Patient dosimetry
X-ray imaging
Factors that increase dose:
- Beam properties
- Higher tube current (mA) and exposure time (s)
- Wider collimation (reduces scatter and irradiated area)
- Larger field of view (FOV)
- Higher kVp (if we stick with the same mAs, more photons overall and more with higher energy. However, some higher energy photons pass straight through the patient)
- Scanner properties
- No filtration
- Use of a grid
- Reduced receptor sensitivity
- Patient properties
- Closer to focal spot (x-ray source)
- Larger patient habitus (larger skin surface to absorb maximum dose)
Fluoroscopy
Factors that increase dose:
- Beam properties
- Lower kVp (a less penetrating beam means more radiation absorbed, particularly on skin)
- Continuous (vs pulsed)
- Using a higher dose level setting
- Larger area of collimation
- Keeping x-ray tube over same anatomical area (maximum skin dose can be reduced by rotating and penetrating patient from different angles, called "dose spreading")
- Whether fluoroscopy or acquisition mode is selected
- Scanner properties
- Use of a grid
- Increased electrical magnification (zoom modes)
- Increased geometric magnification (i.e. moving patient closer to source)
- Decreased distance between the tube and the detector/II
- Patient properties
- Larger patient habitus (as for x-ray imaging)
CT imaging
Factors that increase dose:
- Beam properties
- Higher tube current (mA)
- Higher kV
- Longer exposure time
- Not using mA modulation
- Wider collimation (however, if collimation too small system will compensate for reduced signal by increasing mAs / kVp)
- Scanner properties
- Decreasing pitch (normally dose and pitch inversely proportional. However, some scanners automatically correct for pitch by maintaining same
- Use of noise reduction algorithm allows lower dose to be used
- Patient properties
- Smaller patient (more x-rays will penetrate to the centre and deposit a higher dose N.B. a larger patient will receive more total x-rays but dose is measured per unit mass)
Nuclear imaging
Factors that increase dose:
- Increased amount of injected radioactivity
- Each radioisotope will deposit different doses
- Reduced drinking and urination results in slower loss of activity from the bladder, increasing patient dose
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Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.
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