The Kr81m and Rb81 are in secular equilibrium on the generator. This means that the activity of Kr81m is the same as the activity of Rb81.

When the air flow is switched on the Kr81m gas is immediately removed from the generator. The generator is often not 100% efficient. If the elution efficiency is f and the Rb81 activity is A MBq then there is an initial spike of Kr81m activity of

                    Kr81m activity spike    =     f.A MBq
(We have called this value of A.f MBq the effective Kr81m activity)

After this spike of Kr81m activity, Kr81m is continuously produced from the decay of Rb81 on the generator. This Kr81m is also swept out by the air flow. The rate of flow  of Kr81m from the generator is given by

Kr81m activity flow rate    =     #.f.A MBq.s^-1
                                                =    0.053.f.A MBq.s^-1
where # is the decay constant of Kr81m, = 0.053s^-1

When the air flow is switched off at the end of the study the Kr81m activity builds up to its equilibrium value. This takes about 5 half-lives or about 75 seconds. If the generator is switched on during this time then the initial spike of activity will be smaller than normal. This can affect activity measurements. In fact for activity measurements it is preferable to wait at least 3 minutes for accuracy in case of leakage.

It is important that the initial spike of activity is breathed in by the patient for efficient images. A reservoir breathing technique ensures this.

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Last modified: May 17, 2011