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Individual differences |
Methods | Statistics | Clinical | Educational | Industrial | Professional items | World psychology |
Biological: Behavioural genetics · Evolutionary psychology · Neuroanatomy · Neurochemistry · Neuroendocrinology · Neuroscience · Psychoneuroimmunology · Physiological Psychology · Psychopharmacology (Index, Outline)
X-rays are a form of electromagnetic radiation with a wavelength in the range of 10 to 0.01 nanometres, corresponding to frequencies in the range 30 to 30 000 PHz (1015 hertz). X-rays are primarily used for diagnostic radiography and crystallography. X-rays are a form of ionizing radiation and as such can be dangerous. In many languages it is called Röntgen radiation after the investigator of the radiation, Wilhelm Röntgen.
X-rays are a type of electromagnetics radiation with wavelengths of around 10-10 meters. When medical X-rays are being produced, a thin metallic sheet is placed between the emitter and the target, effectively filtering out the lower energy (soft) X-rays. The resultant X-ray is said to be hard. Soft X-rays overlap the range of extreme ultraviolet. The frequency of hard X-rays is higher than that of soft X-rays, and the wavelength is shorter. Hard X-rays overlap the range of "long"-wavelength (lower energy) gamma rays, however the distinction between the two terms depends on the source of the radiation, not its wavelength; X-ray photons are generated by energetic electron processes, gamma rays by transitions within atomic nuclei.
The basic production of X-rays is by accelerating electrons in order to collide with a metal target (usually tungsten, but sometimes molybdenum). Here the electrons suddenly decelerate upon colliding with the metal target and if enough energy is contained within the electron it is able to knock out an electron from the inner shell of the metal atom and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted. This causes the spectral line part of the wavelength distribution. There is also a continuum bremsstrahlung component given off by the electrons as they are scattered by the strong electric field near the high Z (proton number) nuclei.
The X-ray photographic plate or film is used in hospitals to produce images of the internal organs and bones of a patient. The part of the patient to be X-rayed is placed between the X-ray source and the photographic receptor to produce what is a shadow of all the internal structure of that particular part of the body being X-rayed. The X-rays are blocked by dense tissues such as bone and pass through soft tissues. Those areas where the X-rays strike the photographic receptor turn black when it is developed. So where the X-rays pass through "soft" parts of the body such as organs, muscle, and skin, the plate or film turns black. Contrast compounds containing barium or iodine, which are radiopaque, can be injected in the artery of a particular organ, or given intravenously. The contrast compounds essentially block the X-rays and hence the circulation of the organ can be more readily seen.
For some procedures, the contrast can have a syrupy consistency, which can be thinned by warming, and is introduced with a power injector, such as the Nemoto Injector.
Photostimulable Phosphors (PSPs)Edit
An increasingly common method of detecting X-rays is the use of Photostimulable Luminescence (PSL), pioneered by Fuji in the 1980's. In modern hospitals a PSP plate is used in place of the photographic plate. After the plate is X-rayed, excited electrons in the phosphor material remain 'trapped' in 'colour centres' in the crystal lattice until stimulated by a laser beam passed over the plate surface. The light given off during laser stimulation is collected by a photomultiplier tube and the resulting signal is converted into a digital image by computer technology, which gives this process its common name, computed radiography. The PSP plate can be used over and over again.
Initially, most common detection methods were based on the ionisation of gases, as in the Geiger-Müller counter: a sealed volume, usually a cylinder, with a polymer or thin metal window contains a gas, and a wire, and a high voltage is applied between the cylinder (cathode) and the wire (anode). When an X-ray photon enters the cylinder, it ionises the gas. These ions accelerate toward the anode, in the process causing further ionisation along their trajectory. This process, known as an avalanche, is detected as a sudden flow of current, called a "count" or "event".
Ultimately, the electrons form a virtual cathode around the anode wire drastically reducing the electric field in the outer portions of the tube. This halts the collisional ionizations and limits further growth of avalanches. As a result, all "counts" on a Geiger counter are the same size and it can give no indication as to the particle energy of the radiation, unlike the proportional counter. The intensity of the radiation is measurable by the Geiger counter as the counting-rate of the system.
In order to gain energy spectrum information a diffracting crystal may be used to first separate the different photons, the method is called wavelength dispersive X-ray spectroscopy (WDX or WDS). Position-sensitive detectors are often used in conjunction with dispersive elements. Other detection equipment may be used which are inherently energy-resolving, such 