So now let's discuss interstitial disease.
Interstitial disease is filling of interstitial spaces
which are different than consolidation which is filling of the alveolar spaces.
Interstitial disease can be either focal or diffuse.
It's not confluent so you don't see patches of consolidation.
You see specific linear areas within the lungs which don't coalesce
and you won't see air bronchograms in interstitial disease.
Let's take a look at the three major patterns of interstitial disease.
You could have a reticular pattern and we'll look at each of these
in a little bit more detail.
You can have a primarily nodular pattern
or you can have a reticulonodular pattern, so a mix of the two.
So let's take a look first at the reticular pattern.
Reticular patterns can be divided into two different types.
You could have a coarse reticular pattern
or you could have a fine reticular pattern
and this shows you the difference between the two.
So the left image shows you what a coarse reticular pattern looks like
and it's just a little bit more dense, it's a little bit more irregular
than a fine reticular pattern is.
Fine reticular patterns can be caused by a multiple different reasons,
primarily interstitial edema and that's the most commonly seen
and that is caused by fluid in the interstitial spaces.
Infection can also cause a fine reticular pattern
and that's usually a result of an atypical infection
such as a viral infection, mycoplasma, or PCP.
It can be caused by lymphangitic spread of tumor.
The most common tumors to cause this pattern are breast,
pancreatic, lung, or stomach,
or it can be idiopathic such as an interstitial pneumonia
and usually in these cases the end stage is pulmonary fibrosis.
So this is an example of interstitial edema or fluid in the interstitial spaces.
You can see that there is a very fine reticular pattern,
so you can see these very linear opacities that are present within the lungs.
You don't see any area of focal consolidation
just these very fine prominence of the interstitial spaces.
Infection is another example as we said usually caused by an atypical infection
and so this is again a very similar finding
with prominence of the hilum bilaterally
and again the very fine reticular pattern that we saw
similar to that interstitial edema. Often it's difficult to distinguish
between the different types of fine reticular patterns
and normally what we say is that there's interstitial disease
and then clinically the referring physician can decide
whether or not this is infection or edema
or any other possibility that may be present in this patient.
So this is an example of lymphangitic spread of tumor.
Again, we said the most common tumors that can cause this pattern are breast,
pancreatic, lung, or stomach and this results
usually in a more of a mixed reticulonodular pattern.
So this is an axial CT scan of the chest and lung windows
demonstrating prominence of the interstitial spaces.
You can see the difference between the right lung which has this prominence
and the left lung which is actually clear.
So you can see this diffused haziness and again linear markings
branching throughout that lung and then you also see small areas
of nodularity which is typical of this mixed reticulonodular pattern.
This is an example of an idiopathic pneumonia.
So chronic interstitial pneumonia,
the most common is the usual interstitial pneumonia
but there are actually many different types of idiopathic pneumonias
and as we mentioned end stage is usually pulmonary fibrosis.
So you can see here predominantly in the periphery,
you have these areas of prominence of the interstitium
and you have a little bit of thickening of the pleura
which is commonly seen in these pneumonias.
On the radiograph it's actually a little bit less visible
because it is somewhat focal to the periphery.
So here we see a sharp margin at the costophrenic angle
and it's really the CT that tells us what's going on.