RAD 350 Chapter 8 X-Ray Emission

RAD 350 Chapter 8 X-ray emission

X-ray QUANTITY is directly controlled by mA and time (mAs). The relationship of mAs and quantity of radiation is directly proportional – i.e. – if you double the mAs, the amount of radiation is doubled. X-ray QUALITY (intensity/hardness/penetrability of the beam) is controlled by kVp. As kVp increases, the intensity increases (shorter wavelength photons). X-ray quality/penetrability may also be expressed as HALF VALUE LAYER (HVL). As kVp increases, the tube becomes SLIGHTLY more efficient in producing x-rays.

Filtration – two types of filtration are usually associated with the x-ray tube/unit. Inherent (the tube itself – exit window. etc.) and is about .5 mm al equivalent. ADDED filtration is based upon the kVp of the unit (about 2 mm al equivalent at 70 kVp). The SUM of the two is called TOTAL FILTRATION.

COMPENSATING FILTERS may also be used to attenuate/absorb a portion or all of the beam at certain areas to more evenly apply the radiation. A WEDGE filter can absorb radiation at the toe end of the foot on an AP foot, but allow more radiation to penetrate the heel end thus giving a nice exposure instead of “burning out” the toes or having the heel end be light. A TROUGH filter is most often used in dedicated chest x-ray units/rooms. A “BOW TIE” filter is used in some CT scanners. Conic filter – usually for digital fluoro units.

X-ray tube : 2 primary components – Filament (thoriated tungsten) and anode. A dual filament = dual F.S. (controlled by mA station L or S); the filament can vaporize somewhat with use and gradually increase the inherent filtration over the .5mm al equivalent. Anode rotates 3400 – 10,000 rpm LOWER anode angle = HIGHER heel effect

Protective tube housing = insulating oil and shielding Leakage radiation = less than 100 mR/hr at 1 m 1/2 and full wave rectified = 100% ripple 3-phase power 6 or 12 pulse = 13% or less ripple Hi frequency = 1 % ripple Heat units (HU’s) = mA X time X kVp 3-phase = additional factor to multiply

Tube rating charts MUST match the anode angle/speed to get the correct chart!

Housing cooling chart

Tube cooling chart

How many HU’s if full wave rectified unit uses 5 consecutive exposures of 200 mA, ½ sec, and 80 kVp?

200 X ½ X 80 = 800 X 5 = 4000 HU