…….CT Physics Continued
V.G.Wimalasena
Principal
School of radiography
Correction for star artifacts - Filtering


A filter function is applied to each point along the
attenuation profile to eliminate these artifacts
Different filter functions are used to create sharper
(higher resolution) or smoother (lower noise) images
Attenuation
profile
Filter function
Filtered profile
Convolution?

The process of applying the filter function to
the attenuation profile is called
CONVOLUTION
(Super imposition of the filter function and the
attenuation profile)
Result of convolution

The convolution minimizes the artifacts by changing
the back projected information .

The dark and white bands in the illustration add up in
such a way as to yield an image which accurately
represents scanned object
Impulse signals from
linear traverses following
angular rotations
3rd back
projection
2nd back
projection
1st back
projection
Final back projection
Practical method of Generation of
Attenuation profiles


Attenuation profiles are a recording of X-ray attenuation
verses position in the object.
Attenuation is related to the ratio between incident
intensity (I0) and transmitted intensity (It)
I0
It



X-ray intensity is measured by a DETECTOR which
converts x-ray photons to electrical current
The reference (Input) detector measures the incident
intensity (I0)
The output detector measures the transmitted intensity
(It )
I0= incident intensity = tube output
It


I0 should remain constant
It will change depending on the attenuation of the
object scanned (e.g. centered, round object of uniform
density)
It=1000
Io=1000
It
Calibration profile

If the object scanned is a centered, round water bath each
attenuation profile is called a CALIBRATION PROFILE.
and the set of profiles are called the Calibration File or Cal
file
DATA Calibration


Subtraction of
calibration file
from the
attenuation
profiles of the
object called data
calibration
(consider an
object containing
two areas of
different
densities as
shown)
Scan Data File



The difference profile
are stored as numerical
values as a function of
position in the profile.
This data is used in the
image reconstruction
process
The set of difference
profiles for a complete
scan is called SCAN
DATA FILE or raw data
Back projection of scan data
1.
Back projection of
the corrected
attenuation profiles is
accomplished by
feeding the numerical
values of each point
along the profile into
a matrix. (Since the
scanning motion is
circular, the matrix is
usually round)
--
2.
3.
4.
Close rays are averaged
to attain a pixel value
from the ray that passes
closest to its center
Every pixel is assigned
one value per view.
The final value is the
sum of these divided
by the number of
views (as shown)
6.
When all the measurements have been projected back
into the matrix and all pixel values determined, the
image can be displayed
The number of pixels
in the image
corresponds to the
MATRIX SIZE.
This is given as 256 x 256,
320x 320, 512 x 512.
this refers to the
number of pixels across
a vertical or horizontal
diameter of the image.
7.
8.
The actual size of a
pixel is equal to the
size of the area
scanned, called the
FIELD OF VIEW
(FOV) divided by
the matrix size.
( Small pixel sizes
helps to display
small objects that
can be resolved by
the system)
CT Numbers

CT numbers are calculated from the measured
attenuation values using the equation given below.
Range of CT numbers

Tremendous range of attenuations that might be
displayed in a CT image requires a large number scale.
+/- 1000 is commonly used and some systems have
extended this range further on the high end to
differentiate bone densities
Limitation in display of CT numbers


The human eye can
distinguish only about 20
levels of gray in the gray
scale and cannot
appreciate the large range
of information
contained in CT scan
This is solved by using
the ‘window width’ &
‘window level’
Window width



This is the range of CT
numbers that is displayed
using the useful levels of
gray scale
Small window separates
one CT number from
another
Large window condenses
more than one CT
number in each of gray
level
Window level


This is the centre of the
range of CT numbers
displayed by the window
The level control moves
the visible gray scale up
and down the CT
number scale
Hounsfield unit system & CT
numbers



As an honour to Hounsfield who was the
pioneer of invention of CT the CT numbers are
called Hounsfield units.
The range of Hounsfield units Is from -1000 to
+3000
Standard reference points are -1000 for Air and
0 for water.
END
THANK YOU !