The classic appearance of a PE is that you have a raggedy lung perfusion (Tc-MAA can't get into certain areas) and a pristine lung perfusion with Technegas.
It only gets hard when both the lung images are moth-eaten. Then you have to decide which is more moth-eaten.
The PIOPED criteria are:
To label someone as HIGH probability they have to have 2 big (>75% of the segment) unmatched defects (or 4 moderate ones) and - this being the kicker - a NORMAL CXR in that region (i.e. defects are unmatched on ventilation and on CXR).
To be LOW probability they have to have:-
- an infinite number of matched defects with the CXR normal at those sites
- triple match (V abnormal, Q abnormal, CXR abnormal) in the upper and middle lobes
- match in the lower lobes with a large pleural effusion
- non-segmental defects (e.g., aortic impression, hila, cardiomegaly)
- any perfusion defect where there is a larger CXR abnormality in that region
- STRIPE SIGN (defect surrounded by normally perfused lung)
- an infinite number of small unmatched perfusion defects but a normal CXR at those sites
To be VERY LOW probability they have to have up to 3 small (<25% of the segment) perfusion defects but in those areas the CXR is normal and the ventilation is normal.
So, to be INTERMEDIATE probability:
- 1 large unmatched perfusion defect with a normal CXR
- 1-3 moderate unmatched persuion defects with a normal CXR
- matched defects with a small pleural effusion
- 1 moderate matched defect inperfusion and ventilation but a normal CXR at that site
A pitfall to be aware of is that when you have a pneumothorax or pleural effusion, that whole lung looks underperfused and underventilated, meaning that you might mistake it for a large PE to that side.
If they've forgotten to label which is ventilation and which is perfusion, then you just look to see which has tracer in the trachea - that's the ventilation image!
Other things to know about VQ scans:
Uses Tc-labelled albumin microspheres. Therefore not given in pulmonary HTN unless you are giving less than 200,000 particles. Not given with right-left shunts.
If it is given inadvertantly with a r-l shunt then you will see:
Here is a way to remember the lung segments:
The way to start with this overwhelming image is with the RML.
The MIDDLE lobe can be remembered to have Medial and Lateral segments.
Then, because the middle lobe is a square with a medial segment, the lower lobe is also a square with a medial segment.
Finally, because there are 3 lobes in the Right lung, the upper lobe is remembered to have 3 segments.
The L(eft) lung has lobes that look like L's.
Thus, the lingula, because it cannot be a square, has superior and inferior segments.
And the LLL, because it too looks like a triangle, has a base and a superior segment.
Finally, because there are 2 lobes in the left lung, the upper lobe has 2 segments with the demarcating line (-------), like it is for the other lobes, being oblique.
The report is written thus:
"Severe hypoventilation in the x segments with the corresponding perfusion defects being less marke. No unmatched perfusion defects seen"
...also important to say that no other matched defects seen because matched defects could be chronic PE unless the hypoventlation is worse than the hypoperfusion.
The best conclusion is to say: "No evidence for pulmonary emboli (instead of saying "low probability"). The changes in the x lobe are most consistent with bronchopulmonary disease".