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Computed Tomography (CT) is a powerful nondestructive evaluation (NDE) technique for producing 2-D and 3-D cross-sectional images of an object from flat X-ray images. Characteristics of the internal structure of an object such as dimensions, shape, internal defects, and density are readily available from CT images.
Computed tomography (CT) is a medical imaging method employing tomography. Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.
The test component is placed on a turntable stage that is between a radiation source and an imaging system. The turntable and the imaging system are connected to a computer so that x-ray images collected can be correlated to the position of the test component. The imaging system produces a 2-dimensional shadowgraph image of the specimen just like a film radiograph. Specialized computer software makes it possible to produce cross-sectional images of the test component as if it was being sliced.
HOW IT WORKS
The imaging system provides a shadowgraph of an object, with the 3-D structure compressed onto a 2-D plane. The density data along one horizontal line of the image is uncompressed and stretched out over an area. This information by itself is not very useful, but when the test component is rotated and similar data for the same linear slice is collected and overlaid, an image of the cross-sectional density of the component begins to develop. To help comprehend how this works, look at the animation below
'Computed tomography (CT) is a radiographic inspection method that uses a computer to reconstruct an image of a cross sectional plane of an object. In conventional radiography, information on the slice plane P projects into a single line, A-A; whereas in the associated CT image, the full spatial information is preserved... . The CT image is derived from a large number of systematic observations at different viewing angles, and an image is then reconstructed with the aid of a computer. If an internal feature is detected in conventional projection radiography, its position along the line of sight between the source and the film is unknown. Somewhat better positional information can be determined by making additional radiographs from several viewing angles and triangulating. This triangulation is a rudimentary, manual form of tomographic reconstruction. In essence, a CT image is the result of triangulating every point in the plane from many different directions
BASIC COMPONENTS OF CT SCANNER
There are four basic components in a CT Scanner. They are as follows:
1. GANTRY-
The first major component of a CT system is referred to as the scan or imaging
system. The imaging system primarily includes the gantry and patient table or
couch. The gantry is a moveable frame that contains the x-ray tube including
collimators and filters, detectors, data acquisition system (DAS), rotational
components including slip ring systems and all associated electronics such as
gantry angulation motors and positioning laser lights. In older CT systems a
small generator supplied power to the x-ray tube and the rotational components
via cables for operation. This type of generator was mounted on the rotational
component of the CT system and rotated with the x-ray tube.
Some generators remain mounted inside the gantry wall. Some newer scanner designs utilize a generator that is located outside the gantry. Slip ring technology eliminated the need for cables and allows continuous rotation of the gantry components. The inclusion of slip ring technology into a CT system allows for continuous scanning without interference of cables. A CT gantry can be angled up to 30 degrees toward a forward or backward position. Gantry angulation is
determined by the manufacturer and varies among CT systems. Gantry
angulation allows the operator to align pertinent anatomy with the scanning
plane. The opening through which a patient passes is referred to as the gantry
aperture.
2. X-RAY TUBE-
CT procedures facilitate the use of large exposure factors, (high mA and KvP
values) and short exposure times. The development of spiral/helical CT allows
continuous scanning while the patient table or couch moves through the gantry
aperture. A typical spiral/helical CT scan of the abdomen may require the
continuous production of x-rays for a 30 to 40 second period. The stress caused
by the constant build up of heat can lead to a rapid decrease of tube life. When
an x-ray tube reaches a maximum heat value it simply will not operate until it
cools down to an acceptable level.
CT systems produce x-radiation continuously or in short millisecond bursts or pulses at high mA and KvP values.CT x-ray tubes must possess a high heat capacity which is the amount of heat that a tube can store without operational damage to the tube. The x-ray tube must be designed to absorb high heat levels generated from the high speed rotation of the anode and the bombardment of electrons upon the anode surface. An x-ray tubes heat capacity is expressed in heat units. Modern CT systems utilize x-ray tubes that have a heat capacity of approximately 3.5 to 5 million heat units(MHU). A CT x-ray tube must possess a high heat dissipation rate. Many CT x-ray tubes utilize a combination of oil and air cooling systems to eliminate heat and maintain continuous operational capabilities. A CT x-ray tube anode has a large diameter with a graphite backing. The large diameter backed with graphite allows the anode to absorb and dissipate large amounts of heat.
3. DETECTORS-
When the x-ray beam travels through the patient, it is attenuated by the
anatomical structures it passes through. In conventional radiography we utilize
a film-screen system as the primary image receptor to collect the attenuated
information. The image receptors that are utilized in CT are referred to as
detectors. The CT process essentially relies on collecting attenuated photon
energy and converting it to an electrical signal, which will then be converted to
a digital signal for computer reconstruction. A detector is a crystal or ionizing
gas that when struck by an x-ray photon produces light or electrical energy.
The two types of detectors utilized in CT systems are scintillation or solid state and xenon gas detectors.
Scintillation detectors utilize a crystal that fluoresces when struck by an x-ray photon which produces light energy. A photodiode is attached to the scintillation portion of the detector. The photodiode transforms the light energy into electrical or analog energy. The strength of the detector signal is proportional to the number of attenuated photons that are successfully converted to light energy and then to an electrical or analog signal.
The second type of detector utilized for CT imaging system is a gas detector.
The gas detector is usually constructed utilizing a chamber made of a ceramic
material with long thin ionization plates usually made of Tungsten submersed
in Xenon gas. The long thin tungsten plates act as electron collection plates.
When attenuated photons interact with the charged plates and the xenon gas
ionization occurs. The ionization of ions produces an electrical current. Xenon
gas is the element of choice because of it's ability to remain stable under
extreme amounts of pressure. Utilizing more gas in a detector increases the
number of molecules that can be ionized therefore, the strength of the detector
signal or response is increased. The long thin tungsten plates of the gas detector
are highly directional. Ionization of the plates and the resultant detector signal
rely on attenuated photons entering the chamber and ionizing the gas. If the
xenon gas detectors are not positioned properly there is a chance that the ability
of the detector to produce an accurate signal is compromised because the
photons may miss the chamber.
4. DATA ACQUSITATION SYSTEM (DAS)-
Once the detector generates the analog or electrical signal it is directed to the
data acquisition system (DAS). The analog signal generated by the detector is a
weak signal and must be amplified to further be analyzed. Amplifying the
electrical signal is one of the tasks performed by the data acquisition system.
The DAS is located in the gantry right after or above the detector system. In some modern CT scanning systems the signal amplification occurs within the detector itself. Before the projection or raw data, which is currently in the form of an electrical or analog signal, goes to the computer it must be converted to digital information. The computer does not "understand" analog signals therefore, the information must be converted to digital information. This task is accomplished by an analog to digital converter which is an essential component of the DAS. The digital signal is transferred to an array processor. The array processor solves the statistical information using algorithmic calculations essential for mathematical reconstruction of a CT image.
An array processor is a specialized high speed computer designed to execute mathematical algorithms for the purpose of reconstruction . The array processor solves reconstruction mathematics faster than a standard microprocessor. It is important to note that special algorithms may require several seconds to several minutes for a standard microprocessor to compute. Recently, processors that compute CT reconstruction mathematics faster than an array processors have been utilized to solve reconstruction mathematics essential to the development of CT fluoroscopy. The term image or reconstruction generator is used to describe this type of computer.
IMAGE RECONSTRUCTION
Since the Fourier Transform plays a major role in the understanding of CT
reconstruction, we introduce it here to define the appropriate terms. If the image is blurred with a function whose FT is well behaved, we should be able to construct a de-blurring function. It turns out that the 2-D FT of 1/r is 1/. Since the inverse of 1/is | |, then we should be able to compute the 2D FT of the blurred image, multiply the FT of the result image by | | , and then calculate the inverse FT.
The previous approach is certainly one approach, but not necessarily the most
efficient. There are in fact a number of different ways to view the
reconstruction process.
One of the most fundamental concepts in CT image reconstruction if the
“Central-slice” theorem. This theorem states that the 1-D FT of the projection
of an object is the same as the values of the 2-D FT of the object along a line
drawn through the center of the 2-D FT plane.