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Y2 Nuclear Medicine - MPY8009 > Computed Tomography > Flashcards

Computed Tomography Flashcards

1
Q

What are the steps to reconstuct a simple object?

A
  1. Measure set of projections
  2. Back-project across image plane
  3. Repeat for large set of projections
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2
Q

What is the disadvantage of back-projection?

A

Back-projection produces blur with a 1/r point spread
function that is the same at all points in the image.

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3
Q

How can you counteract the 1/r blur?

A
  1. Filter the projections first
    with a convolution kernel that counteracts the 1/r blur
  2. Then backproject the filtered projections: hence filtered
    backprojection
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4
Q

What are the advantages of iterative reconstruction?

A
  • Lower noise images > Potential for lower dose scanning
  • Potential for artefact reduction
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5
Q

What is the disadvantage of iterative reconstruction?

A

Computer intensive

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6
Q

What are the steps to iterative reconstruction?

A
  1. Projection data acquired
  2. Initial guess made at the image
  3. Comparison of initial guess to actual projections
  4. Initial guess is corrected to match the measured projections
  5. Process repeated
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7
Q

What is the equation for the Hounsfield Units?

A

HUmaterial = ((μmaterial - μwater)/μwater ) x 1000

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8
Q

What is the HU of:
(a) Water
(b) Air

A

(a) HUwater = 0 (μmaterial=μwater)
(b) HUair = -1000 (μmaterial=0)

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9
Q

What is the HU dependent on?

A
  • Composition of tuissue or material
  • Tube voltage
  • Temperature
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10
Q

What are the features of a modern CT scanner?

A
  • 3rd Generation, cone-beam, multi-detector banks
  • 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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11
Q

What are the main components of a CT beam?

A
  • X-ray tube
  • Main collimator
  • Secondary Collimator
  • Post patient collimator
  • Detectors
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12
Q

What is the ideal design of CT Filtration?

A
  • To approximate a monoenergetic beam, need very heavily filtration
  • Typical filtration around 6 mm Al
  • Heavy filtration can be compensated with reconstrcution algorithms
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13
Q

What are ‘Bow-Tie’ Filters and their advantages?

A

‘Bow-Tie’ filters are shaped to provide additional attenuation at edge of field of view.

Advantages:
* Matches noise levels across patient cross-section
* Reduces peripheral dose
* Reduce beam-hardening artifacts

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14
Q

What are 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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15
Q

What are the two main types of CT Detectors?

A
  • Xenon ionisation chambers (single slice scanners)
  • Solid state detectors (scintillant with embedded silicon photodiode)
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16
Q

What are the scanning aspects of modern CT scanners?

A
  • Multislice scanning (wider beam)
  • Helical scanning (continuously rotating X-ray tube)
  • Automatic dose modulation
  • Dual energy scanning
  • ‘Dose Reduction’ features
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17
Q

What is the nominal beam width?

A

Nominal beam width = N x T
* Single slice thickness = T
* Number of slices = N

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18
Q

What is the difference between the pitch and helical pitch?

A

PITCH (Beam) = Couch movement per rotation/ Nominal beam width
PITCH (Helical/Detector) = Couch movement per rotation/ Slice thickness

Beam PITCH = Detector PITCH/ N

PITCH describes how far the couch is moving relative to the rotation of the X-ray tube.

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19
Q

What are the differences with flying focus scanners?

A
  • Two-position focal spot with rapis switching during gantry rotation
  • Doubles the number of slices reconstructed
  • Beam width is half the nominal detector width
20
Q

What is overbeaming?

A

The actual width of the x-ray beam > nominal width

21
Q

How can you quantify overbeaming?

A

Z-axis geometric efficiency = Area under dose profile within active detectors/ Area under dose profile

Requirement for the geometric efficiency to be displayed on the console when less than 70%.

22
Q

What are the considerations for overbeaming?

A
  • Very dependent on detector geometry
  • Mostly an issue when using small number of thin detectors, but not always predictable
  • Requirement to display on console when less than 70%
  • Should be checked for all slice width combinations at commissioning
23
Q

What is over-ranging (or over-scanning)?

A

Due to image reconstruction algorithms based on interpolation, the actual scan range exceed the
nominal range planned by the console.

24
Q

What are the considerations for over-ranging?

A
  • Number of additional rotations each end of scan volume is
  • scanner/manufacturer dependent
  • Over-ranging distance will increase as pitch increases
  • Modern scanners offer features to reduce (but not
  • completely remove) effect of over-ranging
25
Q

What are the two different dose reduction algorithms and briefly explain them?

A

Angular Modulation
* X-rays are much less attenuated in the anteriorposterior direction than in the lateral direction.
* Automatic current moduclation using data from the scanogram to evaluate lateral dimension of the patient

Z- Axis Modulation
* Current is modulated depending on the thickness of the different sections of the patient
* The information for the automated mA is available from the scanogram.

26
Q

What are the main features of automated mA alogorithms?

A
  • May use scanogram data or data from previous rotation
  • May be based on constant noise (typically GE & Toshiba scanners) or may use size-dependant noise
  • Actual value used will depend on reference value set in protocol (may be noise ref or IQ ref or ref mA)
27
Q

What is a Scanogram (or Topogram/Scout View)?

A
  • Used to be purely for positioning, and selecting scan range
  • Now used for mA (&kV) modulation
  • Usually 2 views
  • AP/PA & Lateral
  • Parameters usually preset, little operator flexibility
  • Dose depends on slice-width, table speed & mA
28
Q

What dose reduction features are available for CT?

A

Superficial organ shielding
* mA reduced or switched off for restricted angle of rotation
* Used to reduce exposure to breasts or eyes
* May make mA modulation not possible

Software algorithms for specific situations
* Contrast enhancement
* Edge enhancement

Automated kV
* Appropriate kV selected for scan in smilar fashion to mA
* Selected once for scan
* kV frequently lowered from 120kV, particularly for smaller patients

29
Q

What is CTDI?

A

Computed Tomography Dose Index is defined by a dose measurement made using a pencil ionisation chamber placed through a scanner. Measuremnt of a slingle slice/roation of teh scanner, CTDI is defined as the integral of the dose profile divided by the nominal beam/slice width.

30
Q

What is the equation for CTDIair?

A

CTDIair = 1/s ∫ D(x) dx

  • D(x) is the dose profile across a slice
  • s is the nominal slice width
31
Q

What is the equation for CTDI100?

A

CTDI100 is measured in a 100mm chamber corrected by L/(nT).
* L is the length of teh chamber
* n is the number of simulatneous slices
* T is the nominal slice thickness

32
Q

What is CTDIw?

A

Weighted Computed Tomography Dose Index
* Weighted average dose within the standard phantoms
* Measured within a cylindrical perspex phantom, four measurements at the periphery, one at the centre
* Separate head (16cm diameter) and body (32 cm diameter) phantoms used

33
Q

What is the equation for CTDIw?

A

CTDI w = 1/3 CTDI (100, centre) + 2/3 CTDI (100, periphery)

34
Q

What is CTDIvol and give the equation?

A

CTDIvol is CTDIw corrected for pitch or couch increment and for the mAs used in the scan.

CTDI vol = CDTI w /PITCH

35
Q

What is DLP?

A

Dose Length Product = CTDIvol x Scan Length

36
Q

What are the problems associated with CT dosimetry?

A
  • CTDIw approximates average dose to point from 100mm scan length
  • Not clinically representative
  • For beam widths upto 40mm, this is good approximation
  • For beam widths > 40mm, significant amount of scatter is neglected
37
Q

How to do CT dosimetry for wide-beam scanner?

A
  • Measure CTDIw at reference beam width (20mm)
  • Measure CTDIair at both reference and actual beam width to determine correction factor
  • CTDIair measured for wide beam widths by ‘stepping’ chamber through beam
38
Q

What factors effect CT Spatial Resolution?

A
  • Limited by pixel size, which depends on matrix size & field of view
  • Improved with high resolution algorithms which generally increases noise
  • In x-y direction, no significant difference between axial & helical
  • In z direction, reduced with increased pitch for helical scanning, less so with multislice
39
Q

What are the three main sources of noise and their causes?

A

Quantum noise
* Random variations in number photons detected
* Reduced by increasing mAs, kV or slice width
* Pitch does not affect noise for single slice scanners
* Noise increases with pitch for multislice scanners

Electronic noise
* From measuring system

Structural noise
* From reconstruction algorithm

40
Q

What are the main causes of motion artefacts and how can it be improved?

A
  • Patient motion during scan
  • Cardiac motion
  • Moving structure occupies different voxels during scan
  • Improved with fast rotation times; wide beam; gating etc
41
Q

What are the main causes of streak artefacts and how can it be imporved?

A
  • High-attenuation objects in scan volume
  • Errors in reconstruction lead to streak effects
  • It can be improved with software if available (metal correction algorithm MAR)
42
Q

What causes photon starvation?

A

It arises from high attenuating objects which cause streaks behind artefacts.

43
Q

What are the main causes of beam hardening and how can it be corrected?

A
  • Change in energy as beam passes through patient
  • CT numbers vary along path
  • Lower CT numbers in centre as crossed by hardened beams
    produces cupping (& possible dark streaks)
  • It can be corrected with beam hardening algorithm
  • Effect reduced with bow-tie filter
44
Q

What are the main causes of ring artefacts?

A
  • Detector malfunction
  • Rotational symmetry as faulty detector moves around patient
  • Rings visible on image – position indicative of which detector is faulty
45
Q

What is the partial volume effect?

A
  • Occurs when the feature is smaller thnn the voxel size
  • Causing the object to appear larger/displaced from the actual position
  • Causing distorion of small/narrow details
46
Q

Name 5 CT applications

A
  1. CT Fluroscopy
  2. Gated imaging
  3. CT Angiography
  4. CT Perfusion
  5. CT in Radiotherapy

Y2 Nuclear Medicine - MPY8009 (8 decks)

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