Reference air kerma
Air kerma of the primary x-ray beam measured under specific conditions and expressed as the equivalent value at the patient entrance reference point (IEC, 2004; 2010). For C-arm fluoroscopic systems, the patient entrance reference point is a point along the central ray of the x-ray beam, 15 cm back from the isocenter toward the focal spot. Reference air kerma does not include backscatter and is measured in units of Gy. It is a special case of the quantity with the ICRU notation Ka,i, and has the NCRP notation Ka,r.
(ICRU, 2005; NCRP, 2010)
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from ICRP Publication 120, 2012
Air kerma of the primary x-ray beam measured under specific conditions and expressed as the equivalent value at the patient entrance reference point (IEC, 2004, 2010). It is the air kerma accumulated at a specific point in space relative to the fluoroscopic gantry (see Patient entrance reference point) during a procedure. Reference air kerma does not include backscatter and is measured in units of Gy. It is a special case of the quantity with the ICRU notation Ka,i, and has the NCRP notation Ka,r (ICRU, 2005; NCRP, 2010). Reference air kerma is sometimes referred to as ‘reference dose’ or ‘cumulative air kerma’. Earlier publications used the term ‘cumulative dose’ and the abbreviation ‘CD’ for this quantity (Stecker et al., 2009).