Pulmonary embolism (PE) is a partial or complete obstruction of one or more pulmonary arterial branches by a clot, sometimes leading to lung infarction. 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 pulmonary embolism, each with its own strengths and weaknesses. Usually, more than one test is required to establish a diagnosis. Below is a description of each test and then a synopsis of possible imaging strategies.
A chest radiograph is the initial examination. It serves to identify a possible alternative diagnosis (e.g. pneumothorax, lobar collapse) and is necessary for the meaningful interpretation of ventilation perfusion (V/Q) scans. On occasion, the chest radiograph is helpful in suggesting the diagnosis of PE. The majority of chest radiographs in patients with PE are abnormal. However, most of the abnormalities are minor and nonspecific such as an elevated hemidiaphragm, a small pleural effusion, long bands of focal atelectasis (Fleischner lines) or air space consolidation (Fig.1). Westermark's sign, Hampton's hump and a wedge-shaped, pleural-based density are more suggestive but uncommon manifestations of pulmonary embolus. Westermark described focal oligaemia distal to the occluded vessel. The obstructed feeding vessel is often enlarged. Focal peripheral consolidation may be due to haemorrhage or infarction and may be based on the pleura. When focal consolidation occurs in the costophrenic angle, this is deemed a Hampton's hump.
Ventilationperfusion scintigraphy is the traditional imaging examination following a chest radiograph. Unfortunately, it provides only indirect evidence as to the presence or absence of a PE. A negative perfusion scan virtually eliminates a PE. A high-probability ventilation-perfusion scan, in conjunction with a high clinical probability, is accurate in diagnosing over 95% of PE. However, less than 50% of patients with proven PE have a high probability scan. The accuracy rate of V/Q scan decreases in patient with underlying cardiopulmonary disease. A low-probability scan in a patient with a low clinical suspicion also makes PE unlikely. In the remainder (low or intermediate probability scans in patients with high clinical suspicion), additional testing is required.
Pulmonary angiography has long been recommended as the procedure of choice in the patient with a suspected diagnosis of PE. It is both sensitive and specific and is associated with a low mortality and morbidity. Unfortunately, angiography is invasive and has achieved only limited patient and physician acceptance (Fig.2).
Because pulmonary angiograms are underutilized, alternative tests have been sought. Lower extremity venous examinations will detect venous thrombi and may substitute for proof of a PE. Serial negative lower extremity studies in patients with low probability V/Q scans are seldom associated with clinical evidence of subsequent PE. Unfortunately, 50% of patients with PE have negative lower extremity studies.
Contrast-enhanced helical CT, obtained in a single breath-hold or during shallow breathing, as in angiography, can directly display the emboli. Both will show a clot obstructing a vessel or contrast conservative approach includes V/Q scan for patients without underlying pulmonary disease, to be followed by additional testing if not diagnostic and a combination of CT and lower extremity Doppler ultrasonography in the remainder of patients. Angiography would be reserved for cases where imaging results are equivocal and a strong clinical suspicion persists.
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Pulmonary embolism, Fig.1
Pulmonary embolism, Fig.2
Pulmonary embolism, Fig.3
Pulmonary embolism, Fig.4
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