Secondary shield

 

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The generator may need a secondary shield when operated near the camera. Check!

Background Count Rate Check
  1. 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.
  2. 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)
  3. 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
  4. 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.

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.

Typical 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.

 

Typical Secondary Shield
(modify to suit your own generator)
lead wall thickness 50mm
height 350mm
diameter (external) 250mm
weight 80kg
average dose reduction 98%
typical external surface dose rate 3ÁSv

 

 

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Last modified: November 29, 2002