Okla. Admin. Code § 310:281-11-20

Current through Vol. 42, No. 7, December 16, 2024

Section 310:281-11-20 - Determining skin entrance exposures

The following is the method for measuring and calculating patient entrance skin exposures in diagnostic x-ray examinations.

(1) Have the operator set the x-ray tube at the source to image receptor distance (SID) routinely used, and adjust the collimation as though a patient was in the beam. Measure the distance from the x-ray source to the surface against which the patient rests. Subtract the thickness of the patient part to obtain the source to skin distance (SSD).

(2) Place the ionization chamber in the center of the x-ray field and measure the source to chamber distance (SCD). Use of a test stand (such as the CDRH stand) to position the chamber at a reasonable distance from the table will reduce backscatter contribution. Placing the chamber at the actual SSD will accomplish this and allow direct reading of the ESE.

(3) Record the routine technical factors used by the facility for the standard patient thickness (Table l) and make exposures utilizing these techniques.

(4) For phototimed procedures a phantom must be utilized to control the exposure time and achieve an accurate exposure estimate. When utilizing a phantom, the measuring chamber should be positioned in the beam between the phantom and the x-ray tube (but not placed over an active photocell), and should be located approximately nine inches from the phantom to reduce backscatter contribution.

(5) Calculate the entrance skin exposure as follows if a direct result was not obtained: ESE = (Raw mR) x (SCD) ² / (SSD) ²

(6) Compare the results of these measurements for the speed of the imaging system used by the facility with the ESE guides published by the Conference of Radiation Control Program Directors, Publication 88-5 "Average Patient Exposure Guides - 1988", or a similarly derived group of values, (allowing for the speed of the imaging system used by the facility).

(7) There are many different techniques for measuring ESE, which may result in significant differences in measured values. Factors that can cause variations include instrument calibration, backscatter, collimation, estimation of focal spot location, choice of phantom, and location of chamber in the primary beam.

Okla. Admin. Code § 310:281-11-20

Added at 8 Ok Reg 3503, eff 8-15-91 (emergency); Added at 9 Ok Reg 1643, eff 5-29-92; Amended at 21 Ok Reg 1037, eff 5-13-04

Amended by Oklahoma Register, Volume 38, Issue 24, September 1, 2021, eff. 9/11/2021