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CT scanning â€œ computed tomography is a mechanism of getting the internal details of a section. It is a diagonostic imaging procedure in which anatomical information is digitally reconstructed from X-ray transmission data obtained by scanning an area from many directions in the same plane to visualize information in that plane. CT is a fast patient friendly and has the unique ability to image a combination of soft tissue, bone, lungs and blood vessels.
There are two main limitations of using conventional x-rays to examine internal structures of the body. Firstly superimpositions of the 3-dimensional information onto a single plane make diagnosis confusing and often difficult. Secondly the photographic film usually used for making radiographs has a limited dynamic range and therefore only object that have large variation in the x-ray absorption relative to their surroundings will cause sufficient contrast differences on the film to be distinguished by the eye. Thus the details of bony structures can be seen, it is difficult to discern the shape and composition of soft tissue organ accurately.
CT uses special x-ray equipment to obtain image data from different angles around a body and then shows a cross section of body tissues and organs. i.e., it can show several types of tissue-lung,bone,soft tissue and blood vessel with great clarity. CT of the body is a patient friendly exam that involves little radiation exposure.
In CT scanning, the image is reconstructed from a large number of absorption profiles taken at regular angular intervals around a slice, each profile being made up from a parallel set of absorption values through the object. ie, CT also passes x-rays through the body of the patient but the detection method is usually electronic in nature, and the data is converted from analog signal to digital impulses in an AD converter. This digital representation of the x-ray intensity is fed in to a computer, which then reconstruct an image.
The method of doing of tomography uses an x-ray detector which translates which translates linearly on a track across the x-ray beam, and when the end of the scan is reached the x-ray tube and the detector are rotated to a new angle and the linear motion is repeated. The latest generation of CT machines use a Ëœfan-beamâ„¢ geometry with an array of detectors which simultaneously detect x-rays on a number of different paths through the patient.
CT scanner is a large square machine with a hole in the centre, something like a doughnut. The patient lies still on a table that can move up/down and slide in to and out from the centre of hole. With in the machine an X-ray tube on a rotating gantry moves around the patientâ„¢s body to produce the images.
In CT the film is replaced by an array of detectors which measures X-ray profile. Inside the scanner, a rotating gantry that has an X-ray tube mounted on one side an arc â€œshaped detector mounted on opposite side. An X-ray beam is emitted in a fan beam as the rotating frame spins the X-ray tube and detector around the patient. Each time the X-ray tube and detector make a 360 degree rotation and X-ray passes through the patientâ„¢s body the image of a thin section is acquired. During each rotation the detector records about 1000 images (profiles) of the expanded X-ray beam. Each profile is then reconstructed by a dedicated computer into two time.
DIFFERENCE BETWEEN X-RAY IMAGE AND CT SCANNED IMAGE
PHYSICS OF TOMOGRAPHY
X-ray photons interact with material in there principal ways: pair production, photoelectric absorption and scattering .Pair production only occurs if the photon energy is Âº1.022mev,which is much higher than the energies used in medical tomography. Photoelectric absorption occurs when the photon is completely absorbed and transfers its energy to an electron .The electron then passes through the material giving up its energy until it comes to rest.
Scattering has two components-coherent or Raleigh scattering in which the direction of the photon is changed ,but it does not change frequency. The other is that Compton or incoherent scattering, where the photon gives up some of its energy to an electron and continues on in a different directions at lower energy. The combined effects of scattering and absorption results in an exponential attenuation of a beam of photons as it passes through a material. A mono energetic beam with an input intensity of I0 photons passing through a length of material has an output intensity of
The project and implimentationion p(x) depends on measurements of both the transmitted X-ray intensity I(0) and the incident X-ray intensity I0(x). The intensity variations with time can be measured by putting a reference X-ray detector in a portion of beam which does not intersect the patient, usually at the edge of the beam and sampling this detector at the same time as the measurement of the beam transmitted through the patient is sampled. The spatial fluctuations can be measured during an initial calibration run using a known object, such as a water filled cylinder in the place of patient.
All computed tomography system consists of four major subsystems.
Â¢ Scanning System â€œ takes suitable reading for a picture to be reconstructed. This includes x-ray source and detectors.
Â¢ Processing Unit â€œ converts these readings into intelligible picture information.
Â¢ Viewing System â€œ presents this information in visual form and includes other manipulative aids to assist diagnosis.
Â¢ Storage Unit â€œ here picture is stored in digital form.
The purpose of the scanning is to acquire enough information to reconstruct a picture for an accurate diagnosis. In basic scanning process, a collimated x-ray beam passes through the body and its attenuation is detected by a sensor that moves on a gantry along with the x-ray tube. The tube and the detector moves in a straight line.
Inorder to get a clear image, rotation machines have been designed in which only the x-ray source rotates within a full circle of stationary detectors arranged around a patient. The individual detectors are lined up practically without gaps so that the radiation which has penetrated the patient is optimally used. The system permits calibration during scanning, which eliminates the problem of detector drift.
In CT scanners, the highest image quality free from disturbing blurring effects is obtained with the aid of pulsed x-ray radiation. During rotation, high voltage is applied at all times. A grid tube prevents the electron current from striking the anode except when desired allowing the x-rays to be emitted in bursts. As the gantry rotates an electric signal is generated at certain positions of rotating system.
For a good image quality, it is important to have a stable system response and in that detectors play a significant role. There are three types of detectors commonly used in CT scanning. They are xenon gas ionization detector, scintillation crystal and photomultiplier and scintillarc. A good detector is a pre-requisite to obtain optimal image quality, the measuring electronics must have a large dynamic range to backup the detector.
The information received by the computer from the scanning gantry needs processing for reconstructing the pictures. The data from the gantry contains information on the following parameters.
Â¢ Positional information-such as which traverse is being performed and how far the scanning frame is along its traverse.
Â¢ Absorption information-the values of attenuation coefficient from the detectors.
Â¢ Reference information-obtained from the reference detector that monitors the X-ray tube.
Â¢ Calibration information-Obtained at the end of each traverse.
The first stage of computation is to analyze and convert all the collected data in to a set of profiles. However the main part is of processing the profiles to convert the information which can be displayed as a picture and used for diagnosis. In general the reconstruction method can be classified in to three major techniques.
Â¢ Back project and implimentationion-which is analogous to graphic reconstruction.
Â¢ Iterative methods-which implement some form of algebraic solution.
Â¢ Analytical methods-where an exact formula is used. Two of these are filtered back project and implimentationion, which incoperates the convolution of the data and fourier filtering of the image, and two dimensional fourier reconstruction technique.
The method of back project and implimentationion without any further processing is simple and direct. In This method each of the measured profiles is project and implimentationed back over the image area at same angle from which it was taken. At the same time each project and implimentationion not only contributes to the point that originally formed the profile but also to all the other points in its paths. The technique in fact produces starred images and blurring and this makes it totally unsuitable for providing pictures of adequate clarity for medical diagnosis.
The earlier scanners used iterative technique which took a succession of back project and implimentationion correcting at each stage until an accurate reconstruction was achieved. The method requires several steps to modify the original profiles in to a set of profiles which can be project and implimentationed back to give an un blurred image. This technique however tends require long computation time.
Current commercial scanners use a mathematical technique known as convolution of filtering. This technique employes a spatial filter to remove the artifacts.
In most of the CT system the final picture is available on a television type picture tube. The picture is constructed by a number of elements in a square matrix wherein each element has a value representative of the absorption value of the point in the body which it represents. This technique enables to have a much larger dynamic range than the eye can possibly have.
STORING AND DOCUMENTATION
For subsequent processing or evaluation of a CT picture, various methods of storage are used. The picture is stored in the digital form so that the evaluation is convenient on a computer assisted program. For this purpose the data carries generally employed are magnetic disc ,magnetic tape and floppy disc. The magnetic disc normally hold a small number of pictures. So it cannot be employed as a long term storage medium. Most manufactures of CT units use magnetic tape and floppy disc and floppy disc provide medium storage range. For long term storage magnetic tapes are performed.
Computed tomography scans are a very powerful tool in medicine. X-rays passing through an object can be absorbed or scattered and the resulting loss in intensity is given by
Where Ã‚Âµ is the linear attenuation coefficient
X is the distance the X-ray has traveled.
The initial ground work for computed tomography was laid by Radon, and he demonstracted that an object could be reconstructerd from an infinite number of project and implimentationions through that object .In a modern CT scanner an X-ray fan beam and dector sweep around the patient obtaining thousands of project and implimentationions at different angles .The CT scanner measures the intensity of the X-ray beam which pass through the object .The average linear attenuation coefficient along the project and implimentationed line through the object is given by
where Ã‚Âµ is the average attenuation coefficient
I/I0 is the normalized intensity
Delta t is the product of step size
Nt is the number of steps
RECONSTRUCTION There have been many different algorithms developed to accomplish this task and while they have all been shown to be fundamentally identical the actual techniques appear quite different. One of the most popular algorithm is the filtered back project and implimentationion technique .As its name implies this technique involves two parts. Back project and implimentationing along the project and implimentationion lines used ,and filtering the image.
Back project and implimentationion is a relatively elementary process .One simply assigns the mean attenuation coefficient given by the equation
ln(I/I0)/Nt(delta t) to each point along that line.
This back project and implimentationion is repeated for all angles .The attenuation coefficient for a particular point will be built up from all project and implimentationion passing through that point .In imaging jargon, each of these points pixel ,that is an element of the final image or picture.
In reality ,this process is not quite trival .The image under reconstruction is not continuous ,but is composed of discrete pixels. The project and implimentationion lines will not pass perfectly through the centre of each pixels in their path and it is necessary to establish a method for describing the project and implimentationion lines in terms of individual pixel with in a matrix.
By establishing an N/N matrix reconstruction matrix â€œg(x,y)-where N is the number of translation pixels in the t axis. For a given angle in measurement space a value at can be calculated for each pixel in the N/N matrix as follows.
This pixels in the N/N matrix can be assigned attenuation coefficient values from measurement
This process is repeated for each angle.
And the matrices are summed and divided by the number of angles to obtain the final back project and implimentationed image.
Back project and implimentationion alone results in a blurred reconstruction image .Filtering must be applied to correct for this and obtain an accurate image of the object.
There are a number of choices in the type of filter to use .The simplest and most rigorous one is the ramp filter.
The most commonly used filter is the Shepp-Logan filter, which combines a sine function with ramp filter.
This filter results in a small amount of blurring, but is much less sensitive to noise.
Filtering and back project and implimentationions are both linear operations. Filtering is performed by multiplying the fourier transform of a wave form by the filter function ,the result is then inverse fourier transformed to produce the filtered waveform. In fourier transforms, multiplication in Fourier space is equivalent to a convolution in normal space.
where G() and H() are fourier transforms of g(t),f(t) and h(t) respectively and * represents a convolution integral. The convolution integral is defined by
Once both filtering and back project and implimentationions have been performed ,the result is a two dimensional array of attenuation coefficient.
For historical reasons ,the attenuation coefficients are converted into CT numbers in units of Houvsfield.
Where Ã‚Âµw is the attenuation coefficient for water.
The main problem with CT has been the potential danger it represents because of radiation exposure .The developments in CT imaging have made marked improvements in its technological capabilities ,the radiation effects problem has not received the same degree.
The new processing data method reduces the amount of radiation exposure needed while maintaining ,CTâ„¢s high resolution. The method is based on an algorithm that reconstructs the wavelet coefficients of an image from the radon transform data. The properties of wavelets are used to localize the radon transform and reconstruct a local region of the cross section of a body, using almost completely local data. This significantly reduces the radiation exposure and less computation time. The variance of the elements of the null space is negligible in the locally reconstructed image. An upper bound for the reconstruction error in terms of data used is also determined by the algorithm, which for example requires 2% of full exposure data to reconstruct a local region 16 pixels in radius in a 256*256 pixel image.
After scanning the patient the operator can go straight to the wavelet transform without having to first reconstruct the image. To obtain wavelet transform the algorithm can be applied to full data or local data. Local image reconstruction is achieved with superior definitions in shortest time and with less radiation exposure to the patient.
That is in summary
Â¢ Reconstructs with high accuracy and with few computations the wavelet transform of an image directly from the tomographic measurements.
Â¢ Computes to high accuracy a small region of the image from measurements on line passing only through the region reducing computation time and radiation exposure.
Â¢ Reconstructs the density at a point using only line integral data on lines that pass through a small region containing that point ,achieving reduced radiation exposure.
BENEFITS AND RISKS
Â¢ Unlike other imaging methods CT scanning offers detailed view of many types of tissues , including lungs, bones, soft tissues and blood vessels.
Â¢ CT scanning is painless , noninvasive and accurate.
Â¢ CT examinations are fast and simple.
Â¢ Diagnosis made with the assistance of CT scan eliminate the need for invasive exploratory surgery and surgical biopsy.
Â¢ CT scanning can identify both normal and abnormal structures, making it a useful tool to guide radiotherapy , needle biopsies and other minimally producers.
Â¢ CT has been shown to be a cost-effective imaging tool for a wide range of clinical problems.
Â¢ CT does involve exposure to radiation in the form of X-rays , but benefits of an accurate diagnosis far outweighs the risks .The effective radiation does from this procedure is about 10mv , which is about the same as the average person receives from background radiation in 3 years.
Â¢ Special care is taken during X-ray examination to ensure maximum safety for the patient by shielding the abdomen and pelvis being imaged.
Â¢ The risk of serious allergic reaction to iodine containing contrast material is rare and radiology departments are well equipped to deal them.
Limitations of CT scanning of the body
Very fine soft tissues details in areas such as a shoulder or knee can be more readily and clearly seen with MRI. In some situations soft tissues are may be unclear by near by bone structures. The exam is not generally indicated for pregnant women.
The current trends in the industry appear to be improve picture quality primarily resedation improvement and artifact reduction, and to lower the cost with existing quality. In the future, there is much interest in developing a real time heart tomography machine, allowing radiologists to observe sequences of heat functioning throughout its cycle of operation.
Industrial tomography is another future direction in which tomography is heading. Tomography allows detailed inspection of complex and critical parts. Two important potential applications are inspection of jet engines and machine parts such as blades and disks & inspection of rods in nuclear reactives.
When CT scanners first appeared it used to take four minutes to scan a section, thus making it impossible to image moving organs like the heart. But the machines of today can complete the scan in few seconds. Special machines are being developed for heart scanning which completes scans in milliseconds. Also the developments in CT imaging have made marked improvements in its technological capabilities the radiation effects problem has not received the same degree. The properties of wavelets are used here. This significantly reduces the radiation exposure and less computation time.
Â¢ Hand book of biomedical instrumentation â€œ R.S. Khandpur, Tata Mc Graw Hill
Â¢ Medical Instrumentation â€œ John G. Webster
Â¢ Biomedical Instrumentation and Measurements- Lesliecromwell
1 INTRODUCTION 1
2 BASIC PRINCIPLE 2
3 CT SCANNER 3
5 PHYSICS OF TOMOGRAPHY 5
6 CALIBRATION 6
4 SYSTEM COMPONENTS 7
7 RECONSTRUCTION METHOD 12
8 FUTURE DEVELOPMENTS 19
9 CONCLUSION 20
10 REFERENCES 21
I express my sincere gratitude to Dr.Nambissan, Prof. & Head, Department of Electrical and Electronics Engineering, MES College of Engineering, Kuttippuram, for his cooperation and encouragement.
I would also like to thank my seminar and presentation guide Mrs. Haseena.P.Y (Lecturer, Department of EEE), Asst. Prof. Gylson Thomas. (Staff in-charge, Department of EEE) for their invaluable advice and wholehearted cooperation without which this seminar and presentation would not have seen the light of day.
Gracious gratitude to all the faculty of the department of EEE & friends for their valuable advice and encouragement.
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