Activity Measurements

 

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Activity Measurements

There are two methods which can be used to measure Kr81m gas activity directly. One uses the gamma camera system and one uses an ionisation chamber. Other calibration methods based on measuring the activity in a section of the outlet tube from the generator can be used but rely on an accurate flow rate.

The principle of both measurements is that the Kr81m activity in a sealed system connected to the generator output is constant and is the effective Kr81m activity on the generator. The value is the secular equilibrium activity value of the Kr81m on the generator multiplied by the gas elution efficiency of the generator.

NB
There must be a period of at least 75s between each measurement. This is the time it takes for the Kr81m activity on the generator to approach equilibrium value again (5 half-lives). In fact the generator and lines may hold some residual pressure after the air flow is switched off causing some Kr81m leakage, so it is adviseable to wait at least 180s.

 

Gamma Camera Measurement
  1. Put a medium energy collimator on the camera, even if you do not use this for imaging. This is essential for the activity measurement to avoid septal penetration affecting the measurement.
  2. Connect a sealed expandable bag to the generator and place this on the camera face. An anaesthetic bag can be used.
  3. Set up a 10s acquisition using the Kr81m energy band
  4. Ensure that the generator is in equilibrium by waiting for at least 3min since the last elution and then switch on the generator and a few seconds later start the acquisition. At the end of the acqusition switch off the generator
  5. Calculate the count-rate of the Kr81m bag (use the total frame counts)
  6. Count a known activity of Tc99m on the camera (using Tc99m settings). This could simply be about 100MBq in a syringe. Calculate the count rate.

The calculation compares the count rates of the two radionuclides. There are a number of issues.

  1. The photopeak efficiency will be different at 140kev and 191kev. This will depend on the camera crystal thickness. Assumed values are

    NaI(Tl) Photopeak Efficiency
    Tc99m Kr81m
    3/8" (9mm) 98% 90%
    1/4" (6mm) 95% 86%
  2. The different abundance of the gamma rays from Tc99m and Kr81m has to be considered

  3. The equation for Kr81m effective activity for a 3/8" crystal is

Kr81m activity

Worked Example

IGE camera (3/8" crystal) with Medium Energy Collimator

Tc syringe on camera surface gave 77 000 counts / 10 sec / 51 MBq = 151 cps. MBq-1

Kr bag on camera surface @ 9am gave 160 000 counts / 10 sec = 16 000 cps

 

Patient administered krypton (one view) for 110 seconds @ 11 am

Rbeff @ 11am (2 hrs decay ) = 176 x 0.74 = 130 MBq

Administered Activity (one view) = 130 (1 + .053 x 110) = 892 MBq

Administered Activity (four views) = 892 x 4 = 3 569 MBq / study

Collect data on a range of patients (young, old, large, small etc.). If your measurements exceed the ARSAC limit modify the protocol. Provided the protocol remains the same, it should be accurate enough to record this estimate of the administered activity on to the request card.

 

 

Ionisation Chamber Measurement

The principal of this method is the same as the above. Feed the generator output to a sealed system and the Kr81m activity in the sealed system is the effective Kr81m activity.

ionchamber.gif (3756 bytes)It is difficult and probably dangerous to fit a sealed bag inside the ionisation chamber. However a baffled perspex insert can be used to produce a compartment in the ionisation chamber to measure the Kr81m. The baffles should be a good fit in the chamber although some gas leakage can occur which helps prevent any pressure build up. Two small leakage holes are also included however.  With a lower flow rate (about 800 ml.min-1 ) then the decay of Kr81m means that only a small amount escapes before the measurement is completed.

The ionisation chamber may need to be calibrated for Kr81m gas. Check with the manufacturer.The setting we use for a Capintec system is 915(x10).  Alternatively determine the setting which gives the expected activity for that generator, or compared to the camera measurement. Then carry out a few measurements on different generators over a period of time and check the consistency of the values. If these are consistent continue with the measurement factor, although bear in mind it is a guideline not a true calibration.

Remember to wait at least 3 minutes between measurements to allow the Kr81m on the generator to reach equilibrium value.

 

 

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Last modified: November 29, 2002