• Clinical science

Radiography (X-ray imaging)

Summary

Radiography is an imaging technique that employs x-rays (high-energy electromagnetic radiation of a wavelength between UV light and gamma rays) to visualize internal structures of the body for diagnostic purposes. Conventional (projectional) radiography produces two-dimensional images of the object studied. It involves an x-ray generator projecting an x-ray beam towards an object. Depending on its density and structure, the object absorbs or scatters a portion of the x-rays. A detector situated behind that object captures the x-rays that pass through the object on photographic film or a digital medium. Computed tomography, which employs rotating x-ray generators and detectors to produce three-dimensional images, is covered in a separate article. Radiography plays a key role in the evaluation of thoracic and abdominal organs, bony structures, the breast (mammography), blood vessels (angiography), and the urinary system (cystourethrography, urography). Contrast radiography uses a contrast agent to highlight certain structures not clearly distinguishable from other structures on plain x-ray (e.g., blood vessels). Because of the health risks involved in exposure to ionizing radiation (cell death, teratogenicity, carcinogenicity), radiographical studies are bound to high safety standards (e.g., proper shielding) and should only be performed when medical need and benefit exceed the health risks associated (see also nuclear medicine and radiation protection). Accordingly, the threshold for indication is higher for children and pregnant women, while, generally, the radiation dose should be maintained as low as reasonably possible (ALARA principle).

Indications

Indications for x-ray vary greatly, depending on the problem, patient history, guidelines used, and institution/physician preference. The American College of Radiology offers ACR Appropriateness Criteria®, which are evidence-based guidelines intended to help healthcare providers in making clinical decisions regarding imaging for a wide variety of diagnostic and interventional topics. They can be found at https://acsearch.acr.org/list. [1] Some examples of when x-ray is important include:

Contraindications

There are no absolute contraindications for x-ray studies. However:

  • The FDA recommends that all exams involving ionizing radiation should be performed only when medically necessary, i.e., for diagnosis, treatment, or guiding an invasive procedure. Keeping the radiation dose "As Low as Reasonably Achievable" (ALARA) should be the guiding principle in determining equipment settings.
  • X-ray should especially be avoided in children and during pregnancy because there is a greater risk of negative consequences involved in exposure to ionizing radiation (cell death, teratogenicity, carcinogenicity).

We list the most important contraindications. The selection is not exhaustive.

Technical background

There are 3 main parts involved to create an x-ray image:

  1. Generate a beam of x-rays from an x-ray tube.
  2. Project x-rays toward an object with a detector behind.
    • Some x-rays are absorbed by the object, dependent on its density and structural composition.
    • Remaining x-rays pass through the object and are absorbed by the detector.
  3. Generate image from detector (either digital or photographic film).

Generation of x-rays

X-rays are a type of ionizing radiation that is generated when electrons that have been accelerated to great velocity hit a metallic anode.

  • The heating voltage of a cathode energizes electrons until they are ejected from the metal atoms of the cathode (usually wolfram). The high voltage between cathode and anode (anode voltage) then accelerates the electrons towards the positive pole. While colliding with the metal of the anode, the electrons are deflected and/or slowed down. During this process, energy (bremsstrahlung radiation) is released and emitted in the form of x-rays. Since the intensity of x-rays depends on the voltage and material of the anode, modification of these parameters allows for generation of a wide spectrum of radiation (from soft to hard).
  • An x-ray generator produces a beam of x-rays projected toward the object. A certain amount of x-ray is absorbed by the object, dependent on its density and structural composition. The x-rays that pass through the object are captured behind the object by a detector (either photographic film or a digital detector).

Absorption of x-rays

  • General
    • The denser the tissue and the softer the x-rays, the more radiation is absorbed by the tissue and blocked from reaching the film. Such areas of tissue appear light (i.e., radiopaque or radiodense) on the radiograph, in contrast to areas of tissue that allow x-rays to pass through and appear dark (i.e., radiolucent).
    • Absorbed x-rays release their energy into the surrounding tissue, leading to the formation of free oxygen radicals. This effect is the reason why x-rays are harmful.
  • Soft x-ray radiation (< 100 keV): low-kilovoltage technique
    • Soft radiation is lower in energy
      • The lower the energy level of x-rays, the greater the effect of the atoms and their atomic number (rather than density) in the examined tissue will be on the rate of x-ray absorption.
      • The higher the atomic number, the higher the rate of absorption
    • Assessment is easier in tissues with a high percentage of atoms with high atomic numbers (e.g., bone or calcium).
    • Applications include bone scans and mammography.
    • Not well-suited for the assessment of lung parenchyma
  • Hard x-ray radiation (100–1000 keV): high-kilovoltage technique
    • Hard radiation is higher in energy
      • The higher the energy level of x-rays, the greater the effect of the examined tissue's density (rather than the atomic number) will be on the rate of x-ray absorption.
      • Increased radiolucency (transparency) of bones
    • Well suited for x-ray analysis of nonhomogeneous structures
    • Applications include conventional x-ray chest and x-ray abdomen.
    • Not well-suited for the assessment of bony structures

Because soft x-rays are absorbed in tissue at a higher rate than hard x-rays, their radiation burden is greater despite being lower in energy!

Recording of x-rays

X-ray detectors

  • Methods
    • Originally, x-rays were recorded on x-ray films directly positioned behind the object to be examined.
    • Nowadays, digital radiography with x-ray sensitive plates that can directly record and transfer data to a computer system, has largely replaced photographic films.
  • Exposure
    • Radiopaque: nonexposed regions appear light or white in color
      • E.g., x-rays that hit the femur are largely absorbed, leading to a whitish appearance of the femur on radiographs.
    • Radiolucent: exposed regions of x-ray appear dark or black in color and are directly proportional to the intensity of incident radiation.
      • E.g., normal lung tissue appears dark on x-ray since it is filled with air, which absorbs very few x-rays.

Radiography creates negative images (radiographs):
- RadioPaque Prevents x-rays from getting through and appear Pale.
- RadioLucent Lets x-rays through and Lacks color (bLack).

Image quality

  • Quality: Definition and contrast determine the quality of an image.
    • Definition:
      • Decreases with increasing distance between x-ray tube and examined object.
      • The distance between an object and the x-ray detector determines the size of the object's projection onto the detector: the closer the object to the x-ray detector, the more realistic the size of the object.
    • Contrast: depends on radiation dose, filters employed, and degree of scatter radiation

  • Problem: scatter radiation
    • When x-rays hit tissue, they are partially deflected and, consequently, hit the detector at a slanted angle, leading to a distorted visualization of anatomical structures.
    • Scatter radiation can be reduced by placing a grid between the x-ray detector and the object to be examined.

Procedure/application

Plain radiography

  • Description: projectional radiography without contrast agent use
  • Procedure
    • Patients should be positioned with the region to be examined as close to the x-ray detector as possible. This ensures highest image quality by reducing blur and size distortion (i.e., magnification as a result of projection).
      • Posterior-anterior (PA) projection is preferred for radiographs of the chest to avoid size distortion of the heart!
    • X-ray images should generaly be taken in (at least) two planes so that the two-dimensional images collectively provide better visualization of an otherwise three-dimensional structure.
      • Possible exception: in children or pregnant patients to reduce the radiation burden
      • Advantages of multiplanar radiography
        • Accurate spatial allocation of visible structures
        • Reduced risk of missing anomalies that may not be visible in certain projections
      • Classic x-ray studies include a frontal- and a sagittal-plane projection.
      • Certain x-ray studies require special projections (e.g., Lauenstein projection for the assessment of hip joints).

Due to size distortion, the heart may appear enlarged in radiographs of chest taken in the supine position (anterior-posterior projection)!

Contrast radiography

Interpretation/findings

Diagnostic radiology of thoracic organs

Normal x-rays

Pneumonia

Pleural effusion

Cardiac insufficiency

Sarcoidosis

Tuberculosis

Pneumothorax

Diagnostic radiology of abdominal organs

Ileus

Perforation of hollow organs

Assessment of bony structures

Bone fractures

Bone tumors

Bone cysts

Complications

Radiography involves exposure to harmful ionizing radiation!

X-rays are a form of ionizing radiation, meaning they can detach electrons from atoms and molecules (ionization), disrupting molecular bonds and damaging organic material in the process. The effects can be deterministic or stochastic.

  • Deterministic effects
  • Stochastic effects
    • Ionizing radiation damages DNA and other cellular components directly or indirectly (via radical formation). Damaged cells retain their ability to divide and can transfer their genetic alterations and the risk of degeneration to daughter cells.
    • The probability of cell changes and genetic mutations occurring increases with the dose of radiation, though the severity of the negative consequences is independent of the dose.
    • Stochastic effects of exposure to ionizing radiation include radiation-induced cancer and teratogenesis.

We list the most important complications. The selection is not exhaustive.

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last updated 11/16/2020
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