What is Nuclear Imaging?

Nuclear image technique is a new technique that combines the science of nuclear technology with modern image theory. Including X-ray tomography (XCT), positron tomography (PET), nuclear magnetic resonance computer tomography (NMRCT), single photon emission tomography (SPECT), and Compton scattering tomography (CST) )Wait.

Nuclear image technique is a new technique that combines the science of nuclear technology with modern image theory. Including X-ray tomography (XCT), positron tomography (PET), nuclear magnetic resonance computer tomography (NMRCT), single photon emission tomography (SPECT), and Compton scattering tomography (CST) )Wait.

Chinese name

Nuclear imaging

Foreign name

nuclear image

Principles of nuclear imaging

The common principle of various nuclear imaging technologies is to use the attenuation law or distribution characteristics of physical quantities related to nuclear radiation in the measured object to obtain detailed information inside the research object, and then use a computer to process this information at high speed, and finally reconstruct the Study the internal image of the object. The information acquisition parts of various nuclear imaging technologies, including the physical principles and specific measurement methods that they rely on, can be different, but their data processing parts are based on computer information processing and image reconstruction techniques. [1]?

XCT. The principle is based on the X-ray absorption characteristics of the object. When an X-ray beam with intensity I0 passes through a uniform object with density and thickness D, its intensity is weakened to I, then

I = I0eD

Where is the mass absorption coefficient of the object to X-rays. If it is a non-uniform object, the above formula becomes

I = I0e (x) (x) dx

When an object is rotated 360 ° around a device composed of an X-ray source and a detector, and the transmission intensity of a certain fault plane of the object is measured at a certain angle, the value of ln (I / I0) can be obtained, which is called the The density projection of the angle. In this way, a density projection matrix composed of all angles can be obtained. Solving this matrix can give the density of each point, so as to determine the specific positions and sizes of different components and defects in the fault, and finally draw the fault of the object under study. image. XCT generally consists of a ray source (X-ray tube for medical CT, electron accelerator for industrial CT or high-intensity isotope source), detector, collimator, scanning device, electronics system and computer. [1]?

PET. The principle is based on the annihilation effect of positrons and electrons. When a positron and an electron meet, the masses of the two annihilate and produce a pair of -rays with energy of 511KeV in opposite directions. Together with conforming measurement technology, a positron annihilation event can be detected. The main part of the PET device is a regular hexagonal detection ring. The detector is mainly made of bismuth germanate (BGO) crystals. It is not easy to deliquesce and has a high density. The linear attenuation coefficient of gamma rays to 511KeV is 0.92 cm-1, so the detection efficiency is high. The disadvantage is that the light emission attenuation constant is large, which leads to poor time resolution and also affects the spatial resolution of the imaging. For this reason, a cesium iodide crystal can be selected. Its luminescence time is very short, only 2.5 nanoseconds (300 nanoseconds for BGO), which is enough to distinguish the time difference between the two annihilated gamma rays reaching the corresponding two detectors, thereby greatly improving The spatial resolution makes the image very clear. [1]?

NMR-CT. Its principle is based on the determination of NMR electromagnetic information of non-zero spin atomic nuclei. The hydrogen nuclei present in the human body are the simplest non-zero spin nuclei, so NMRCT is extremely useful in medical diagnosis. It uses a static magnetic field and a gradient magnetic field with the same magnetic field strength to determine the spatial position of the nuclear magnetic signal generation point. It is mainly composed of a main magnet that generates a static magnetic field, a gradient coil, a radio frequency coil, a nuclear magnetic resonance spectrometer, and an image reconstruction and display system. NMR-CT imaging is fast, which can realize three-dimensional space imaging, and can also be used to make two-dimensional tomographic images from different angles. In addition to giving information on non-zero spin nucleus density, it can also provide information about the surrounding media of non-zero spin nucleus, showing information such as biochemical functions and metabolic processes. With the increase of the magnetic field strength, in addition to 1H, it is also possible to measure 13C, 15O, 19F, 23Na and 31P nuclides, thus providing more information. NMR-CT has good resolution for soft tissues, and is especially suitable for the diagnosis of brain tissue (see Nuclear Medicine). [1]?

SPECT. There are two types, one is a multi-probe ring type, similar to PET and XCT. The other type is a gamma camera type, which uses a general-purpose gamma camera to rotate around the human body to sample the whole body or a limited part of the body to achieve tomographic imaging. The commonly used radionuclide is 99mTc, which enters the human body in a specific chemical form and accumulates in the organs or tissues to be studied, and then uses a rotating gamma camera to measure the intensity of the gamma rays emitted by the 99mTc, which is transformed into an image by mathematical methods. SPECT has been widely used in clinical diagnosis due to its low price and convenient use. [1]?

CST. It is a method of displaying tomographic images by using the Compton scattering effect (see Compton effect). It is based on the Compton scattering of incident gamma photons and electrons. The intensity of the scattered photons at a given scattering angle depends on the probability of the Compton scattering. When the incident photon energy and the scattering angle are determined, the intensity of the scattered photon is only related to the electron density at the place where the scattering occurs, from which the mass density and distribution of the object under investigation can be inferred. The specific method is to collimate the incident photon beam and the scattered photon beam, and the intersection of the two is the "focusing element" in the object under study, so that the incident and scattering parts move up and down or left and right along the object, so that the entire object can be obtained The density information was finally processed by computer to draw the tomographic image. Since the CST device does not need to rotate the measured object 360 °, it is suitable for the detection of large objects, such as airport runways, house components, large concrete parts, etc. [1]?

Nuclear imaging application editing

Nuclear imaging technology is widely used, especially for medical diagnosis and non-destructive testing of industrial products. In medical diagnosis, XCT, PET and NMR-CT methods are mainly used. Industrial applications have developed rapidly, and the imaging methods used are mainly XCT and CST, referred to as industrial CT. Due to its fast detection speed, non-destructiveness, high spatial resolution and density resolution, it cannot be replaced by conventional X-ray inspection technology and ultrasound technology in many occasions. For example, XCT is used to measure 0.25 mm pores and inclusions in gas turbine blades, overall inspection of rockets up to 2 meters in diameter, non-destructive testing of nuclear fuel element rods, determination of gunpowder density in artillery shells, online real-time detection on industrial production lines and participation in automatic Control process, etc. ^[1] .