Energy imparted to the patient in diagnostic radiology: calculation of conversion factors for determining the energy imparted from measurements of the air collision kerma integrated over beam area.

The energy imparted to the patient in diagnostic radiology, related to radiation risk in examinations of the trunk and head, can be deduced from a measurement of the air collision kerma (or exposure) of the incident primary photons integrated over beam area by using a thin, flat ionisation chamber covering the entire roentgen beam. Factors for converting the integral of the air collision kerma to energy imparted to the patient have been calculated using a Monte Carlo method. The patient is simulated by laterally infinite water slabs with thicknesses from 100-300 mm. Calculations are performed for monoenergetic photons (5-300 keV) and energy spectra commonly used in diagnostic radiology (40-130 kV acceleration potential differences and values of the half-value thickness of air collision kerma in aluminium from 0.9 to 9.9 mm). Correction factors which take into account the additional escape of scattered photons from the sides of a laterally finite water slab as a function of field size and focal distance are also given.