Radiography

Jai Ganesh · Yesterday 18:05:19

Radiography

Gist

Radiography is a medical imaging technique that uses X-rays to create static images of the inside of the body to diagnose fractures, infections, or locate foreign objects. During the procedure, an X-ray beam is passed through the body, and the remaining radiation is captured on film or a digital detector to form an image. These images are used by doctors to visualize internal structures and aid in diagnosis and treatment planning.

It is used to diagnose or treat patients by recording images of the internal structure of the body to assess the presence or absence of disease, foreign objects, and structural damage or anomaly. During a radiographic procedure, an x-ray beam is passed through the body.

Summary

X-ray or radiography uses a very small dose of ionizing radiation to produce pictures of the body's internal structures. X-rays are the oldest and most frequently used form of medical imaging. They are often used to help diagnosed fractured bones, look for injury or infection and to locate foreign objects in soft tissue. Some x-ray exams may use an iodine-based contrast material or barium to help improve the visibility of specific organs, blood vessels, tissues or bone.

Details

Radiography is an imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical ("diagnostic" radiography and "therapeutic radiography") and industrial radiography. Similar techniques are used in airport security, (where "body scanners" generally use backscatter X-ray). To create an image in conventional radiography, a beam of X-rays is produced by an X-ray generator and it is projected towards the object. A certain amount of the X-rays or other radiation are absorbed by the object, dependent on the object's 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). The generation of flat two-dimensional images by this technique is called projectional radiography. In computed tomography (CT scanning), an X-ray source and its associated detectors rotate around the subject, which itself moves through the conical X-ray beam produced. Any given point within the subject is crossed from many directions by many different beams at different times. Information regarding the attenuation of these beams is collated and subjected to computation to generate two-dimensional images on three planes (axial, coronal, and sagittal) which can be further processed to produce a three-dimensional image.

Industrial radiography

Industrial radiography is a method of non-destructive testing where many types of manufactured components can be examined to verify the internal structure and integrity of the specimen. Industrial Radiography can be performed utilizing either X-rays or gamma rays. Both are forms of electromagnetic radiation. The difference between various forms of electromagnetic energy is related to the wavelength. X and gamma rays have the shortest wavelength and this property leads to the ability to penetrate, travel through, and exit various materials such as carbon steel and other metals. Specific methods include industrial computed tomography.

Image quality

Image quality will depend on resolution and density. Resolution is the ability of an image to show closely spaced structure in the object as separate entities in the image while density is the blackening power of the image. Sharpness of a radiographic image is strongly determined by the size of the X-ray source. This is determined by the area of the electron beam hitting the anode. A large photon source results in more blurring in the final image and is worsened by an increase in image formation distance. This blurring can be measured as a contribution to the modulation transfer function of the imaging system.

Additional Information

X-ray, electromagnetic radiation of extremely short wavelength and high frequency, with wavelengths ranging from about {10}^{-8} to {10}^{-12} metre and corresponding frequencies from about {10}^{16} to {10}^{20} hertz (Hz).

X-rays are commonly produced by accelerating (or decelerating) charged particles; examples include a beam of electrons striking a metal plate in an X-ray tube and a circulating beam of electrons in a synchrotron particle accelerator or storage ring. In addition, highly excited atoms can emit X-rays with discrete wavelengths characteristic of the energy level spacings in the atoms. The X-ray region of the electromagnetic spectrum falls far outside the range of visible wavelengths. However, the passage of X-rays through materials, including biological tissue, can be recorded with photographic films and other detectors. The analysis of X-ray images of the body is an extremely valuable medical diagnostic tool.

X-rays are a form of ionizing radiation—when interacting with matter, they are energetic enough to cause neutral atoms to eject electrons. Through this ionization process the energy of the X-rays is deposited in the matter. When passing through living tissue, X-rays can cause harmful biochemical changes in genes, chromosomes, and other cell components. The biological effects of ionizing radiation, which are complex and highly dependent on the length and intensity of exposure, are still under active study (see radiation injury). X-ray radiation therapies take advantage of these effects to combat the growth of malignant tumours.

X-rays were discovered in 1895 by German physicist Wilhelm Konrad Röntgen while investigating the effects of electron beams (then called cathode rays) in electrical discharges through low-pressure gases. Röntgen uncovered a startling effect—namely, that a screen coated with a fluorescent material placed outside a discharge tube would glow even when it was shielded from the direct visible and ultraviolet light of the gaseous discharge. He deduced that an invisible radiation from the tube passed through the air and caused the screen to fluoresce. Röntgen was able to show that the radiation responsible for the fluorescence originated from the point where the electron beam struck the glass wall of the discharge tube. Opaque objects placed between the tube and the screen proved to be transparent to the new form of radiation; Röntgen dramatically demonstrated this by producing a photographic image of the bones of the human hand. His discovery of so-called Röntgen rays was met with worldwide scientific and popular excitement, and, along with the discoveries of radioactivity (1896) and the electron (1897), it ushered in the study of the atomic world and the era of modern physics.

Fundamental characteristics:

Wave nature

X-rays are a form of electromagnetic radiation; their basic physical properties are identical to those of the more familiar components of the electromagnetic spectrum—visible light, infrared radiation, and ultraviolet radiation. As with other forms of electromagnetic radiation, X-rays can be described as coupled waves of electric and magnetic fields traveling at the speed of light (about 300,000 km, or 186,000 miles, per second). Their characteristic wavelengths and frequencies can be demonstrated and measured through the interference effects that result from the overlap of two or more waves in space. X-rays also exhibit particle-like properties; they can be described as a flow of photons carrying discrete amounts of energy and momentum. This dual nature is a property of all forms of radiation and matter and is comprehensively described by the theory of quantum mechanics.

Though it was immediately suspected, following Röntgen’s discovery, that X-rays were a form of electromagnetic radiation, this proved very difficult to establish. X-rays are distinguished by their very short wavelengths, typically 1,000 times shorter than the wavelengths of visible light. Because of this, and because of the practical difficulties of producing and detecting the new form of radiation, the nature of X-rays was only gradually unraveled in the early decades of the 20th century.

In 1906 the British physicist Charles Glover Barkla first demonstrated the wave nature of X-rays by showing that they can be “polarized” by scattering from a solid. Polarization refers to the orientation of the oscillations in a transverse wave; all electromagnetic waves are transverse oscillations of electric and magnetic fields. The very short wavelengths of X-rays, hinted at in early diffraction studies in which the rays were passed through narrow slits, was firmly established in 1912 by the pioneering work of the German physicist Max von Laue and his students Walter Friedrich and Paul Knipping. Laue suggested that the ordered arrangements of atoms in crystals could serve as natural three-dimensional diffraction gratings. Typical atomic spacings in crystals are approximately 1 angstrom (1 × {10}^{-10} metre), ideal for producing diffraction effects in electromagnetic radiation of comparable wavelength. Friedrich and Knipping verified Laue’s predictions by photographing diffraction patterns produced by the passage of X-rays through a crystal of zinc sulfide. These experiments demonstrated that X-rays have wavelengths of about 1 angstrom and confirmed that the atoms in crystals are arranged in regular structures.

In the following year, the British physicist William Lawrence Bragg devised a particularly simple model of the scattering of X-rays from the parallel layers of atoms in a crystal. The Bragg law shows how the angles at which X-rays are most efficiently diffracted from a crystal are related to the X-ray wavelength and the distance between the layers of atoms. Bragg’s physicist father, William Henry Bragg, based his design of the first X-ray spectrometer on his son’s analysis. The pair used their X-ray spectrometer in making seminal studies of both the distribution of wavelengths in X-ray beams and the crystal structures of many common solids—an achievement for which they shared the Nobel Prize for Physics in 1915.

Particle nature

In the early 1920s, experimental studies of the scattering of X-rays from solids played a key role in establishing the particle nature of electromagnetic radiation. In 1905 German physicist Albert Einstein had proposed that electromagnetic radiation is granular, consisting of quanta (later called photons) each with an energy hf, where h is Planck’s constant (about 6.6 × {10}^{-34} joule∙second) and f is the frequency of the radiation. Einstein’s hypothesis was strongly supported in subsequent studies of the photoelectric effect and by the successes of Danish physicist Niels Bohr’s model of the hydrogen atom and its characteristic emission and absorption spectra (see Bohr atomic model). Further verification came in 1922 when American physicist Arthur Compton successfully treated the scattering of X-rays from the atoms in a solid as a set of collisions between X-ray photons and the loosely bound outer electrons of the atoms.

Adapting the relation between momentum and energy for a classical electromagnetic wave to an individual photon, Compton used conservation of energy and conservation of momentum arguments to derive an expression for the wavelength shift of scattered X-rays as a function of their scattering angle. In the so-called Compton effect, a colliding photon transfers some of its energy and momentum to an electron, which recoils. The scattered photon must thus have less energy and momentum than the incoming photon, resulting in scattered X-rays of slightly lower frequency and longer wavelength. Compton’s careful measurements of this small effect, coupled with his successful theoretical treatment (independently derived by the Dutch scientist Peter Debye), provided convincing evidence for the existence of photons. The approximate wavelength range of the X-ray portion of the electromagnetic spectrum, 10−8 to 10−12 metre, corresponds to a range of photon energies from about 100 eV (electron volts) to 1 MeV (million electron volts).

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Radiography

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