the spect acquisition in lung perfusion scan provides higher sensitivity and specificity to detect pulmonary thromboemboli ( pte ) compared to planar imaging ( 1 , 2 ) . because of higher specificity , there are hopes that the lung perfusion spect could be interpreted without a ventilation study . the value of higher specificity is more highlighted in complicated cases with vascular abnormality and shunts . in this report , a case with tetralogy of fallot ( tf ) and multiple cardiovascular abnormalities and corrective shunts are presented underscoring the importance of spect imaging in complicated patients . a 22-year - old female patient was admitted for surgery to revise her waterston central aortopulmonary shunt . she had the history of tf , pulmonary valve stenosis , ventricular septal defect and major aortopulmonary collateral artery . her shunt was placed at the age of 5 years then complicated with stenosis for which a stent was placed and re - dilated 1 year ago . after echocardiography reported minimal aorto - pulmonary shunt flow , a decision was made to operate the patient for revision of the central shunt . midsternal incision was done and a polytetrafluoroethylene ( ptfe ) tube was inserted between the ascending aorta and right pulmonary branch , where the previous gortex was located . patient recovery and rehabilitation after surgery was optimal until the patient experienced abrupt left - sided pleuritic chest pain and dyspnea . the patient was referred to the department of nuclear medicine for perfusion and ventilation lung scanning . perfusion lung scan was done with 4 mci 99 m technetium labeled macroaggregated albumin ( total particle count 120,000 ) in planar anterior , posterior , lateral and multiple oblique projections ( figure 1 ) . additionally , the brain , renal parenchyma and the thyroid demonstrated uptake due to the right to left shunt ( figure 2 ) . ventilation scan was not performed due to logistic problems ; the available ventilation tracer in iran is 81kr , which is distributed only on wednesdays . there was a large wedge - shaped peripheral defect in the apicoposterior segment of the left upper lobe . the finding was interpreted as either acute / chronic pte or secondary to developmental / iatrogenic vascular abnormality . meanwhile , the patient underwent ct angiography reporting postoperative atelectatic changes in the bilateral lower lobes without evidence in support of pte . the iv heparin was changed to warfarin after 5 days when the symptoms improved gradually and the arterial o2 saturation increased to 92% in room atmosphere . tetralogy of fallot is a congenital cyanotic heart disease in about 3.5% of infants with congenital heart diseases . there are a wide spectrum of anatomic defects with varying severity including ventricular septal defect , overriding aorta , right ventricular outflow tract obstruction , and right ventricular hypertrophy ( 3 ) . diagnosis of pulmonary emboli is a great diagnostic challenge and it should be suspected in any patient with unexplained dyspnea , tachypnea , or chest pain ( 4 , 5 ) . lung perfusion and ventilation spect is superior to planar scan in detecting pulmonary embolism with a high sensitivity and specificity ( 6 ) . perfusion lung scan without ventilation scan has also been shown to have a high specificity of 92% and sensitivity of 87% ( 7 ) . the advantages of lung perfusion spect imaging include higher contrast for better detection of the shape and location of defects and higher sensitivity ( 6 ) . in complicated cases similar to the patient described here , this is of foremost importance . because pulmonary angiography was not performed and ct angiography was negative , pte was not confirmed in this case . ct angiography is a sensitive and optimal technique for diagnosing large pulmonary emboli , but it has a relatively low sensitivity ( below 80% ) for sub - segmental ptes ( 4 ) .