Quantitation in planar renal scintigraphy: which mu value should be used?

The attenuation coefficient value mu used by different authors for quantitation in planar renal scintigraphy varies greatly, from the theoretical value of 0.153 cm-1 (appropriate for scatter-free data) down to 0.099 cm-1 (empirical value assumed to compensate for both scatter and attenuation). For a 6-cm-deep kidney, such variations introduce up to 30% differences in absolute measurement of kidney activity. Using technetium-99m phantom studies, we determined the mu values that would yield accurate kidney activity quantitation for different energy windows corresponding to different amounts of scatter, and when using different image analysis approaches similar to those used in renal quantitation. With the 20% energy window, it was found that the mu value was strongly dependent on the size of the region of interest (ROI) and on whether background subtraction was performed: the mu value thus varied from 0.119 cm-1 (loose ROI, no background subtraction) to 0.150 cm-1 (kidney ROI and background subtraction). When using data from an energy window that could be considered scatter-free, the mu value became almost independent of the image analysis scheme. It is concluded that: (1) when performing background subtraction, which implicitly reduces the effect of scatter, the mu value to be used for accurate quantitation is close to the theoretical mu value; (2) if the acquired data were initially corrected for scatter, the appropriate mu value would then be the theoretical mu value, whatever the image analysis scheme.