IJCS | Volume 32, Nº2, May/June 2019

307 Figure 2 - Adapted from Waxman, et al. Appropriate Use Criteria for Ventilation-Perfusion Imaging in Pulmonary Embolism. In cases in which the ventilation cannot be performed, only the perfusion phase may be performed, keeping good accuracy, with a sensitivity of 86% and a specificity of 93% compared to CTA; 17 and 80% and 96%, respectively, when combined with the interpretation of the chest x-ray. 18 Evaluation of right to left shunts: The presence of right to left shunts is detected through the presence of a tracer in extrapulmonary tissue, primarily in the brain. An image of the head complements the evaluation and helps detect small shunts. The presence of the radiopharmaceutical in the brain helps distinguish between a right-to-left shunt and free Tc-99m pertechnetate due to kit unlabelling, for instance, since the free Tc-99m pertechnetate would not be present in the brain. The calculation of percentage shunts can be assessed using regions of interest (ROIs) drawn over the whole body and the lungs, in both anterior and posterior images, and through comparison of the radioactive activity in the lungs in relation to the whole body, resulting in a percentage number. Usually, it is considered as positive if the difference between the activity present in the lungs compared to the whole body is greater than 10%, following the formula: 19 (total number of whole body counts, including the background – total counts in the lungs) / total number of whole body counts, including the background x 100 = percentage of R-L shunt. Attention should always be drawn to radiopharmaceutical quality Rigolon et al. Guideline for lung scintigraphy Int J Cardiovasc Sci. 2019;32(3)302-309 Guideline