Computed Tomography
CT, CAT
tomos = slice, graphein = to write
Magdalena Bazalova
1. What is a CT scanner?
• an X-ray device capable of cross-sectional
imaging
• creates images of slices through the patient
What is a CT scanner?
• doughnut shaped gantry with moving patient
table
Why CT?
• conventional radiography suffers from
collapsing of 3D structures onto a 2D image
• although the resolution of CT is lower, it has
extremely good low contrast resolution
enabling the detection of very small changes in
tissue type
• CT gives accurate diagnostic information
about the distribution of structures inside the
body
CT scanning applications
• very wide ranging – good for imaging bone
and soft tissue
– diagnostic imaging
– radiotherapy planning
• 3D applications
CT imaging applications
CT imaging applications
CT imaging applications
Why CT for radiotherapy?
• Radiation therapy planning is done on the
basis of patient CT images and is therefore
patient specific
– the target and organs at risk are delineated in CT
images (possibly with help of other imaging
modalities – PET)
– dose calculation algorithms use CT images for
determination of dose delivered to the patient
during treatment
Why CT for radiotherapy?
• Tissue inhomogeneities can be taken into account in
most treatment planning systems
• Dose to soft tissue is different than dose to cortical
bone - mass density variations between tissue types
are the most important factor
• Therefore, mass densities of tissues have to be known
for an accurate dose calculation
• CT images do not represent mass densities of patient
body directly but they can be converted into mass
densities using a calibration curve
2. CT scanner components
X-ray tube
X-ray beam
detector ring
X-ray journey
X-ray tube
Beam shaping filter
Detectors
Detector arrangement
Philips CT simulator
Questions on CT apparatus
• How do we call the device that produces X-ray
beam?
– (X-ray tube )
• What have the X-rays pass through on their
way to the detector ring?
– (beryllium window, Al filters, bow-tie filter, patient,
anti-scatter grid)
3. CT image definition and formation
What are we measuring in CT?
• the linear attenuation
coefficient, µ, between the
X-ray tube and
the detector
• the linear attenuation
coefficient is a measure
of how rapidly are X-ray
attenuated
I  I 0 exp(x)
2D-projection data set - sinogram
I 
• projections p   ln 
 I0 
-d
x-ray source
-d
• I, I0 - intensities
d
p
d
Projection angle
Reconstruction algorithms
•
•
•
•
Computer based
simple back-projection
filtered back-projection
iterative techniques
Simple back-projection
• reverse the process of measurement of
projection data to reconstruct an image
• each projection is uniformly distributed across
the reconstructed image
Simple back-projection
1/r blurring
Filtered back-projection
• simple back-projection produces blurred
images
• projection data need to be filtered before
reconstruction
• different filters can be applied for different
diagnostic purposes
– smoother filters for viewing soft tissue
– sharp filters for high resolution images
• back-projection is the same as before
Filtered back-projection
Image reconstruction
Simple back-projection
Filtered back-projection
FBP
Patient image reconstruction
Patient filtered back-projection
CT number scale
HU represents the linear attenuation of a material.
CT number window
CT number window
CT for radiotherapy – calibration,
HU to mass density conversion
• HU do not represent mass density, needed for
dose calculation, directly. To obtain mass
densities of each voxel:
• A set of tissue equivalent
materials with known
mass densities is scanned
and a calibration curve
is created
Calibration curve for treatment
planning
Mass density calibration curve
1.800
3
mass density [g/cm ]
1.600
y = 5.78E-04x + 1.05E+00
1.400
1.200
1.000
y = 8.84E-04x + 9.83E-01
0.800
0.600
y = 1.03E-03x + 1.03E+00
0.400
0.200
0.000
-1000
-500
0
500
HU
1000
Questions on reconstruction
• How do we call picture and volume elements?
– (pixels and voxels)
• What do CT images represent?
– (linear attenuation coefficients of voxels)
• How do we call raw detector data?
– (a sinogram)
• Name two reconstruction techniques?
– (simple and filtered back-projection)
4. CT technology
Third generation CT scanners
Fourth generation CT scanners
Fifth generation CT scanners
Helical CT scanning
Advantages of helical mode
Questions on CT technology
• How many CT generations exist?
– 5 (maybe more)
• Which one is the third one?
– rotate/rotate
• What are the advantages of helical scanning?
– arbitrary image position, faster scanning
5. CT image quality
Image noise
Image noise
Image contrast
Image contrast
Factors affecting image noise
Reconstruction filters
Factors affecting detector signal
Radiation dose
Questions on image quality
• Name three factors that influence image quality.
– kVp, mA, time, filteration of the beam, slice thickness,
reconstruction filter, pitch
• Name three parameters that describe image quality
– spatial resolution, contrast, noise
• What is noise?
– variation in HU in a uniform image
• What is contrast
– ability to resolve details without blurring
6. Artefacts in CT
If not recognized, CT artifacts can cause misdiagnosis
and incorrect outcomes of radiotherapy treatment planning.
Definition of CT artefacts
Types of CT artefacts
Origin of artefacts
Beam hardening: cupping
Beam hardening: correction
Beam hardening: correction
Reduction of streaks and bands
Partial volume artefacts
Partial volume artefacts
Minimizing partial volume artefacts
Photon starvation
Avoidance using mA modulation
Metal artefacts
Metal artefact reduction
Patient motion artefacts
• Voluntary and
involuntary motion
cause artefacts in the
reconstructed image
Minimizing motion artefacts
Motion artefact correction
Detector sensitivity: ring artefacts
Ring artefacts
CT artefacts: summary
Questions on CT artifacts
• Remedy for photon starvation?
– mA modulation, adaptive filtering
• What can cause metal artifacts in patients?
– dental implants, surgical clips, electrodes,
prostheses
• In which CT generation can occur ring
artifacts?
– in the 3rd where detector ring rotates with X-ray
tube