In conventional CT, the only way to evaluate a substance is the density of the substance, expressed by the CT value. It is known that two different substances, such as a bottle of salt water and a bottle of sugar water, can be measured with exactly the same CT value, which cannot show the difference in their composition. In conventional examinations, the only way to distinguish different substances with the same CT value is the expensive MRI. When passing through a specific object, its CT value changes regularly as the energy of the X-ray beam increases or decreases, forming a characteristic spectral line, which is only related to the nature of the substance itself and often differs from substance to substance. Based on this discovery, as long as the energy spectrum curve of a substance of interest can be plotted, and then compared with the energy spectrum curve of a known substance, the composition of the substance can be understood. In other words, energy spectrum CT goes beyond the limitation of describing a substance by its CT value alone, and begins to detect its exact composition and improve the level of differential diagnosis, so some people also call it chemical CT and pathological CT. The Discovery CT 750 HD innovated the core component, the detector, using garnet as the base material, and developed a gemstone detector, which has improved the key indicators such as response speed and afterglow effect by 4 times or even thousands of times, hence the name Gemstone CT, and its imaging name GSI ( Gemstone Spectral Imaging). The key to drawing the energy spectrum curve is to have a single energy X-ray to detect the object. In practice, X-rays are mixed energy images composed of photons of different energies, and it is impossible to pursue single energy X-rays. The realization of the energy spectrum CT imaging of gemstone CT is first based on a solid physical theoretical foundation. ct is imaged by measuring the absorption of x-rays in objects, and the absorption of substances varies with the energy of x-rays, such as soft tissues and blood, which do not vary much with energy; on the contrary, high atomic weight substances, such as bone and contrast agents used in CT (mainly iodine), vary more strong. Secondly, any substance has a corresponding characteristic absorption curve, and this absorption curve can be fully expressed by two energy points. Therefore, when one images the same object with two different energies of X-rays, it is possible to determine an absorption curve and thus find the substance corresponding to this absorption curve. It is this different variation with energy that allows one to distinguish different substances by the energy CT imaging method. Gem CT has an instantaneous high and low voltage switching dual energy bulb, which allows two sets of different energy X-rays to penetrate the body within 0.5 ms by instantaneous switching of the high voltage generator, obtaining two sets of data, and then through complex mathematical operations, it is possible to obtain 101 sets of single energy images at different X-ray energies and calculate the energy spectrum curve of the scanned object, which cleverly solves this problem. kVp: kilovolt peak kilovolt peak (unit of output capacity of X-ray equipment); KeV: 1000 electron volts, is the energy required to accelerate the electrons through the 1000 v voltage difference. Physical experiments have shown that the absorption of X-rays by any one substance can be expressed by the absorption of any two other substances (base substance pairs), just as any point on a map can be expressed on an X-Y coordinate. Because the X-ray absorption coefficient of any substance can be determined by the X-ray absorption coefficients of any two base substances, the attenuation of one substance can be converted into the density of two substances that produce the same attenuation, thus enabling the separation of substance composition analysis and substance separation. Water and iodine are usually chosen as the base substance pair because they are closer to the common soft tissue and iodine contrast agents in medical imaging, which will facilitate analysis and understanding. Of course, one can choose any substance pair as the base material pair, and in fact for some specific clinical applications, one would like to use a base material pair different from water and iodine to reflect the tissue composition of the unknown object in a more intuitive and quantitative way. Advantages of Gemini Spectral CT: 1. We know that in order to separate the lesion from the surrounding tissue, the effect of different X-ray energy is different, for example, mammography can only use the low energy X-ray generated by molybdenum target, but not the ordinary energy. Now we can get 101 groups of different energy mono-energy images in one scan, and we can choose the best observation image for a specific lesion at will, which not only can observe the details of the lesion more clearly, but also can find the lesion that cannot be detected by conventional examination. Preliminary studies have shown very satisfactory results in improving the detection rate of low-contrast parenchymal organs such as liver and pancreas. 2.Remove sclerosis and metal artifacts. The above artifacts have been insurmountable since the birth of CT, which are essentially related to the mixed-energy X-rays, and when mono-energy images are obtained, this problem can be largely solved. The greatest clinical benefit will be the review of various types of metal placements in orthopedics. Previously, CT artifacts were obvious and affected observation, X-ray images overlapped and details were limited, MRI must ensure that metal objects are not magnetic, and gemstone CT can give good play to its advantages of thin layer and reconstruction after removing this interference.