If part of the lung is radiodense (alveolar pattern, consolidated, water density, airless), it can affect our ability to see adjacent structures. We can use these changes to help us both detect and localize disease in the lung. This chapter discusses how disease in different lobes affects the appearance of adjacent organs.
1
There are four basic radiographic densities. In order of increasing radiodensity, they are gas, _______________, _______________, and _______________.
A.
soft tissue (water)
B.
fat
C.
metal (bone)
1
B. fat
A.
soft tissue (water)
C.
metal (bone)
Figure 6-1 shows an upright test tube containing, from top down, air, oil (fat), water, and metal. Calcium is the prime example of metal density normally found in the body. Note the sharp interface between each density. ( Arrow = air/fat interface; arrowhead = fat/water interface.)
2
Anatomic structures are recognized on an x-ray by their density differences. These four basic densities keep the radiologist in business. Test yourself by identifying the following on Figure 6-2 , a normal posteroanterior (PA) radiograph. (The fat planes between the muscles are barely visible.)
Figure 6-2
2
a.
air (lung)
b.
metal (bone)
c.
water (heart)
d.
air (stomach bubble)
e.
water (liver)
f.
air (pure air beyond patient)
g.
water (muscle)
→
metal (lead marker)
a.
_____________________
b.
_____________________
c.
_____________________
d.
_____________________
e.
_____________________
f.
_____________________
g.
_____________________
→
_____________________
air
soft tissue (water)
metal (bone)
In Figure 6-2 , the heart, aorta, and superior surfaces of the diaphragms have sharp margins because they are all water density, adjacent to air density. The inner stomach wall is visible because air contacts the inner wall, but the outer wall is not seen because it is surrounded by other soft tissue. Conversely, the liver and under surface of the right diaphragm are not seen separately because they are both of water density.
The heart, aorta, and blood—as well as the liver, spleen, and muscles—all are soft tissue density. So is diseased airless lung. Two substances of the same density, in direct contact, cannot be differentiated from each other on an x-ray. This phenomenon, the loss of the normal radiographic silhouette (contour), is called the silhouette sign.
3
Let’s reinforce this concept. Figure 6-3 shows three x-rays of a model of the heart and aorta. In Figure 6-3A , the heart and ascending aorta are in one empty container anteriorly, and the descending aorta is in a second empty container, behind the first. Their margins are sharp. In Figure 6-3B , some water has been poured into the anterior box. In Figure 6-3C , the water has been removed and placed in the posterior box.
The lower aortic border is not visible in Figure 6-3C because _______________. The name silhouette sign is counterintuitive. Logic suggests “we see the aortic silhouette,” but it means “we don’t see it.” So much for logic!
4
aorta now contacts water, rather than air
In Figure 6-4 , the left diaphragm is visible, but the right is not because the adjacent right lower lobe is consolidated (airless)—the silhouette sign. The right heart border, still in contact with aerated right middle lobe, is visible. The left heart border is normal.
5
In Figure 6-4 , the trachea, which is of _______________ density, can be differentiated from the mediastinum, which is of _______________ density.
A.
air
B.
fat
C.
water (soft tissue)
D.
metal
5
A. air
C.
water (soft tissue)
The liver and under surface of the diaphragm cannot be separated because both are of _______________ density. The upper surface of the left diaphragm is visible because it abuts _______________.
The left heart border is not visible. An interface is not visible when two areas of _______________ radiodensity touch.
A.
similar
B.
different
C.
any
A.
similar
7
Now that you know what the silhouette sign is, what are you going to do with it? The silhouette sign helps diagnose and localize lung disease. If you know the position of intrathoracic structures, you can precisely localize the lung disease.
7
The heart and ascending aorta are _______________ structures.
A.
anterior
B.
posterior
C.
central
A.
anterior
Conversely, the descending aorta is a(n) _______________ structure.
A.
anterior
B.
posterior
C.
central
B.
posterior
The aortic arch crosses the middle mediastinum from _______________ on the _______________ to _______________ on the _______________.
A.
anterior
B.
posterior
C.
right
D.
left
A.
and C. anterior on right
B.
and D. posterior on left
Figure 6-6A is a lateral view of the chest with an atherosclerotic (calcified) aortic wall. The heart and ascending aorta ( A ) are anterior, and the descending aorta ( D ) is posterior. Figure 6-6B is a nonenhanced computed tomography (CT) scan taken through the aortic arch as it passes from right anterior to left posterior. Figure 6-6C is contrast enhanced. Note: The calcified aortic wall is visible on Figure 6-6A and Figure 6-6B because calcium is denser than the aorta. In Figure 6-6C , the calcium density is similar to the contrast in the aorta and not visible.
8
There is even a “normal” silhouette sign on the lateral chest x-ray. In Figure 6-6A , we see two diaphragms posteriorly, but only one anteriorly.
8
The heart sits on the _______________ diaphragm, obscuring the anterior part of that diaphragm.
A.
right
B.
left
B.
left
The diaphragm seen overlapping the heart is the right diaphragm. It is adjacent to air all the way.
How can this be useful? _______________.
It helps to distinguish the left and right diaphragms on the lateral.
9
Let’s wrap this up. Match the location of each of the following:
9
A.
anterior
B.
posterior
C.
anterior
D.
anterior
E.
central
A.
right heart border _______________
B.
descending aorta ________________
C.
left heart border _________________
D.
ascending aorta _________________
E.
aortic knob (arch) _______________
anterior
posterior
central
10
Each lobe produces a characteristic silhouette sign that we can use. The right middle lobe and lingula lie in anatomic contact with the _______________.
A.
ascending aorta
B.
descending aorta
C.
aortic knob
D.
heart
10
D. heart
All are anterior structures.
In the chest x-ray in Figure 6-7 , there is a silhouette sign of the _______________.
A.
right diaphragm
B.
right heart border
C.
left heart border
D.
aortic knob
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