Radiological imaging tests such as chest X-rays and CT (computerized axial tomography) scans can reveal the shadow of the heart. Commonly called "cardiac shadow" and "cardiac silhouette" in medical terminology; the meaning of this expression can sometimes be confusing. In simple terms, the cardiac shadow refers to the visible portion of the heart in a processed chest X-ray or imaging scan.
X-rays were discovered in 1895 by Wilhelm Conrad Rontgen, a German physicist studying cathode rays. The discovery was accidental and Roentgen named the invisible ray "X" because its properties were unknown at the time. Scientists soon learned this electromagnetic radiation had significant penetrating powers and that substances absorbed X-rays in direct relation to their specific density and atomic weight. Within a few years, this knowledge was being used in medicine to study and analyze the human anatomy.
When a human is exposed to the electromagnetic radiation of an X-ray, the radiation penetrates the body. Just as with other substances, according to the Radiological Society of North America, "Different body parts absorb the X-rays in varying degrees." After conventional X-ray film is processed, this is evident in the results. Bones appear as white, air is black, and soft tissues are in graduated degrees of gray, depending on the respective density of the tissue.
The resulting cardiac shadow on a radiological imaging test is used to assist medical personnel in the diagnosis of particular illnesses and disease manifestations. For example, an evaluation of the progression of such diseases as chronic obstructive pulmonary disease (COPD) can be supplemented by certain indicators present or, in some cases, lacking in the cardiac shadow. An image showing an enlarged or narrowed cardiac shadow can also be used in the diagnosis and evaluation of a number of other cardiopulmonary illnesses.
A normal human heart has a particular density that results in predictable amounts of radiation absorption within a given range. Variants in the normal rate of absorption are displayed on a processed film as atypical shades of gray in unpredicted areas of the image. Radiologists are trained to interpret the often minute differentiations in these monochrome results. Cardiologists too are particularly adept at identifying normal or abnormal photographs of the cardiothoracic region and the cardiac shadow.
Improper positioning of a patient during the X-ray procedure can alter the results shown on the processed film. An inappropriate amount of exposure to X-ray radiation also may modify the shape or size of the cardiac shadow on a processed image. An experienced radiologist will know that the film is substandard and repeat the test when warranted. Patients' questions and concerns about all medical procedures and results, including the cardiac shadow, should be addressed directly to their doctors.