The generator may need a secondary shield when operated near the camera. Check!
Typically, some form of secondary shield will be needed. This needs to cut
down the high energy emissions from the Rubidium radioisotopes in the generator so may
need to be about 5cm thick. It is therefore heavy !. Our shield is constructed of
overlapping rings of lead mounted on a wheeled trolley. This allows for easy
transportation of the generator and shielding and the pump for the generator is also
mounted on the trolley.
- Check your camera background on both Tc99m and Kr81m energy settings with no generator
or other sources in the camera room. This gives a baseline figure for reference.
- With the generator in its operating position in the camera room, placed inside any extra
shielding if you are already using this, but with no patient in front of the camera
measure the background count rate on both Tc99m and Kr81m energy windows.(NB do not
blow air through the generator but simply have it in position)
- The background count rate will probably be above the baseline values. However it is
important that the levels are not likely to form a significant proportion of the counts in
any clinical study
- A useful benchmark is that the background count rate should be less than 200 c.s-1 in
each channel. This means that for a 100sec clinical image of say 300k counts the
background level from the generator contributes about 7% of the counts.
Specification for a Secondary Shield |
The secondary shield we use is constructed from interlocking lead rings, the top
ring and the lid having 4% antimony added to the lead to give a harder wearing surface.
The rings are sleeved in a steel tube and the whole unit is mounted on a steel plate which
is in turn mounted on four casters. A handle is fixed on to the steel plate reaching up to
waist height. The two gas tubes connecting the generator are led out through a cut-out in
the top lid.
(modify to suit your own generator)
surface dose rate