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Introduction to Radiography, Fluoroscopy, and Tomosynthesis
Radiography
1
What is radiography?
Radiography is an imaging technique that uses x-rays to create projectional (2D) images of a region of interest in the body. It is performed by shining x-rays on a film or other image detector with a patient placed in front of it in a certain orientation. Different types of tissues in the patient attenuate x-rays to different degrees, leading to formation of a composite of x-ray shadows that will ultimately create the radiographic image. It is most commonly used to evaluate the bones and joints, the chest (especially the lungs), the abdomen and pelvis (especially the bowel), and the breasts (in which case it is called mammography).
2
When were x-rays discovered?
Wilhelm Conrad Roentgen is credited with the discovery of x-rays in 1895, and was the first to systematically study them. He was also the first to obtain an x-ray photograph of part of the human body, his wife's hand, discovering its potential medical use. In 1901, he received the first Nobel Prize in physics. X-rays are sometimes referred to as Roentgen rays.
3
How do x-rays differ from other types of electromagnetic radiation?
X-rays have higher energies, higher frequencies, and shorter wavelengths in the electromagnetic spectrum than ultraviolet light, visible light, infrared light, microwaves, and radio waves, and lower energies, lower frequencies, and longer wavelengths than gamma rays. Diagnostic x-rays typically have energies between 20 and 150 keV.
4
How do x-rays interact with matter?
X-rays may either pass through matter unaffected, may be absorbed, or may be scattered (the latter of which leads to decreased image quality). Key factors that influence which interactions occur include the incident x-ray photon energy and the physical density, thickness, and atomic number of the material being imaged.
5
What is an x-ray tube?
An x-ray tube is a device that is used to create x-rays for diagnostic imaging. It contains a negatively charged cathode that contains a filament (usually made of coiled tungsten wire) to produce electrons. The electrons are accelerated toward a positively charged anode that contains a target (usually made of tungsten or made of molybdenum and rhodium in mammography) where x-rays are produced. Both the cathode and anode are housed within a vacuum tube.
6
How are x-rays for diagnostic imaging produced?
In the x-ray tube, about 90% of x-rays are created when electrons emitted from the cathode pass close to positively charged atomic nuclei within the anode target and change direction, resulting in a loss of energy in the form of x-ray photons known as bremsstrahlung (“braking”) x-rays. An additional 10% of x-rays are produced when inner shell atomic electrons of the anode target are ejected by incident electrons, followed by relaxation of outer shell atomic electrons to fill the inner shell vacancies, leading to emission of energy in the form of x-ray photons known as characteristic x-rays.
7
What is a focal spot?
A focal spot is the apparent source of x-rays in an x-ray tube. Smaller focal spots (such as those used in mammography) produce sharper images, whereas larger focal spots (such as those used in fluoroscopy) tolerate greater amounts of heat.
8
What is a collimator?
A collimator is a device used to narrow the x-ray beam as it leaves the x-ray tube prior to entering the patient, reducing x-ray scatter and improving image contrast.
9
What happens to most of the energy entering the x-ray tube?
Most (99%) of the energy is converted to heat, whereas about 1% is converted to x-rays.
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