Radiation Biology Chapter 7
Patient Immobilization:
Reasons for immobilization
Blurring of radiograph if there is motion
Reduction of radiation exposure due to reduction of repeat films
Infant or small children will not hold still
Combative adults (Alzheimer’s, mental illness, intoxication)
Types of motion on film
Voluntary – on a non combative patient, good communication should limit this
Involuntary – image unsharpness due unavoidable motion of anatomy (cardiac contraction/relaxation, peristolsis). Try to reduce exposure times to limit motion.
Beam Restriction / Collimation:
You MUST collimate. Effects of collimation include lower tissue dose, less scatter radiation, and improvement of image quality.
Two factors affecting the amount of scatter: kV and material irradiated.
kV: As kV increases, fewer atoms interact with the tissue, and more pass through to end up on the film. If kV increases and mAs stays the same, scatter will increase. But if kV increases and mAs is lowered to maintain the same density, scatter will decrease.
Irradiated Material: Scatter increases as volume and atomic number of irradiated material increases. When using larger film, you will have more scatter radiation (volume of irradiated material increases). Use collimation to limit amount of scatter.
Beam Limiting Devices: help reduce absorbed and scattered radiation by the patient
Aperture diaphragms: flat plate of lead that attaches to collimating device – usually used in special procedures.
Cones: circular attachments to the tube housing of varying and sometimes adjustable lengths that can be flared or straight. Typically used with skull or dental work.
Collimator: variable aperture collimator…adjustable set of apertures within the tube housing, accompanied by a lamp and mirror projecting the exposed field. Can have automatic collimation to film size in the bucky. Should be accurate within 2% of the SID
Filtration: filters assist in reduction of patient exposure by eliminating low energy photons. This improves beam quality and increases the average energy of the beam (beam hardness).
Inherent filtration: Glass housing around xray tube and cooling oil surrounding the housing.
Added filtration: any filtration added on the outside of the film housing (compensating filter)
Total filtration = inherent filtration + added filtration.
Minimum filtration requirements for fixed radiographic equipment are:
Below 50kV = 0.5mm Al
50 – 70kV = 1.5mm Al
above 70kV = 2.5mm Al
Minimum requirements for mobile xray equipment or C-arm is 2.5mm Al equivalency
HVL – Thickness of material that will reduce the xray beam’s intensity to have of its original value. Measures beam quality.
Shielding:
Gonadal – to be used when gonads are within 5cm of primary beam on anyone of reproductive age. Shaped contact gonadal shielding is the best type to use during sterile procedures– worn by males inside a support around the gonads. Other types include flat contact shields… flat rubberized lead strips, shadow shields – usually used in surgery... radiopaque material draped over a portion of the tube housing.
Reduction of Radiation Exposure:
kV: an increase in kV will increase patient exposure, unless it is accompanied by a reduction in mAs. Law of reciprocity – to maintain the same density on film
mAs: an increase in mAs will increase patient exposure. Inversely related to kV when maintaining radiographic density. The use of high kV and low mAs will result in less patient exposure. When determining proper technique for your radiographs, films must display adequate part penetration, acceptable density, and proper contrast.
Film/Screen combination: a higher screen speed will reduce patient exposure. Approx. 95% of film exposure is from visible light emitted from the intensifying screen. Also, as kV increases, the speed at which the screen converts xray to light increases.
Patient positioning: especially when phototiming, part of interest over photocell. This will reduce amount of repeat radiographs.
Grids: Placed between the patient and the image receptor. They absorb primary and scatter radiation. You need an increase in exposure factors while using grids, and there will be an increase in patient exposure. The result will be less scatter radiation and better film quality. The lower the grid ratio, the lower the patient dose will be.
Pregnant patient: 10 day rule – xray exams should be postponed on women of child-bearing age until 10 days after the onset of menstruation. Ultimately up to the physician ordering the exam. Always ask if pregnant, if unknown, ask date of last menstruation. If pregnant, use shielding and collimation. Some facilities have a pregnancy consent form for the patient to fill out.
Repeat radiographs: Most common reason for repeat radiographs is operator error. Operator errors include improper exposure techniques, poor positioning, failing to align beam to bucky, multiple exposures on film. Other factors include dirty screens, incorrectly loaded cassettes, light leaks, chemical fog, processor artifacts, and grid errors.
Image intensification fluoroscopy: Conversion of live xray to visible light. Conventional fluoroscopy required adjustment of the eyes to the dark, and image was faint. Now it is easily visible through intensified fluoro as image brightness increases.
Intermittent fluoroscopy: Radiologist should periodically activate flouro instead of standing on the pedal… you’ll notice this happens with surgeons. This also prolongs tube life.
Reduction of field size: Collimating shutters should be utilized to include only the area of interest.
Cumulative timing device: Records fluoroscopic time and sounds an audible alarm every 5 minutes. Due to strict radiation dose guidelines, California requires the technologist to inform the physician every 5 minutes of fluoro used. Federal regulations state exposure rate at tabletop must be no greater than 10R/min for intensified fluoroscopic units and no greater than 5R/min without intensified fluoro.
Minimum source to tabletop distance for stationary fluoro equipment must be at least 15 inches, and 12 inches for mobile fluoroscopic equipment.