1. Computed Tomography Basic principles V.G.Wimalasena Principal
School of Radiography

2. 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.

3. 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 and it was publicly announced in 1972.
Allan McLeod Cormack of Tufts University in Massachusetts independently invented a similar process, and both Hounsfield and Cormack shared the 1979 Nobel Prize in Medicine.

4. Prototype CT scanner

5. Historic EMI Scanner

6. Modern CT scanner

7. Label gantry aperture (720mm diameter) microphone
sagittal laser alignment light
patient guide lights
x-ray exposure indicator light
emergency stop buttons
gantry control panels
external laser alignment lights
patient couch
ECG gating monitor

8. CT Gantry –Internal structure

9. Label x-ray tube filters, collimator, and reference detector
internal projector
x-ray tube heat exchanger (oil cooler)
high voltage generator (0-75kV)
direct drive gantry motor
rotation control unit
data acquisition system (DAS)
detectors
slip rings

10. 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
Scattered x-rays
Transmitted X-ray beam
Incident x-ray beam

11. 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 transmitted x-rays
More transmitted x-rays
Less dense material

12. Applications of X-ray attenuation & detection
Conventional X-ray (Radiography)
Conventional Tomography
Computed Tomography

13. Conventional X-Ray
Conventional x-ray produces a compression of a volume to a plane
The detector is the Silver halide crystal on a x-ray film
The degree of blackening represents the total attenuation through the path of x-ray photons

14. The higher the attenuation the lesser is the blackness
The structure which results more attenuation or more transmission predominates in the image

15. Conventional Tomography
The source and detector moves
Produces Images of coronal or sagittal sections (cuts) of areas of interest
Eliminates the superimposition of structures above and below

16. CT Scan 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

17. 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
To be continued ….CTComplementary2