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Biologic Effects of Ionizing Radiation (BEIR) Committe VII risk model for exposure to low-level radiation
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Pt protection
Patient protection
| Question | Answer |
|---|---|
| characteristics of radiation risks | invisible, long-term, cumulative |
| Biologic Effects of Ionizing Radiation (BEIR) Committe VII risk model for exposure to low-level radiation | approx. 1 in 100 people are likely to develop solid cancer or leukemia from an exposure of 100 mSv above background dose |
| Important for patient safety | Good communication and quality assurance programs |
| Beam restriction | single most important factor in keeping patient dose to a minumum, only tissues of diagnostic interest should be iradiated -reduces pt dose, reduces production of scattered radiation, improves quality of image |
| Beam restrictor types | aperture diaphragm, cone/cylinder, collimator |
| Collimator | most efficient, attached to tube head, first set of shutters placed close to x-ray tube port window to control amt of image degrading "off-focus" radiation leaving tube, lead shutters (two adjustable)for length and width |
| Aperture diaphragm | elementary -dedicated chest, dental unit, trauma imaging equipment -flat piece of lead having a central opening with a size and shape that determines the size and shape of x-ray beam |
| Cone/cylinder -used for relatively small field size (paranasal sinuses) | circular, lead-lined, slide into place in the tube head or onto the collimator housing Straight-prox and dist diameters identical Flare-dist diameter greater than prox diameter -limited use, more efficient bcs beam restriction is closer to anatomy |
| Disadvantages of aperture and cone beam restrictors | have a fixed opening size, only one field size at a given distance |
| mAs | controls quantity, no affect on quality |
| kVp | controls quality, affects quantity |
| NCRP guideline for manual collimation must be within _____ of the SID. | 2% |
| Positive beam limitation-PBL | sensors located in Bucky tray or other IR holder signal the collimator to open or close accordingly to the IR size being used, will provide a small unexposed border on all sides of the finished image -required by the NCRP to be withi |
| PBL requirements by NCRP | -required by the NCRP to be within 3% of SID for a single side and within 4% of SID for all 4 sides |
| NCRP guidelines for manual collimation | within 2% of the SID |
| Collimator assembly-light localization apparatus | small light bulb to illuminate the field and a 45-degree angle mirror to deflect the light. |
| T/F For the x-ray light field and x-ray field to correspond accurately, the x-ray tube focal spot and the light bulb must be exactly the same distance from teh center of the mirror. | TRUE |
| exposure factors for patient safety | high kVp, low mAs -also good bcs heat delivered to x-ray tube is lower and tube life is extended |
| Three-phase or high frequency generators | stationary radiologic equipment, constant potential waveform, reduced patient dose |
| Inherent filtration | built in, permanent -window of glass envelope (.5 mm Al) -thin layer of oil coolant/insulation surrounding the x-ray tube -tends to increase as tube ages, tungsten deposits |
| Filtration summary | less than 50 kV = .5 mm Al 50-70 kV = 1.5 mm Al more tha 70 kV = 2.5 mm Al |
| Radiosensitive organs | gones, lens, blood-froming organs |
| Whose gonads to shield | all women under 55, all men under 65 |
| Gonad shielding | easier and more effective on males than on females |
| When to shield the gonads | -when diagnostic objectives permit -when pt is of reasonable reproductive age -when gonads lie in or within 5 cm of the collimated field |
| Contour shaped shields | effective for gonads, shaped to enclose male reproductive organs, held in place by disposable briefs, effective for oblique, erect, and fluro exams |
| Shadow shields | leaded material casts a shadow within the illuminated field that corresponds to shielded area, inially expensive but one time expense, can be used without contacting sterile fields, NOT TO BE USED IN FLUORO. |
| Mammography equipment with a molybdenum target has how much filtration? | .025-.03 mm molybdenum filtration |
| Glass window in mammo is made of what? | beryllium-low atomic number (4) |
| Added filtration | thin sheets of aluminum that are added, collimator and its mirror |
| Why use PA instead of AP chest or spine? | decrease dose to breast tissue to .1% of that received in AP, magnification considerations are minimal |
| T/F Entrance skin dose is significantly greater than exit dose. | True -place radiosensitive organs away from the entrance beam |
| Perform these exams in PA projection when possible. | Skull, chest, abdomen, scoliosis series, spine |
| good communication | helps gain patient's trust and confidence with a thorough explanation |
| lead aprons during fluoro | placed under patient |
| Minimum response/reaction time | length of the shortest exposure possible with a particular AEC |
| If the minimum response time is too high, how is this fixed. | needs fixed or will have excess density -preferable to decrease ma, but may decrease kV |
| Rare earth phosphors | film/screen radiography -four times faster than calcium tungstate phosphors |
| Advantage of digital fluoro | lower patient dose due to x-ray beams are pulsed not continuous |
| grids | function to remove a large percentage of scattered radiation mostly from Compton scatter, improve radiographic contrast |
| air-gap technique | limited to imaging fairly thin parts (thick and dense tissues would require excessive and impractical radiation exposures) |
| equipment must be properly calibrated to give predictable results. What two values need to be checked? | linearity and reproducibility |
| linearity | adjacent mA stations should be consistent 200 mA should be 2x exposure rate as 100 mA, any variation in output intensity must not exceed 10% |
| Reproducibility | consistency in exposure output during repeated exposures at a particular setting, any variation in output intensity must not exceed 5% |
| High ratio grids with low kV | BAD, discouraged because of unnecessary exposure required |
| high ratio grids affect on positioning | decreases positioning latitude |
| low ratio grids | used in mobile radiography, more positioning latitude |
| 8:1 grid good for what kV range? | up to 90 kV |
| 16:1 grid good for what kV range? | 100 kV or more |
| 10:1 or 12:1 grid used in what type of equipment? | general fixed equipment |
| T/F Moving grids call for less exposure than stationary grids. | False. They require more exposure. |
| Ways to decrease patient fluoro dose | -decrease length of exposure -use last image hold -keep pt as close to II as possible -use automatic brightness control setting with high kV, low mA -minimize "boost" and "mag" modes, low pulse rate -collimate to smallest field of view |
| Ways to decrease patient fluoro dose cont. | -use lowest practical pulse rate -change tube angle or patient position to spread dose over larger area |
| NCRP guidelines for x-ray tube housing leakage | less than 100 mR/hr 1 meter from tube |
| Source-Skin distance for all radiographic procedures (NCRP) | no less than 12 in -12 inches in mobile radiography -15 inches in stationary radiography |
| (NCRP) Fluoro tabletop intensity | fewer than 10 R/min |
| Where should the focal spot location be indicated? (NCRP) | outside of tube housing |
| How often should intensifying screens be cleaned and checked? (NCRP) | every 6 months |
| (NCRP)What type of exposure switches are used? | dead-man switch |
| (NCRP) How visible should the patient be to the radiographer? | RT should be able to see and communicate with the patient at all times |
| NCRP | national council on radiation protection and measurements |
| Principal function of x-ray tube filtration | reduce patient skin dose |
Created by:
mmarnold2