Today, X-ray imaging is an important and indispensable part of medical device diagnostics. As the first imaging technique, X-ray diagnostics revolutionized the possibilities of medicine and paved the way for modern procedures such as computed tomography (CT), magnetic resonance imaging (also called MRI, NMR, or magnetic resonance imaging), and today’s radiation therapy in cancer treatment. The discovery of X-rays on November 8, 1895 at the University of Würzburg can be traced back to the German physicist Wilhelm Conrad Röntgen, who was awarded the Nobel Prize in Physics for this discovery in 1901. In the following years, the X-ray method was already being used for skeletal diagnostics. The discovery and documentation of radiation-induced damage to human tissue opened up the possibility of treating malignant tumors. Technological development today is at the level of digital X-ray diagnostics, which enables rapid and efficient evaluation or reporting of images.
The procedure
Generation of X-raysX-rays are electromagnetic waves that lie between UV light and gamma radiation in the electromagnetic spectrum. They are generated with the help of an X-ray tube, which has a special structure: two electrodes (cathode – tungsten wire; and anode) are located in a glass cylinder in which there is a vacuum. In order to generate X-rays, the tungsten wire is now made to glow, so that electrons are released from the material, which are then accelerated towards the anode. When the electrons hit the anode, energy is released, one percent of which is converted into X-rays. The rest of the energy is lost as heat. The place (anode) where the electrons from the cathode hit is called the focal spot. The resulting X-rays consist of two distinct components:
- Bremsstrahlung – This X-ray radiation is produced when the electrons decelerate and consists of a continuous energy spectrum whose low-energy radiation is strongly absorbed by tissue, so there is radiation exposure here. For this reason, the radiation must be removed by a filter required by law.
- Characteristic radiation – This radiation forms a line spectrum and superimposed on the Bremsstrahlung.
Depending on the voltage applied to the X-ray tube, different radiation quality is produced, which is expressed in electron volts. Soft radiation has a strength of less than 100 keV (kilo-electron volts) and produces soft beam images that can show the finest tissue differences, but also result in high radiation exposure. Hard radiation has a strength of 100 keV to 1 MeV (mega-electron volts) and produces hard-beam images whose contrast is lower than soft-beam images, as is the radiation exposure. Formation of X-ray imagesThe X-rays produced propagate divergently (away from the center) from the focal spot of the anode and strike the patient’s body. After passing through the tissue, the rays strike the X-ray film. The X-ray film is coated with light-sensitive silver bromide crystals and housed in a cassette. So-called film-foil combinations are used: The films (intensifying screens) consist of phosphors that fluoresce on contact with the X-rays and cause 95% of the X-ray film blackening, while the X-rays themselves cause only 5% of the film blackening. The intensifying screens are glued to the back and front of the cassette and, depending on the sensitivity class, determine the necessary radiation dose for a sharp image. Criteria that determine the quality of an X-ray image are as follows:
- Contrast – Contrast is degraded primarily by scattered radiation: this occurs as the radiation passes through the tissue and can be mitigated by a scattered radiation grid.
- Blur – motion blur, geometric blur, film-foil blur.
Diagnostic radiologyDiagnostic radiology is a collective name for imaging procedures that use X-rays to produce a representation of changes inside the human body.Important procedures in diagnostic radiology are:
- Conventional X-ray diagnostics (projection radiology).
- Computed tomography (CT)*
- Angiography
* Computed tomography is described in a separate chapter.The following chapter mainly presents methods of conventional radiography. Native radiographs are assessed according to different criteria. The person making the assessment views the X-ray image as if it were a patient facing him or her, which means that the left and right sides are reversed. Complex anatomical conditions require an image in at least two planes. This means that the body is x-rayed from different angles. Since an X-ray image is the negative of the actual tissue, white structures are referred to as shading and black structures as brightening. Pathological changes often present themselves as only a small nuance of a different type of shadowing or brightening. The denser a tissue, the stronger the absorption of X-rays and the brighter the area on the X-ray image. For orientation, four density groups are distinguished:
- Bone – Low image blackening (very bright on the X-ray image), which is due to the strong absorption of X-rays.
- Water – Allows delineation of gaseous and fatty structures and may also appear pathologically in body cavities such as ascites (abdominal fluid).
- Fat – High image blackening (dark on the X-ray) caused by the low absorption of X-rays. Especially in the mamma (female breast) fat tissue is clearly visible in the X-ray image.
- Air – Very high image blackening (almost completely black), which is due to the almost non-existent absorption of X-rays. Physiologically, air is particularly well visible in the intestine and lungs in the X-ray image.
A dynamic version of X-ray diagnostics is the so-called fluoroscopy. Here, the region to be examined is displayed on a monitor in real time. The images are individually adjusted and thus allow viewing from different angles. In addition, moving structures, such as contractions of the heart, can be better observed. Fluoroscopy is particularly useful for contrast examinations. Fluoroscopy is performed for:
- Localization of unclear findings
- Setting of target images
- Functional shots such as in a gastrointestinal passage.
- Radiographic control during the placement of catheters, probes and guide wires.
- Targeted puncture for histological extraction of material (histology – the study of tissues).
- Assessment of the contrast medium flow in hollow organs or vessels.
- Reduction of fracture fragments (bone parts that are misplaced after a fracture and need to be repositioned)
During a fluoroscopic examination, the patient is on a table, usually tilting, under which the X-ray tube is located. In front of or above the patient are detectors that collect the incoming X-rays after traveling through the body and translate them into electrical pulses. The detectors can be moved by the radiologist (specialist in diagnostic imaging) in all three spatial axes, so that a variety of imaging directions are possible. In addition, the table can be tilted from the standing position to the horizontal position or even beyond, so that a head-down position is created. X-ray examination with contrast mediumContrast media are used to increase the density differences so that the organ to be depicted can be optimally distinguished from its surroundings. Since contrast media can cause potentially severe intolerances, the patient must be informed beforehand.X-ray contrast media are used in:
- Bronchography
- Vascular imaging
- Imaging of the bile ducts, e.g., during ERCP (endoscopic retrograde cholangiopancreatography).
- Representation of the gastrointestinal tract.
- Myelography
X-ray positive contrast agents absorb X-rays more intensely, thus enhancing contrast. An example of this is barium sulfate, which is used, for example, in gastrointestinal passage. Iodine compounds such as triiodobenzoic acid are also used. X-ray negative contrast media reduce the absorption of X-rays by the tissue. These are usually gases such as air or carbon dioxide. As already mentioned, undesirable effects are not negligible.First and foremost, intolerance reactions occur in the form of an anaphylactic (allergic) reaction, which require immediate interruption of the contrast medium administration. Impairment of kidney function up to acute renal insufficiency (kidney weakness) as well as an influence on thyroid function by an iodine-containing contrast medium are possible. Special examination variants of X-ray technology (conventional X-ray diagnostics) are presented subsequently in the separate subchapters:
- Abdominal empty image (native image of the abdomen, ie, without contrast medium) or abdominal overview (X-ray image of the abdomen while standing, lying or in the left lateral position).
- Angiography
- Arthrography
- Bronchography
- Small intestine imaging according to Sellink
- ERCP
- Colonic contrast enema
- Myelography
- Gastrointestinal passage
- Mammography
- Esophageal swallow
- X-ray thorax
- X-ray abdomen or abdomen empty image / abdomen overview.
- X-ray of bones and joints
- I. v. pyelogram
- Phlebography
