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Flashcards in CT Deck (45)
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Image reconstruction process FBF

1. Measure a set of projections
2. Filter the set of projections
3. Back-project across image plane
4. 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

1. Projection data acquired
2. Initial guess made at the image
3. Compare back projection of the guess to the initial data
4. Correct the guess to better match the original data
5. 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

1. Translate-rotate - single det
2. Translate-rotate - bank of dets
3. 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