CT Scanning:
Dosimetry and Artefacts
Dr. Craig Moore
Medical Physicist & Radiation Protection Adviser
Radiation Physics Service
CHH Oncology
Operator Controlled Variables and
their effects on Image Quality and
Patient Dose
&
Imaging Performance
• Spatial Resolution
– Significance
– Factors affecting resolution
• Z-sensitivity
– Significance
– Factors affecting z-sensitivity
Modulation Transfer Function (MTF):
• Details contrast in image relative to contrast in object
• MTF50, MTF10, MTF2, MTF0 are often quoted
• MTF2 approximates to the limit of visual resolution
In-plane Spatial Resolution
• Maximum that can be achieved is about
20 lp/cm (but usually less)
• Typical matrix size is 512 x 512
• In the scan plane limited by pixel size
– Pixel size = FOV/matrix
– i.e. if FOV = 40cm and matrix = 512 then pixel
size = 0.8 mm
– Pixel size determines limit of resolution but
there are other factors that affect resolution on
a scanner
In-plane Spatial Resolution
• Other factors affecting
resolution:
– Filter used for backprojection
– Size of focal spot,
geometry of the scanner
and size of detectors
– Sampling frequency
(number of times the x-ray
beam is sampled as it
rotates around the patient)
• Spatial resolution the
same for axial and helical
scanning
z-plane Resolution
• Spatial resolution in
the z-direction
(parallel to patient)
dependent on pitch
• The greater the pitch
the lower the
resolution
Summary of spatial resolution
• In-plane spatial resolution and z-resolution are
usually thought of as different parameters
• In fact, z-resolution is an extension of spatial
resolution in third dimension
• Multi-slice scanning is moving CT away from
slice based medium to a truly 3D modality
• Structural features are 3D, so resolution should
be equal in all dimensions
Image Noise
Sources of noise (1)
• Quantum Noise:
– Randomness of
photon detection
– This type of noise is
the most dominant in
CT
FOV
Sources of Noise (2)
• Structure noise:
– Affected by backprojection filters
Sources of Noise (3)
• Electronic noise
– small compared to
other sources
Image Contrast & Noise
• Contrast is equivalent to the difference in CT number between an
object and its surrounding tissue
Increased contrast
• When viewing objects which have CT numbers close to background
noise can mask detail
Pixel number
Increased noise
Low Contrast Resolution:
• Measure of how well a system can differentiate between an object and
its background having similar attenuation coefficients
Low
Medium
High
• Low contrast resolution is important when small contrast differences
are crucial for early detection of disease
• Low contrast exams account for approximately 90% of CT scans
• Affected by all parameters that influence noise
• Minimum detectable contrast is <0.5%
Low contrast
of a few HU
Principles of CT Dosimetry
Dose in CT
• One of the highest dose techniques used
in medical imaging
• Within the UK it has been shown to
contribute to 40% of the total dose
attributable to medical exposure
• But only 4% of the total number of exams
Radiation Units
• Absorbed dose in CT
– CT Dose Index (CTDI) (mGy)
• Radiation risk in CT
– Dose Length Product (DLP) (mGy cm)
– Effective dose (mSv)
Dose Distribution in CT
• Absorbed dose not a single value
• Dose values vary with position in patient in
scan plane and along z axis
Dose Distribution
• Depends on
–
–
–
–
Filtration
Beam shaping
Scanner geometry
Size of patient
• More uniform for
– Higher filtration
– Optimised beam shaping
– Smaller patient
• Periphery:centre
– Body 2:1
– Head 1:1
CT Dose Index (CTDI)
• Measure of dose in
the scan plane from a
single rotation
• CTDI defined as:
– Dose at position z, Dz
is integrated over the
complete dose profile
and divided by slice
thickness T
D( z ).dz
 T
Dose Profile along z
patient
Measurement of CTDI
• Routinely measured with
air filled pencil ion
chambers
• Use PMMA phantoms to
simulate patient
• Single rotation with
chamber:
– At the centre
– At the edges
• Calculate CTDIcentre and
CTDIedge
Complete cross
section of dose
‘MEDIUM’ DOSE
LOWER DOSE
HIGHER DOSE
Pitch typically between 1 and 2
LOW DOSE
HIGHER DOSE
HIGHER DOSE
(mA typically 100 to 200)
HIGH DOSE
LOW DOSE
MUST INCREASE mA
and/or s
kV typically between 80 and 140 kV
For the same
pitch
Effective Dose
• To estimate the stochastic risk to the
patient, must consider scan length (DLP)
and anatomical location of scan
• Effective dose used
• This is the equivalent whole body dose
x7
Effective Dose (mSv)
• CT is one of (if not THE), highest dose x-ray modalities
used in the hospital
– Head – 2 mSv
– Chest – 8 mSv
– Abdomen and pelvis – 10 mSv
– CT KUB – 10-15 mSv
• Planar chest – 0.015 mSv
• CT chest is 600 times the dose of planar chest
• Planar IVU has effective dose 1.5 – 3 mSv
• CT KUB is up to 10 times the dose of planar IVU
MUST BE CAREFUL TO OPTIMISE DOSE – HAVE SPECIFIC
INFANT PROTOCOLS
Overview of dose reduction
methods
•
•
•
•
•
•
•
X ray beam filtration
– Harden the beam so to reduce low energy photons
– Shape the beam to the relevant anatomical region
X ray beam collimation
– The beam should be limited to the minimum dimensions required
Tube potential, current and time
Tube current modulation and AECs
– Tube current is chosen to maintain predetermined level of noise
Size or Weight Based Technique Charts
– CT image never appears overexposed
– Standardise kVp, mA and scan time
Detector efficiency
– No detector is 100% efficient at converting photon into signal
– Modern scanner have 90% efficiencies or above
Noise reduction algorithms
– Smooth noise without reducing fine detail
Contrast and Special Uses of CT
• CT is being used increasingly for interventional work
– Special sequences can allow 3D images at 5 frames per second for
biopsy needle placement
– Alternatively needle can be moved between scans in a ‘step and shoot’
method
• Contrast given to enhance the visibility of certain structures (i.e.
increase the CNR)
– Should be given with caution or avoided altogether in patients with poor
kidney function
– eGFR 30 – 60 with caution
– Below 30 significant risk
– Doctor must be available on-call in case of anaphylactic reaction
(treatment of which includes intramuscular adrenaline, coticosteroids
and antihistamines)
• Modern scanners can scan the heart in less than 0.5s is one
rotation, thus avoiding motion artefacts of the beating heart
Image Artefacts
• What are artefacts?
– Systematic discrepancies between the CT
numbers in the reconstructed image and the
true attenuation coefficients of the object
– Non-random, or structured image noise
Artefact Origins
• Physics based
–
–
–
–
Beam hardening
Partial volume effects
Photon starvation
Undersampling
• Patient based
– Presence of metal
– Motion
• Scanner based
– Detector sensitivity
– Mechanical instability
Non-random, or structured, image noise
Beam Hardening Artefacts
• As beam passes through patient, low energy
photons are filtered and the beam becomes
harder
• This causes the attenuation coefficient and CT
number for a given tissue to decrease along the
beam path.
• Reconstruction process assumes a
monoenegetic beam
• CT numbers are lower in the centre than they
should be
– Cupping
– Streaks
Avoiding beam hardening artefacts
• Adequate beam filtration
– Flat filter
– Bow tie filter
• Correction factors built in to calibrate
• Beam hardening correction software
• Avoidance of bony regions when possible
– Patient positioning and gantry tilt
Objects smaller than a
voxel its density will be
averaged across the whole
voxel so it will appear
bigger and less dense
Can still be a significant
problem in images even with
software correction
Mechanical Instability
• X-ray tube rotor wobble
–
–
–
–
rotates at ~10,000 rpm
high temperatures
high frequency rotor wobble causes beam deviations
streak artefacts result
With rotor wobble
Without rotor wobble
Artefacts in Helical Scanning
• In general, the same as in conventional
scanning
• Additional artefacts occur due to
interpolation
• Worse at higher pitches
• Due to changing structure in z-direction
(e.g. skull)
Helical artefact of spherical
phantom at pitch 2
Bilateral subdural haemorrhage
or helical artefact?
Minimization
• Reduce effects of variation along z-axis
– Pitch of 1 rather than higher pitch
– 180° rather than 360° interpolation
– Thin slices
Conclusions
• Artefacts originate from a range of sources and
can degrade the diagnostic quality of an image
• Some can be partially corrected for in software
• Good scanner design, careful positioning of
patient and optimum selection of scan
parameters can minimise the artefacts present in
an image
CT Summary
•
•
•
•
•
•
•
•
•
•
•
•
•
CT scanner generate images in transaxial slices
CT number represents average linear attenuation coefficient in the voxel
Contrast in the displayed image is enhanced by windowing
The scanner gantry carries the X-ray tube and generator and a curved bank
of detectors
The image is reconstructed by filtered back projection
Slip ring technology allows the gantry to rotate continuously
In helical scanning, the patient is moved through the gantry while the gantry
rotates
Helical pitch is defined as ratio of table movement per rotation to slice
thickness
Multislice scanners have several rows of detectors that collect data
simultaneously
Multislice scanning can produce 3D images
In-plane resolution is at best 20 lp/cm
Quantum noise is the main limit ti low contrast resolution
Dose is generally higher for CT than other X-ray modalities