Dr: Gamal Fathallah

1. ‫تعلم‬ ‫تكن‬ ‫مالم‬ ‫وعلمك‬
‫هللا‬ ‫فضل‬ ‫وكان‬
‫عظيما‬ ‫عليك‬

2. Basics Of Computed
Tomography (CT)
Lecture One
Gamal Fathalla M.
Mahdaly
gamal_mahdaly@hotmail.com

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

4. Tube-Patient- Film array

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

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

7. Lateral view to minimize Diagnostic
Pitfalls due to Superimposition

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

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

10. Conventional Tomography

11. Tube-Film axis repositioning for
Conventional Tomography

12. Tube Movements
There are five basic types of movement :
 linear
 Elliptical
 Circular
 Spiral
 Hypocycloidal

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

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

15. Excellent Contrast Resolution

16. CT Scanner main components

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

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

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

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

21. Attenuation Value
 The data needed to reconstruct the image are
transmission measurement through the patient.
 The transmitted intensity is given by:
 Where Iout= transmitted intensity, Iin= measured
intensity, e= Euler’s constant, µ= Linear
Attenuation Coefficient & x = thickness
x
e
in
I
out
I




22.  Since x, Iout, Iin and e are known M can be
calculated:
 M = 1/x . log (Iin / Iout)
M (x,y)
I (detector)
I0 (source)
object
x
y

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

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

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

26. Any Question???.
Take your Tiiime!!! .
Again Any Question???.
Otherwise, I’m going to ask!!!.
Should I Ask???.

27. Have a nice day