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Medical Physics 2: Radiology > CT > Flashcards

CT Flashcards

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1
Q

Image reconstruction process FBF

A
  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
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2
Q

Limitation of basic back projection

A

1/r Blurring - need to filter using convolution or FT multiplication

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3
Q

Benefits and limitations of Iterative reconstruction

A
  • Potential for lower dose scanning
  • Lower noise images
  • Computer intensive
  • Potential for artefact reduction
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4
Q

Iterative reconstruction process

A
  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
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5
Q

Hounsfield Units

A

Attenuation of materials expressed relative to the linear attenuation coefficient of water at room temperature (μwater)

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6
Q

HU of water, air and bone

A 
water  = 0
air = -1000
bone = 1000
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7
Q

CT types over time

A
  1. Translate-rotate - single det
  2. Translate-rotate - bank of dets
  3. Rotate-Rotate - fan beam

electron beam scanners

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8
Q

Modern CT features

A
  • 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
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9
Q

Bow-tie filter

A

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

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10
Q

CT detector requirements

A 
Small  (spatial resolution)
High detection efficiency
Fast response with negligible after-glow
Wide dynamic range
Stable noise-free response
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11
Q

Types of CT detector

A
  • Xenon ionisation chamber

- Solid state detector (scintillant with embedded Si photo-diode)

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12
Q

Modern scanner features

A 
Multislice scanning
Helical scanning
Automatic dose modulation
Dual energy scanning
‘Dose Reduction’ features
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13
Q

Multislice X-ray beam width

A

Acquisition slice width -> total nominal beam width

Nominal beam width = Single slice thickness x tot no. slices

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14
Q

Types of pitch

A

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

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15
Q

Describe flying focus

A

Two-position focal spots with rapid switching

Effectively double no. of slices for a given det bank

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16
Q

Define overbeaming

A

Actual width x-ray beam > nominal width

Should be checked at commissioning

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17
Q

Define z-axis geometric efficiency

A

Measure of overbeaming

= Area under dose profile within active detectors / area under total dose profile

Must be greater than 70%

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18
Q

Define overranging

A

Interpolation from Im recon algorithms causes the actual scan range to exceed the nominal range planned by the system.

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19
Q

Overranging considerations

A
  • No. additional rots is manufacturer dep
  • Overranging increases as pitch increases
  • Modern scanners have overranging reduction features
20
Q

Methods of dose reduction

A
  • X-y plane vurrent modulation - based on scanogram atten map
  • Z-axis current mod - aims to obtain similar noise level along the patient
21
Q

Types of mA algorithms

A
  • 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)
22
Q

Define a scanogram

A 
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
23
Q

Types of kV automation

A
  • Selection based on scanogram
  • Or comp to ref values
  • Usually start at 120kV and lower based on patient size
24
Q

Dose reduction techniques

A

Auto kV
Auto mA
Superficial organ shielding (breast/eyes)
Software algor - contrast /edge enhancement

25
Q

Other CT features

A

Cardiac gating
Image processing - noise reduction
Dual energy scanning - tissue visualisation

26
Q

Define CTDI

A

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

27
Q

Define CTDI100

A 
CTDI measured in a 100mm chamber 
Corrected for L/nT 
L=length of chamber
n=no. simult slices
T= nominal slice width
28
Q

Define CTDIw

A

Weighted CTDI measured in a cylindrical head or body phantom

CTDIw = 1/3 CTDI100, centre + 2/3 CTDI100,periphery

29
Q

Define CTDIvol and DLP

A

CTDIw corrected for pitch

CTDIvol = CTDIw x NT/l

DLP is CTDIvol x irrad length

30
Q

Limitations of CT Dosimetry

A

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

31
Q

Discuss correcting for wide-beam dosimetry

A

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’)

32
Q

Spatial resolution characteristic of CT

A
  • 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)
33
Q

Sources of noise

A

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

34
Q

What are CT Motion artefacts?

A
  • patient / cardiac motion
  • Causes b/w bands
  • moving struct occupies different voxel
  • Decrease with inc rot speed, wide beam, gating
35
Q

What are streak artefacts?

A
  • Caused by high atten obj
  • Recon errors
  • Software correction available
36
Q

What is photon starvation?

A
  • High atten obj cause streaks behind artefacts
37
Q

What are beam hardening artefacts?

A
  • 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
38
Q

What are ring artefacts?

A
  • The result of detector damage
  • Ring caused by rotational symmetry
  • can indicate which det is faulty
39
Q

What is the partial volume effect?

A
  • 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
40
Q

Describe CT fluoroscopy

A
  • Used for interventional work
  • Uses repeated conv. CT
  • High resolution but high staff/patient skin dose
  • Low dose protocols available
41
Q

Describe CT gating

A

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

42
Q

Describe cardiac CT

A
  • 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
43
Q

Describe CT angiography

A

Test bolus often used to determine correct delay time
Likely to be multi-phased procedure
Potential for high patient skin doses

44
Q

Describe CT perfusion

A

CT Perfusion
Repeat scanning in one area until correct perfusion level reached
High patient skin doses are possible

45
Q

Describe dual energy imaging

A

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

Medical Physics 2: Radiology (10 decks)

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