Accuracy should be checked at installation, annually, and after repair or when the instrument is moved to a new location within the clinic. Accuracy is a measure of the readings of the dose calibrator in comparison with the well-accepted standards. Long-lived radionuclides such as ⁵⁷Co (half-life 270 days) and ¹³⁷Cs (half-life 30 years) are measured repeatedly in the dose calibrator, and average readings are compared with the values issued by National institute of Standards and Technology (NIST). If the reading differs by more than 10% of standards, the instrument should not be used.¹

Constancy should be checked daily. For the constancy test, a reference source such as (⁵⁷Co, ¹³³Ba, ⁶⁸Ge, or ¹³⁷Cs) is placed in the dose calibrator, and the activity reading on each scale is recorded; day-to-day readings should agree within 10%. The readings at each predefined setting for different radionuclides used in the nuclear medicine department (^99mTc, ¹³¹I, ¹²³I, ¹¹¹In, etc.) are recorded and compared with previous readings.

Linearity should be checked at the installation of the device, quarterly, when the instrument is moved from one place to another, and after repairs. For the quarterly check of linearity by the decay method, a high activity (˜37 GBq) of Tc-99m is independently calibrated and then the measurements are repeated at 12-hour intervals over three consecutive days. Over that time, which is equivalent to 12 half-lives of ^99mTc, the activity decays to about 11 MBq. The measured activities are then plotted versus time on a semilogarithmic graph, and the best-fit straight line drawn through the data points is plotted. For each data point, the difference between the measured activity and the activity on the best-fit straight line at that point should be less than 10%. To save time, a more rapid technique known as the shield method is used. In this technique, lead sleeves of increasing thickness are placed in the dose calibrator with a ^99mTc source (Fig. 9.2). The thickness of sleeves is such that when used both individually and in combination they effectively reproduce the decline in the activity of Tc-99m over 96 hours.

The effect of sample geometry should be checked at installation and after repairs. The apparent activity of a dose will vary with the volume and shape of the container, as well as the position of the dose placed within the chamber of a dose calibrator. Dose measurements should not vary by more than 10% when the sample is measured in different types of containers such as vials, syringes, or bottles. The sample geometry can also be checked by placing a small amount of activity at the bottom of the container and then progressively adding a diluent such as water.

The battery should be checked daily, with a display indicating whether the voltage supplied by the battery is within the acceptable operating range.

Background check should be performed daily to ensure that the survey meter has not been contaminated, and the background exposure or counting rate is performed daily in an area well away from the radioactive sources within the department.

A long-live radionuclide should be checked daily to ensure that the survey meter reading of the source remains constant, that is, within 10% of its original value. The survey meter should be recalibrated if the reading differs by more than 10% of original value.

Calibration should be performed at installation, after repairs, and annually. All survey meters should be checked for accuracy using long-lived radionuclides. The readings are taken from two long-lived radionuclide sources at incremental distances from the sources (such as every 10th of a meter up to 1 m). The readings must be within 20% of their expected measurement. Many nuclear medicine facilities have their survey meters calibrated by the institutional radiation safety office or by a commercial calibration laboratory. The measurement procedure, the measured and expected exposure rates, and a dated sticker summarizing the calibration results are affixed to the meter itself.

Routine checks of the photopeak energy window (i.e., energy peaking) are performed if the counter is equipped with a multichannel analyzer (MCA) and a background check. Before counting samples containing a particular radionuclide, the energy spectrum is checked to verify that the counter is properly peaked so that the photopeak of the radionuclide coincides with the preset photopeak energy window. Isotope-specific radionuclide counting or imaging with a scintillation detector is commonly done using a 20% photopeak energy window, equivalent to an energy range of Eγ ± 10% (i.e., 0.9 to 1.1 Eγ), where Eγ is the X- or γ-ray energy of the radionuclide. This is performed daily.

Energy resolution (percentage full width at half-maximum) should be checked at least quarterly using a reference-source radionuclide such as ⁵⁷Co. Energy resolution is the ability of a counter to discriminate between light pulses caused by gamma rays of different energies. The full width half-maximum (FWHM) of the photopeak should be less than 10% of the energy of the photopeak.

Calibration is performed on a daily basis using a long-lived radionuclide source (such as ¹³⁷Cs), which is placed in the well counter or in front of the probe.

Efficiency check is performed annually. Efficiency check is performed using reference standard sources with emission sources with similar energy emissions to the radionuclides measured routinely in the counter or in front of the thyroid/organ probe. The efficiency of the thyroid probe or well counter can be calculated using the following equation: