Answer to Question #8273 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Shielding

The following question was answered by an expert in the appropriate field:


We are using 18F for PET (positron emission tomography) scanning. We have several tungsten syringe shields, all with windows. The windows are cracked. It makes visibility an issue, but my concern is, Is the radiation protection compromised when windows of syringe shielding are cracked?


It really isn't possible to determine if a cracked window on a syringe shield compromises the shielding properties of that syringe shield by its appearance; however, there is a relatively simple way to determine if a cracked syringe-shield window results in elevated radiation levels. Any facility performing PET scans should have a portable survey instrument, usually either a Geiger-Mueller counter or possibly a portable ion chamber. If one has a syringe shield with an intact window that is of similar design, simply measure the radiation level at a specific distance (e.g., 30 cm) from the window of both syringe shields. If the syringe shield with the cracked window exhibits a higher exposure rate than the undamaged one, then that would be an indication that the shielding properties of that syringe shield have been compromised due to the cracked window.

Another way to determine a possible problem with the syringe-shield window would be to take measurements from different points around the syringe shield. To do this, one would position the syringe shield in a vertical configuration (e.g., hang the shielded syringe from a ring stand or similar type of support or stick it upright in a Styrofoam block) and take measurements at approximately four locations around the syringe shield (e.g., 12 o'clock, 3 o'clock, 6 o'clock, and 9 o'clock). If the reading at the position corresponding to the window position is significantly higher than the readings at the other locations, the cracked window could be the reason. Since not all syringe shields are necessarily created equal, it would be helpful to take similar measurements from an undamaged syringe shield for comparison.

Regardless of which measurement method one chooses, it is important to take comparative measurements at exactly the same distance due to inverse square considerations. It is also important to use the same amount of radioactivity for comparative measurements. Simply switch the dose between the two syringe shields or "normalize" the radiation level by dividing the measured values by the activity in the dose (e.g., mrem/GBq).

Mack L. Richard, MS, CHP

Answer posted on 29 April 2009. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.