What is projection radiography?
Projection radiography is basically just a fancy term for an X-ray. Also known as diagnostic radiography, it is the practice of producing two-dimensional images using X-ray radiation. Diagnostic radiography is the medical science of producing images of the human body, which can be used to make a diagnosis of an injury or disease. Diagnostic radiographers play an important role in the diagnosis and management of disease and in the screening programs for early detection of cancer.
Radiographic exams are typically performed by radiographers. Therefore, projection radiography really looks at what most people think of when they think of an X-ray of a bone. It is a beam of radiation that goes across a body part and captures the differences in density of the tissues.
What happens during projection radiography?
Projection radiography is the cornerstone of modern medical imaging, and can be used to image almost every part of the human body. When an exposure is made, X-ray radiation exits the tube, which is known as the primary beam. When the primary beam passes through the body, some of the radiation is absorbed in a process known as attenuation. Anatomy that is denser has a higher rate of attenuation than anatomy that is less dense, so bone will absorb more X-rays than soft tissue.
What remains of the primary beam after attenuation is known as the remnant beam. The remnant beam is responsible for exposing the image receptor. Areas on the image receptor that receive the most radiation, or portions of the remnant beam experiencing the least attenuation, will be more heavily exposed and therefore will be processed as visibly darker. Conversely, areas on the image receptor that receive the least radiation, or portions of the remnant beam that experience the most attenuation, will be less exposed and will be processed as being lighter.
Attenuation is the reduction of the intensity of an X-ray beam as it traverses matter. The reduction may be caused by absorption or by deflection of photons from the beam and can be affected by different factors such as beam energy and atomic number of the absorber.
The standard chest radiograph is acquired with the patient standing up, and with the X-ray beam passing through the patient from posterior to anterior (PA). The chest X-ray image produced is viewed as if looking at the patient from the front, face-to-face. The heart is on the right side of the image as you look at it. Sometimes, it is not possible for radiographers to acquire a PA chest X-ray. This is usually because the patient is too unwell to stand. The chest X-ray image is still viewed as if looking at the patient face-to-face. This is why bone, which is very dense, process as being white on radiographs, and the lungs, which contain mostly air and is the least dense, shows up as black.
Radiographic density is the measure of overall darkening of the image. Density is a logarithmic unit that describes the ratio between light hitting the film and light being transmitted through the film. A higher radiographic density represents more opaque areas of the film, and lower density shows as more transparent areas of the film. With digital imaging, however, density may be referred to as brightness. The brightness of the radiograph in digital imaging is determined by computer software and the monitor on which the image is being viewed.
If your doctor tells you that you need projection radiography, don't be too alarmed. He or she just means you need an X-ray.