Introduction to CT Physics

Shahid Younas

Introduction to CT Physics

Computed Tomography

X rays

Electromagnetic

0.01- 10 nm

30 PHz to 30 Hexa Hz

PET CT / Cyclotron- the Cycle

PET CT / Cyclotron- the Cycle

Highly energetic electrons interact

with matter and convert their

kinetic energy into

electromagnetic radiation.

X ray Production Device

Electron source (Filament)

Glass tube (Evacuated)

Target (Cathode)

Bremsstrahlung X Rays

Bremsstrahlung X Rays

The electron accelerates then

suddenly de-accelerates, causing

electromagnetic radiation equal

to that of the change in kinetic

energy of the electron.

Bremsstrahlung X Rays

The energy of the E= mc2 has

a continuous spectrum up to a

maximum, equal to the

maximum kinetic energy of

the electron.

Bremsstrahlung X Rays

The production of

Bremsstrahlung radiation

increases with the atomic

number of the target atom;

thus, it increases electrostatic

attraction.

Bremsstrahlung X rays

Bremsstrahlung X rays

Bremsstrahlung X Rays

Do you know peak voltage (Vp).

Characteristic X Rays

Brem + Char; = Continuous

Characteristic X Rays

Characteristic X Rays

Introduction to CT Scanner History

Roentgen was

beginning …..

Introduction to CT Scanner History

He was actually working

on properties of “cathode

rays”.

Introduction to CT Scanner History

Roentgen, like many husbands,

wanted to get rid of …… so ………

Introduction to CT Scanner History

‘‘X-ray mania’’

Introduction to CT Scanner History

… and then since 1896 she ….

Introduction to CT Scanner History

This is an era of “planer radiology”.

The discovery of X-rays was a necessary but insufficient

condition for the rise of computed tomography.

Its design also depended on the development of

computational techniques.

Introduction to CT Scanner History

Allan MacLead Cormack (a), Godfrey Newbold Hounsfield (b)

Introduction to CT Scanner History

The EMI Mark I scanner (a) and a transverse image of the brain (b)

The EMI scanner with instrumentation and images obtained with it (a), and the front cover of a company brochure describing the new

technology

EMI CT Scanner

The basic parameters of the scanner used at that time were

as follows:

scan time: about 4.5 min,

reconstruction time: 20 s,

cross-section thickness: 13 mm,

image matrix: 80 x 80 pixels, where each pixel

represented an area 3 9 3 mm.

X Ray Tube

Do you know which algorithm was used by Hounsfield in the

first CT Scanner?

Algebraic Reconstruction Technique (ART)

X Ray Tube

The x-ray tube provides an environment for x-ray

production via bremsstrahlung and characteristic radiation

mechanisms.

Major components are the cathode, anode, rotor/stator,

glass (or metal) envelope, and tube housing.

X Ray Tube

The major components

of a modern x-ray tube

and housing assembly.

X Ray Tube

For diagnostic imaging, effective peak voltage ranging

from 20 to 150 kVp.

The tube current is the rate of electron flow from the

cathode to the anode.

1 mA = 6.24 X 1015 electrons/see.

X Ray Tube

Do you know the difference between KVp and KeV?

Peak Voltage (Vp) is the pushing force whereas Electron Volts is a unit of energy

X Ray Tube

For continuous fluoroscopy the tube current is typically 1

to 5 mA, and for projection radiography tube currents

from 100 to 1,000 mA are used with short exposure times

(less than 100 msec).

X Ray Tube

The kVp, mA, and exposure time are the three major

selectable parameters on the x-ray generator control panel

that determine the x-ray beam characteristics (quality and

quantity). These

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