Comparison of (81m)Krypton and (99m)Tc-Technegas for ventilation single-photon emission computed tomography in severe chronic obstructive pulmonary disease

INTRODUCTION: Ventilation and perfusion single-photon emission computed tomography combined with computed tomography (SPECT/CT) is a powerful tool to assess the state of the lungs in chronic obstructive pulmonary disease (COPD). (81m)Krypton is a gaseous ventilation tracer and distributes similarly to air, but is not widely available and relatively expensive. (99m)Tc-Technegas is cheaper and has wider availability, but is an aerosol, which may deposit in hot spots as the severity of COPD increases. In this study, (81m)Krypton and (99m)Tc-Technegas were compared quantitatively in patients with severe COPD. METHODS: The penetration ratio, the heterogeneity index (with and without band filtering for relevant clinical sizes) and hot spot appearance were assessed in eleven patients with severe COPD that underwent simultaneous dual-isotope ventilation SPECT/CT with both (99m)Tc-Technegas and (81m)Krypton. RESULTS: Significant differences were found in the penetration ratio for the medium energy general purpose (MEGP) collimators, but not for the low energy general purpose (LEGP) collimators. The difference in the overall and the band filtered heterogeneity index was significant in most cases. All patients suffered from (99m)Tc-Technegas hot spots in at least one lung. Comparison of MEGP (81m)Krypton and LEGP Technegas scans revealed similar results as the comparison for the MEGP collimators. CONCLUSION: Caution should be taken when replacing (81m)Krypton with (99m)Tc-Technegas as a ventilation tracer in patients with severe COPD as there are significant differences in the distribution of the tracers over the lungs. Furthermore, this patient group is prone to (99m)Tc-Technegas hot spots and might need additional scanning if hot spots severely hamper image interpretation.