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MRS432- CT > week 3, lec 1; dose > Flashcards

week 3, lec 1; dose Flashcards

1
Q

what are the two main types of dose

A

absorbed dose

effective dose

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2
Q

general properties of radiation

A

Occupational exposure shall not exceed 20 mSv per annum averaged over five years, with a maximum of 50 mSv in any one year (radiographers hardly reach 1mSv)
Average dose to Australians from natural background radiation (sun) per year is 1.5 and 3 mSv

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3
Q

list 3 facts regarding CT dose

A
  • Comprise only 17% of all radiological examinations, but…
  • Contributes to 49% of the effective dose all radiological examinations
  • Not doing an unnecessary CT scan reduces the radiation dose by 100%; suggesting alternative modality without using radiation
  • CT accounts for 25% of the worlds radiation
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4
Q

what are 5 considerations to think about when doing a CT examination on paediatrics
(there are 9)

A
  • Life time to manifest the bioeffects
  • More radiosensitive tissues
  • Risk is higher for females and younger age groups
  • Optimization of image quality and patient dose
  • Selection of appropriate technical parameters
  • Use of shielding devices in paediatric CT
  • Dose management strategies in paediatric
  • Requirements for staff: experience and training
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5
Q

what is absorbed dose

A

-Refers to the amount of energy that radiation imparts to the human body

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6
Q

what is effective dose

A

A measure of the equivalent effect to the whole body of the absorbed dose (direct and indirect- scattered radiation)
-Effective dose takes into account the type of radiation (gamma, isotope etc.) and the radio sensitivity of the tissues

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7
Q

how is the effective dose calculated

A
  • CT dose index; adjust for spatial variation of dose
  • Weighted CTDI; adjust for pitch
  • CTDI volume; adjust for distance scanned
  • Dose Length Product; adjust for organ sensitivity and radiation type
  • Effective dose
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8
Q

what is CT dose index (CTDI)

A

a measure of the radiation dose output of a CT scanner

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9
Q

how is CTDI volume obtained?

A

by dividing CTDIw by pitch factor

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10
Q

what is weighted CTDI

A

the dose isn’t equal across the scan plane; need to adjust for this

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11
Q

what is volume CTDI

A

takes into account the pitch

higher pitch = lower dose

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12
Q

what is dose length product

A

totsl dose to patient along the distance scanned

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13
Q

factors affecting dose in CT

A 
Exposure factors 
Collimation 
Detectors 
Type of reconstruction algorithm used 
Pitch 
Over –Ranging
Patient Centring and positioning
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14
Q

what are some factors relating to tube potential (kVp)

A
  • high tube potential; without a reduction in tube current (mA) will lead to a significantly higher dose
  • lowering kVp enhances contrast
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15
Q

when to use lower tube current (mA)

A

-Lower tube current (mA) should be used for scanning children according to body weight

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16
Q

what is automatic tube current modulation and why is it used

A
  • Automatic variation of mA in x-y and z directions across scan range
  • Automatically adjusts exposure (mA) for attenuation differences of tissue types and thicknesses
17
Q

what is collimation used for; pre patient

A
  • Pre patient collimation defines the slice thickness

- Accurate pre-patient collimation reduces dose

18
Q

what is a bowtie filter

A
  • Shape’s and harden the beam increasing uniformity

- Removes low energy x-rays and increases average energy beam; lower absorbed dose to patient

19
Q

How does noise affect dose

A
  • Changing CT dose primarily affects images by altering image noise
  • Higher dose results in decreased image noise
  • Overcome noise by increasing dose: balance dose and image quality (call this dose optimisation)
20
Q

dose optimisation

A
  • Dose delivered to patients adheres to ALARA principal
  • Maintain image quality needed to make diagnosis
  • Benefit from scan to justify risk
  • Optimise scan to suit individual patient
  • Minimise dose to the patient
21
Q

benefits of ATCM

A
  • Provides consistent image quality
  • Lower doses used in thinner and less dense areas
  • Overall patient dose is reduced
22
Q

what is post patient collimation used for

A

Post-patient collimation picks up unhelpful scatter and improves image quality

MRS432- CT (15 decks)

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