1. Will increasing the air flow will give more Kr81m gas?
No! The activity of Kr81m gas obtained from the generator is fixed by the decay of Rb81. Increasing the air flow cannot alter that. The activity of Kr81m is essentially constant over a wide flow rate, eg 200 - 3000 ml/min. There are several important points. One is to use a reservoir breathing system and the other is to use thin bore tubing to minimise decay. At too low a flow rate there is likely to be decay due to transit time between the generator and the patient. At too high a flow rate there is likely to be a greater volume of delivery than the patient can breath in.
2. Do we need a special collimator?
Not necessarily. A number of Tc99m collimators are capable of giving good images at 191kev for Kr81m. The quality may not be as good as for Tc99m due to some septal penetration but is often quite acceptable for lung ventilation imaging. It is easy to test your collimator with a simple phantom experiment. Sometimes the only Tc99m collimator suitable for Kr81m may be a high resolution collimator which can give long acquisition times. It is worth checking the medium energy collimator. These can often give acceptable Tc99m perfusion images as well as Kr81m ventilation images, so avoiding a change of collimator. Alternatively you could consider getting a collimator designed for Kr81m. If so it is important to take into account the fact that for imaging the Kr81m 191kev gamma rays some resolution can be traded off for sensitivity to a greater degree than would apply for a standard Tc99m collimator design.
3. Will staff breathe in Kr81m gas that escapes to the atmosphere?
This is not an issue for concern. Some Kr81m gas will escape into the camera room either from the patient as they breathe out during the study or from a poor facemask or mouthpiece seal. In fact the dose to a person in a cloud of a noble gas such as Kr81m is almost entirely external exposure. Less than 1% is due to 'breathing in' the Kr81m gas. Even this component of external exposure is extremely low and much less than the small radiation exposure to staff from the Tc99m and Kr81m within the patient during imaging.
4. Why can't I place the generator behind the camera to ensure it will not interfere with the images?
This does not work (although we have seen and heard this). The generator gives off high energy gamma rays from the Rb radionuclides. Depending on the generator strength and the composition of the Rb radioisotopes it may be neccessary to have a secondary shield when the generator is near the camera. Placing the generator behind the camera does not remove the problem. In fact the shielding to the rear of the camera may not be as thick as the side-shielding. Essentially the high energy gamma rays will penetrate the shielding of the camera from any direction, hit the NaI(Tl) crystal and create a background level. The background level should be checked with no patient present and the generator in place. A value of a few hundred counts per second is acceptable. If higher, then either extra shielding should be used around the generator or the generator needs to be positioned further away.
5. Is it safe to carry out the study on a patient who is pregnant?
Sometimes it is important to rule out PE in a patient who is pregnant. Generally in this situation the clinical benefit of carrying out the investigation is such that it will outweigh the aspects of radiation exposure involved. In fact the radiation exposure to the fetus from both the Kr81m and the Tc99m is very small. Because of this, we recommend that there is no need to consider adjusting the standard protocol for lower levels of activity, or to consider carrying out the Tc99m views first. The Kr81m dose to the fetus is less than 3% the dose from Tc99m.
6. Can we elute the generator with a compressed air cylinder?
Yes. The problem is that cylinder air is very dry. The generators based on zirconium phosphate will rapidly loose the ability to give off Kr81m with such air. This is reversible if air with some water vapour content is then used. So you must fit a humidifier in line if you use an air cylinder. Beware, humidifiers are a large source of gas leaks. We prefer to use a small air pump nowadays. We have found in the UK that the humidity of ordinary room air is good enough even in summer and so no humidifier is necessary. (this may relate to the climate we have in the UK of course!)
7. Can we elute the generator with a compressed O2 cylinder?
Yes, but there are two problems. The first is that cylinder oxygen will be very dry and so will also need a humidifier in the same way as compressed air above. The other main problem is that oxygen cannot be recommended for general clinical use. There is a danger particularly in patients with COAD that you will disturb the oxygen partial pressure situation in the lung and that the patient could potentially stop breathing!. If you have a very ill patient who is on oxygen then see our suggestions.
8. How safe is it to give krypton gas to poorly patients?
Depends what you mean by safe and krypton gas! We have been asked if the inert krypton gas could be a problem for patients. The answer to that is no. The amount of elemental krypton from the generator is a tiny fraction ( about 10-5 ) of the amount of krypton gas that is already in air. The main issues relate to ensuring that the patient can breath correctly through the apparatus you use and noting the comment above regarding cylinders of O2 as opposed to compressed air.
Last modified: July 14, 2003