CT image quality

1. CT IMAGE QUALITY
DR. Pradeep Patil
Prof. Department of Radio-diagnosis,
DY Patil medical college, hospital & research institute Kolhapur

9. MODULATION TRANSFER FUNCTION
• MTF is the ratio of accuracy of the image to the actual object
scanned.
• Let us assume a series of lines with decreasing space as we go by.That
is as we go by the number of lines per same mount of space will be
increasing (nothing but spatial frequency).

10. • Let us assume these series of
lines have been scanned parallel
to them.
• The rectangular profile
represents the density profile of
an object composed of dense
lines
• The density profile of
reconstructed image is shown as
rectangles with rounded corners

11. • The ratio of the height of the rectangles to the height of the valley
between lines become smaller as the lines become closer in the
reconstructed image but obviously the ratio is fixed in the object.
• This decrease in ratio is a manifestation of decreasing ability of
system to resolve small separation of objects.
• As the frequency increases that is more lines per cm the ability of the
system to reproduce the lines and valleys accurately is decreased and
thus MTF is also decreased.

13. • If an object is smaller than a
pixel its density will be averaged
with the density of other tissues
contained in the pixel creating a
less accurate image
• When the pixel size is smaller it
is less likely that they will
contain different densities
therefore decreasing the less
likelihood of volume averaging.

14. NYQUIST SAMPLING THEOREM
• Because an object may not entirely lie
within a pixel ,so the pixel size should
be half the size of the object to
increase the likelihood of object being
resolved.
• In (A),(B),(C) the object to be
displayed is the same as the pixel.The
three figures show different scenarios
as to how the object could be
reconstructed each resulting in a
different level of volume averaging.
• In (D),(E) a smaller pixel size is used
and the scenarios regarding the
likelihood of volume averaging
improve.

21. FOCAL SPOT SIZE
• Use of small filament size concentrates the FOCAL SPOT,
reducing the penumbra,but cannot tolerate mA as it
causes damage to the anode
• Larger filament resolution
NUMBER OF PROJECTIONS
• To reconstruct the image,data must be acquired to enough projection
angles to provide independent measurements through each desired pixel.
• If one wishes to reconstruct an image with a matrix of n columns and n
rows then n square measurements must be obtained
• This criterion is easily met in present CT machines.

26. CURRENT
• Current is the rate at which electrons flow past a point.
AMPERE
• one coulomb of charge travels across a given point in 1 second.
COLUMB
• amount of electric charge carried by 6.25X1018 electrons.
So mAs represents the amount of electric charge that is nothing
but number of electrons.

27. VOLTAGE
• Potential difference between two points in an electric circuit.
• If we increase potential difference between two points the flow of
electrons is increased from one point to the other.
• So the flow of electrons will have a much higher energy.
• Hence the x rays produced by higher voltage will have higher energy.

28. TUBE CURRENT(mA) and SCAN TIME
• The total x ray beam exposure in CT is dependent on a combination of
mA setting,scan time and kVp setting .
• mA and scan time together are referred to mAs and defines the
quantity of the x ray energy.
• kVp setting defines the quality(average energy) of the x ray beam.
• Increasing the mA increases the number of electrons that will
produce produce x ray photons

30. SCAN TIME
• Scan time is the time the x ray beam is on for the collection of data
for each slice(The time it takes for the gantry to make a complete 360
degrees rotation)
• A short scan time is critical in avoiding image degradation as a result
of patient motion.
• Short scan times are used to avoid involuntary movement such as
peristalsis and cardiac motion

31. KILOVOLTAGE PEAK
• Kilovoltage peak (kVp) is the peak potential applied to the x-ray tube, which
accelerates electrons from the cathode to the anode in radiography or computed
tomography.
KVP ON PENETRATIBILITY AND CONTRAST
• Particularly in obese people lower energy photons are absorbed completely without
contributing to image formation. In such situations, higher kVp is employed to
improve the x-ray intensity reaching the receptor, thus increasing the signal to noise
ratio on the images.
• Moreover, higher energy photons are more likely to travel through the patient
without any tissue interaction. Consequently, higher energy x-ray beams generate
images with poorer contrast .

32. TUBE VOLTAGE or KILOVOLT PEAK
• In CT,kVp does not change contast as directly as it does in film screen
radiography.
• kVp beam intensity penetrability
IMPACT OF mAs and kVp SETTINGS ON RADIATION DOSE
1) mAs + kVp constant
2) Constant mAs + kVp
So the appropriate selection of mAs and kVp is critical to optimize radiation
dose to the patient and image quality

33. • Two reasons to change mAs rather than kVp
First the coice of mA is more flexible.
Effect on image quality is more straight forward and predictable.
AUTOMATIC TUBE CURRENT MODULATION
• Software that automatically adjusts the tube current to fit specific
anatomic regions is increasingly used in clinical practice
• These automatic exposure control techniques report a 15% to 40%
reduction in dose.

34. PIXEL SIZE
• If the object size is smaller than the pixel then its
density will be averaged with the density of other
tissues contained in the pixel so hence affecting the
contrast resolution
• When the pixel size is smaller than the object size it is
less likely that they will contain different densities so it
will show better contrast resolution.

37. • The appropriate reconstruction algorithm depends on which parts of
the data should be enhanced or suppressed to optimize the image for
diagnosis.
• For example – the temporal bone in which tiny of inner ear are
displayed the image can be reconstructed for spatial rather than
contrast fidelity.

42. • QUESTION
Given here are two graphs of MTF
with increasing spatial frequencies
of two CT scanner which scanner
has got better spatial resolution?

43. • QUESTION
Given here are two graphs of MTF
with increasing spatial frequencies
of two CT scanner which scanner
has got better spatial resolution?