RAD 354 Chapt. 8 X-ray emission

• X-ray output in AIR is measured in ROENTGENS and refers to x-ray QUANTITY– 1 R = 2.58 X 10-4 C/kg

4 Factors affect the QUANTITY of RADIATION: mAs, kVp, distance & filtration

• mAs – DIRECTLY proportional to the x-ray quantity

• kVp – x-ray quantity varies proportional to the SQUARE of the RATIO of the kVp

• Distance – Inverse square law• Filtration – Absorbs/Attenuates the LOW

energy LONG wavelength photons – INCREASES average hardness of the beam

X-ray QUALITY = Penetrability

• INCREASED hardness = INCREASED penetration and DECREASED wavelength

• Measured via HALF VALUE LAYER (HVL = BEST measure of x-ray QUALITY)

• Penetrometer/step wedge• Filtration – used to harden the beam and

absorb the “useless, long wave photons”– Inherent (.5 mm al) Added = 2 mm al (70-80 kVp)

“Special” Filters

• Trough – for CXR’s• “Bow Tie” – for some generations of CT• Wedge – for use with an AP foot, etc.• Conic – for digital fluoro

• Don’t reduce patient dose, but improve the image