Projectional radiography


Projectional radiography, also known as conventional radiography, is a form of radiography and medical imaging that produces two-dimensional images by X-ray radiation. It is important to note that projectional radiography is not the same as a radiographic projection, which refers specifically to the direction of the X-ray beam and patient positioning during the imaging process. The image acquisition is generally performed by radiographers, and the images are often examined by radiologists. Both the procedure and any resultant images are often simply called 'X-ray'. Plain radiography or roentgenography generally refers to projectional radiography. Plain radiography can also refer to radiography without a radiocontrast agent or radiography that generates single static images, as contrasted to fluoroscopy, which are technically also projectional.

Equipment

X-ray generator

Projectional radiographs generally use X-rays created by X-ray generators, which generate X-rays from X-ray tubes.

Grid

An anti-scatter grid may be placed between the patient and the detector to reduce the quantity of scattered x-rays that reach the detector. This improves the contrast resolution of the image, but also increases radiation exposure for the patient.

Detector

Detectors can be divided into two major categories: imaging detectors and dose measurement devices.

Shielding

is the main material used by radiography personnel for shielding against scattered X-rays.

Image properties

Projectional radiography relies on the characteristics of X-ray radiation and knowledge of how it interacts with human tissue to create diagnostic images. X-rays are a form of ionizing radiation, meaning it has sufficient energy to potentially remove electrons from an atom, thus giving it a charge and making it an ion.

X-ray attenuation

When an exposure is made, X-ray radiation exits the tube as what 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 will be more heavily exposed, and therefore will be processed as being darker. Conversely, areas on the image receptor that receive the least radiation will be less exposed and will be processed as being lighter. This is why bone, which is very dense, process as being 'white' on radio graphs, and the lungs, which contain mostly air and is the least dense, shows up as 'black'.

Density

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 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.

Contrast

Contrast is defined as the difference in radiographic density between adjacent portions of the image. The range between black and white on the final radiograph. High contrast, or short-scale contrast, means there is little gray on the radiograph, and there are fewer gray shades between black and white. Low contrast, or long-scale contrast, means there is much gray on the radiograph, and there are many gray shades between black and white.
Closely related to radiographic contrast is the concept of exposure latitude. Exposure latitude is the range of exposures over which the recording medium will respond with a diagnostically useful density; in other words, this is the "flexibility" or "leeway" that a radiographer has when setting his/her exposure factors. Images having a short-scale of contrast will have narrow exposure latitude. Images having long-scale contrast will have a wide exposure latitude; that is, the radiographer will be able to utilize a broader range of technical factors to produce a diagnostic-quality image.
Contrast is determined by the kilovoltage of the x-ray beam and the tissue composition of the body part being radiographed. Selection of look-up tables in digital imaging also affects contrast.
Generally speaking, high contrast is necessary for body parts in which bony anatomy is of clinical interest. When soft tissue is of interest, lower contrast is preferable in order to accurately demonstrate all of the soft tissue tones in these areas.

Geometric magnification

Geometric magnification results from the detector being farther away from the X-ray source than the object. In this regard, the source-detector distance or SDD is a measurement of the distance between the generator and the detector. Alternative names are source/''focus to detector/image-receptor/film distance.
The estimated radiographic magnification factor is the ratio of the source-detector distance over the source-object distance. The size of the object is given as:
where Sizeprojection is the size of the projection that the object forms on the detector. On lumbar and chest radiographs, it is anticipated that ERMF is between 1.05 and 1.40. Because of the uncertainty of the true size of objects seen on projectional radiography, their sizes are often compared to other structures within the body, such as dimensions of the vertebrae, or empirically by clinical experience.
The source-detector distance is roughly related to the source-object distance and the object-detector distance'' by the equation SOD + ODD = SDD.

Geometric unsharpness

Geometric unsharpness is caused by the X-ray generator not creating X-rays from a single point but rather from an area, as can be measured as the focal spot size. Geometric unsharpness increases proportionally to the focal spot size, as well as the estimated radiographic magnification factor.

Geometric distortion

Organs will have different relative distances to the detector depending on which direction the X-rays come from. For example, chest radiographs are preferably taken with X-rays coming from behind. However, in case the patient cannot stand, the radiograph often needs to be taken with the patient lying in a supine position with the X-rays coming from above, and geometric magnification will then cause for example the heart to appear larger than it actually is because it is further away from the detector.

Scatter

In addition to using an anti-scatter grid, increasing the ODD alone can improve image contrast by decreasing the amount of scattered radiation that reaches the receptor. However, this needs to be weighted against increased geometric unsharpness if the SDD is not also proportionally increased.

Imaging variations by target tissue

Projection radiography uses X-rays in different amounts and strengths depending on what body part is being imaged:
  • Hard tissues such as bone require a relatively high energy photon source, and typically a tungsten anode is used with a high voltage on a 3-phase or high-frequency machine to generate bremsstrahlung or braking radiation. Bony tissue and metals are denser than the surrounding tissue, and thus by absorbing more of the X-ray photons they prevent the film from getting exposed as much. Wherever dense tissue absorbs or stops the X-rays, the resulting X-ray film is unexposed, and appears translucent blue, whereas the black parts of the film represent lower-density tissues such as fat, skin, and internal organs, which could not stop the X-rays. This is usually used to see bony fractures, foreign objects, and used for finding bony pathology such as osteoarthritis, infection, cancer, as well as growth studies.
  • Soft tissues are seen with the same machine as for hard tissues, but a "softer" or less-penetrating X-ray beam is used. Tissues commonly imaged include the lungs and heart shadow in a chest X-ray, the air pattern of the bowel in abdominal X-rays, the soft tissues of the neck, the orbits by a skull X-ray before an MRI to check for radiopaque foreign bodies, and of course the soft tissue shadows in X-rays of bony injuries are looked at by the radiologist for signs of hidden trauma.

    Projectional radiography terminology

NOTE: The simplified word 'view' is often used to describe a radiographic projection.
Plain radiography generally refers to projectional radiography. Plain radiography can also refer to radiography without a radiocontrast agent or radiography that generates single static images, as contrasted to fluoroscopy.
  • AP - Antero-Posterior
  • PA - Postero-Anterior
  • DP - Dorsal-Plantar
  • Lateral - Projection taken with the central ray perpendicular to the midsagittal plane
  • Oblique - Projection taken with the central ray at an angle to any of the body planes. Described by the angle of obliquity and the portion of the body the X-ray beam exits; right or left and posterior or anterior. For example, a 45 degree Right Anterior Oblique of the Cervical Spine.
  • Flexion - Joint is radiographed while in flexion
  • Extension - Joint is radiographed while in extension
  • Stress Views - Typically taken of joints with external force applied in a direction that is different from main movement of the joint. Test of stability.
  • Weight-bearing - Generally with the subject standing up
  • HBL, HRL, HCR or CTL - Horizontal Beam Lateral, Horizontal Ray Lateral, Horizontal Central Ray, or Cross Table Lateral. Used to obtain a lateral projection usually when patients are unable to move.
  • Prone - Patient lies on their front
  • Supine - Patient lies on the back
  • Decubitus - Patient lying down. Further described by the downside body surface: dorsal, ventral, or lateral.
  • OM - occipito-mental, an imaginary positioning line extending from the menti to the occiput
  • Cranial or Cephalad - Tube angulation towards the head
  • Caudal - Tube angulation towards the feet