CT Physics
V.G.Wimalasena
Principal
School of radiography
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
Matrix



The image is represented
as a MATRIX of
numbers.
Matrix :- A two
dimensional array of
numbers arranged in
rows and columns.
Each number represents
the value of the image at
that location
VOXEL

Each individual element
or number in the image
matrix represents a three
dimensional volume
element in the object,
called a VOXEL
PIXEL

The VOXEL is
represented in the image
as a two-dimensional
element called PIXEL (picture element)
CT numbers

The numbers in the
image matrix are called
CT numbers.

Each pixel has a number
which represents the xray attenuation in the
corresponding voxel of
the object
Visual image & Gray Scale


To obtain a visual image,
the CT numbers are
assigned different shades
of gray on a gray scale.
Each shade of gray
represents the x-ray
attenuation within the
corresponding voxel
CT Image
Phases of CT imaging
1.
2.
Scanning the patient
Data Acquisition
I.
II.
3.
4.
5.
Tube or tube and detector move
Multiple attenuation measurements are taken
around the object
Image reconstruction
Image Display
Image archival (recording)
Scanning the patient - Evolution of CT
scanners
1st Generation
3rd Generation
2nd Generation
4th Generation
DATA ACQUISITION

Basic components





X-ray tube
Collimators
Detector/s
Collimated x-ray beam
traverses the object and
enters the detector.
The attenuation is
determined from the
difference between
incident intensity and
transmitted intensity
X-ray
tube
Collimators
Detector
Translate rotate system

In the basic CT system
the x-ray tube and
detector are translated
linearly so that the
beam scans the object
(eg. A disc of uniform
density)
Ray, Ray sum, View & Attenuation Profile
Ray – Imaginary line
between Tube &
Detector
Ray Sum – Attenuation
along a Ray
View – The set of ray
sums in one direction
The attenuation for each ray
sum when plotted as
function of its position is
called an attenuation
profile
Ray
View
Ray sums
Attenuation
profile
Attenuation profile of different structures

Attenuation of objects
with different densities
will change the
attenuation profile
Object with low
attenuation
Object with high
attenuation
Attenuation profile at different positions

In a translate –rotate CT,
after a view is recorded,
the tube and detector
rotate a small angle and
the entire process is
repeated until many
views have been
recorded for the same
slice
Views & attenuation profiles for a slice
1
2
4
3
Image reconstruction


The image is created by
reflecting the attenuation
profiles back in the same
direction they were
obtained
This process is called
BACK PROJECTION
BACK PROJECTION
Drawbacks in Back projection
(Artifacts - Star shape & streaks)



The resultant image
closely resembles the
original object
But it shows star shaped
patterns around objects
and streaks
These are called ‘Star’
and ‘streak’ artifacts
Formation of Star artifact and streaks

Consider a
scan of a single
high density
object
suspended in
air

The attenuation profile for this object has a single
impulse signal

The back projections take the form of a stripe through
the center of the object

Back projections are crated for each profile

Addition of the attenuation profiles create an image
with star and streak artifacts
Final back projection
To be continued – CT Complementary 3