The document discusses the basics of computed tomography (CT) scanning. It explains some limitations of conventional X-rays like superimposition of structures and poor contrast resolution. CT provides cross-sectional images of the body with no superimposition of tissues and excellent contrast resolution. A CT scan uses X-rays combined with computer technology to produce internal images of the body. An X-ray tube rotates around the patient, emitting a thin beam of X-rays that pass through the body and are measured by a detector. The computer analyzes the data to generate two-dimensional cross-sectional images of the inside of the body.

1. ‫تعلم‬ ‫تكن‬ ‫مالم‬ ‫وعلمك‬‫تعلم‬ ‫تكن‬ ‫مالم‬ ‫وعلمك‬
‫ال‬ ‫فضل‬ ‫وكان‬‫ال‬ ‫فضل‬ ‫وكان‬
‫عظيما‬ ‫عليك‬‫عظيما‬ ‫عليك‬

2. Basics Of
Computed
Tomography (CT)
Lecture One
GGamalamal FFathallaathalla MM..
MMahdalyahdaly
gamal_mahdaly@hotmail.comgamal_mahdaly@hotmail.com

3. Limitations ofLimitations of Conventional X-RaysConventional X-Rays
 Poor Contrast Resolution.Poor Contrast Resolution.
 Superimposition.Superimposition.
 Qualitative rather that quantitative.Qualitative rather that quantitative.

4. Tube-Patient- Film arrayTube-Patient- Film array

5. SuperimpositionSuperimposition
You can’t locate the exact position of the tumor

6. SuperimpositionSuperimposition
Note the superimposition of several anatomies in this PA Chest radiograph

7. Lateral view to minimize DiagnosticLateral view to minimize Diagnostic
Pitfalls due to SuperimpositionPitfalls due to Superimposition

8. Poor Contrast ResolutionPoor Contrast Resolution
You can’t see the soft tissue

9. Qualitative rather than QuantitativeQualitative rather than Quantitative
2 different structures can have the same appearance despite different composition

10. Conventional TomographyConventional Tomography

11. Tube-Film axis repositioning forTube-Film axis repositioning for
Conventional TomographyConventional Tomography

12. Tube MovementsTube Movements
There are five basic types of movement :There are five basic types of movement :
 linearlinear
 EllipticalElliptical
 CircularCircular
 SpiralSpiral
 HypocycloidalHypocycloidal

13. Conventional TomographyConventional Tomography
 Superimposition in radiograph can beSuperimposition in radiograph can be
lessened. Yet, some degree remains.lessened. Yet, some degree remains.
 Poor contrast resolution.Poor contrast resolution.
 Hazy outline due to moving projections.Hazy outline due to moving projections.

14. Computed TomographyComputed Tomography
 1- A highly collimated X-Ray beam is transmitted through a specific1- A highly collimated X-Ray beam is transmitted through a specific
cross-section of the patient.cross-section of the patient.
(No superimposition- Minimal scattered radiation resulting in better(No superimposition- Minimal scattered radiation resulting in better
Contrast Resolution).Contrast Resolution).
 2- This beam is variably attenuated in the body of the patient by2- This beam is variably attenuated in the body of the patient by
different anatomies with different densitiesdifferent anatomies with different densities
 3- The attenuated beam strikes special detectors , these detectors3- The attenuated beam strikes special detectors , these detectors
are quantitative and measure subtle attenuation differencesare quantitative and measure subtle attenuation differences
resulting in high Spatial Resolution.resulting in high Spatial Resolution.
 4- The data from the detectors are processed by digital computer4- The data from the detectors are processed by digital computer
(ADC) that uses special algorithms to reconstruct an image of cross-(ADC) that uses special algorithms to reconstruct an image of cross-
section in different ways & different filters.section in different ways & different filters.
 5- The computed motion of the cradle together with the collimation5- The computed motion of the cradle together with the collimation
helps determine the slice thickness.helps determine the slice thickness.

15. Excellent Contrast Resolution

16. CT Scanner main components

17. Data AcquisitionData Acquisition
The Data Acquisition System (DAS) =The Data Acquisition System (DAS) =
Tube + Detector array + ADCTube + Detector array + ADC

18. Formation of a CT image - Tomographic
Acquisition
A series of rays that pass through the patient at the same
projection or view .
All modern CT scanners incorporate fan beam geometry.

19. Formation of CT imageFormation of CT image
You could have approx. 800 rays taken at 1,000 different projection angles
giving 800,000 transmission measurements

20. AttenuationAttenuation
 Attenuation= Number of Photons produced byAttenuation= Number of Photons produced by
tube – Number of Photons striking thetube – Number of Photons striking the
detectors.detectors.
 Some photons are absorbed, but others areSome photons are absorbed, but others are
scattered.scattered.
 Attenuation depends on the electrons perAttenuation depends on the electrons per
gram, atomic number and type of radiationgram, atomic number and type of radiation
(homogeneous or heterogeneous).(homogeneous or heterogeneous).

21. Attenuation ValueAttenuation Value
 The data needed to reconstruct the image areThe data needed to reconstruct the image are
transmission measurement through the patient.transmission measurement through the patient.
 The transmitted intensity is given by:The transmitted intensity is given by:
 Where IWhere Ioutout= transmitted intensity, I= transmitted intensity, Iinin= measured= measured
intensity, e= Euler’s constant, µ= Linearintensity, e= Euler’s constant, µ= Linear
Attenuation Coefficient & x = thicknessAttenuation Coefficient & x = thickness
x
einIoutI
µ−
=

22.  SinceSince xx, I, Ioutout, I, Iinin and e are knownand e are known MM can becan be
calculated:calculated:
 MM = 1/x . log (I= 1/x . log (Iinin / I/ Ioutout))
MM (x,y)
I (detector)
I0 (source)
object
x
y

23.  The total distribution of attenuationThe total distribution of attenuation
coefficient in the object iscoefficient in the object is MM ((xx ,, yy).).
 The role of CT system is to calculateThe role of CT system is to calculate
MM ((xx ,, yy) from a set of projections specified) from a set of projections specified
by the angle theta.by the angle theta.
 Remember always that if we use a fanRemember always that if we use a fan
beam, we have multiple theta angles onebeam, we have multiple theta angles one
for each ray.for each ray.

24. CT NumbersCT Numbers
 Numerical value forNumerical value for
each pixel.each pixel.
 The CT number isThe CT number is
directly related todirectly related to
the x-ray linearthe x-ray linear
attenuationattenuation
coefficient for thecoefficient for the
tissue contained intissue contained in
the voxel.the voxel.
Where K constant =1000
µ = Tissue attenuation
value
CT number = HU
CT Number = KCT Number = K [[
μ tissue - μ water
____________________
μ water
]]

25. Linear Attenuation Coefficients for Various BodyLinear Attenuation Coefficients for Various Body
TissuesTissues
Tissue Linear Attenuation Coefficient
Bone 0.528
Blood 0.208
Grey matter 0.212
White matter 0.213
CSF 0.207
Water 0.206
Fat 0.185
Air 0.0004

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27. Have a nice dayHave a nice day