Multiple window spatial registration error of a gamma camera: 133Ba point source as a replacement of the NEMA procedure

Abstract

Background: The accuracy of multiple window spatial resolution characterises the performance of a gamma camera for dual isotope imaging. In the present study we investigate an alternative method to the standard NEMA procedure for measuring this performance parameter.

Methods: A long-lived 133Ba point source with gamma energies close to 67Ga and a single bore lead collimator were used to measure the multiple window spatial registration error. Calculation of the positions of the point source in the images used the NEMA algorithm. The results were validated against the values obtained by the standard NEMA procedure which uses a liquid 67Ga source with collimation.

Results: Of the source-collimator configurations under investigation an optimum collimator geometry, consisting of a 5 mm thick lead disk with a diameter of 46 mm and a 5 mm central bore, was selected. The multiple window spatial registration errors obtained by the 133Ba method showed excellent reproducibility (standard deviation < 0.07 mm). The values were compared with the results from the NEMA procedure obtained at the same locations and showed small differences with a correlation coefficient of 0.51 (p < 0.05). In addition, the 133Ba point source method proved to be much easier to use. A Bland-Altman analysis showed that the 133Ba and the 67Ga Method can be used interchangeably.

Conclusion: The 133Ba point source method measures the multiple window spatial registration error with essentially the same accuracy as the NEMA-recommended procedure, but is easier and safer to use and has the potential to replace the current standard procedure.

Figure 1

Cylindrical lead collimator for multiple window spatial registration measurement according to [5] showing liquid ⁶⁷Ga source inside.

Figure 2

Mini collimator made of lead showing ¹³³Ba point source in plastic disc on top. Collimator hole sizes were 2, 3, 4 and 5 mm.

Figure 3

Counts per minute in low and high energy windows for different collimator hole diameters of mini collimator. noColl is for uncollimated ¹³³Ba source. Detector thickness for results shown is 15.9 mm.

Figure 4

Full width at half maximum (FWHM) of point source image for different collimator hole diameters of mini collimator. noColl is for uncollimated ¹³³Ba source. Detector thickness for results shown is 15.9 mm.

Figure 5

Vector plot of differences in positions between low (marked x) and high energy positions for the nuclides ⁶⁷Ga and ¹³³Ba for a collimator hole size of 5 mm and the 9.5 mm thick detector. For better visibility the differences are scaled by a factor of 40.

Figure 6

Regression for differences between low and high energy positions of the ¹³³Ba point source images with 4 and 5 mm collimation on the corresponding differences for the ⁶⁷Ga source images.

Figure 7

Bland-Altman analysis of Fig. 6. The solid line indicates the mean difference between the methods, and the 95% confidence intervals for the differences are indicated by dashed lines.