CT PHysics for the medical gradutaes in india

CT

1. CT PHysics JR RAdiodiagnosis

2. Basics OF CT – Part 2 Xray tubes Collimators Detectors Xenon gas ionization chambers Detectors Image reconstruction CT Artifacts 3D imaging

3. Computed : use of computer Tomography: Greek word  tomos   means " slice ",   graphy   means " write ". Computed Tomography is the process of generating a two-dimensional image of a slice or section through a 3-dimensional object (a tomogram) Computed tomography (or computerized axial tomography) is an examination that uses X-ray and computer to obtain a cross-sectional image of the human body.

4. Xray tubes Ideally, the radiation source for CT would supply a monochromatic x-ray beam(i.e., one made up of photons all having the same wavelength). With a monochromatic beam, image reconstruction is simpler and more accurate. New fan beam units have a diagnostic type x-ray tube with a rotating anode and a much smaller focal spot, in some units down to 0.6 mm. These tubes have large heat loading and heat dissipation capabilities to withstand the very high heat loads generated when multiple slices are acquired in rapid sequence.

5. Collimator The x-ray beam is collimated at two points, one close to the x-ray tube and the other at the detector(s). Perfect alignment between the two is essential. The collimator at the detector is the sole means of controlling scatter radiation. Each detector has its own collimator. The collimators also regulate the thickness of the tomographic slice (i.e., the voxel length).

6. Detectors There are two types of detectors used in CT scanners. These are: -Scintillation crystals -Xenon gas ionization chambers Xenon gas ionization detectors are limited in use to rotate-rotate (i.e., "third-generation") type scanners. Rotate-fixed scanners ("fourth generation") use scintillation crystal detectors.

7. Scintillation crYstals Scintillation crystals are any of an extremely large number of materials that produce light as a result of some external influence. More specifically, these materials are those that will produce light (scintillate) when ionizing radiation reacts with them. This is just what an x-ray intensifying screen does. The difference is that an intensifying screen consists of many tiny crystals imbedded in a matrix. Scintillation crystals are normally single crystals of the material.

8. Xenon gas ionization chambers Those rotate-rotate CT units that do not use scintillating crystals use xenon gas ionization chambers. In all single gas filled detector. there must be: 1. An anode and a cathode 2. A counting gas (inert gas) 3. A voltage between the anode and cathode 4. Walls that separate the detector from the rest of the world 5. A window for the radiation photon to enter the detector

9. Suppose a photon enters a detector. The photon interacts with a gas atom by ionizing the atom into an electron-ion pair. The voltage between the anode and cathode will cause the electron (negative ion) to move toward the anode, and the positive ion (atom minus an electron) to move toward the cathode. When the electrons reach the anode, they produce a small current in the anode. This small current is the output signal from the detector.

10. Image reconstruction In computed tomography a cross-sectional layer of the body is divided into many tiny blocks such as the ones shownin Figure 19-10, and then each block is assigned a number proportional to the degree that the block attenuated the x-ray beam. The individual blocks are called "voxels." Their composition and thickness, along with the quality of the beam, determine the degree of attenuation.

11. Algorithms An algorithm is a mathematical method for solving a problem. Thousands of equations must be solved to determine the linear attenuation coefficients of all the pixels in the image matrix. Several different methods, or algorithms, are in current use. They all attempt to solve the equations as rapidly as possible without compromising accuracy. The original EMI scanner took 4 to 5 minutes to process the data for a single CT section. This long processing time seriously limited the number of patients that could be handled in a working day. More recent algorithms are much faster and can display a picture immediately after the tomographic slice is completed.

12. The following three mathematical methods of image reconstruction are applied: 1. Back-projection/summation method(oldest ) 2. Iterative methods 3. Analytical methods The object of all the methods is to produce an accurate cross-sectional display of the linear attenuation coefficients of each element in the image matrix.

13. The CT scanner calculates, from the collected data, the linear attenuation coefficient (IJ.) of each pixel. After the CT computer calculates a value for the linear attenuation coefficient of each pixel, the value is converted to a new number called a "CT" number. It is rather easy to see that the CT number for air (we will assume air stops little or no x rays) will be equal to - K, and that the CT number for water will be 0. If dense bone has a linear attenuation coefficient of about 0.38 cm-1 and water has a linear attenuation coefficient of about 0.19 cm-1,then the CT number for this bone will be +K

14. In currently available CT units, K has a value of 1000. With this value of K, the CT number for air becomes - 1000, for water the CT number is still 0, and for dense bone becomes 1000. To honor Hounsfield, CT numbers based on a magnification constant of 1000 are also called Houns field units (H). Since new CT scanners use a magnification constant of 1000, CT numbers and H units are equivalent for these scanners.

15. Image Display A CT image is usually displayed on a television monitor for immediate viewing, and recorded on film for interpretation and permanent storage. O ne usually selects a CT number that will be about the average CT number of the body tissue being examined. Such a CT number might be – 200 for air (lung), + 20 to + 40 for retroperitoneal soft tissues, or + 200 for bone. The computer may then be instructed to assign one shade of gray to each of the 128 CT numbers below and each of the 128 CT numbers above the baseline CT number. For example, a general view of the abdomen will center at CT number 20, and display CT numbers - 108 to + 148 as 256 gray levels. The center CT number is called the " window level," and the range of CT numbers above and below the window level is called the "window width ." It is possible to set the window level at any desired CT number. Window width is also variable to any width desired by the operator.

16. Windowing obviously limits viewing to a narrow portion of the total information available. In practice, multiple window levels and multiple window widths may be examined in an effort to extract maximum diagnostic information from each examination.

17. Classification of artifacts Patient based- motion , clothing , jewelry, transient contrast interruption. Physics based- beam hardening ( cupping , streak , metal ) , partial volume averaging , quantum mottle( noise) , aliasing , truncation. Hardware based- ring , tube arcing , air bubble artifact , helical ( windmill , conebeam ) , multiplanar artifact ( zebra , stair step)

18. Artefacts - It is a flaw in an image that can make it difficult to visualize whats being imaged or can lead to potential misdiagnosis. CT artefact is any systematic discrepancy between the CT numbers in the reconstructed image and the true attenuation coefficients of the object.

19. IMPortant ArtifacTs in CT Motion Artifacts: Patient motion has a devastating effect on image quality. This is the primary reason for developing faster scan units, including the 50-f,Lsec unit used to image cardiovascular structures. When the patient moves during scan acquisition, the reconstruction program has no ability to make appropriate corrections because motion is random and unpredictable. The reconstructed image will display an object in motion as a streak in the direction of motion. On reconstruction the scanner will average the density of the pixels covering the motion area. In some fashion, the intensity of the streak artifact will depend on the density of the object in motion. Motion of objects that have densities much different from their surroundings produces more intense artifacts.

20. Streak artifact One of the basic assumptions in CT scanning is that each detector, at every position, will observe some transmitted radiation. If a high density material severely reduces thetransmission , the detector may record notransmission . This violates the basic assumption, and the reconstruction program will not account for such a violation.Streaks will appear in the image. Even for high density materials that do transmit some x ra ys) the filter in the reconstructionprogram is not designed to perfectly reproducethe high density to average densityinterface . Some CT units expand the CT numbers well above those numbers associated with dense bone in order to assign a linear absorption coefficient to materials that are denser than bone.

21. Beam hardening artefact Unfortunately, the x-ray beam producing the CT image is not a monochromatic beam. The beam contains x rays with many energies, although we speak of the average energy of a CT x-ray beam as being about 70 keV. As a heterogeneous x-ray beam passes through the patient, the low energy protons are rapidly absorbed. This means the x-ray beam exiting the patient containsa lower percentage of energy photons than the beam had when it entered the patient. This effect is called "beam hardening.“ The linear attenuation coefficient of a tissue is directly related to the average energy of the x-ray beam.

22. The measured linear attenuation coefficient (t-L) of a tissue near the beam entry site will be higher than the measured 1-L of the same tissue after the beam has been hardened by passage through a volume of tissue. Reconstruction programs anticipate and correct for variation in linear attenuation coefficients caused by beam hardening, but such corrections are not precise. Generally, beam hardening artifacts are not a problem.

23. In the head, a so-called "cup artifact" may be produced. This results from reconstructing an image from a 360° tube rotation. All points in the periphery of the brain will Figure 19-26 Streak artifact produced by a shotgun pellet (B). Scout film (A) shows location of pellet (arrow) "see" both a hardened and a non-hardened beam, but the central area of the brain will always "see" a beam that has been partially hardened. Linear attenuation coefficients from the center of the brain will be decreased compared to the periphery and the reconstructed image will be less dark centrally than peripherally.

24. Ring artefacts Ring artifacts are the result of miscalibration of one detector in a rotate-rotate geometry scanner. (Of course, detector failure would also produce a ring artifact.) If a detector is miscalibrated , it will record incorrect data in every projection. This misinformation is reconstructed as a ring in the image, with the radius of the ring determined by the position of the faulty detector in the detector array. Faulty detectors in rotate-fixed units also record false information. However, this information is not visible in the reconstructed image because the faulty detector collects data from many angles (rings may appear in rotate-fixed geometry if the x-ray tube is not aligned correctly). Ring artifacts have virtually disappeared in contemporary CT units.

25. 3 d imaging Three-dimensional reconstruction of CT scan data is an option now available on many contemporary units. Whether the technology will prove to be of significant clinical usefulness is still being evaluated. There are two modes of 3-D reconstruction: 1. Surface Fej = Onstruction shows only the surface of the object. 2. Volumetric reconstruction shows the surface of the object in relation to its surroundings. Three-dimensional reconstruction is simply the result of computer manipulation of CT scan data obtained in a routine manner. That is, there is no additional information obtained for the specific purpose of obtaining a 3-D image. It is true that a high resolution set of CT scans is obtained for good 3-D reconstruction; a typical scanmight consist of 4-mm-thick slices with no gap between slices.

26. summary The basic principle behind CT is that the internal structure of an object can be reconstructed from multiple projections of an object. Today, CT units use either rotate- rotate or rotate-fixed geometry. Scan times of slightly less than 1 sec are possible.

27. Two types of detectors are used. Scintillation crystals must be coupled with photodiodes, and may be used with rotate-rotate or rotate-fixed geometry. Xenon gas ionization detectors are the most common detector in contemporary units, and must be used in rotate-rotate geometry only. Image reconstruction requires adequate non-redundant data. Analytic methods use 2-D Fourier transform or filtered backprojection to reconstruct the image. CT numbers are also called Hounsfield units (H). CT numbers are derived by comparing the liner attenuation coefficient of a pixel with the linear attenuation coefficient of water. Most CT units cover a range of at least 4000 CT numbers.

28. CT numbers are also called Hounsfield units (H). CT numbers are derived by comparing the liner attenuation coefficient of a pixel with the linear attenuation coefficient of water. Most CT units cover a range of at least 4000 CT numbers. Resolving power of CT units may be as high as 14 line pairs per em when used in a high resolution mode. Spatial resolution is the ability of the CT scanner to display separate images of high contrast objects placed close together. Spatial resolution is determined by scanner design, computer reconstruction, and the display. Contrast resolution is the ability of the system to display an image of a relatively large object only slightly different in density from its surroundings. Contrast resolution is limited by noise. Patient dose from a CT scan is intimately related to contrast resolution and spatial resolution. Typical doses range from about 2 cGy to 4 cGy . Computer programs can produce 3-D reconstruction of CT images.

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