week 3, lec 1; dose Flashcards
what are the two main types of dose
absorbed dose
effective dose
general properties of radiation
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
list 3 facts regarding CT dose
- 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
what are 5 considerations to think about when doing a CT examination on paediatrics
(there are 9)
- 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
what is absorbed dose
-Refers to the amount of energy that radiation imparts to the human body
what is effective dose
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
how is the effective dose calculated
- 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
what is CT dose index (CTDI)
a measure of the radiation dose output of a CT scanner
how is CTDI volume obtained?
by dividing CTDIw by pitch factor
what is weighted CTDI
the dose isn’t equal across the scan plane; need to adjust for this
what is volume CTDI
takes into account the pitch
higher pitch = lower dose
what is dose length product
totsl dose to patient along the distance scanned
factors affecting dose in CT
Exposure factors Collimation Detectors Type of reconstruction algorithm used Pitch Over –Ranging Patient Centring and positioning
what are some factors relating to tube potential (kVp)
- high tube potential; without a reduction in tube current (mA) will lead to a significantly higher dose
- lowering kVp enhances contrast
when to use lower tube current (mA)
-Lower tube current (mA) should be used for scanning children according to body weight
what is automatic tube current modulation and why is it used
- 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
what is collimation used for; pre patient
- Pre patient collimation defines the slice thickness
- Accurate pre-patient collimation reduces dose
what is a bowtie filter
- Shape’s and harden the beam increasing uniformity
- Removes low energy x-rays and increases average energy beam; lower absorbed dose to patient
How does noise affect dose
- 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)
dose optimisation
- 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
benefits of ATCM
- Provides consistent image quality
- Lower doses used in thinner and less dense areas
- Overall patient dose is reduced
what is post patient collimation used for
Post-patient collimation picks up unhelpful scatter and improves image quality