X-ray Fundamentals

5/5/1999

Who gets x-rays?

• Fractures
• Subluxations—do you need to prove its presence? Not really
• Tumors—not good for early tumors—30-50% of bone has to be destroyed before visual on film
• Deformities
• Infections—osteomyelitis and joint infections—have to be advanced to show up on x-rays
• Arthritis—certain times—rheumatoid in hands, gout, ankylosing spondylitis
• Aortic aneurysm
• X-ray children when it is clinically necessary

What films are you going to take?

• Need at least 2 views at right angles to each other
• Depending upon clinical indications may need others
• Full spine x-rays—scoliosis

Equipment

• Upright Buckey—loosely used here
• Upright receiver
• Grid cabinet—stationary; newer models, very small lead lines that cannot be seen under normal viewing distances
• Grid
• takes scatter x-ray and absorbs so it doesn't hit the film to improve contrast
• Developed early on
• Lead strips w/spaces in b/w
• Strips catch scatter x-ray
• Spaces allow good x-ray to go through

• Tray to hold the film (cassette)
• Dumb colmnator—you have to set
• Automatic colmnator—adjusts the size of the beam depending upon the size of the film (controls the shutter)
• Colmnator attaches to the x-ray tube
• X-ray tube
• Evacuated
• Has a lead cover to help control x-ray beam

• Tube stand
• Attach to the floor (free standing needs at least 12in more wider)
• Attach to the wall
• Attach to ceiling (depends upon construction)

• Locks
• Hold the tube in space
• Electric locks

• Table
• Needed for extremity shoots and other situations
• Moveable—have to physically move the patient into place
• Floating top table—can adjust the patient easily on the table

• Transformer
• 20,000V up to 60,000V
• box w/copper wiring
• box 2ft by 2ft—put in room drawing

• operator's stand
• secondary barrier
• primary beam does not hit this wall
• the primary beam hits the wall behind the patient and this is the primary barrier
• state and federal wall does not say that the wall has to be lead containing it only has to be safe
• scatter source is the patient that we have to protect from

• KVP—Kilovolts peak—controls penetrating power of x-ray photons

• How long will release x-ray photons—40 different time stations 1/20-6 secs—keep time under 1 sec if you do not plan on tying down the patient

• MA—milliamps 25, 50, 100, 200, 300

X-ray Production

• Electron-target interaction
• MA controls electrical current to filament on cathode (cathode block)
• Filament—similar to the spring on the end of a push pen (one is small and one is large)—glows hot w/electricity
• Prep button spins rotor and sends electrictiy to filament—rotating anode effectively increases the area the electrons bombard by a factor of 2*Pi*radius over a stationary anode (spinning of the anode allows for the spreading out of the heat)
• Resistane causes filament to heat up
• Electrons are "boiled off" filament"—thermionic emission
• Exposure switch is depressed
• KVp is applied across the cathode/anode
• Electrons are accelerated from cathode to anode
• Projectile electrons collide w/anode—electrons that go from anode to cathode
• 1A = 1C/s = 6.3x10¹⁸ electrons/sec - @ 100mA , 6.3x10¹⁷electrons/second
• line focus principle—tungston wheel has an angled surface (anode wheel)—allows to spread out heat over anode
• effective focal spot—part of beam that goes toward the patient—smaller than actual
• actual electron flow is large going towards the anode
• anode heat
• the majority of energy used (99%) results in heat
• outer shell electrons of target material are temporarily excited
• as they fall back into orbit they release energy as infrared radiation (heat)
• heat is directly related to mAs and kVp
• heat units (hu) are calculated by multiplying kVp by mAs (single phase only, x1.41 for HF){high frequency has a quality factor}
• mAs = mA * time

• anode heel effect
• intensity of the beam is non-uniform in the long of the tube
• if the central ray = 100%, intensity progressively decreases toward the anode
• if the central ray =100% intensity increases then decreases toward the cathod; gets stronger then starts to taper off a little

• decreased at longer FFD (focal film distance)

• Characteristic radiation
• Bremsstrahlung radiation—slowing down radiation

• X-ray emission spectrum
• Factors affecting x-ray emission
• X-ray tube drawing
• Leaded housing of the tube—part that you put your hand on
• Leading has to be at a specific amount

• Oil—thermal characteristics; electrical insulating
• Glass envelope—same glass as in a light bulb—needed to provide an evacuated environment
• Vacuum helps facilitate the flow of the electrons from the cathode
• X-rays that go through the floor and the glass is very thin so the x-rays can get through
• Diode
• Cathode—source of electrons
• Anode—rotating wheel is the target of the x-rays
• X-ray production—isotropic; go every in every direction