Reducing radiation exposure in fluoroscopy

• Collimate to smallest area possible (improves image quality, minimizes scattered radiation reaching input, reduces exposure)
• Minimize patient-to-detector distance
• Voltage/current settings:
• Use high voltage (usually 125-150 kVp (Joules per charge))
• high electron volts = better absorption by tissue; decreases patient dose but increases scatter proportionally
• Use low amperes/current (usually 1-3 mA (charge per second))
• Directly proportional to patient radiation dose
• Whereas radiography (cassettes) uses 100 mA compared to < 5 mA for fluoroscopy
• Allowable exposure rate (XR intensity) is 5 rads/min
• Automatic brightness control (ABC) - keeps brightness constant by regulating kVp and mA, depending on density/thickness of patient. 
• Brightness varies directly with mA and with square of kVp - if kV goes from 80-88 (10% change, brightness goes up by 100% or 2-fold, so ABC turns down mA by half)
• High kVp for decreased dose; low kVp for better contrast
• Automatic exposure control (AEC) - controls length of exposure, shuts off at 5 rads/min
• System can have potential to generate 10 rads/min if has AEC; 5 rads/min if no AEC
• Tube current and potential must be monitored weekly
• 'Cumulative manual-reset timer' - makes a sound every 5 min to make fluoro operater aware
• During C-arm - reduce exposure by > 50%:
• 'low dose' setting (but can increase noise/decrease contrast)
• pulsed (1-3/s); synchronized to when shutter is open so that you don't irradiate when shutter is closed during cine/video
• always use pedal yourself rather than have tech pushing button

Mechanisms

• Filter - at least 2.5 mm of aluminum to absorb low-energy Xray before it reaches the patient
• XR intensity at tabletop should not exceed 2.2 rads/min for each mA at 80 kVp
• Target - fast-moving electrons from tungsten cathode hit the target/anode and create Xrays. Target is the source of the X rays.
• XR photon produced by bremmststrahlung process - electrons interact with nuclear electric field and decelerate - lost energy becomes an XR photon
• 5% of incident XR photons emerge from patient unaffected (remnant radiation) and form the XR image
• Target to tabletop distance should be at least 12-18 inches
• Inverse square law - intensity of radiation inversely proportional to square of distance from source, so further distance = less radiation
• XR beam should be under the table to minimize exposure
• Table must be low atomic number (carbon fiber, aluminum, Bakelite) to avoid absorbing XR
• Scatter is from patient - directly proportional to patient exposure; thickness of body part
• Scatter is mainly at 45, then 60, 90 degrees from vertical under the table (eg 45 degrees from vertical = 135 degrees from primary beam)
• 99% is from Compton scatter - XR interacts with outer shell electron and ejects electron, AND re-emits secondary photon which is scattered
• Whereas photoelectric absorption means that XR photon interacts with inner shell electron and loses all energy, ejects electron
• Thompson effect/coherent scatter - not enough energy to eject electron; electron bumped up to higher orbital and falls back down to emit a photon -> 'fog'
• Image reception
• Intensifier = vacuum tube device that converts XRs to visible image
• Input phosphor absorbs XR photons and converts to light photons
• This is the primary barrier for the C arm
• Photocathode converts light photons to electrons (photoemission)
• Electrostatic lens focuses electrons into a fine beam (~ 25 kV)-> accelerating anode -> output phosphor which converts electrons back into light photons
• Phosphors should be checked periodically for degradation by measuring conversion factor = intensity of output phosphor/input exposure rate
• Brightness is achieved by electron acceleration (flux gain usually ~ 100) and minification of output image (how much it is focused - ratio of input area (eg radius squared) to output area of image intensifier; compresses photons, usually 12 in -> 1 in ~ 144x)
• Flat panel
• Quantum mottle - random variation of photons incident on the radiation detector; depends on number of photons used to produce an image. Doubling radiation reduces fluctuations by 2^0.5 (41%). Correct by increasing kVp. Usually use 2-5 mA for KUBs.
• Shape distortion caused by geometric problems in shape of image intensifier - can have 'pincushion distortion' at edge because convex input is projected to flat output (edge is magnified)
• Vignetting - periphery is less bright
• Camera - vidicon is most common. Plumbicon used for cath lab (very low lag), less exposure
• Resolution of image intensifier measured in line pairs/mm (usually ~ 4; higher with cesium iodide)
• Monitor is usually 525-lines per image regardless of monitor size; higher res is 1000 lines/image
• Kell factor = ratio between vertical resolution (lp/mm) and # horizontal scan lines
• Larger film size is more exposure
• Protection - 0.25 mm lead-equivalence needed to protect from scatter
• 0.25 mm blocks 97%, 0.5 mm reduces 99.9%
• Minimum is 0.25; shields/aprons are 0.5 mm, stationary shields 1 mm, glasses 0.35 mm

Doses

• 1 Gy = 100 rad (measurement of radiation absorbed; absorbed dose)
• 1 Sv = 100 rem (measurement of biological effect; absorbed dose equivalent) = J/kg = ~ 100 Roentgen (unit of exposure)
• 10 mSv = 1 rem
• 1 gm rad = 100 ergs (integral dose; amount of energy absorbed by tissue)
• Short term - 25 rads or less acute dose is safe
• Cumulative lifetime dose - 10 x age (in mSv)
• Occupational workers - limit 50 mSv/year = 5 rem (< 0.5 rem/month)
• eye dose equivalent - < 150 mSv/year
• Random people - limit 1 mSv/year
• Pregnant - < 5 mSv or 0.5 rem/year = 0.05 rem or 0.5 mSv/month
• If a embryo already has had 5 mSv by the time pregnancy is declared, cannot have more than 0.5 mSv for remainder of pregnancy
• Spontaneous abortion - 50 rads
• Weeks 1-2 spontaneous abortion; 2-6 -> organogenesis effects. Risk of malformations significantly increased above 15 rad
• Cataracts - several hundred rads acute dose
• 50Gy - death in a couple days, 10Gy - hematopoietic syndrome, 6Gy - GI syndrome (death in 3-10 days)
• Tissue sensitivity depends on mitotic activity - spleen/bone marrow > GI (epithelial cells) -> developing embryo
• Radiation induced cancer - breat > thyroid > bone marrow > lung > GI > bone
• High radiation area - could receive > 1 mSv/hr = 100 mRem/hr
• Radiation area - could receive > 0.05 mSv/hr = 5 mRem/hr
• 1 foot away from source - 500 mR/hr, 2ft 100, 3 ft 50

Fluoro Formulas

• Image intensifier
• Brightness gain = minification gain x flux gain
• Minification gain = (input screen diameter)² / (output screen diameter)²
• Flux gain = measurement of increase in light photons due to conversion efficiency of the output screen (not input screen) → value given by manufacturer
• Conversion factor (the measurement of brightness gain on an II) = intensity of output phosphor (candelas/m²) / input exposure rate
• Magnification: increase in radiation dose = (normal mode size)² / (magnification mode size)²
• Increasing mag increases radiation dose to brighten the image
• General
• V = IR
• Noise = 1 / √# x rays (to decrease noise, increase x rays by half)
• Dose = Energy deposited / mass(dose ex// air kerma; units = Gy, rad)
• DAP (Dose Area Product) = Dose x Area of beam (Gy x cm²)
• Oxygen Enhancement Ration = radiation dose to cause response w/o O² / dose to cause response w O²
• Magnification = SID / SOD