CT Flashcards
Image reconstruction process FBF
- Measure a set of projections
- Filter the set of projections
- Back-project across image plane
- Repeat for a large set of filtered projections
Limitation of basic back projection
1/r Blurring - need to filter using convolution or FT multiplication
Benefits and limitations of Iterative reconstruction
- Potential for lower dose scanning
- Lower noise images
- Computer intensive
- Potential for artefact reduction
Iterative reconstruction process
- Projection data acquired
- Initial guess made at the image
- Compare back projection of the guess to the initial data
- Correct the guess to better match the original data
- Process is repeated
Hounsfield Units
Attenuation of materials expressed relative to the linear attenuation coefficient of water at room temperature (μwater)
HU of water, air and bone
water = 0 air = -1000 bone = 1000
CT types over time
- Translate-rotate - single det
- Translate-rotate - bank of dets
- Rotate-Rotate - fan beam
electron beam scanners
Modern CT features
- 3rd Generation, cone-beam, multi-detector
- Modified volume filtered back projection or iterative image reconstruction
- Continuous rotation to 0.25 s
- Volume dataset with image display in all planes
- High heat capacity x-ray tube for high throughput
Bow-tie filter
Inc attenuation at edge of field of view
Matches noise accross patient cross section
Reduces peripheral dose
Reduce beam-hardening artefacts
Different size for head and body
CT detector requirements
Small (spatial resolution) High detection efficiency Fast response with negligible after-glow Wide dynamic range Stable noise-free response
Types of CT detector
- Xenon ionisation chamber
- Solid state detector (scintillant with embedded Si photo-diode)
Modern scanner features
Multislice scanning Helical scanning Automatic dose modulation Dual energy scanning ‘Dose Reduction’ features
Multislice X-ray beam width
Acquisition slice width -> total nominal beam width
Nominal beam width = Single slice thickness x tot no. slices
Types of pitch
P = couch move per rot / slick thickness
P (helic) = couch move per rotation / slice thick (d)
Px = couch per rot / Nom beam width
beam pitch = det pitch / N
Describe flying focus
Two-position focal spots with rapid switching
Effectively double no. of slices for a given det bank
Define overbeaming
Actual width x-ray beam > nominal width
Should be checked at commissioning
Define z-axis geometric efficiency
Measure of overbeaming
= Area under dose profile within active detectors / area under total dose profile
Must be greater than 70%
Define overranging
Interpolation from Im recon algorithms causes the actual scan range to exceed the nominal range planned by the system.
Overranging considerations
- No. additional rots is manufacturer dep
- Overranging increases as pitch increases
- Modern scanners have overranging reduction features
Methods of dose reduction
- X-y plane vurrent modulation - based on scanogram atten map
- Z-axis current mod - aims to obtain similar noise level along the patient
Types of mA algorithms
- Use scanogram data from previous rotations
- Aim for constant noise or size-dep noise
- Use a reference value set (for noise, ref IQ or mA)
Define a scanogram
Often called a scout view A quick scan purely for positioning Now used for kV and mA modulation Dose dep on slice-width and table speed Front or lateral projection
Types of kV automation
- Selection based on scanogram
- Or comp to ref values
- Usually start at 120kV and lower based on patient size
Dose reduction techniques
Auto kV
Auto mA
Superficial organ shielding (breast/eyes)
Software algor - contrast /edge enhancement
Other CT features
Cardiac gating
Image processing - noise reduction
Dual energy scanning - tissue visualisation
Define CTDI
CTDI = 1/S int(D(x)dx)
D(x) dose profile across a slice
s = nominal slice width
relates machine output in air
Measured via partially irrad pencil chamber
Define CTDI100
CTDI measured in a 100mm chamber Corrected for L/nT L=length of chamber n=no. simult slices T= nominal slice width
Define CTDIw
Weighted CTDI measured in a cylindrical head or body phantom
CTDIw = 1/3 CTDI100, centre + 2/3 CTDI100,periphery
Define CTDIvol and DLP
CTDIw corrected for pitch
CTDIvol = CTDIw x NT/l
DLP is CTDIvol x irrad length
Limitations of CT Dosimetry
CTDIw assumes a clinical scan length of 100mm which is not clinically representative for a beam width >40mm
At that point it neglects a signif amount of scatter
Discuss correcting for wide-beam dosimetry
Measure CTDIa at both ref and actual beam width to find a correction factor
Measure by stepping the chamber through the beam (measure in each 100mm ‘section’)
Spatial resolution characteristic of CT
- Limited by pixel size - mat/FoV
- Can use high res algor but they increase noise
- No difference in axial and helical for x-y plane
- Z dir res is reduce with increased pitch for helical
(less so with multislice)
Sources of noise
1) Quantum noise:
reduced by increasing mAs, kV or slice width
pitch does not affect noise for single slice scanners
noise increases with pitch for multislice scanners
2) Electronic noise
from measuring system
3)Structural noise
from reconstruction algorithm
What are CT Motion artefacts?
- patient / cardiac motion
- Causes b/w bands
- moving struct occupies different voxel
- Decrease with inc rot speed, wide beam, gating
What are streak artefacts?
- Caused by high atten obj
- Recon errors
- Software correction available
What is photon starvation?
- High atten obj cause streaks behind artefacts
What are beam hardening artefacts?
- Lower CT No. in centre due to hardened beams crossing
- cause by changes in energy as beam passes through patient
- Cupping or streak artefacts
- Reduced by bow tie filter
What are ring artefacts?
- The result of detector damage
- Ring caused by rotational symmetry
- can indicate which det is faulty
What is the partial volume effect?
- Occurs when an object is smaller than the voxel size
- a distortion occurs as the voxel is assigned a value based on the average CT value within the voxel
- can occur with contrast filled blood vessels
- Depends on transaxial slice thickness
Describe CT fluoroscopy
- Used for interventional work
- Uses repeated conv. CT
- High resolution but high staff/patient skin dose
- Low dose protocols available
Describe CT gating
Used in Cardiac or respiratory CT
Scanning linked to ECG or respiratory monitor = select based rest phase to im
Reduces dose at times of little interest
Reduces motion artifacts
Reduces amount of data collected
mA modulation not always possible when gating utilised
Describe cardiac CT
- Connect CT to ECG
- Record during phase of least motion (diastole, or end diastole)
- can be retro gated or ECG triggered
- retro gating uses helical overpitching then bins the raw data using the ECG results
- ECG triggered reduces dose as ‘step and shoot’ acqs.
- Single heart beat im possible with dual-source CT or wide cone-beam CT
Describe CT angiography
Test bolus often used to determine correct delay time
Likely to be multi-phased procedure
Potential for high patient skin doses
Describe CT perfusion
CT Perfusion
Repeat scanning in one area until correct perfusion level reached
High patient skin doses are possible
Describe dual energy imaging
May be 2 tubes (Siemens)or rapid kV switching
Enables more information on component tissues
Contrast enhancement possible
May be dose implications – no mA modulation with kV switching
