K.E.M. Radiology

Lessons from radiography in vitro

Armaandeep Singh Aulakh, 2nd year Resident

Conventional radiology is the oldest modality of peeking inside the human body, which still finds its place (and relevance) in the ocean of modern imaging. With continuous improvements in technology, we have started imaging the body to submillimeter (mm) levels with great details, yet the fundamentals of radiography still hold true.

In this article, I wish to revisit some of the classic radiographic principles in vitro. Bear with me, and I promise you won't be disappointed. 

The “skiagram”, is defined as a picture or image created from shadows, especially with X- rays. The images essentially are formed by the x-rays reaching the photographic film (target) after passing through the obstacles(subject) in the path, between the source and target. Depending on the density of the subject, a fraction of x-rays get absorbed or deflected by the subject. It's only those mighty few, who continue their journey, penetrate the subject, and end up hitting the target. Denser the subject, fewer are the rays that are able to penetrate it.  

Yes, all this happens as we say “shoot” and hit the button.

Have an unbiased and keen look at the image below (Fig 1). Try and list various radiographic principles at play.

Whenever there is fluid and air (which in fact also is a fluid technically) in an enclosed cavity, air occupies a non-dependent position and forms a level (air-fluid interface). This can be extrapolated to describe the appearance of lung abscess, hydropneumothorax, and free intraperitoneal air on an erect chest radiograph (Fig 2-4). Similarly, fat-fluid levels may be also be seen.

If we look at the bottle on the right side of the film, its left half is much more radio-opaque as compared to the right (please don’t assume it to be a patient’s radiograph, “right” and “left’ are used as they lie on the film). This is also applicable to the fluid within the bottle. This demonstrates another principle, whenever two subjects or structures overlap each other, the image shows a summation of their densities. In our image, the left half of the bottle on right side of the film is overlapping the right half of the bottle in the center. Areas of overlap show summation of their densities and give rise to a higher radio-opacity on the film.

In vivo, this is commonly noted in the anterior and posterior ribs, with the area of overlap showing higher radio-opacity, which may mimic a sclerotic bone lesion for an untrained eye (Fig 5a,b).

In the case of left lower lobe consolidation, increased opacity of the retrocardiac region is explained by this principle (Fig 6).

In a supine chest radiograph, a unilateral faint increase in density of hemithorax is explained by the pleural fluid encasing the lung, leading to the summation of alveolar air and fluid density of pleural space (Fig 7).

If you really have looked closely, you would notice that the bottle on right side (of film) looks larger than the one on left. Actually, both soap bottles are of the same size. The apparent difference is due to their position with respect to the film(target). As X-rays come out of the source, they diverge to some extent. Hence, the object placed further away from the film casts a bigger shadow on the film as compared to the one lying close to the film. You may experience this in real life if you remember making funny shadows of animals using your hand by keeping it in front of a torchlight (source). The closer you are to the source, the larger is the image formed.

In vivo, this explains the apparent cardiomegaly in an AP frontal radiograph as compared to PA view (Fig 8). 

The heart is an anterior structure that comes away from the film in the AP radiograph leading to magnification. Hence, cardiomegaly should not be diagnosed on an AP radiograph. This also explains prominent hilum on a rotated film. 

Look at one of the bottles. We can see a well-defined wall of the bottle only in the upper half- region where there is air within the bottle. The fluid in the bottle obscures the wall adjacent to it. Hence, two densities can be differentiated on a radiograph only when they have a significant difference in their densities. 

This forms the basis of silhouette sign, originally described by Benjamin Felson - “An intrathoracic lesion touching a border of the heart, aorta, or diaphragm will obliterate that border on the roentgenogram. An intrathoracic lesion not anatomically contiguous with a border of one of these structures will not obliterate that border”[1]. The majority of other signs of chest radiography are applications of the silhouette sign.

“Silhouette” essentially means the margin of a shadow against a bright background (Fig 10). 

Hence, the silhouette sign is actually an obliteration of the silhouette because of the loss of contrast adjacent to the subject.      

Here, the thin wall of the plastic bottle has a density almost similar to the fluid that it contains, hence gets obscured when it comes in contact with it. In the region where it contains air, its wall can be seen distinctly because of significant differences in their densities. 

This observation can be extrapolated further, the wall of the bottle is seen clearly because it is outlined by air on both sides (air within the bottle and outside it). This explains the appearance of a well-delineated bowel wall in a case of pneumoperitoneum (Rigler Sign) (Fig 11).

Normally, the margins of renal shadows and inferior margin of the liver are well defined because of their interface with adjacent fat. They can be even more well defined when they are outlined by retroperitoneal and intraperitoneal air in cases of pneumoretroperitoneum and pneumoperitoneum respectively (Fig 12).

Few lucencies seen in the fluid in the left bottle represent bubbles of soap.

There could be a few more observations that I may have missed. Feel free to mail me at armaangsmc@gmail.com in case you find any.

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