The entrance dose of x-rays is the dose received at the body's surface, where the x-ray beam enters. The exit dose, which is what results in an image, is very much lower. The body absorbs the difference between the entrance and exit doses.

          TLDs (ThermoLuminescent Dosimeters) can measure the x-ray entrance dose received by any patient during an x-ray imaging procedure. TLDs are small, nearly flat crystals (about the diameter of a pencil eraser), sometimes with a self-adhesive back.

          On the patient's skin, in the field where the x-ray beam will hit, the x-ray technician just tapes the TLD. The TLD does not interfere with the image. After the x-ray procedure, the irradiated TLDs are sent to a "reading" machine which reveals the entrance dose.

          X-ray practitioners can obtain the TLDs, the instructions, and the reading service by mail from an accredited laboratory at the University of Wisconsin:

Radiation Monitoring by Mail
Univ. of Wisconsin Radiation Calibration Lab
1530 Medical Sciences Center
1300 University Avenue
Madison WI 53706-1532.

Tel: 608-262-6320
Dr. Larry Dewerd, or Keith A. Kunugi.
E-mail: kakunugi@facstaff.wisc.edu

          TLDs are inexpensive (probably less than $3 each), but they are useless without a laboratory's services (instructing, shipping, reading, record-keeping, supplying and reading the control-TLD which measures the radiation exposure accumulated from natural sources by each batch of TLDs before and after the x-ray exposure). These services cost more than the TLDs themselves. There are probably reduced fees for volume.

          For fluoroscopy machines, "real-time" (immediate) dose-measuring equipment has been available for decades, and is highly recommended, because of the high doses delivered by an x-ray beam which stays "on" continuously for many seconds, even for many minutes. Such equipment tells the fluoroscopist the x-ray dose already accumulated by the patient's surface from fluoroscopy at any instant during a procedure.

X-rays and Health
Committee for Nuclear Responsibility, Inc.
January 2001