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Foundation Training - DRRP > CT Imaging > Flashcards

CT Imaging Flashcards

1
Q

When and where are CT scanners used clinically?

A

Radiology Departments:
- General and A&E
- NM departments: PET-CT
- Radiotherapy

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2
Q

What is the controlled area around the CT scanner?

A

The entire room when the equipment is on

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3
Q

What legislation makes it a requirement to carry out routine tests of CT?

A

IRMER17

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4
Q

Describe the construction of a modern multi-slice CT scanner. Describe the use of each component

A
  1. Bow tie filter:
    - Thicker in the sides than the middle
    - Takes into account the shape of the patient
    - Reduces dynamic range but keeps equal distribution of x-rays and reduces beam hardening
    - Also removes low energy x-rays that do not have clinical value, but contributes to patient dose
  2. Collimator:
    - Pre- and post-patient
    - Limits extend of beam and scatter radiation post-patient
  3. Tube:
    - Anode-cathode running parallel to Z-axis
    - Creates X-rays
  4. Detector:
    - Commonly scintilation detectors
    - Creates the image
  5. Slip ring:
    - Provides power as equipment rotates around continuously
  6. Gantry:
    - Houses the detector components and also the tube to go around the patient
  7. Patient Couch:
    - Where patient lies during exposure
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5
Q

What are the advantages and disadvantages of using CT scanners over e.g. planar x-rays?

A

Advantage: Cross-sectional slices are obtained (more, high resolution information: 3D)

Disadvantage: Typically higher dose, with longer waiting times

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6
Q

What is the difference between Helical and Axial scanning?

A

Axial: Also known as step-and-shoot. The couch is translated between exposures, but not during a fill rotation of the assembly

Helical: Couch moves through the gantry as the tube continuously goes around the patient

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7
Q

Can you describe how CT images are constructed?

A

Filtered Back projection:
- Basic principle is due to the varying degrees of attenuation of the x-ray beam that occurs as it transverses the patients body
- Image is reconstructed based on the intensity of the detected beam for each transmission line
- A composate image based on the attenuation coefficients is obtained through FBP of the data
- A mathmatical filter is applied prior to FBP to reduce star artefacts

Iterative reconstruction methods are becoming more common.
- These use algorithms that use multiple repititions to improve on the current solution, converging to the most optimum

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8
Q

What is a scan projection radiograph (or scoutview?)

A

A 2D image of the patient which is acquired by passing them through the gantry while the tub and detector remain stationary

Used to plan patient scan by allowing scan range to be defined and provides information regarding attenuation
- Used for AEC current modulation

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9
Q

How often do DRRP test the CT scanners as part of their equipment surveys?

A

Annually

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10
Q

How do AECs work in CT scanners?

A

Info from the scoutview provides information for z-modulation

Info on x/y-modulation is given from first half irradiation of the full circle

Canon = SD?

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11
Q

How is pitch defined?

A

Pitch = Table travel per rotation / nominal detector width

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12
Q

What is meant by isocentre?

A

The middle of the gantry: This is the point which keeps the same relative position at all angles

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13
Q

What is the main photon attenuation process in CT imaging?

A

Compton scanntering. With some photoelectric effect at lower energies

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14
Q

What is the key phantom that you used for CT image quality tests?

What aspects of image quality does each module in the phantom test?

A

The Catphan

  1. HU, scaling and axial slice thickness (CTP404)
  2. slice thickness & modulation transfer function (MTF) (CTP591)
  3. High contrast resolution (CTP 528)
  4. Low contrast resolution (CTP 515)
  5. Noise and uniformity
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15
Q

Can you describe what a Hounsfield unit is?

A

HU are a measure of ‘CT number’ where:
Hs(HU) = 1000*[(LAC-sample - LAC-water)/LAC-Water]

By definition Hs = 0 (water), & -1000 (air)

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16
Q

Can you provide typical HU values for:
1. Water
2. Air
3. Soft-tissue
4. Fat
5. Muscle
6. Bone
7. Fluid

A
  1. 0
  2. -1000
  3. 35
  4. -70
  5. 50
  6. 300
  7. 0-20
17
Q

What factors can affect the HUs?

A

HU are dependant on:
- kV
- bowtie filter
- FOV
- collimation
- reconstruction algorithm

18
Q

Describe how you would test HU consistensy

A

There is a HU module within the catphan: Module CTP404

Scan this module so that the CT number of the 8 homogenous materials can be determined:
1. Acrylic
2. Water
3. Polysterene
4. LPDE (low density polyethylene)
5. Delrin
6. Teflon
7. Air
8. PMP (polymethylepentene)

These materials are used as they represent the full range of linear attenuation found in the body.

Ensure during testing:
Water = 0 +/- 5 HU
Air = -1000 +/- 10 HU
Rest within 10 HU of baseline or average value for that type of scanner

19
Q

Why is HU CONSISTENCY important clinically for scans?

A

HU can be used to diagnose disease
e.g., to determine if adrenal mass is benign or not (>10 HU suggests malignancy)

HU is used for appropriate attenuation correction in SPECT-CT and PET-CT and electron density information is used in radiotherapy treatment planning systems

20
Q

How is slice thickness measured for axial and helical scans?

Why is the same method not employed for both?

A

Axial:
- CTP404 catphan module has wire rampes inclined at 23o to scan plane
- This enables axial slice thickness to be determine where the length of the wire is dependant on slice thickness:
sw = d*tan(theta) (d = FWHM of the plot of CT number agains distance across the imaged wire, theta = angle of wire to scan plane)

Helical:
Two methods can be used:
- Image a disk or tungsten carbide bead: This gives point spread function and the FWHM is measured (used for small slice increment, ~10% of slice width)

Cannot use the same method for helical scanning due to artefacts that would arise from interpolation of higher contrast objects inclined at an angle to the image plane when spiral scanning

21
Q

How are noise and uniformity defined in CT, and how are they tested?

A

Noise = Random fluctuation in CT numbers, mainly from variation in the number of photons reaching the phantom (quantum noise), but also structure noise (reconstruction process and electronic noise)
- Noise = (SD-water/CT-scale) *100, CT-scale = diff of CT no. in water and air

Uniformity = variations in CT number across an image of homogenous diameter
- Uniformity = ([CT-No:centre - CT-No:periph]/[CT-No:Water - CT-No:Air])

Noise and uniformity are measured using the same images acquired of the homogenous module of the catphan (CTP486)

22
Q

How does noise change in CT if you increase the kV?

A

Decreases (Difficult to correct however due to it being polychromatic)

23
Q

How does noise change in CT if you increase the slice thickness?

A

Decreases (inversely proportional to Sqrt)

24
Q

How does noise change in CT if you use a sharper reconstruction kernel?

A

Increases

25
Q

How does noise change in CT if you use a lower mAs?

A

Increases (inversely proportional sqrt)

26
Q

How does the noise in CT change if you change the pixel pitch?

A

Noise inversely propotional to pixel pitch

27
Q

How does noise in CT change if you change the pitch?

A

Increase pitch = more noise (difficult to correct for)

28
Q

Why do you acquire repeat imaging for noise assessment?

A

Noise values have been found to vary up to 15% on images from consecutive scans (IPEM report 32 pt III)

29
Q

What factors of the CT scanner affect uniformity?

A
  • Phantom size
  • FOV
30
Q

What paramaters of the CT scanner affect resolution?

A
  • Reconstruction kernel
  • geometry
  • focal spot size
  • flying focal spot
  • detector element size
31
Q

What are the different methods that can be used to assess resolution?

A
  • Counting line pairs in test object in CTP 528 catphan module
  • Measuring MTF : Quoting MTF10 & MTF50 = This is a more quantitative test which describes how well spatial frequencies are produced in the image
32
Q

Can you list some CT artefacts

A
  • Beam hardening
  • Cupping
  • Ring artefacts
  • Photon starvation
  • Partial volume effect
  • Streaks
  • Helical artefacts
33
Q

What is CTDI and what does it stand for?

A

CTDI = CT Dose Index

Gives standardised measure of the output of a CT scanner, is a measure of dose for a single axial slice

CTDI = area under dose profile(mGy.mm)/nominal slice width (mm)

34
Q

What is CTDI-air?
How yould you measure it?
What is the value used for?

A

CTDI value measured in air, measured routinely to give an indication of tube output

Measured in air using a 100mm pencil chamber (CTDI-100, and normalised to 100mAs)

CTDI-air is sensitive to changes in a number of system parameters and is a useful QC measurement

35
Q

What is CTDI-W?
How would you measure it?
What is the value used for?

A

CTDI-W is the weighted average CTDI-100 in the scan plane for a single slice of a cylindrical perspex phantom (IEC). Quoted as absorbed dose in air

CTDI-W = 1/3CTDI-100:Cenre + 2/3CTDI-100:Periphery
- Perspex phantoms of 16 and 32 cm are used to simulate the head and body respectively.
- Each phantom placed at isocentre
- Pencil chamber used to measure the dose in:
- Centre
- left, right, top, bottom of the phantom

36
Q

What is CTDI-Vol?

A

Average dose over scanned volume (pitch corrected)

CTDI-Vol = CTDI-W/pitch

37
Q

What is DLP?

A

The dose length product

A measure of the total energy absorbed

Simple way of comparing dose for different patients and setting DRLs

DLP = CTDI-Vol * Length of scan

38
Q

What information do you need to calculate effective dose?

A
  • kV
  • Output
  • FOV
  • Length
  • Region

Foundation Training - DRRP (6 decks)

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