2. Introduction
The purpose of a Computed tomography (CT) acquisition is to
measure x ray transmission through a patient for a large
number of views.
3. Computed tomography (CT)
Different views are achieved in computed tomography primarily by using :
1- detectors (generally 800-900 detector elements)
2- rotating x-ray tube ( rotate around the patient)
4. Principles of CT
o The process of CT image involves the rotation of the X ray tube and detector row
around the patient, a large number of views can be obtained.
o The detector row is an array of several hundred small separate detectors. Readings
from the detectors are fed in to a computer.
( X ray projection, attenuation and acquisition of transmission profiles)
5. Principles of CT
( X ray projection, attenuation and acquisition of transmission profiles)
6. Principles of CT
The acquired transmission profiles are used to reconstruct the CT
image, composed of a matrix of picture elements (pixels)
The values that are assigned to the pixels in a CT image are
associated with the linear attenuation coefficient μ (m–1) of the
corresponding tissue.
( X ray projection, attenuation and acquisition of transmission profiles)
7. Principles of CT
Different tissues are encountered with different linear attenuation
coefficients. So each element in the matrix have a different value of
the associated linear attenuation coefficient.
calculate the average linear attenuation coefficient (μ) for each voxel
is called reconstruction.
Image reconstruction techniques is applied to derive the matrix of
linear attenuation coefficients, which is the basis of the ct image.
( X ray projection, attenuation and acquisition of transmission profiles)
8. Hounsfield unit (HU)
In the ct image, the matrix of reconstructed linear attenuation coefficients (μmaterial ) is transformed into a
corresponding matrix of Hounsfield units
o Hounsfield scale - tissue density is expressed in different shades of grey in relation to its x-ray
absorption.
9. (HU) is given by the following equation linear attenuation
coefficient of water
at room
temperature
linear attenuation
coefficient of voxel
Hounsfield unit (HU)
The Hounsfield unit (HU) : is a quantity commonly used in computed tomography
(CT) scanning to express CT numbers in a unified and convenient form.
10. The CT Imaging System
The gantry contains all the system components that are
required to record transmission profiles of the patient.
Since transmission profiles have to be recorded at
different angles, these components are mounted on a
support within the gantry that can be rotated.
Gantry and table
11. The CT Imaging System
Gantry and table
Gantry component
X- ray tube
High voltage
generator Tube cooling
system
Collimator
Beam shaping
filters
Detector arc
Data acquisition system
12. The CT Imaging System
Gantry and table
Recorded projection profiles are generally
transmitted from the gantry to a computer
by means of wireless communication
technologies.
13. The CT Imaging System
Gantry and table
The design and engineering of the table, as with the
gantry, are critical to allowing accurate acquisition of
data at high rotational speeds.
The table must also be able to withstand heavy
weights without bending.
14. The X ray tube and generator
An x ray tube (with a rotating tungsten anode) and high voltage generator
are used for generating the x ray beam.
16. Beam shaping filters
Beam shaping filters are being used to create a gradient in the
intensity of the X-ray beam
• They are sometimes called “bow-tie” filters
• They are mounted close to the X-ray tube.
The purpose of the beam shaping filter is to:
1• reduce the dynamic range of the signal recorded by the CT detector
2• Reduce the dose to the periphery of the patient
17. Detectors
Essential physical characteristics of the CT detectors are :
1- Good detection efficiency
2- Fast response (and little afterglow)
Detectors are used to capture energy that has not been attenuated by the
patient.
