The document provides a comprehensive overview of computed tomography (CT), detailing the principles, components such as x-ray tubes, collimators, and detectors, and the process of image reconstruction. It discusses different types of detectors used in CT, such as scintillation crystals and xenon gas ionization chambers, along with the algorithms for reconstructing images. Additionally, the document addresses various CT artifacts, their causes, and the advancements in 3D imaging technology.

1. CT PHYSICS
JR RADIODIAGNOSIS

2. CT scan

3. BASICS OF CT – PART 2
• Xray tubes
• Collimators
• Detectors
• Xenon gas ionization chambers
• Detectors
• Image reconstruction
• CT Artifacts
• 3D imaging

4. 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.

5. 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.

6. 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).

7. 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.

8. 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.

9. 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

10. 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.

11. 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.

13. 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.

14. 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.

15. 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

16. 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 densebone becomes 1000.
To honor Hounsfield, CT numbers based on a magnification constant
of 1000 are also called Hounsfield units (H). Since new CT scanners
use a magnification constant of 1000, CT numbers and H units are
equivalent for these scanners.

17. IMAGE DISPLAY
A CT image is usually displayed on atelevision monitor for immediate
viewing, and recorded on film for interpretationand permanent storage.
One 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.

18. 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.

19. 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)

20. 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.

21. 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
cardiovascularstructures. When the patientmoves 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.

23. 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 rays) 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.

25. 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.

26. 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.

27. 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.

28. 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.

29. 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.

30. 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.

31. 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.

32. 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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