Pulmonary Embolism: Diagnosis

00:01 The lung scan is a nuclear-medicine technique. It consists of two components: a ventilation component and a perfusion component.

00:12 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 lung.

00:29 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.

00:41 Normal patients will have a uniform lung ventilation scan and patients with pulmonary embolism will also have a normal ventilation scan.

00:51 And what’s demonstrated here is a normal ventilation scan with xenon gas.

00:59 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.

01:19 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.

01:30 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 perfusion scan.

01:39 So a positive scan consists of a normal ventilation scan and an abnormal perfusion scan.

01:47 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.

02:37 Again, many lung diseases will result in both abnormalities in both perfusion and in ventilation.

02:46 But they match. In other words, the area of poor perfusion is also the area of poor ventilation.

02:51 In pulmonary embolism, there is a so-called ‘mismatch.’ Normal ventilation, abnormal perfusion. And I’m going to show you an example of that.

03:02 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.

03:15 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.

03:27 Now let’s look at the perfusion scan.

03:31 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.

03:45 That’s the ventilation scan.

03:46 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 total non-uniformity.

03:59 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.

04:15 The perfusion scan, to reiterate, is obtained by taking macroaggregated albumin and attaching a radionuclide to it.

04:25 In the normal situation, there’s uniform distribution of the radionuclide intensity throughout both lungs.

04:32 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.