Energy resolution

5.1.4. Energy resolution

5.1.4.1. Aim This test is relevant only for tomography equipment using singles-based

attenuation correction and calibration. Measurement of energy resolution allows an assessment of proper photomultiplier calibration and ensures that the efficiency of light collection is within the specifications.

5.1.4.2. Frequency This test, if applicable, must be done by the qualified medical physicist

at the time of acceptance testing, as part of end-of-warranty tests and whenever it is suspected that the detector system performance may have changed significantly.

5.1.4.3. Materials

A point source of 18 F is employed, less than 1 mm in extent in both the transaxial direction and the axial direction. An adequate source can be prepared using a capillary tube, with an inside diameter of less than 1 mm and an outside diameter of less than 2 mm. The axial extent of the radioactivity in the capillary tube must be less than 1 mm.

A source of the same type used for spatial resolution measurements can

be used for this test. The source should be placed at the centre of the FOV, suspended in air, to minimize the effect of scattered radiation. The radioactivity of the source shall be such that the per cent dead time or randoms loss is less than 5%, as for the spatial resolution test. Typically, this can be obtained with a radioactivity of approximately 1 MBq. The radioactivity concentration of the starting radioactive solution should thus be about 1000 MBq/mL (or 27 mCi/mL) or less. As for other tests, manufacturer recommended activities, if available, may depart from these general recommendations and should be used instead.

5.1.4.4. Data acquisition The manufacturer’s procedure should be followed for energy testing or

for energy spectra collection and display (an example of such a spectrum is

FIG. 20. Example of an acquired energy spectrum; energy resolution can be calculated from the FWHM of the energy peak distribution.

shown in Fig. 20). A time sufficient to obtain not less than 10 000 counts in the peak of the energy distribution should be taken.

5.1.4.5. Analysis Using the manufacturer’s procedure for energy testing, obtain the per

cent energy resolution of the system. Alternatively, if a predefined procedure is not available, the energy spectra should be analysed to obtain the FWHM of the energy peak distribution.

An approximate energy calibration factor can be obtained by calculating the peak position using a parabolic fit to the top of the peak. Using this factor, the FWHM can be converted into units of energy (keV). Energy resolution can then be calculated using the following relation:

R E = 1000 FWHM/500 (27)

5.1.4.6. Suggested tolerances The values of energy resolution measured should not exceed those given

in the vendor’s specification.

The user should set reference values, tolerances and action levels (i.e. to trigger the decision to place a call for maintenance). An appropriate tolerance criterion for FWHM is:

R E measured < 1.05R E expected

5.1.4.7. Corrective action If tolerance criteria are not met for this test, it should be verified that all

procedures were correctly followed. If the cause cannot be identified, the manufacturer should be notified and corrective action taken.