Transcript Image Quality in CT

CT Image Quality
Dr M A Oghabian
Medical Physics Department,
Tehran University of Medical
Sciences
Sinogarm
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Reconstruction Filters
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CT Number
CT number=1000×(mpixel-mwater)/mwater
Window width & window level
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Quality criteria for CT images
Image Quality
1) Spatial Resolution
 2) Density Resolution
 3) Noise
 4) Artifact

Partial volume effect
 Beam Hardening
 Star Artifact
 Slice profile

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Spatial Resolution (high contrast)
Spatial Resolution is defined as:
The number of line pairs per cm just visible in image
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Spatial Resolution
high contrast resolution

The minimum size of detail visualized in the
image with a contrast >10%.
It is affected by:
 Reconstruction algorithm
 Detector width
 Slice thickness
 Object to detector distance
 X-ray tube focal spot size
 Matrix size.
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Spatial Resolution
Low contrast resolution

The size of detail
that can be
visualized when
there is only a
small difference in
density
 It limited by
noise.
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Slice Thickness and Pixel size

Thicker slice or bigger pixel size causes
worse Spatial Resolution, but better
SNR

Thicker Slice improve Density resolution

Higher mAs and more patient dose
leads to better SNR
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Noise
Noise is obtained
from the standard
deviation in CT
number in a
region of interest
(ROI) placed
within the image
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Noise



It affects the low contrast resolution
Noise dependents on the radiation dose
The medical problem is: to obtain an image
with an acceptable level of noise while keeping
the patient dose as low as reasonably
achievable
Noise =
1
dose
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Scatter!!
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Typical Scatter Survey
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CT Number Accuracy
Measured CT number should be < ± 4 HU in
the central ROI
 CT number of water is by definition equal to 0
 CT number depends on tube voltage, filtration,
object thickness

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CT number uniformity
CT number of
each pixel in
the image of
an
homogeneous
object should
be the same
over various
regions
•The difference in the CT number between a peripheral and a
central region should be < 8HU
•Differences are largely due to beam hardening phenomenon
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CT number linearity
Linear relationship between the calculated CT number and
the linear attenuation coefficient of each element of the
object. Deviations from linearity should be < ± 5 HU
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Artifacts
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Partial Volume Artifact
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Star Artifact
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Ring Artifact
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The slice sensitivity profile (SSP)
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Z-Sensitivity (Imaged slice width)
Plan view of a test object used to measure imaged slice widths for
axial scans, to assess the accuracy of post patient collimation,
and to calculate the geometric efficiency for the scanner
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Alignment of indicating lights with
scan, coronal and sagittal planes
 Several methods can be
used to perform these tests
The wrapped film is placed flat
on the table and illuminated
by the external scan plane
light
The position of the light is
marked on the film envelope
and the table is moved
automatically to the scan
plane
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Couch travel accuracy
To assess the distance indicator accuracy, a ruler or tape
measure placed alongside the table, to check that the
degree of couch movement indicated on the gantry
agrees with the actual distance moved.
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Axial scan incrementation
accuracy
 Verification of incrementation accuracy
between successive axial slices can be
achieved by placing envelope-wrapped film
on the couch (in the isocentre plane) and
exposing it to an axial scan sequence
 Narrow slices separated by a couch
increment greater than 1 slice width can be
used, and the distance between the lines on
the film measured
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Couch travel accuracy for
helical scans
 In helical scanning, to assess imaged distance
accuracy, a Perspex test object containing two small
radio-opaque markers, separated by a fixed distance
(ex:20 cm) is used.
 A helical run is planned to start at the first marker and
to end at a distance x from the first marker
 If couch travel is accurate during the helical scan, the
markers should be clearly seen on the first and final
images of the series.
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Couch travel accuracy for
helical scans
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Assessment of accuracy of gantry tilt
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A film must be held vertically, so that it is parallel to the sagital plane
Three axial exposures with 0, max superior and max inferior gantry
tilt are made using the same film:
The three scan planes should then be visible on the developed film
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Dosimetry - CTDI in air (helical)
Axial slice positions
Helical scan (pitch 1)
The Computed Tomography Dose Index (CTDI) in air
can be measured using a 10cm pencil ionization
chamber, bisected by the scan plane at the isocentre.
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Dosimetry - CTDI in Perspex
Phantoms
Insert to plug holes
Head phantom
Body phantom (or annulus
to fit over haed phantom)
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Dosimetry - CTDI in Perspex
Phantoms
 Central and peripheral CTDI’s are used to calculate
weighted CTDI, CTDIw:
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n CTDI w =
C
(
1
2
CTDI100,c + CTDI100,p
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3
)
 CTDIws can be compared against diagnostic
reference levels for standard scan examinations
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