [ 11 ] Furthermore, as there are many values of ( typically one for each point of the scanned portion of the organic structure ) , it is non possible to cipher their values with one scorch step. However, if steps of the same plane by many different waies are made, all the coefficients may be calculated, and that is what CT does. As Figure 4 shows, a narrow X-ray beam that is produced by the beginning in the way of a sensor, which means that merely a narrow piece of the organic structure is imaged and the value of strength recorded by the sensor depends on all the stuff crossed by the X ray in its manner. That is the ground why it is called imaging it derives from the Greek tomos which means to cut or subdivision. Many informations of X-ray transmittal through a plane of an object ( an organ or a party of the organic structure ) from several waies are recorded and are so used to retrace the object by signal processing techniques. These techniques will be discussed subsequently in this monog raph ( Section 6 ) . The tightly collimated X-ray beam ensures that no important spread is present in order to guarantee a low signal/noise ratio ratio ( SNR ) , a necessary premiss to obtain a faithful image of the scanned object. For that ground, unlike conventional imaging, in CT, patient s constructions located outside the country that is being imaged do non interfere. [ 17 ] , [ 9 ] , [ 12 ] 5. Instrumentality The X-ray system is composed by an X-ray beginning, collimators, sensors and a data-acquisition system ( DAS ) . X-ray beginning is doubtless the most of import portion, because it is what determines the quality of the image. [ 10 ] , [ 8 ] 5.1 The X-ray beginning The footing of the X-ray beginning ( called X-ray tubing ) is to speed up a beam of negatrons between two electrodes against a metal mark and is shown in Figure 5. The cathode is a coiled wolfram fibril, which is crossed by a current which causes the fibril to heat up. At high temperatures ( 2220 A ; deg ; C ) , the wolfram releases negatrons, a procedure called thermionic emanation. A 15 to 150 kilovolts possible difference is applied between the cathode and the anode, which forces the released negatrons to speed up towards the anode. [ 10 ] When the negatrons hit the anode, they produce X raies by two ways. On the one manus, when an negatron base on ballss near the wolfram karyon, it is deflected by an attractive electric force ( because the karyon is positively charged and the negatron has a negative charge ) and loses portion of their energy as X raies. As there are an tremendous figure of possible interactions and each one leads to a partial loss of kinetic energy, the produced X raies have a great scope of energies, as Figure 5 shows. This procedure is called bremsstrahlung ( i.e. braking radiation ) . On the other manus, if an negatron from the cathode hits and penetrates an atom of the anode, it can clash with an interior negatron of it, doing the negatron to be ejected and the atom to hold a hole , which is filled by an outer negatron. The difference of adhering energy of these two negatrons is released as an X ray. This procedure is called characteristic radiation, because its energy depends on the adhering en ergy of the negatrons, which is characteristic of a given stuff. [ 10 ] , [ 9 ] , [ 15 ] The tubing current represents the figure of negatrons that pass from the cathode to the anode per unit of clip. Typical values for CT are from 200 up to 1000 ma. The possible difference between the electrodes is by and large of 120 kilovolts, which produces an energy spectrum runing from 30 to 120 keV. The tubing end product is the merchandise between the tubing current and the electromotive force between the electrodes and it is desired to hold high values because that permits a shorter scan clip, which reduces the artefacts due to motion ( such as for bosom scans ) . [ 10 ] , [ 8 ] Production of X raies in these tubings is an inefficient procedure and most of the power supplied to the tubing is converted in warming of the anode. So, a heat money changer is needed to chill the tubing. This heat money changer is placed on the revolving gauntry. Spiral CT in peculiar requires high chilling rates of the X-ray tubing and high heat storage capacity. [ 8 ] 5.2 Collimators The negatron beam released from the beginning is a spread beam, usually larger than the coveted field-of-view ( FOV ) of the image. Normally, the fan beam breadth is set for 1 to 10 millimeters ( although recent CT scanner allow submilimetric preciseness ) , with determines the breadth of the imaged piece. The collimator is placed between the beginning and the patient and is composed by lead sheets to curtail the beam merely to the needed waies. An X-ray beam larger than the FOV leads to a larger figure of X raies emitted than the 1s needed to the scan and that has two jobs: the radiation dose given to the patient is increased unnecessarily ; and the figure of Compton-scattered radiation additions. [ 10 ] , [ 8 ] 5.3 Antiscatter grids An ideal CT system merely with primary radiation ( x-rays emitted from the beginning ) making the sensor does non be and Compton spread is ever present. As this spread is indiscriminately distributed and has no utile information about the distribution of denseness of the scanned object, it merely contributes to the decrease of image contrast and should be minimized to the upper limit. This, because unlike photoelectric consequence, Compton consequence has a low contrast between tissues. As referred above, collimators are utile to restrict the X-ray beam to the FOV. However, even with a collimator, 50 % to 90 % of the radiation that reaches the sensor is secondary radiation. To cut down the Compton spread, antiscatter grids can be placed between the sensor and the patient. [ 10 ] An antiscatter grid consists of strips of sheets oriented parallel to the primary radiation way combined with a support of aluminium, which drastically reduces the spread radiation that has non the way of the primary one, as illustrated in Figure 6. In order to non take down the image quality because of the grid shadiness, the strips should be narrow. There is, nevertheless, a trade-off between the decrease of spread radiation ( that better the image contrast ) and the dosage that must be given to the patient to hold the same figure of detected X raies. [ 10 ] 5.4 Detectors At the beginning, single-slice CT scanners with merely one beginning and one sensor were used. However, these took much clip to get an image, ground why the development brought us single-source, multiple-detector machinery and multislice systems. The 3rd and 4th coevalss added a broad X-ray fan beam and a larger figure of sensors to the gauntry ( typically from 512 to 768 ) , which permitted to get more information in a smaller clip. The sensors used in CT must be extremely efficient to minimise the dosage given to the patient, have a big dynamic scope and be really stable over the clip and over temperature fluctuations inside the gauntry. Three factors contribute to overall efficiency: geometric efficiency ( fraction of the entire country of sensor that is sensitive to radiation ) , quantum efficiency ( the fraction of incident X raies that is absorbed to lend to signal ) and transition efficiency ( the ability to change over the captive X raies into electrical signal ) . These sensors can be of two types ( shown in Figure 7 ) : solid-state sensors or gas ionisation sensors. Solid-state sensors consist of an array of scintillating crystals and photodiodes, while gas ionisation sensors consist of an array of compressed gas Chamberss to which is applied a high electromotive force to garner ions produced by radiation in inside the chamber. The gas is kept under a high force per unit area, to maximise interactions between X raies and gas molecules, which produce electro-ion braces. [ 10 ] , [ 8 ] 5.5 Data-Acquisition System The familial fraction of the incident X-ray strength ( I/I0 in equation 1 ) can be every bit little as 10-4, ground why DAS must be really accurate over a great scope. The function of DAS is to get these informations and so encode it into digital values and convey these to computing machines for Reconstruction to get down. DAS make usage of many electronic constituents, such as preciseness preamplifiers, current-to-voltage convertors, parallel planimeters, multiplexers and analog-to-digital convertors. The logarithmic measure needed in equation 3 to acquire the values of  µi can be performed with an parallel logarithmic amplifier. Data transportation is a important measure to guarantee velocity to the whole procedure and used to be done by direct connexion between DAS and the computing machine. However, with the visual aspect of revolving scanners in 3rd and 4th coevalss, these transportation rate, which is every bit high as 10 Mbytes/s is now accomplished by optical senders placed on the revolving gauntry that send information to repair optical receiving systems. [ 8 ] 5.6 Computer system The information acquisition of the projections, the Reconstruction of the signal, the show of the reconstructed informations and the use of tomographic images is possible by computing machine systems used to command the hardware. Current systems consist of 12 processors which achieve 200 MFLOPS ( million floating-point operations per second ) and can retrace an image of 10241024 pels in less than 5 seconds. [ 8 ] 6. Signal Processing and Analyzing Techniques As informations are acquired in several waies ( e.g. with increases of 1 A ; deg ; or even less ) and each way is split in several distinguishable points ( e.g. 160 or more ) , at least 28 800 points are stored, which means that there must be efficient mathematical and computational techniques to analyse all this information. A square matrix stand foring a two-dimensional map of the fluctuation of X-ray soaking up with the place is so reconstructed. There are four major techniques to analyse these informations, which we will discourse later. [ 12 ] 6.1 Coincident additive equations As it was referred above ( Section 4 ) , there is a step of for each pel, which means that modern CT scanners deal with 1 048 576 points for each piece ( nowadays the matrixes used are 10241024 ) . As a consequence, to bring forth the image of one individual piece, a system of at least 1 048 576 equations must be solved ( one equation for each unknown variable ) , which means that this technique is wholly unserviceable. In fact, imagine that in 1967, Hounsfield built the first CT scanner, which took 9 yearss to get the information of a individual piece and 21 hours to calculate the equations ( and by the clip, the matrix had merely 28 000 entries ) . Besides, nowadays CT scanners get about 50 % more steps than it would be needed in order to cut down noise and artefacts, which would necessitate even more computational resources. [ 16 ] , [ 11 ] , [ 8 ] 6.2 Iterative These techniques try to cipher the concluding image by little accommodations based on the acquired steps. Three major fluctuations of this method can be found: Algebraic Reconstruction Technique ( ART ) , Coincident Iterative Reconstruction Technique ( SIRT ) and Iterative Least-Squares Technique ( ILST ) . These fluctuations differ merely in the manner corrections are made: ray-by-ray, pixel-by-pixel or the full information at the same time, severally. In ART as an illustration, informations of one angular place are divided into every bit separated elements along each beam. Then, these informations are compared with correspondent informations from another angular place and the differences between X-ray fading are added every bit to the fitting elements. Basically, for each step, the system tries to establish out how each pel value can be modified to hold with the peculiar step that is being analyzed. In order to set steps with pel values, if the amount of the entries along one way is lower than the experimental step for that way, all the pels are increased. Otherwise, if the amount of the entries is higher than the mensural fading, pels are decreased in value. By reiterating this iterative rhythm, we will increasingly diminish the mistake in pels, until we get an accurate image. ART was used in the first commercial scanner in 1972, but it is no longer used because iterative methods are normally slow. Besides, this method implies th at all informations must be acquired before the Reconstruction begins. [ 9 ] , [ 16 ] 6.3 Filtered backprojection Backprojection is a formal mathematical technique that reconstructs the image based merely on the projection of the object onto image planes in different waies. Each way is given the same weight and the overall additive fading coefficient is generated by the amount of fading in each X-ray way that intersects the object from different angular places. In a simpler mode, backprojection can be constructed by smearing each object s position back trough the image plane in the way it was registered. When this processed is finished for all the elements of the anatomic subdivision, one obtains a incorporate image of the additive fading coefficients, which is itself a rough Reconstruction of the scanned object. An illustration of this technique is represented in Figure 8. By its analysis, it is besides clear that the concluding image is blurred, which means that this technique needs a small betterment, which is given by filtered backprojection. [ 12 ] , [ 9 ] , [ 16 ] Filtered backprojection is hence used to rectify the blurring end point from simple backprojection. It consists of using a filter meat to each of the 1-Dimensional projections of the object. That is done by convoluting a deblurring map with the X-ray transmittal informations before they are projected. The filter removes from data the frequences of the X-ray responsible for most of the blurring. As we can see in Figure 8, the filter has two important effects. On the one manus, it degrees the top of the pulsation, doing the signal uniform within it. On the other manus, it negatively spikes the sides of the pulsation, so these negative vicinities will neutralize the blurring consequence. As a consequence, the image produced by this technique is consistent with the scanned object, if an infinite figure of positions and an infinite figure of points per position are acquired. [ 16 ] , [ 9 ] Compared with the two old methods this procedure has besides the advantage that Reconstruction can get down at the same clip that informations are being acquired and that is one of the grounds why it is one of the most popular methods presents. [ 9 ] 6.4 Fourier Reconstruction The last signal processing technique that will be discussed in this monograph is the Fourier Reconstruction which consists of analysing informations in the frequence sphere alternatively of the spacial sphere. For this, one takes each angular orientation of the X-ray fading form and decomposes it on its frequence constituents. In the frequence sphere, the scanned image is seen as a two-dimensional grid, over which we place a dark line for the spectrum of each position, as Figure 9 shows. To retrace the image, one has to take the 1-Dimensional Fast Fourier Transform ( FFT ) . Then, harmonizing to the Fourier Slice Theorem, each position s spectrum is indistinguishable to the values of one line ( piece ) through the image spectrum, guaranting that, in the grid, each position has the same angle that was originally acquired. Finally, the reverse FFT of the image spectrum is used to accomplish a Reconstruction of the scanned object. 7. Datas Display As it was said earlier ( Section 6 ) , additive fading coefficients give us a rough image of the object. In fact, they can be expressed in dB/cm, but as they are dependent on the incident radiation energy, CT scanning does non utilize the fading coefficients to stand for the image, but alternatively it uses integer Numberss called CT Numberss. These are on occasion, but on the side, called Hounsfield units and have the undermentioned relation with the additive fading coefficients: where  µ is the additive fading coefficient of each pel and  µw is the additive fading coefficient of H2O. This CT figure depends clearly on the medium. For human applications, we may see that CT figure varies from -1000 for air and 1000 for bone, with CT figure of 0 for H2O, as it is easy seen from equation 5. [ 9 ] , [ 13 ] , [ 4 ] , [ 12 ] The CT Numberss of the scanned object are so presented on the proctor as a gray graduated table. As shown in Figure 10, CT Numberss have a big scope and as human oculus can non separate so many types of greies, it is normally used a window to demo a smaller scope of CT Numberss, depending on what it is desired to see. The Window Width ( WW ) identifies the scope of CT Numberss and accordingly alters the contrast ( as Figures 11 and 12 show ) , whereas Window Level ( W