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
Out now in paperback and on Kindle
Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.