The lung scan is a nuclear-medicine technique.
It consists of two components: a ventilation
component and a perfusion component.
For the ventilation component, the patient
inhales radioactive xenon gas. And what happens
there is that the lung fills with radioactivity
and we take a scanning picture to see whether
the xenon gas gets uniformly throughout the
If patients have pneumonia or if they’ve
had a lung resection or if they have a tumour
in the lung, often you’ll see an area of
the lung that doesn’t get xenon gas because
it’s been replaced.
Normal patients will have a uniform lung ventilation
scan and patients with pulmonary embolism
will also have a normal ventilation scan.
And what’s demonstrated here is a normal
ventilation scan with xenon gas.
The imaging of the perfusion scan consists
of looking at where small particles of either
technetium or another radioactive substance
that’s been attached to albumen goes in
the lung after it’s been injected intravenously.
So you mix this up very thoroughly with some
blood, you squirt it in. It should be spread
uniformly throughout the lung showing that
there’s uniform perfusion.
When there’s pulmonary embolism, there’s
an area that’s not being perfused, not getting
blood flow. And you will see a hole on the
So a positive scan consists of a normal ventilation
scan and an abnormal perfusion scan.
So, again, reiterating: phase 1 and 2. In
the ventilation scan, there’s uniform distribution
of the radionuclide throughout both lungs
in ventilation. In the perfusion scan, there
is abnormality in that there’s asymmetric
distribution of perfusion when there are diseases
that affect ventilation – for example a
tumour or pneumonia. You will see an abnormal
ventilation scan. And that tells you, “Oh!
This is very unlikely to be pulmonary embolism.”
The scan that shows you pulmonary embolism
shows uniform ventilation but abnormal perfusion.
Again, many lung diseases will result in both
abnormalities in both perfusion and in ventilation.
But they match. In other words, the area of
poor perfusion is also the area of poor ventilation.
In pulmonary embolism, there is a so-called
‘mismatch.’ Normal ventilation, abnormal
perfusion. And I’m going to show you an
example of that.
So you’ve seen just before, this is an enlarged
component of the ventilation scan. And you’ll
notice how nice and uniform the radioactivity
is across the lung and how it washes out uniformly.
So this patient doesn’t have any evidence
of any form of lung disease. I don’t see
any sign that there’s a pneumonia there
or a tumour which would cause a hole in the
uniform ventilation scan.
Now let’s look at the perfusion scan.
This is a very abnormal perfusion scan. Compare
it – let me go back. Here’s uniform. Look
at the upper left-hand corner. The first initial
one. Notice how smooth and full the lung is.
That’s the ventilation scan.
Now look at the perfusion can. Look at how
abnormal it is. Look in particular at the
right lung. Much of the right lung is not
perfused at all. The left lung is okay but
there are cuts, there are holes. There’s
So this patient had a normal ventilation scan,
a markedly abnormal perfusion scan. In other
words, a ventilation-perfusion mismatch. This
is highly suggestive of pulmonary embolism.
The perfusion scan, to reiterate, is obtained
by taking macroaggregated albumin and attaching
a radionuclide to it.
In the normal situation, there’s uniform
distribution of the radionuclide intensity
throughout both lungs.
In the abnormal setting, there are holes in
the distribution of the radionuclide where
there’s no perfusion. And at the same time
there’s normal ventilation scan. So aeration
is normal but perfusion is abnormal: strongly
suggests pulmonary embolism.