| Pulmonary embolism,
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(PE), 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).
Pulmonary embolism, Fig. 1
Chest radiograph shows bilateral pleural effusion and long linear
bands of atelectasis (Fleischner lines) (arrows).
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.
Ventilationperfusion scintigraphy,
radionuclide imaging study of pulmonary circulation and
ventilation. It is mainly indicated in the detection of pulmonary embolism (Fig.1); monitoring the natural history or treatment of thromboembolic
disease; quantitative evaluation of distribution of obstructive pulmonary
disease; and preoperative evaluation of patients with emphysema, lung cancer;
and bronchiectasis
Ventilationperfusion scintigraphy, Fig.1b
Ventilation (A) and perfusion (B) scans in a patient with
pulmonary embolism. There are several bilateral segmental perfusion
defects without matched ventilation defects in the same territories
(mismatch).
Techniques for assessing pulmonary ventilation involve the
inhalation of a radioactive gas (Xe-133 or Kr-81m) or a nebulized
aerosol of a radioactive material (albumin labelled with I-131 or
Tc-99m).
For studies of the pulmonary circulation with particulates, the
commonest vehicle for the radionuclide is macroaggregated albumin
prepared by heating human serum albumin. The resultant particle
sizes range from 10 to 100 in diameter. The albumin may be labelled
with I-131, Tc-99m, or I-113m. Other radiopharmaceuticals include
Tc-99 sulphur colloid macroaggregated albumin, radioactive albumin
microspheres. The procedure of perfusion scanning is based on
trapping particles within the small pulmonary arteries of the lung.
A particle size of 10 50 in diameter is optimal. The optimum number
of particles is about 100,0000. Disappearance time from the lungs is
nearly exponential, biological half life ranging from 4 to 20 hours.
There is no significant toxicity or morbidity related to the
injection of human serum albumin. The radiation dose to the lungs is
quite acceptable.
The gaseous pharmaceutical most commonly used for studying
pulmonary circulation is Xe-133. The gas is dissolved in saline and
injected intravenously. The gamma camera records perfusion while the
patient holds his breath. The xenon passes almost immediately into
the alveoli. As a result, during subsequent respiration the
distribution of xenon in all areas of ventilation can be recorded.
Xenon is cleared from the lungs in 3 4 minutes in normally
ventilated areas, but is delayed in regions poorly ventilated (air
trapping)
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).
Pulmonary embolism, Fig. 2
Multiple intravascular filling defects in the right pulmonary
artery on angiogram.
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 (see CT angiography
chest), 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
Pulmonary embolism, Fig. 3
CT scan shows an intraluminal filling defect in the right lower
lobe pulmonary artery (arrow). Bilateral pleural effusion is also
present.
Pulmonary embolism, Fig. 4
CT demonstrate wedge-shaped, nonenhancing pulmonary infarction in
the anterior and posterior basal segment. Clot is visible in
anterior and posterior basal segment arteries (arrows). Right
pleural effusion is also present.
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