Staff Dosimetry

 

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One concern that staff often have is that they might be breathing in any Kr81m gas which leaks into the room. We have calculated staff doses that might result from this. In fact the main issues of dosimetry for staff relate to external exposure from the patient. This will generally be due to the Tc99m and Kr81m in the patient's lungs during the study.   This would also apply in any considerations if a technologist became pregnant. We have therefore included dosimetry for Tc99m MAA as well.

It may be that the technologist has to hold the mask onto the patient's face if they are poorly or infirm. The radiation dose during this will still be small, perhaps increasing the dose by about 1ÁSv. With a seated patient it may be possible for the technologist to stand at one side of, or partly behind, the camera which will provide excellent shielding.

Kr81m

There are two main components to consider

  1. radiation dose due to Kr81m breathed out by the patient into the room

  2. external radiation exposure from the Kr81m in the patient's lungs during the study

 

Free Kr81m in the room

There is no doubt that some Kr81m goes into the atmosphere during the study. The patient exhales Kr81m when they breath out so that even with a perfect mask seal there will be some Kr81m in the room.

With any noble radioactive gas, if you are in a cloud of this gas then the radiation exposure you receive will be 99% due to external radiation from the gas cloud, not from breathing in the gas. There are models for this radiation exposure. Based on these models we have calculated that the radiation exposure to a technologist from Kr81m released into the room during one patient study of 4 views is approximately -

technologist dose from free Kr81m in the imaging room   =  0.xx  ÁSv per study

Kr81m in the patient

In addition there is an external exposure from the Kr81m content in the lungs of a patient during the study. We estimate that this may contribute about 0.7ÁSv to a technologist per 4 view study.

technologist dose from Kr81m in the patient  =   0.7  ÁSv per study

It can be seen the the technologist radiation dose from free Kr81m in the imaging room is a very small component of the radiation dose. There will be other aspects of radiation dose due to unpacking and setting up the generator. However these should also be small compared to the imaging aspects.

 

Tc99m

Considering the aspect of the Tc99m in the patient's lungs, then we have estimated the radiation dose to the technologist from this to be 0.7ÁSv

technologist dose from Tc99m in the patient  =   0.7  ÁSv per study

This excludes any radiation dose due to dispensing and injecting which are about the same order of magnitude.

Pregnant Technologist

It can be seen from the above that the aspect of free kr81m in the imaging room is actually a very small component of the radiation exposure of technologists, pregnant or not! The main components derive from simply carrying out the study on the patient. Therefore there are no reasons to consider restricting work with Kr81m than with any other nuclear medicine study. The only issues are that it would perhaps be adviseable for someone else to set up the generator system for use (it is heavy !) and someone else could hold the mask if that was required for a patient.

There is of course a need  to consider the technologist's workload in general. The ICRP now recommend that the radiation dose to the fetus of a radiation worker should not usually exceed the dose limit for a member of the public, or 1mSv, during the declared period of the pregnancy. This may possibly require some limitation of the pregnant technologist's workload, but this will be obvious from radiation dose records.    

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Last modified: November 29, 2002