In this example 12.5% of the Tc99m image counts are due to
downscatter from the Kr81m in dual acquisition mode . In fact , this value is generally
acceptable. A value approaching 25% may even give acceptable Tc99m perfusion pictures.
This implies that a generator strength of double the activity might be used in dual
However a generator with twice the activity means that the
Kr81m view will finish (ie 400k) in 50s. This time is too short for the Tc99m view.
Therefore the stop condition needs to be set to allow the perfusion image to be collected
long enough (90s in the above example). This will mean that the Kr81m image may also have
to be collected for up to 90s unless separate stop conditions are possible.
With the above conditions it is always possible to stop the
air flow to the generator and remove the patient mask after the 50s but continue to
collect the dual energy study until the time of 90s for the Tc99m view. This will in fact
cut down the downscatter as well. The main problem is ensuring that the patient does not
move when the mask is removed.
Some departments acquire the posterior views first
separately on single acquisition. This gives a guide to Kr81m and Tc99m times for the
other views to be counted in dual acquisition mode.
In the above example, If the generator activity on another
day happened to be much greater than 400MBq then it would be important to stop the
Kr81m view early in the dual acquisition study.
Of course, if you have a high activity Kr81m generator then
why bother with dual energy acquisition? The Kr81m images will be very quick, so why not
just collect single views? The images will be better quality and you avoid the risk of too
The final issue is that later in the day or with a low
activity generator then it may become important to use dual energy acquisition to speed
things up. But make sure the Kr81m view now has enough counts with any stop condition that
With all the above issues, a patient protocol will depend
on local circumstances and equipment. However we have given a typical protocol for