To quantify the degree of the perfusion defect, the RPO and LPO projections of each lung were analyzed, since the major lobes (ie, right upper lobe [RUL], right middle lobe, right lower lobe [RLL], lingula, and left lower lobe [LLL]) could be separated from other lobes of the same lung, and from the counts from the contralateral lung, as illustrated in Figure 1
. Using a normal perfusion scan, the uninvolved segment of lung or lobe was count-normalized to obtain an expected perfusion percentage for each lobe. The degree of perfusion reduction was calculated as follows: (normal lobe perfusion − affected lung perfusion)/total lung perfusion. This yielded a percentage of lobar perfusion reduction that was correlated with location and the degree of PV stenosis, as measured on the pulmonary venogram, CT scan, and hemodynamic measurement. The counts were normalized to the most normal segment of the lung in patients with multiple lobar involvement. In patients with multiple perfusion defects, the reductions were relative to the most normal segment of the involved lobes.