Answer to Question #701 Submitted to "Ask the Experts"

Q

I would like to know the formula(s) used to calculate (1) direct exposure from a source (x-ray tube) and (2) exposure from scatter, i.e., from a patient. Thank you!

A

Radiation from an x-ray generator consists of photons with a variety of energies. The term kVp is usually used to describe the maximum photon energy, but x rays may be produced at various energies up to the maximum energy.

If an x-ray beam were monoenergetic, it might be possible to calculate the energy per unit area per unit of time, an energy fluence rate, and convert that value into a unit such as the Gy or rad. There is some discussion of this concept in Johns and Cunningham's The Physics of Radiology in the Chapter on "The Measurement of Radiation." Because the photons have a variety of energies, the exposure from an x-ray tube is usually measured using an instrument such as an ionization chamber. Values may be expressed in units of exposure per unit of current per unit of time, e.g., R/mAs.

There is a general relationship between current and exposure which is linear. For example, raising the tube current from 100 mA to 200 mA doubles the dose. The relationship between dose and kVp is not linear. Measurements of these values are usually made by a staff or consultant physicist. Again, there is no formula for the calculation of scatter from a patient.

There is a "rule of thumb" that the scatter at 1 meter from the center of a patient field is approximately 0.1% of the primary beam dose rate at 1 meter from the center of the beam. Thus if the exposure rate in the center of a primary beam were 1 R/min, the scattered exposure rate would be 1 mR/min at 1 meter from that point.

Please note that the inverse square law generally applies and the exposure rates from scatter can be significant, especially from patients undergoing fluoroscopic procedures. Reduction of scattered exposure rates are achieved using leaded barriers or leaded garments.

Jean St. Germain, CHP