Finding-the-Path: Ventilation-perfusion scintigraphy

Perfusion scintigraphy

Ventilation scintigraphy

Repeat perfusion scan 5 days later

Planar lung views of the perfusion study using Tc-99m MAA (macroaggregated albumin) shows absent perfusion to most of the right lung (arrows). There are also large segmental perfusion defects in the lingula, left basal segments, and anteroposterior segment of the left upper lobe (arrows). Ventilation imaging with Xe-133 gas in the posterior projection shows normal ventilation of the lungs, and therefore, a significant ventilation-perfusion mismatch. These findings indicate a high probability (greater than 80%) for a pulmonary embolism. The patient was treated with systemic thrombolysis and a repeat perfusion study in 5 days showed near-complete resolution of segmental perfusion defects.

Discussion

Pulmonary embolism is a potentially fatal disorder that is often difficult to diagnose clinically. Fewer than one-fourth of patients show the classic signs and symptoms of tachypnea, pleuritic chest pain, dyspnea, cough and hemoptysis. The predisposing risk factors for pulmonary embolism include postoperative or immobilized state, cancer, congestive heart failure or a hypercoagulable condition. Given the nonspecific clinical presentation, pulmonary emboli require a radiographic diagnosis. Pulmonary arteriography is the only definitive means of establishing a diagnosis of pulmonary embolus. However, it is an invasive and expensive technique that should not be used as a screening test. A radionuclide ventilation-perfusion scan is a highly sensitive and accurate test for detection of pulmonary emboli. The V/Q scan shows pulmonary emboli as segmental regions of reduced or absent pulmonary perfusion associated with normal ventilation, known as a ventilation-perfusion mismatch. Patients are classified as having low (0-19%), intermediate (20-79%) or high (80-100%) probability of pulmonary embolus based on the size and number of perfusion defects in comparison with the ventilation pattern and radiographic findings. Pulmonary arteriography is generally reserved for patients with an intermediate probability of pulmonary embolism based on the V/Q scan or those in whom a definitive diagnosis is required prior to any contemplated interventional treatment.

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