computed tomography

2. Content :
1-Some definition
- CT.
-radiology .
2-Introduction .
3- Interaction of radiation with matter .
4- X-ray production and X-ray tube.

3. CT device
CT:
is the device that used in radiology .
Radiology:
A branch of medicine concerned with the use of
radiant energy as x-rays or radioactive material in the
diagnosis and treatment of disease .

4. INTRODUCTION
-CT developed into a versatile 3D whole the body
imaging modality for oncology , cardiology and
vascular radiology this when was CT introduced
but now is use for radiology planning and CT
scanner can integrated with a PET scanner .
Purpose :
-Measure x-ray transmission through patient.

5. The essential physics of medical
image
-Radiation may be ionizing radiation or non-ionizing
radiation
Ionizing radiation may be :
1—Directly ionizing radiation (charged particles):
electrons, protons, a particles and heavy ions.
2—Indirectly ionizing radiation
(neutral particles):
photons (X rays and g rays), neutrons.

6. photon interactions with matter :
During the interaction the photon may;
1-completely disappear (photoelectric effect, pair
production, triplet production).
2-scattered coherently (coherent scattering)
or incoherently (Compton effect).

7. 1_Photoelectric effect:
A photon interact with an atom and ejects one of the
orbital electrons from the atom , the incident photon
energy is transferred to the atomic electron. The kinetic
energy of the ejected photoelectron(Ee) is equal to the
incident photon energy (Eo) minus the binding energy of
the orbital electron (Eb)
Ee = E0 – Eb

8. After the electron has been ejected from the
atom, a vacancy is created in the shell (atom in
excited state ) .this vacancy can be filled by an
outer orbital electron with the emission of the
characteristic x-rays .

9. 2-Coherent (Rayleigh) scattering:
An electromagnetic wave passing near the electron and
setting it into oscillation ,this electron radiates the
energy at the same frequency as the incident
electromagnetic wave . thse scatter x-ray that
have the same wavelength as the incident
beam .

10. 3-Compton Effect (incoherent
scattering):
Photon interaction with a free and stationary’ orbital
electron. the electron receives some energy from the
photon and emitted at an angle ϴ.
“The term of free electron mean that the binding
energy of the electron is much less than the energy of
bombarding photon .”

11. 4-Pair and triplet production
In pair production, an X- or gamma ray interacts with
the electric field of the nucleus of an atom. The
photon's energy is transformed into an electron-
positron pair. The electron and positron lose their
kinetic energy via excitation and ionization. when the
positron comes to rest, it interacts with a negatively
charged electron, resulting in the formation of two
oppositely directed 0.511( The rest mass energy) MeV
annihilation photons..

13. Beam attenuation
The intensity Ix of a narrow monoenergetic
photon beam, attenuated by an attenuator
of thickness x, is given as:
Ix = Io e m x
where I0 is the original intensity of the
unattenuated
beam
m is the linear attenuation coefficient.

14. X-ray Production, X-ray
Tubes
-Superficial and orthovoltage X rays used in
radiotherapy are produced with X ray
machines.
The main components of a radio-
therapeutic X ray machine are:
an X ray tube, a ceiling or floor mount for the
X ray tube, a target cooling system, a
control
console, and an X ray power
generator

16. -The electrons producing the X ray
beams in the X ray tube originate in
the heated filament (cathode) and
are accelerated in a vacuum towards
the target (anode) by a constant
potential electrostatic field supplied
by the X ray generator

17. -With X ray tubes, the patient dose is
delivered using a timer and the
treatment time must incorporate the
shutter correction time which accounts
for the time required for the power
supply components to attain the steady
state operating conditions.

18. -The X ray tube current is controlled by a
hot filament emission of electrons, which,
in turn, is controlled by the filament
temperature (thermionic emission).
-For a given filament temperature the X
ray tube current increases with the tube
(anode) voltage.

19. X-RAY TUBE INSER
:
-The x-ray tube insert contains the cathode,
anode, rotor assembly, and support structures
sealed in a glass or metal enclosure under a
high vacuum.
-X-rays are emitted in all directions from the
focal spot however, the x-rays that emerge
through the tube port constitute the useful
beam.

20. X-Ray Tube Housing
-The x-ray tube housing supports, insulates,
and protects the x-ray tube insert from the
environment.
-x-ray tube housing contains a bellows to allow
for oil expansion as it absorbs heat during
operation.
-Lead shielding inside the housing attenuates
the x-rays that are emitted in all directions, and
of course there is a hole in the shielding at the
x-ray tube port.

21. Special X-Ray Tube Designs
-Grid-biased tubes have a focusing cup that is
electrically isolated from the cathode filament .
-When the bias voltage is large, the resulting
electric field lines shut off the tube current.
- The grid-biased tube is used in applications
such as pulsed fluoroscopy and radiography.
-rays that penetrate this lead shielding, and
therefore it has a high effective energy.

22. presented by :
Rania saeed
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