The scanning part of the CT machine mainly consists of an X-ray tube and a different number of control detectors, which are used to collect the information.The X-ray beam scans the selected level, and its intensity is absorbed and attenuated accordingly due to the interaction with different densities of tissues. The detector converts the collected X-ray signals to electrical signals, which are converted to digital signals by an analog-to-digital converter (A/D converter) and fed into a computer to be stored and processed, thus obtaining the CT number of each unit of volume at that level and arranging it into a digital matrix (Figure 7-2). These figures can be stored in the hard disk (Hard disk), floppy disk (Floppy) and magnetic tape (Magnetic tape, MT), can also be printed with a printer. The digital matrix is converted to an image on a monitor by a digital/analog (D/A) converter, which is a cross-sectional image of the layer. The image can be captured on film by multiple cameras for reading, archiving, and consultation.
Magnetic **** vibration principle
Solid in the constant magnetic field and high-frequency alternating electromagnetic field **** the same action, in the vicinity of a frequency of high-frequency electromagnetic field **** vibration absorption phenomenon. In the constant external magnetic field under the action of solid magnetization, solid in the metamagnetic moment are to be around the external magnetic field into action. Due to the presence of damping, this in-motion is quickly attenuated. But if a high-frequency electromagnetic field perpendicular to the direction of the external magnetic field, when its frequency and the frequency of the same into the action, it will be absorbed from the alternating electromagnetic field in order to maintain its into the action of the solid on the incident high-frequency electromagnetic field energy in the frequency of the above to produce a * * * vibration absorption peak. If the magnetic *** vibration of the magnetic moment is a paramagnetic body in the atomic (or ion) magnetic moment, it is called paramagnetic *** vibration; if the magnetic moment is the atomic nucleus of the spin magnetic moment, it is called nuclear magnetic *** vibration. If the magnetic moment is the electronic spin magnetic moment in a ferromagnet, it is called a ferromagnetic *** vibration. The nuclear magnetic moment is about three orders of magnitude smaller than the electronic magnetic moment, so the frequency and sensitivity of the nuclear magnetic **** vibration is much lower than the paramagnetic **** vibration; similarly, the magnetic **** vibration of weak magnetic material sensitivity is lower than the strong magnetic material. From the viewpoint of quantum mechanics, the magnetic moment of the electron and the nucleus under the action of the external magnetic field is spatially quantized, and accordingly has a discrete energy level. When the applied high-frequency electromagnetic field energy hv is equal to the spacing between the energy levels, the electron or atomic nucleus from the high-frequency electromagnetic field absorbs energy, so that it from the low energy level jump to the high energy level, thus forming an absorption peak at the *** vibration frequency.
The use of paramagnetic *** vibration can study the molecular structure and the electronic structure of defects in crystals and so on. Nuclear magnetic **** vibration spectrum is not only related to the chemical elements of the material, but also affected by the chemical environment around the atom, so nuclear magnetic **** vibration has become an important means to study the structure of solids, chemical bonding and phase transition processes. Nuclear magnetic **** vibration imaging technology with ultrasound and X-ray imaging technology has been commonly used in medical examinations. Ferromagnetic *** vibration is an important method to study dynamic processes in ferromagnets and to measure magnetic coefficients.