Probe Maintenance

Any probe selected for clinical use should have excellent reliability and stability. Quality control consists mainly of a check of the background value and the sensitivity of the probe. The shielding should be looked at as well. Although in our experience stability is not a problem, it is recommended that before each use, the background value and sensitivity are determined. A background count will also detect any contamination with radioactivity of the probe. It is necessary to check the sensitivity regularly, because a gradual decrease may be unnoticed. A 57Co source is convenient to use for a check of the sensitivity, but the readings should be corrected for the half-life of 270 d. Initially the source should not be too strong to avoid dead-time effects, which would cause a reduced count rate. The maximum count rate of the probe (typicaly 10,000 counts per second) should not be exceeded. After 5 years, the count rate will then have decreased to 100 counts per second. In that case, the sensitivity can still be determined with an accuracy of 3% if a counting interval of 10 seconds is used, which is acceptable. Therefore, it is recommended to obtain a new 57Co source at least every 5 years.

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