Nuclear Medicine: VQ Scans Part 2
Part 2 review of VQ scans for the ABR Core Exam. VQ scans are nebulous and I hope these podcast reviews will help you succeed on your board examinations. Also, check out the free downloadable VQ scan study guide available on this website (link here).
What is the definition of a mismatched perfusion defect?
Perfusion defect in an area with normal ventilation OR perfusion defect that is larger than a ventilation defect.
How many small, mismatched defects are required for a high probability scan?
The answer is that no number of small, mismatched perfusion defects will qualify for a high probability scan. Only medium or large mismatched defects can get one to a high probability scan.
What is the definition of a matched defect?
Perfusion and ventilation defects that are similar in size OR a perfusion defect that is smaller than a ventilation defect.
What is a triple matched defect?
A matched perfusion defect with a corresponding opacity on a CXR or CT scan.
What is the probability for PE of a triple matched defect in the upper or mid lungs?
Very low.
What is the probability for PE of a triple matched defect in the lower lung zone?
Intermediate.
What is the probability for PE of a triple matched defect associated with a small pleural effusion with no other defects?
Intermediate
What if the pleural effusion is large (>1/3 of thorax) and no other defects?
Very low.
What is the probability for PE of a single moderate sized triple matched defect?
Intermediate
What is the probability of PE for a single mismatched large defect?
Intermediate
What about 2 moderate mismatched defects?
Intermediate
What is the probability for PE with multiple small, matched defects?
Very low
What is the stripe sign? If you see a stripe sign what is the probability for PE?
Stripe sign means there is a perfusion defect with a peripheral rim of uptake to show there is preservation of peripheral perfusion. This corresponds with a very low probability of PE.
What is the underling mechanism of a stripe sign?
Emphysema is most classic. You have decreased perfusion in the emphysematous lung parenchyma but can see preserved perfusion peripherally.
What is the probability for PE if there are non-segmental defects?
Very low. PE’s present as segmental mismatched perfusion defects.
What is the probability of PE if an entire lobe or entire lung has a mismatched perfusion defect?
High probability.
What if there is a matched defect involving one entire lung?
Low probability.
Why does PE present early with a mismatched defect and later with a matched defect?
Physiologic hypoxic bronchoconstriction takes some time to set in. So early PE’s will have reduced blood flow but preserved ventilation (mismatched defect). Once the hypoxia from absent perfusion has enough time to cause bronchoconstriction you will develop a corresponding ventilation abnormality in the region of absent perfusion (matched defect). If the hypoxic bronchoconstriction mechanism is not robust, you can have a chronic mismatched defect which is a cause of a false positive VQ scan in the setting of known prior PE.
Why might you consider a follow-up VQ scan 3 months after a high probability scan, even if the patient has no residual PE symptoms?
Some advocate to follow-up a high probability VQ scan in 3 months to get a new baseline. The rationale is that the VQ scan usually reverts to normal at 3 months but if defects persist at 3 months they may never completely normalize. So, you can repeat a VQ scan at 3 months to get a new normal in case a future VQ scan is needed. Otherwise, you could assume a future VQ scan is positive when you are only seeing sequela of the prior known PE.
