Download 39.CT Physics Module I

Module I
Computed Tomography
Physics, Instrumentation,
and Imaging
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CT Artifacts and Quality
control
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Seeram’s definition: “a distortion or error in (on)
an image that is unrelated to the subject being
studied”.
CT Artifacts include:
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Foreign objects
Breathing motion
Peristalsis
Cardiac motion
Inaccurate Hounsfield numbers
Equipment artifacts (electronics and mechanics
or algorithms)
Production artifacts
Data Correction Processes
Calibration procedures
 Pre and post patient
operations
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…………have significantly reduced
the occurrence of artifacts related to
the CT imaging process.
CT Artifacts
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Classification is related to the
cause and appearance of the
artifact.
Streaks
 Shading
 Rings
 Bands
 cone
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Streak Artifacts
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Most common classification of artifacts
manifested in CT.
Causes:
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Improper sampling of data because of beam
hardening
Edge-gradient effect
Equipment malfunction
Metal objects
Partial volume averaging
Patient motion
Beam Hardening
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Occurs when there is an increase
in the mean energy of the x-ray
beam as it attenuates through
the patient.
 Low
energy photons are
absorbed, leaving high energy
photons striking the detectors.
 Also called “cupping”
Image taken from Seeram’s
text page 192
Edge-gradient Artifacts
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CT system is unable to transfer
information from the area of interest
accurately due to large differences in
atomic numbers of adjacent structures.
ie: barium / air
 Appear as straight thin black streaks
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Equipment-induced
artifacts
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Poor gantry rigidity
Mechanical misalignment
Radiographic tube rotor wobble
Inadequate sampling of the detector
signals
Corrupt detectors can produce:
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Streaks
Ring artifacts
Streak Artifacts
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Metal inside scan area
In some instances the gantry can
be tilted to avoid streak artifacts
which can not be removed.
MAR – (Metal Artifact Reduction)
software program which assists in
the reconstruction through
interpolation.
MAR – (Metal Artifact
Reduction)
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The raw data is acquired and stored
The CT image is reconstructed
Rough tracing of the prosthesis (metal implant)
with a light pen is performed
Automatic definition of the borders of the
prosthesis within the projection data is
performed.
Using ROI threshold values
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Linear Interpolation is performed
Reconstruction of the artifact reduced image
from the new computed projection data is
accomplished
Partial Volume Averaging
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Occurs when a voxel is occupied by
more than one type of tissue.
The different tissues therefore can cause
the area to be averaged.
Partial volume averaging can cause
partial volume artifacts (bands and
streaks)
– Can be reduced by selecting thinner slices
and using (VAR) volume artifact reduction
technique software.
VAR
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Works by taking the different types of tissue
within a voxel and dividing the tissues into
thinner slices with a table incrimination
between each.
Data sets of the raw data are averaged to
produce a composite thicker slice that will be
free of the partial volume artifact.
Illustration and more information on beam hardening and
partial volume artifact can be found on pages 192-193 in the
Seeram’s 2nd edition text.
Patient Motion
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Voluntary and Involuntary
– Use positioning aids such as pillows, sponges,
myelogram straps.
– Make your patient comfortable before beginning the
exam.
– Shorter scan times
– Explain the exam to the patient
– Motion software
– Illustration for MAC (Motion Artifact Correction) can be found on
page 189, Figure 11.22, Seeram’s 2nd edition text.
Sampling Errors
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Sampling errors cause streak artifacts.
Sampling errors are called Aliasing.
Nyquist Theorem Sampling must occur
at least twice the spatial frequency of
the object scanned.
Shading
– Can be the result of:
 Partial
volume averaging
 Beam hardening
 Spiral scanning
 Scatter radiation
 Off focal radiation
 Incomplete projection profiles
Cone Artifact
– Results when interpolation is not
successful.
– Image appears elliptical instead of
round
– The cone artifact can represent as
two crescent-shaped bands of
increased density
Figure 2b Medicamundi, Volume 43,
Issue 1, March 1999
Figure 2c Medicamundi, Volume 43,
Issue 1, March 1999
Noise
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Produced from an insufficient number of
x-ray photons striking the detectors.
Can appear as graininess or streaks
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Can be caused from:
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Poor centering of the patient within the
gantry
 Incorrect selection of exposure factors
 Aperture size
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Quality Control Tests
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Daily, Monthly, or yearly depending on
the type of test.
Must be done to produce high quality
diagnostic scans.
The scanner must be performing at
optimal levels to insure the accuracy of
the images and Hounsfield numbers.
Some QC tests are done by the
technologist and some are performed
by engineers during PM (preventive
maintenance).
3 Basic Components of
Quality Control
1.
2.
3.
Quality control must be
performed on a regular,
periodic basis.
Prompt interpretation of QC
test measurements.
Faithful and meticulous record
keeping.
Daily QC Tests
1.
2.
Average CT number in water
Standard deviation of CT
numbers in water
CT Testing Phantoms
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CT scanners should never be sold
without providing the QC
phantoms for regular quality
control measures.
FDA – Department of Public Health
– Office of Radiation Control