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Nuclear scanning tests use a special
camera to take pictures of certain tissues in the body after a
radioactive tracer (radionuclide or radioisotope) accumulates in the tissues
to make them visible. Each type of tissue that may be scanned (including bones,
organs, glands, and blood vessels) uses a different radioactive compound as
a tracer. The tracer remains in the body temporarily before it is eliminated
as waste, usually in the urine or stool (feces).
Lung scans are most often done to
detect a blood clot that is preventing normal blood flow to a lung (pulmonary
embolism).
Two types of lung scans can be
done:
Ventilation scan. During a ventilation scan, a tracer gas
(xenon 133 or krypton 85) is inhaled into the lungs. Pictures from this scan
can show areas of the lungs that are not receiving enough air or that retain
too much air. Areas of the lung that retain too much air show up as bright or “hot” spots on
the pictures. Areas where air flow is decreased show up as dark or “cold” spots.
Perfusion scan. During a perfusion scan, the tracer (radioactive technetium attached
to a blood protein) is injected into a vein in the arm. It travels through
the bloodstream and into the lungs. Pictures from this scan can show areas
of the lungs that are not receiving enough blood. The tracer is absorbed in areas of the lung where
the blood flow is normal. These areas show up as hot spots on the pictures. Areas where blood flow
is decreased show up as cold spots.
If the lungs are working normally, blood flow on
a perfusion scan matches air flow on a ventilation scan. A mismatch between the ventilation and
perfusion scans may indicate a blood clot that is preventing normal blood flow to
a lung (pulmonary embolism).
Ventilation and perfusion scans
can be done separately or together to diagnose certain lung diseases. If both
scans are done, the test is called a V/Q scan. In this case, the ventilation
scan is usually done first.
Lung Scan Results
The results of a lung scan are
usually available within 2 days.
Lung scan
Normal:
The radioactive tracer
is evenly distributed throughout the lungs. No areas of decreased accumulation
(cold spots) are seen.
Abnormal:
The tracer is not
distributed evenly and areas of decreased accumulation (cold spots) are seen.
A cold spot seen in
a ventilation scan but not in a perfusion scan may be caused by blockage or
narrowing of an airway leading to a lung.
A cold spot seen in a perfusion scan but not in a
ventilation
scan may be caused by a blood clot that is preventing normal blood flow to
a lung (pulmonary embolism).
Cold
spots seen in both the ventilation and perfusion scans can be caused by certain
types of lung disease, such as pneumonia or
chronic obstructive pulmonary disease (COPD).
Lung scan results
can help a doctor determine the likelihood that pulmonary embolism is present. The results are
generally reported in one of the following ways:
Low probability. The results
indicate that the likelihood of pulmonary embolism is low. No further diagnostic testing is
generally necessary.
Indeterminate or intermediate probability. The lung scan
results indicate an average likelihood of pulmonary embolism. Additional testing, such as
angiography or CT pulmonary angiography, may be necessary.
High probability.
The results indicate that the likelihood of pulmonary embolism is high. No further diagnostic
testing is generally necessary.
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