Conventional (also called analog, plain-film or projectional) radiography is a fundamental diagnostic imaging tool in the detection and diagnosis of diseases. X-rays reveal differences in tissue structures using attenuation or absorption of x-ray photons by materials with high density (like calcium-rich bones).
Basically, a projection or conventional radiograph shows differences between bones, air and sometimes fat, which makes it particularly useful to asses bone conditions and chest pathologies. Low natural contrast between adjacent structures of similar radiographic density requires the use of contrast media to enhance the contrast.
In conventional radiography, the patient is placed between an x-ray tube and a film or detector, sensitive for x-rays. The choice of film and intensifying screen (which indirectly exposes the film) influence the contrast resolution and spatial resolution. Chemicals are needed to process the film and are often the source of errors and retakes. The result is a fixed image that is difficult to manipulate after radiation exposure. The images may be also visualized on fluoroscopic screens, movies or computer monitors.
X-rays emerge as a diverging conical beam from the focal spot of the x-ray tube. For this reason, the radiographic projection produces a variable degree of distortion. This effect decreases with increased source to object distance relative to the object to film distance, and by using a collimator, which let through parallel x-rays only.
Conventional radiography has the disadvantage of a lower contrast resolution. Compared with computed tomography (CT) and magnetic resonance imaging (MRI), it has the advantage of a higher spatial resolution, is inexpensive, easy to use, and widely available. Conventional radiography can give high quality results if the technique selected is proper and adequate. X-ray systems and radioactive isotopes such as Iridium-192 and Cobalt-60 for generating penetrating radiation, are also used in non-destructive testing.

See also Computed Radiography and Digital Radiography.

Image quality is an important value of all radiographic imaging procedures. Accurate measures of both image quality and patient radiation risk are needed for effective optimization of diagnostic imaging. Images are acquired for specific purposes, and the result depends on how well this task is performed. The imaging performance is mainly influenced by the imaging procedure, examined object, contrast agents, imaging system, electronic data processing, display, maintenance and the operator. Spatial resolution (sharpness), contrast resolution and sensitivity, artifacts and noise are indicators of image quality.
A high image contrast provides the discrimination between tissues of different densities.
The image resolution states the distinct visibility of linear structures, masses and calcifications.
Noise and artifacts degrade the image quality. In computed tomography (CT), high spatial resolution improves the visibility of small details, but results in increased noise. Increased noise reduces the low contrast detectability. Noise can be reduced by the use of large voxels, increased radiation dose, or an additional smoothing filter, but this type of filter increases blurring.
An image acquisition technique taking these facts into account maximizes the received information content and minimizes the radiation risk or keeps it at a low level.

See also As Low As Reasonably Achievable.

X-rays contain a range of energies (polychromatic photons), the higher energies pass through the patient, the lower energies are absorbed or scattered by the body. Ideally, the x-ray beam should be monochromatic or composed of photons having the same energy. Strong filtration of the beam results in more uniformity. The more uniform the beam, the more accurate the attenuation values or CT numbers are for the scanned anatomical region.
There are two types of filtration utilized in CT:
Inherent tube filtration and filters made of aluminum or Teflon are utilized to shape the beam intensity by filtering out the undesirable x-rays with low energy. Filtration of the x-ray beam is usually done by the manufacturer prior to installation. The half value layer provides information about the energy characteristics of the x-ray beam. Too much filtration produces a loss of contrast in the x-ray image.
A mathematical filter such as a bone or soft tissue algorithm is included into the CT reconstruction process to enhance resolution of a particular anatomical region of interest.