Findings: The patient's ventilation-perfusion (V-Q) lung scan demonstrated perfusion defects involving the entirety of all three segments of the upper lobe of the right lung. No corresponding ventilation defects were present. A post-contrast spiral CT of the chest showed an intraluminal filling defect within the right main pulmonary artery, extending into the pulmonary artery supplying the right upper lobe.

Diagnosis: Pulmonary embolism (PE).

Discussion: Pulmonary emboli cause 120,000 deaths per year in the United States. Risk factors for PE include immobilization for greater than seventy-two hours, recent hip surgery, cardiac disease, malignancy, estrogen use, and a history of prior deep venous thrombosis (DVT). Possible clinical findings include chest pain, tachypnea, dyspnea, rales, cough, tachycardia, hemoptysis, fever, diaphoresis, cardiac gallop, syncope, and phlebitis. PE's are most commonly sequelae of DVTs. However, other types of embolism include fat embolism in skeletal trauma or surgery, tumor embolism, amniotic fluid embolism, talc embolism in intravenous drug abusers, mercury embolism in thermometer accidents, air embolism, and parasites, especially schistosomiasis.

The goal of diagnostic imaging is, of course, to identify pulmonary emboli when they are present and to direct therapy. Patients diagnosed with a PE undergo therapy to prevent subsequent PE. Chest radiographs are not accurate in diagnosing PE but serve in diagnosing conditions which may mimic PE such as pneumonia, rib fracture, and pneumothorax. They are also necessary for comparison when interpreting a lung scintigram. According to the PIOPED criteria, two or more large perfusion defects or the arithmetic equivalent with normal ventilation and no chest radiograph abnormalities in the corresponding areas are required to interpret a V-Q scan as high probability for the presence of pulmonary embolus. A large defect is defined as greater than 75% of a pulmonary segment. Two moderate defects (25 - 75% of a segment) equal one large defect. High probability V-Q scans have a probability of greater than or equal to 80% for the presence of a PE, regardless of clinical assessment. When there is a high clinical suspicion of PE and a high probability V-Q scan, the probability of a PE being present is 95%.

An alternative to a V-Q scan in diagnosing PE is a post-contrast spiral CT of the chest. A common protocol utilizes 120 ml of contrast power-injected at a rate of 4mL/sec.

A diagnosis of PE can be made when a filling defect can be seen within the lumen of a pulmonary artery. A recent article reported that in a study of normal patients, 85% of segmental arteries in the right lung and 83% of the left lung were adequately visualized for confident analysis. The segmental arteries which were considered nonanalyzable in greater than 20% of cases were the posterior segmental artery of the right upper lobe, the segmental artery supplying the paracardiac segment of the right lower lobe, and the artery supplying the anterior segment of the left lower lobe.

References:
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Nuclear Medicine in Clinical Diagnosis and Treatment, Vol. 1. Edinburgh;
Churchill Livingstone; 1994:29-45.

Schoepf UJ, et al. Segmental and Subsegmental Pulmonary Arteries: Evaluation with
Electron-Beam versus Spiral CT. Radiology Feb. 2000; 214:433-439.

Weissleder R, Rieumont MJ, Wittenberg J: Primer of Diagnostic Imaging,
2nd Edition. Mosby, St. Louis; 1997:62-63, 857.

Worsley DF, Alavi A. Comprehensive Analysis of the Results of the PIOPED Study.
Prospective Investigation of Pulmonary Embolism Diagnosis Study. J Nucl Med
Dec 1995; 36(12):2380-2387.

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