Know the radiological studies used in the diagnosis of
patients suspected of having pulmonary embolism.
You are suspecting that your patient may have pulmonary embolism. What
are the imaging procedures you will consider to make the diagnosis? Which would
be your first imaging procedure and why?
Answer
- Chest x-ray
- lung scan
- spiral CT
- doppler exam of legs
- pulmonary angiogram
Your first imaging procedure should be chest x-ray
- It helps to rule out other causes eg pneumonia, pneumothorax etc
- It is a pre requisite for lung scan interpretation
The normal presentation of findings suggestive of PE in CXR
What is the most common radiological manifestation of pulmonary embolism in
chest x-ray?
Answer
- It is normal in most cases
- Following are some common presentations
- basal infiltrate, elevated diaphragm and blunting of costophrenic
angle
- avascular region (Westermark sign)
- Hampton's hump
The rationale for the need for CXR prior to performing lung
scan or CT chest
Why do you need current chest x-ray before ordering lung scan?
Answer
- Ventilation and perfusion is altered by any lung pathology
eg emphysema, pneumonia.
- The observed defects in ventilation and perfusion scan can
be properly interpreted only when you know the underlying pathologies of
lungs.
Methods for detecting clots in deep veins
What is the common source for pumonary emboism? Which
radiological procedure will help you identify that source?
Answer
- Deep veins of legs
- Doppler examination
Value of negative doppler examination in legs. Senstivity
of diagnosis for deep vein thrombosis by doppler examination
You order doppler exam of legs to rule out deep vein
thrombosis as the source for pulmonary embolism. It is reported negative. How
does it alter your management of the case?
Answer
- Positive scan is extremely useful as a supporting evidence
- However only 30% of patients with PE have positive scan
- Negative scan does not rule out prior deep vein thrombosis.
- It is believed that the clots have already embolized
- If there is sufficient reason one can look for other
sources for pulmonary embolism
Utility of doppler exam of legs in conjunction with lung
scan in deciding on pulmonary angiography
You are suspecting pulmonary embolism in your patient. Chest
x-ray is normal. Ventilation perfusion scan is indeterminate. Doppler exam of
leg is positive for deep vein thrombosis. What will be your management decision?
Answer
- Proceed to anticoagulate and treat patient as if he has
pulmonary embolism.
- There is no need to consider pulmonary angiogram
- Anticoagulation strategy for deep vein thrombosis and
pulmonary embolism is the same.
How is ventilation scan accomplished? Rationale behind
simultaneous evaluation of ventilation and perfusion scan
What is the principle behind ventilation perfusion scan? How
are these procedures performed? Why do you need both ventilation and perfusion
scan?
Answer
- Perfusion image made after IV injection of a
"small" number of isotope tagged micro spheres.
- The micro spheres "embolize" an insignificant
number of capillaries and map perfusion.
- Ventilation images are made during inhalation of
Radioactive Xenon-133.
- Ventilation and Perfusion images are compared in similar
formats.
- Although PE results in characteristic perfusion defects,
many airspace diseases also cause perfusion defects.
- In order to interpret perfusion images; concurrent
ventilation images map airspaces.
- If a perfusion defect matches a ventilation defect, it is
interpreted as non-embolic airspace disease.
- If a perfusion defect is not matched by a ventilation
defect, it is likely due to embolism.
- Since the scan "sees" a absent perfusion but not
the embolus, it is not completely specific.
Probability factor for lung scan interpretation Interpretation
based on probability factor for lung scan
What is the meaning of "probabilty" factor in
interpreting ventilation perfusion scans in the evaluation for pulmonary
embolism?
Answer
- Criteria for the size and number of unmatched perfusion
defects create a "probability" factor in four classes.
- 1.) Normal scan -normal perfusion, normal ventilation
Less than 1% chance.
- 2.) Low Probability - perfusion abnormalities from COPD
or matched defects from pneumonia, masses or pleural effusions. less
than 10-30% chance
- 3.) High Probability - Unmatched segmental Defects,
probability 90%.
- 4.) Intermediate Probability - matched defects (not
from pleural effusions or pneumonia) or unmatched sub segmental defects
probability 10-90%.
- A normal scan basically excludes PE and indicates search
for alternative explanations for the patient's condition.
- A high probability scan is usually sufficient to begin
treatment.
- An low probability scan can be used to withhold treatment,
but if clinical signs are compelling then Angiogram is indicated.
- An intermediate probability scan provides no information
and an angiogram is needed in further evaluation.
Sensitivity and specificity of spiral CT for central and
peripheral clots. Radiological signs of clots in pulmonary vessels in CT. Lung
scan vs Spirat CT for diagnosis of pulmonary embolism
How does spiral CT differ from Ventilation perfusion scan in
the evaluation of pulmonaty embolism?
Answer
- A helical or spiral CT is best for detecting pulmonary
emboli that involve central vessels.
- visualize clots in pulmonary artery
- cut off of vessels
- larger branches only
- high sensitivity and specificity for central blood clots
- provides a alternate method for diagnosis of pulmonary
embolism
In which group of patients you would consider spiral CT over
ventilation perfusion scan for the diagnosis of pulmonary embolism?
Answer
- Patients with chronic obstructive lung disease
- Ventilation perfusion scan is most often indeterminate in
these group fo aptients
What is the significance of negative spiral CT scan for
pulmonary embolism? Does it rule out pulmonary embolism?
Answer
- No
- Spiral Ct only can demonstrate clots in larger vessels
- It does not pick up small vessel emboli.
- Ventilation perfuaion scan can be high probabilty with a
normal spiral CT scan
- a positive scan is significant, but a negative scan
does not rule out PE
Will you order spiral CT or ventilation perfusion scan to rule
out pulmonary embolism?
Answer
- If the patient has COPD, it is best to order spiral CT
- In others it is stadard practice now (2002) to order
ventilation perfusion scan
Gold standard for diagnosis of pulmonary embolism
What is the gold standard diagnostic procedure for pulmonary
embolism?
Answer
Radiological signs of clots in pulmonary vessels in pulmonary
angiogram
What are the radiological findings of pulmonary embolism in
pulmonary angiogram?
Answer
- Clots showing as filling defects
- cut off of vessels
Complications of pulmonary angiogram
How is pulmonary angiogram done? What are the complication
reported with pulmonary angiogram?
Answer
- A catheter is passed into right heart and dye is injected and pulmonary
artery is visualized
- Complications
- During angiography, a catheter passes the right heart
and may induce arrhythmia.
- This is more significant in Left Bundle Branch
Block cases.
- X-ray contrast injection causes some increase in
pulmonary resistance.
- In pulmonary hypertension (RVEDP), further increase
may cause acute decompensation.
- Rarely a catheter may knot or perforate the heart.
- Morbidity 4% and Mortality 0.3%.
Risk of pulmonary angiogram in patients with pulmonary
hypertension
Your patient has physical findings suggestive of pulmonary
hypertension. You have an indeterminate ventilation perfusion scan. a clinical
decision is made to go ahead with pulmonary angiogram. How do you think
radiologist will proceed?
Answer
- The primary concern is worsening pulmonary hypertension and
the risk of arrythmia
- Angiographer will reduce the amount of dyes used
- Resort to more selective angiogram, guided by the
ventilation perfusion scan
Indication for pulmonary angiogram
Do you routinely do pulmonary angiogram in all cases suspected
fo having pulmonary angiogram?
Answer
You are going to be presented with clinical scenarios. Decide
whether you will request pulomonary angiogram or not.
Normal chest x-ray, High probabilty lung scan.. What is your
choice?
- No angiogram
- Order pulmonary angiogram
Answer
No angiogram.
You have enough evidence to treat this patient
Normal chest x-ray, normal lung scan, negative doppler of
legs. What is your choice?
- Spiral CT
- Pulmonary ngiogram
- Neither
Answer
- Neither
- A normal ventilation perfusion scan with negative doppler
is sufficient evidence to rule out the possibility of pulmonary embolism.
Normal chest x-ray, Low probability lung scan, positive
doppler of legs. What is your choice?
- Spiral CT
- Pulmonary ngiogram
- Neither
Answer
- I will proceed with anticoagulation / fibrinolytic therapy
strategy
- I do not see how therpy will be altered by spiral CT or
pulmonary angiogram
Chest x-ray consistant with COPD, Intermediate probability
lung scan, positive doppler of legs. What is your choice?
- Spiral CT
- Pulmonary ngiogram
- Neither
Answer
- I will proceed with anticoagulation / fibrinolytic therapy
strategy
- I do not see how therpy will be altered by spiral CT or
pulmonary angiogram
Chest x-ray consistant with COPD, Intermediate probability
lung scan, negative doppler of legs. What is your choice?
- Spiral CT
- Pulmonary ngiogram
- Neither
Answer
- Spiral CT
- If spiral CT is negative proceed with pulmonary angiogram