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 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

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 back-

projection 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

• When viewing objects which have CT numbers close to background noise can mask detail

Increased noise

Incr

ease

d co

ntra

st

Pixel number

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

T

dzzD ).(

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 DOSEHIGHER DOSE

HIGHER DOSE

(mA typically 100 to 200)

HIGH DOSE LOW

DOSEMUST 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

x 7

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