computed tomography by jay&jay
TRANSCRIPT
Computed Tomography
Presented by :
120130103071-Nandasana Jay
120130103084-Prajapati Jay1
Introduction
Computed tomography (CT) is a medical imaging method employing tomography.
The word "tomography" is derived from the Greek tomos (slice) and graphein (to write).
A large series of two-dimensional X-ray images (slices) of the inside of an object are taken around a single axis of rotation.
Digital geometry processing is used to generate three-dimensional images of the object from those slices.
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History
The first commercially viable CT scanner was invented by Sir Godfrey Hounsfield in Hayes, United Kingdom at EMI Central Research Laboratories using X-rays. Hounsfield conceived his idea in 1967. and it was publicly announced in 1972.
Allan McLeod Cormack of Tufts University in U.S. independently invented a similar process, and both Hounsfield and Cormack shared the 1979 Nobel Prize in Medicine.
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CT Scan
CT scan produces axial sections/cuts /Slices
The CT image is recorded through a SCAN.
Scan?
A scan is made up of multiple X-Ray attenuation measurements around an objects periphery
X-ray tube
Detector
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Understanding Basic factors
Absorption :-stopping of x-rays with transfer of energy
Scatter:- deflection of x-rays
Incident Intensity :- No. of x-ray photons falling on an object
Transmitted Intensity:-No. of photons passing through
Incident x-
ray beam
Transmitted
X-ray beam
Scattered x-rays
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Attenuation
The reduction of the beam intensity on passing through the material due to absorption plus scatter
The degree of attenuation is obtained by measuring and comparing the incident and transmitted intensities
More dense
material
Less dense
material
Less
transmitted
x-rays
More
transmitted
x-rays
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Slice / Cut
The cross sectional portion of the body which is scanned for the production of CT image is called a slice.
The slice has width and therefore volume.
The width is determined by the width of the x-ray beam
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Matrix
The image is represented as a MATRIX of numbers.
Matrix :- A two dimensional array of numbers arranged in rows and columns.
Each number represents the value of the image at that location
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VOXEL
Each individual element or number in the image matrix represents a three dimensional volume element in the object, called a VOXEL
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PIXEL
The VOXEL is represented in the image as a two-dimensional element called PIXEL - (picture element)
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CT numbers
The numbers in the image matrix are called CT numbers.
Each pixel has a number which represents the x-ray attenuation in the corresponding voxel of the object
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Visual image & Gray Scale
To obtain a visual image, the CT numbers are assigned different shades of gray on a gray scale.
Each shade of gray represents the x-ray attenuation within the corresponding voxel
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Tissue Types And Appearance
*Cerebrospinal fluid (CSF)
CT Image
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Phases of CT imaging
1. Scanning the patient
2. Data Acquisition
I. Tube or tube and detector move
II. Multiple attenuation measurements are taken around the object
3. Image reconstruction
4. Image Display
5. Image archival (recording)
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DATA ACQUISITION
Basic components X-ray tube
Collimators
Detector/s
Collimated x-ray beam traverses the object and enters the detector.
X-ray
tubeCollimators
Detector
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Practical method of Generation of Attenuation profiles
Attenuation profiles are a recording of X-ray attenuation verses position in the object.
Attenuation is related to the ratio between incident
intensity (I0) and transmitted intensity (It)
I0It
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X-ray intensity is measured by a DETECTOR which converts x-ray photons to electrical current
The reference (Input) detector measures the incident intensity (I0)
The output detector measures the transmitted intensity (It )
I0= incident intensity = tube output
It
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I0 should remain constant
It will change depending on the attenuation of the object scanned (e.g. centered, round object of uniform density)
Io=1000
It=1000It
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Calibration profile
If the object scanned is a centered, round water bath each attenuation profile is called a CALIBRATION PROFILE.
and the set of profiles are called the Calibration File or Cal file
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DATA Calibration Subtraction of
calibration file from the attenuation profiles of the object called data calibration
(consider an object containing two areas of different densities as shown)
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Scan Data File
The difference profile are stored as numerical values as a function of position in the profile.
This data is used in the image reconstruction process
The set of difference profiles for a complete scan is called SCAN DATA FILE or raw data
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Back projection of scan data
1. Back projection of the corrected attenuation profiles is accomplished by feeding the numerical values of each point along the profile into a matrix. (Since the scanning motion is circular, the matrix is usually round)
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Formation of Star artifact and streaks
Consider a scan of a single high density object suspended in air
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The attenuation profile for this object has a single impulse signal
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Back projections are crated for each profile
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Addition of the attenuation profiles create an image with star and streak artifacts
To be continued – CT Complementary 327
Generations of CT Scan
1st generation: rotate/translate, pencil beam
2nd generation: rotate/translate, narrow fan beam
3rd generation: rotate/rotate, wide fan beam
4th generation: rotate/stationary
5th generation: stationary/stationary
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1st generation: rotate/translate, pencil beam
• Only 2 x-ray detectors used (two different slices)
• Parallel ray geometry
• About 4.5 minutes/scan with 1.5 minutes to reconstruct slice
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2nd generation: rotate/translate, narrow fan beam
• Incorporated linear array of 30 detectors
• More data acquired to improve image quality
• Shortest scan time was 18 seconds/slice
• Narrow fan beam allows more scattered radiation to be detected
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3rd generation: rotate/rotate, wide fan beam
• Number of detectors increased substantially (to more than 800 detectors)
• Angle of fan beam increased to cover entire patient.
• Mechanically joined x-ray tube and detector array rotate together
• Newer systems have scan times of ½ second
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4th generation: rotate/stationary
• Designed to overcome the problem of ring artifacts.
• Stationary ring of about 4,800 detectors
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5th generation: stationary/stationary
• Developed specifically for cardiac tomographic imaging
• No conventional x-ray tube; large arc of tungsten encircles patient and lies directly opposite to the detector ring
• Electron beam steered around the patient to strike the annular tungsten target
• Capable of 50-msec scan times.
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Latest update:-
6th generation: helical
7th generation: multiple detector array
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Modern CT scanner
1. gantry aperture (720mm diameter)
2. microphone
3. sagittal laser alignment light
4. patient guide lights
5. x-ray exposure indicator light
6. emergency stop buttons
7. gantry control panels
8. external laser alignment lights
9. Patient couch
10. ECG gating monitor 36
Advantages
•Quick and painless
•Can help diagnose and guide treatment for a wider range of conditions than plain X-rays
•Can detect or exclude the presence of more serious problems
•Can be used to check if a previously treated disease has recurred
Disadvantages
•Small increased risk of cancer in future from exposure to ionisingradiation (X-rays). Risk is greater for children•Uses higher doses of radiation, so the risks (while still small) are in general greater than other imaging types•Injection of a contrast medium (dye) can cause kidney problems or result in allergic or injection-site reactions in some people•Some procedures require anaesthesia
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Questions?
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Thank You……………
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