1. CT Scanning: Dosimetry and Artefacts
Dr. Craig Moore
Medical Physicist & Radiation Protection Adviser
Radiation Physics Service
CHH Oncology

2. Operator Controlled Variables and their effects on Image Quality and Patient Dose
&

3. Imaging Performance Spatial Resolution Z-sensitivity Significance
Factors affecting resolution
Z-sensitivity
Factors affecting z-sensitivity

5. 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

7. 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

8. In-plane Spatial Resolution
Other factors affecting resolution:
Filter used for back-projection
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

9. z-plane Resolution
Spatial resolution in the z-direction (parallel to patient) dependent on pitch
The greater the pitch the lower the resolution

10. 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

11. Image Noise

15. Sources of noise (1) Quantum Noise: Randomness of photon detection
This type of noise is the most dominant in CT

16. FOV

20. Sources of Noise (2) Structure noise:
Affected by back-projection filters

21. Sources of Noise (3) Electronic noise
– small compared to other sources

22. Image Contrast & Noise
Contrast is equivalent to the difference in CT number between an object and its surrounding tissue
When viewing objects which have CT numbers close to background noise can mask detail
Increased noise
Increased contrast
Pixel number

23. Low Contrast Resolution:
Measure of how well a system can differentiate between an object and its background having similar attenuation coefficients
Low
Medium
High

24. Low contrast exams account for approximately 90% of CT scans
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

26. Principles of CT Dosimetry

27. 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

28. Radiation Units Absorbed dose in CT Radiation risk in CT
CT Dose Index (CTDI) (mGy)
Radiation risk in CT
Dose Length Product (DLP) (mGy cm)
Effective dose (mSv)

29. Dose Distribution in CT
Absorbed dose not a single value
Dose values vary with position in patient in scan plane and along z axis

30. Dose Distribution Depends on More uniform for Periphery:centre
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

31. 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

32. Dose Profile along z
patient

35. 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

37. Complete cross section of dose

38. HIGHER DOSE
‘MEDIUM’ DOSE
LOWER DOSE

39. Pitch typically between 1 and 2

41. HIGHER DOSE
HIGHER DOSE
LOW DOSE
(mA typically 100 to 200)

42. HIGH DOSE
LOW DOSE
MUST INCREASE mA and/or s
kV typically between 80 and 140 kV

43. For the same pitch

48. 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

49. x 7

50. 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 – mSv
Planar chest – 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

58. MUST BE CAREFUL TO OPTIMISE DOSE – HAVE SPECIFIC INFANT PROTOCOLS

63. 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

64. 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

65. 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

66. 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

67. 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

69. 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

72. Objects smaller than a voxel its density will be averaged across the whole voxel so it will appear bigger and less dense

80. Can still be a significant problem in images even with software correction

86. 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

90. 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)

92. Helical artefact of spherical phantom at pitch 2
Bilateral subdural haemorrhage or helical artefact?

93. Minimization Reduce effects of variation along z-axis
Pitch of 1 rather than higher pitch
180° rather than 360° interpolation
Thin slices

98. 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

99. 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