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1. There are 3 basic rules in radiology:
summation-, silhouette phenomenon and shell-sign.

Ultimately, we want contrast. If all rays attenuate at the same level, this results in a homogenous grey area, which means a complete lack of contrast.
We want ‘ray X’ and ‘ray Y’ to be weakened differently.
Let us assume the following: the body to be examined should consist of 2 substances (A and B).
Left part of drawing: the thicker A is, the more ray X is attenuated (in comparison to ray Y);

 

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2. To conclude: the rule is simple. The thicker and/or denser the object is, the greater the attenuation of radiation. This corresponds to a less bright region in the film. We call this "opacity”, the opposite of "translucency”. - In German it is rather more complicated:
a bright structure in the radiograph (e.g. a pleural effusion) is termed "Verschattung"; a dark structure (e.g. a pneumothorax) is referred to as "Aufhellung". This paradox results from fluoroscopy where information provided is inverse: "black" in the image is transparent on a fluoroscopy screen; "transparent” in fluoroscopy  is "black” on the x-ray film.

Here are 4 x-rays of objects and all have been obtained under the same conditions:
same kilovolt (KV),
same mAs,
same distance,
same intensification factor,
same material!

2. A slice is transparent (less summation). We see more details because not too many details are superimposed! But the disadvantage is that there is only one slice, all the other slices are not been recorded.
Truth be told, this is not a regular x-ray; but a composition of CT-slices. X-rays of those objects look exactly the same. To experiment and to photograph is much easier in digital conditions.
A summation of slices can simulate an x-ray precisely. The summarizing ("averaging" is a little different) of a "CT- examination of head" provides one with a nice x-ray of the skull.

3. In what field of radiology is the term "Silhouette" best known?

"Silhouette sign" is well known in thoracic radiology. Heart and diaphragm have clear silhouettes (A = soft tissue, B = air containing lung). Pneumonia leads to a loss of cardiac or diaphragmatic margins, depending on where the change in pulmonary parenchyma is located.

The sign we want to talk about now, deals with the contours, edges, or margins.

The less parallel the border area runs, the more lost, or more imperfect, becomes the delineation sharpness. If this delineation runs very obliquely or perpendicular fashion to the beam path, it is impossible to visualize it.

4. This is a special case of silhouette phenomenon:

a shell of dense material causes a silhouette on its outer and inner shape.

A snail’s outer shell shown here creates a remarkable x-ray. Not only the outer but the inner surfaces are visualized in some parts of the x-ray film. It demonstrates the strengths of the Roentgen method.

5. There are plenty of shell-like structures in anatomy and pathology.