What is the least penetrating radiation

 Question:
How is an X-ray created?


Short answer

The different attenuation of the X-rays in the body is determined by the type of penetrating tissue (composition of atoms, density, thickness) and by the energy of the X-rays (tube voltage, filtering). This creates an image with different transmission properties, the X-ray image. Dense (e.g. bone) and thick tissue weaken X-rays more than loose (e.g. lungs) and thin tissue. As a result, bones and metals are shown as light objects and lungs as dark objects on the X-ray image.

illustration





X-ray image of a broken forearm with an intramedullary nail (metal) in the ulna. The bones and their overlays (at the elbow) are clearly visible.Chest x-ray. The lung tissue is clearly different from the soft tissue and bone structures.

Explanation

The energy distribution of the radiation generated in an X-ray tube is initially changed by the filtering, in particular by reducing the low-energy components. As a result, less radiation is absorbed, especially in the superficial tissue in the body, and protection is thereby achieved. Due to the low energy, this radiation could not penetrate the body anyway and thus also contribute nothing to the imaging. As the X-rays pass through the patient's tissue, they are weakened as a result of absorption and scattering. These interactions take place between an X-ray beam and individual atoms. The effects responsible for this are called classic scattering (scattering), photo effect (absorption) and Compton effect (absorption and scattering). When scattered, the X-ray beam changes its original direction and thus causes problems for radiation protection and image quality.

The number and type of interaction effects depends on the type of atoms in the body, on the density and thickness of the matter. The weakening of the X-rays in the body is thus influenced by the type of tissue (soft tissue, lungs, bones, etc.), its density and thickness. The intensity of the weakening also depends on the energy of the X-rays. With lower energies, the transmission differences with denser tissue (such as bones) are greater compared to higher energies. This increases the contrast of an X-ray image. On the other hand, high-energy X-rays penetrate the lung tissue in a more or less undifferentiated manner, and lung structures can hardly be seen on the X-ray image.

The transmission image represents the examination volume as a summation image; the three-dimensional structures are superimposed to form a two-dimensional image. For the exact determination of the size and the localization of an object, two recordings perpendicular to one another are therefore generally required or a computer tomography in which a sectional image is generated.

In conventional X-ray diagnostics, the transmission image is displayed with the help of radiation-sensitive detectors, an imaging system. The most common is still the X-ray film (analog imaging) in a cassette between two luminescent intensifying screens, which convert the X-rays into visible blue or green light. The so-called X-ray film is not particularly sensitive to X-rays, but - like the film in conventional cameras - to visible light. 95 - 99% of the film blackening is generated by this converted light and only 1 - 5% directly by the X-rays. Intensifying screens and film must be matched to one another with regard to optimal spectral sensitivity. Another imaging system consists of solid-state plates (storage films), which store the incident radiation energy pixel by pixel (similar to thermoluminescence dosimeters). The stored information is then released by means of laser excitation and further processed in digital form to create the X-ray image. Other image systems also measure the radiation intensity pixel by pixel directly in digital form, for example in flat detectors (chip inside the detector with several million photodiodes working in parallel that convert X-ray quanta into electrical signals). In terms of geometric resolution, the film is unsurpassed due to the small, light-sensitive crystal grains. Digital imaging has the advantage that the image information can be processed and optimized with the computer, e.g. changes in brightness and contrast, edge enhancement, enlargements, etc. These images can be saved and sent electronically.

If movement sequences - e.g. swallowing or bowel movements - or the introduction of implants in the body are to be made visible, the fluoroscopic examination is the method of choice. The transmission image impinging on the so-called image intensifier tube is electronically amplified by converting the X-rays into fluorescence rays, then into electrons and finally into visible light again. The small, high-intensity image is generally shown enlarged directly on a screen. The light intensity controls the current in the X-ray tube through feedback, so that the dose rate is continuously optimized. Today, much smaller solid-state detectors (e.g. amorphous silicon) are used instead of the image intensifier tubes.

Jakob Roth, Basel, October 2005