[P084] Tests of down-scatter correction in simultaneously acquired TC-99M perfusion and KR-81M ventilation scans

Purpose Lung scans are basic nuclear medicine studies, often consisting of both ventilation and perfusion studies. A gamma camera acquires the ventilation studies of a radioactive gas or aerosol inhaled by a patient, and the perfusion studies of 99m Technetium macro aggregated albumin injected in the patient’s blood. There are several possible gasses or aerosols for ventilation scintigraphy, but 81m Krypton is usually considered the golden standard. Due to the higher gamma energy of 81m Kr compared to 99m Tc, ventilation and perfusion scans can be acquired simultaneously, which saves time and allows perfect image registration. Unfortunately, down-scatter of 81m Kr often distorts the perfusion images significantly. A widely used method to solve this problem is to subtract a fraction of the ventilation studies from the perfusion studies. However, we suspect this to be an inadequate procedure that may even lead to further degradation of the perfusion scans. Methods In our project, we acquired studies of a patient and of homemade phantoms. While the patient underwent a lung ventilation scintigraphy, an extra energy window at 140 keV was added, leading to a scintigraphy of down-scattered photons. The homemade phantom consisted of a 50 ml 81m Kr filled syringe surrounded by two water boxes, representing respectively lungs and nearby tissue. Four energy windows were acquired simultaneously: 140 keV  ± 10%, 190.5 keV ± 10%, 159 keV ± 2.5% and 168 keV ± 2.5%. Results Both of the methods showed none or little overlap between the scintigraphies with energy windows at 140 keV and 190.5 keV. This supported our theory of the subtraction method being without effect in the best case. Instead, other theoretically conceived procedures were tested, but the results were insufficient. Conclusions In the case of unacceptable quality of perfusion scintigraphies caused by down-scatter from 81m Kr, one has to consider other solution than the typically used subtraction method. The short half-life of 81m Kr (13 seconds) may even allow acquisition of ventilation studies prior to perfusion studies.