Pulmonary Embolism

Pulmonary embolism (PE) is a difficult disease to diagnose. Many patients with PE are never studied and the majority of patients suspected of having PE, do not have the disease. PE is often, but not invariably, associated with lower extremity venous thrombosis. The prompt and accurate diagnosis is of major concern because untreated PE is potentially fatal and unnecessary treatment with anticoagulation has a high degree of morbidity and mortality. There are many imaging approaches to PE, each with its own strengths and weaknesses. Usually, more than one test is required to establish a diagnosis. For many years, ventilation-perfusion (V/Q) scintigraphy has been the main imaging modality for the evaluation of patients with suspected PE. Below is a description of each test and then a synopsis of possible imaging strategies.

Ventilation/Perfusion (V/Q) Scintigraphy
V/Q scan is the traditional imaging examination following a chest radiograph. The tracer is 99mTc-MAA (macroaggregated albumin), which is injected intravenously. Segmental perfusion defects denote block in vessels. Following this, a ventilation study is performed using inhalation of 99mTc-DTPA aerosols. If the segments with perfusion defects show filling up of aerosols, i.e.‘mismatched defects’ is diagnostic of PE. A negative perfusion scan virtually eliminates PE. A high-probability ventilation/ perfusion scan, in conjunction with a high clinical probability, is accurate in diagnosing over 95 per cent of PE.

An example of a high probability ventilation/perfusion lung scan. There are multiple
perfusion defects (arrows) with normal ventilation in these regions