What is the difference between CT and NMR*** vibration principle and what is the scope of application?

CT scanners can be used to scan the whole body, while nuclear magnetic **** vibration scanner is mainly used for the body's soft tissue scanning. Through these two instruments, the doctor can obtain a detailed three-dimensional image of the human body, clearly see the subtle changes in the human body tissues, for scientific diagnosis to provide strong evidence.CT scanner and the nuclear magnetic *** vibration scanner is very similar in appearance, they obtained three-dimensional image is also very similar, but it should be pointed out that these two instruments of the imaging principle is completely different.CT scanner principle is relatively simple. CT scanner principle is relatively simple, it is the use of different densities of human tissue on the X-ray has a different absorption rate of the principle and design. As we all know, X-rays are a kind of electromagnetic wave with short wavelength, which propagates along a straight line, and because of its high energy, it can penetrate all tissues of the human body. Since different tissues of the human body have different densities, they have different absorption rates of X-rays. If X-rays are sent through the body in parallel or at an angle, and then exposed to photographic film, the distribution of bones, ribs and soft tissues of the body can be clearly seen. This is the basic principle of X-ray fluoroscopy, which was invented in the early twentieth century and has provided an extremely important source of information for medical diagnosis. Unfortunately, X-ray is a flat image, and because of the overlapping of body tissues, which causes a superimposed effect on the absorption of X-rays, many details are not visible in the X-ray picture. In order to understand some three-dimensional details, it is necessary to perform X-ray fluoroscopy from different angles, and it is impossible to obtain a three-dimensional image of the human body. In order to obtain the details of human tissue, in order to obtain a three-dimensional image of human tissue, which can only rely on the modern CT scanner and nuclear magnetic **** vibration scanner. CT scanner was invented in 1971 by Hounsfield (Hounsfield), Hounsfield and thus won the Nobel Prize in 1979. CT scanner and X-ray fluoroscopy have a lot of the same place, but there are many different places. CT scanners and X-ray fluoroscopy have a lot in common, but they also have a lot of differences. The same is that they are both based on the basic principle that organs of different densities in human tissue have different absorption rates of X-rays. The ray source they use can be the wavefront surface X-ray surface source, can also be the wavefront is a spherical dispersion of the X-ray point source. The difference between them is that 1) the receiving device for X-ray fluoroscopy is a piece of film, whereas CT scanners use a set of garden-arc-shaped electronic receiving devices, which are generally composed of crystals separated by a collimator. This electronic receiver is located directly opposite the X-ray source. 2) X-ray fluoroscopy work its ray source and film are in a fixed position, while CT scanning in the work of not only the body scanned in the scanner's garden hole will be moved back and forth, and the X-ray source and the electronic receiver will be in the CT scanner's garden ring of high-speed rotation. These two directions of motion are monitored by sophisticated encoders on the CT scanner.3) The final difference between the two instruments is that X-ray fluoroscopy does not require computer processing, whereas the CT scanner requires a computer to perform more complex calculations and processing of the image to form a detailed three-dimensional image of the human body tissues. In order to understand the principles of the CT scanner, it is necessary to understand the absorption of X-rays. If a material has an absorption coefficient of , then the transmission of X-rays through the material after a certain distance is . When the plane of the X-film or receiver is parallel to the plane of X-ray emission, the distribution of X-rays after absorption by various parts of the body, the transmittance of X-rays at various points on the film is: (1) The product of the transmittance and the source intensity of the X-rays is the energy of the X-rays arriving at the photographic film or receiver. Assuming that the wavefront of X-rays is a plane, the original intensity of X-rays is , taking into account the background noise in the receiver is , if the absorption coefficient of the medium is discretized, for the length of each discrete point in the medium, then the radiation intensity of the corresponding point that finally falls on the receiver is: (2) Taking into account the scattering of X-rays and other factors, the formula is a simple transformation of the: (3) Note that when the X-rays for the dispersion of propagation, we must also note that the intensity of X-rays in the propagation of their own strength will continue to decay. X-rays of their own strength and X-rays propagate the distance of the square is inversely proportional to the square. From the above formula, the information generated by X-rays passing through structures with different absorption coefficients can be formed into a linear system of equations.CT scanners can also be used for isotope radiation imaging in general. When an isotope with a short half-life accumulates in a human organ, the decay of the isotope emits rays. At this time, if the body's absorption is not taken into account, the CT scanner receiver in a point so that the radiation obtained as: (4) where is the isotope of the spatial distribution function. The image obtained by the receiver is the projection of the spatial distribution function in one direction. Equations 3 and 4 above illustrate that both CT scanners and isotope imaging are typically problems of coordinate function projection. In the process of coordinate function projection, the three-dimensional image information will be compressed into a two-dimensional image, while the one-dimensional image information will be compressed into a one-dimensional image. With the limited information of an individual one- or two-dimensional image, it is not possible to recover again the two- or three-dimensional information it contains. However, if multiple exposures are made to the same 2D or 3D structure in different directions, it is possible to fully recover all the information of the original 3D structure from multiple 1D or 2D images. In fact, the CT scanner is a source of X-rays that continuously project a 2D section of the human body from different positions, which are imaged on a constantly rotating curved 1D or 2D receiver, resulting in multiple 1D or 2D images of the same section. With the information from these images, it is possible to recover the two-dimensional form of the profile to form a single image of the human body profile. The same principle is used in positron emission tomograh. In this case, radioactive substances are injected into the body, which are distributed according to the characteristics of the various organs of the body. These radioactive substances emit photons, which are concentrated to the same degree as the radioactive substances. When these photons are received, it is possible to learn about the organs of the human body based on the principle of the inverse transformation of projections. The problem of projection of coordinates and coordinate functions is a very simple one and will not be introduced here. However, the problem of inverse projection transformation of coordinates and coordinate functions is a more complex and difficult subject. The former is the basis for today's photographic precision measurements and aerial geodesy, while the latter is the basis for many medical imaging instruments. Photographic precision measurement and aerial geodesy formula is not the content of this paper, here mainly introduces the inverse projection transformation of the coordinate function. There are generally four different ways to solve this inverse transformation problem: 1) a simple inverse projection method; 2) an integral equation method; 3) a Fourier transform method; and 4) a series expansion method. For the sake of brevity, the first and third methods are mainly presented here. The third of these methods is by far the most commonly used in medical imaging. The inverse projection method is very simple and is based on the assumption that any contributing image point in the image has exactly the same contribution along the projection direction. Taking the simplest case, if there is a 2x2 planar image, and the intensities of each of their image points are: 2, 3 and 4, 5. then their projections in the x-direction are 5 and 9, and in the y-direction they are 6 and 8. In carrying out the inverse projection, the values of the projections in the x-direction are first distributed uniformly, so that the intensities of each of the image points obtained are 2.5, 2.5, and 4.5, 4.5. to which are added the values of the the contribution of the projections in the Y-direction, so that the intensity of each image point is obtained as 5.5, 6.5 and 7.5, 8.5. Since we have repeated the assignment of multiple projections, we have to subtract a value N from the intensity of each image point, which is , where is the total number of projections utilized in the inverse transformation, is the total function intensity value in each projection, is the total number of image points in the image, and is the total number of image points in the image. is the total number of image points in the image. In the example here, after subtracting this value, the intensities of all the resulting image points are 2, 3, and 4, 5, respectively, which are identical to the intensity values of the individual image points in the original image. However, the limitations of this method are obvious: 1) when the number of projections increases, each pixel on the image does not correspond well to the pixel on the projection step; 2) this assumption of equal intensity contribution gives the inverse projection method a tendency to remove the highs and add the lows, and the image obtained from the original clear form is blurred and unclear. Therefore this method has been rarely used. Now more widely used are several other methods of inverse transformation. Among them, the Fourier method is one of the most important and most widely used methods. The principle of the Fourier method is to use the frequency distribution of each projection to synthesize the frequency distribution of the original image, the specific method is like this: assuming that the original image is a two-dimensional image , the image will be projected along the direction of the projection of one-dimensional function: (5) If the coordinates rotated by an angle, the coordinates after the rotation is used to indicate that the function of the new projection is: (6) then the projection of frequency analysis, its Fourier transform is: (7) Note that the function above is a one-dimensional function, but it is also a part of a two-dimensional function. This two-dimensional function is the Fourier transform of the original image, or the frequency distribution of the original image: (8) More precisely, the Fourier transform of the one-dimensional projection function of the image along the angle is exactly the Fourier transform of the two-dimensional function of the value of the axis (which is at an angle of the X-axis of the original). This important result is known as the center profile theory (Kak and Slaney, 1988). According to this theory, a necessary and sufficient condition for solving a two-dimensional function by projections is to obtain all the projections in the range and . With these projections, the Fourier transform can be used to find all the values in the Fourier plane of the original function, but of course the density of the values obtained in the Fourier plane is very high. After the Fourier inverse transform of such a one-to-one mapping can be derived from the distribution of the original function. This conclusion can also be easily generalized to the case of three-dimensional images, where the original three-dimensional image can be derived as long as there are enough projections of the three-dimensional form. This theory is the basis for the design of CT scanners and many imaging instruments. there are many other design points in the design of CT scanners, limited to space, in this paper will not be introduced. Nuclear magnetic **** vibration scanners were invented in the 1980s. Although the shape of the MR*** vibration scanner and CT scanner and the three-dimensional image of the human body they obtain is very similar, but the basic principle of the MR*** vibration scanner and CT scanner is completely different. The main body of the NMR***Vibration scanner is a stable magnetic field, the direction of which is the same as the direction of the body's movement in the instrument. Early NM***V scanners used heavy permanent magnets to obtain this stable magnetic field, which were bulky and costly to manufacture. But permanent magnets do not use energy, so they are cheaper to run. Later this magnet was replaced by a large DC coil, which was cheaper, but it was expensive to run, required a lot of electrical energy, and the magnetic field it produced was less intense. However, these have now been replaced by superconducting coils, the use of which has the advantage that once the current has been excited in the superconducting coil, the current supply is no longer required. A typical superconducting coil configuration consists of six main coils, and two coils of somewhat larger diameter, which serve to flatten the magnetic field over the operating range, compensating for the bending of the field. Superconducting coils are generally constructed of niobium titanium alloy encased in a copper skin. The superconducting temperature of these superconductors is below 12 K. For higher current densities, the temperature has to be lower. Therefore, it is necessary to use liquid helium or nitrogen to carry on the cooling, generally the coil is immersed in liquid helium, this time the temperature is 4.3 K. In addition to the low temperature, the current inside the superconductor can not exceed a certain limit value, at the same time the value of the magnetic field on the superconductor should also be low enough. For this reason the requirements in the specific design are very high. If these design requirements are not met, a resistance is created in part of the circuit, causing a rise in temperature. This rise in temperature in turn causes the surrounding superconductors to move out of the superconducting operating range, creating more resistance and thus more heat. This process is an unstable one which causes the magnetic energy to disappear and the liquid helium to evaporate. In order to keep the liquid helium warm and to minimize heat loss, there are two radiation shields outside the liquid helium container, which are at 15 K and 60 K. These shields are supported by thin rods with low thermal conductivity, so special care is needed when transporting the liquid helium. From long term operation, there is always heat going into the liquid helium, and also superconductors are not really zero resistance, so the current in the coils is gradually reduced, which reduces the strength of the magnetic field. So at some point, the magnet must be re-excited. In specific superconducting circuits, normally the superconducting coil is a closed circuit, but when it needs to be excited, a portion of one of the circuits is heated and disconnected so that the coil is directly connected to an external power source, increasing the amount of current in the coil. This is a very slow process, due to the fact that the voltage is equal to the product of the inductance and the rate of change of current. Since the inductance of the coil is very large, it takes a long time for the current to increase at an appropriate voltage. If a niobium tin alloy (niobium tin wire) is utilized as a superconductor, it has a critical temperature of 18K, so it can be used without the use of Asia-Pacific helium. The unit of magnetic field strength is gauss , and a typical nuclear magnetic **** vibration scanner has a magnetic field strength of about one thousand to twenty thousand gauss. In addition to the main magnetic field coils, there are fill field coils in the main body of the MRI scanner to overcome the unevenness of the main magnetic field in the fringe areas and a gradient coil to create a gradient in the strength of the main magnetic field. The function of these gradient coils is described in more detail below. Generally, the strength of the gradient field is about one percent of the strength of the main magnetic field. The principle of the nuclear magnetic **** vibration scanner is more complex, and our discussion must begin with the spin of the proton in the nucleus of an atom. For example, the simplest atomic nucleus, the hydrogen nucleus, has a proton and a neutron in one ****, where the proton carries one unit of positive charge and the neutron has no charge. Because of the spin of the nucleus, a tiny magnetic field is created around it because of the charged proton. Or each atom is equivalent to a separate magnetic moment. But the energy of the magnetic field created by this magnetic moment is so small that one can hardly feel it. At the same time, because of the random nature of the direction of the spins of the individual atoms, the magnetic fields created by their individual magnetic moments cancel each other out, and the total effect is exactly zero. But because of the presence of such tiny magnetic moments, they will reflect somewhat on the magnetic field in the space adjacent to the nucleus. It is this reflection that forms the basis for the imaging of nuclear magnetic **** vibration scanners. In the absence of an external magnetic field, the tiny magnetic field of the hydrogen nucleus in the human body is randomly distributed, so there is no magnetization. But when there is a stable magnetic field outside, the tiny magnetic moments of most nuclei will be neatly aligned in the direction of the external magnetic field. For example, when the human body is in the NMR***Vibration Scanner, the tiny magnetic fields of the hydrogen nuclei in the human body will be aligned in the direction of the main magnetic field, and this is when we say that these magnetic moments are magnetized. In a nuclear magnetic **** vibration scanner, the strength of the main magnetic field is , and the direction of this field is usually noted as being the direction of the axis, while the direction of the axis is noted as pointing in a vertical upward direction. The strength of the magnetization of human tissues is usually expressed in terms of the value of the magnetization, which is usually very small and under normal circumstances cannot be measured. However, after they have been magnetized, if the direction of their magnetic moment is induced to be different from the direction of the main magnetic field, these small magnetic moments will be in a high-energy unstable state, and they will quickly release their energy and return to a low-energy stable state, and in this process, the existence of the magnetic moments may be determined. In order to determine the existence of this tiny magnetic field, there is a second external magnetic field in the plane of the nuclear magnetic *** vibration scanner . This magnetic field is created by means of one or more coils in this direction. This coil can be used both to excite these small magnetic moments and to receive the inductions due to changes in the direction of these small magnetic moments, strictly speaking the projections of the changes in nuclear magnetic direction onto the plane. In practical measurement work, the excitation of this coil takes only a very short time, about a few tens of milliseconds, per excitation. In order to excite the magnetic moment formed by a certain number of atoms, a tiny pulse of a certain frequency must be fed into the coil. The frequency of this pulse is proportional to the strength of the main magnetic field and is related to the electromagnetic properties of the nuclei to be measured. For the hydrogen nucleus, which is commonly used in human examinations, the value of this frequency is: (9) where is called the magnetic coefficient of rotation. The value of this coefficient varies for different nuclei. The frequency of change of the magnetic field must be exactly equal to the value of this frequency; if the frequencies are not equal, the direction in which the nuclear magnetic moment of such an atom points cannot be changed. At the same time the signal of this frequency must have a certain residence time, so that the direction of the magnetic moment is turned by exactly 90 degrees, or 180 degrees. If this time is longer than 180 degrees, the magnetic moment cannot continue to increase in energy, nor can the direction continue to change. In brief: In order for the tiny magnetic field in the molecular hydrogen nucleus of the human tissue to rotate in the direction of the axis, this external magnetic field must firstly appear in the plane, secondly it must rotate constantly in this plane at the frequency calculated in the above equation, and thirdly the duration of this field must be exactly equal to a certain value. Under the action of this additional magnetic field, the tiny magnetic field of the hydrogen nucleus in the human body will rotate along with the magnetic field and become a helical curve that keeps turning over, and finally turns completely to the plane and coincides with the axis. The microwave pulse required in this small coil is called a pulse. If this pulse is doubled, the tiny magnetic field in the body will continue to rotate and eventually turn in the direction of the axis. This longer pulse is called a pulse. These two types of microwave pulses are now used by MR*** vibration scanners for imaging. The tiny magnetic fields in the molecules of human tissues are excited by this additional magnetic field, which increases their energy, resulting in an unstable, high-energy state. When the magnetic field in the molecules of a human tissue is rotated in the direction of an axis or an axis, it is in an unstable state. If the additional magnetic field with a specific frequency is turned off, the magnetic field in the molecules of human tissue will slowly rotate in the direction of the main magnetic field along a spiral curve. As the magnetic field rotates, its energy decreases and additional energy is released. If the magnetic field is received by an inductive coil, a small pulse is generated in the coil at a frequency related to the strength of the magnetic field at the location of the tissue molecule. This relationship between spatial position and pulse frequency can be expressed simply as follows:(10) where is the gradient magnetic field that may be added during reception of the pulsed signal. In nuclear magnetic *** vibration the main magnetic field is in the axial direction, , and the gradient magnetic field is a tensor with 9 components, but generally only one or more of the three gradient direction components are used, i.e. . This formula and the previous formula (9) are the basis for nuclear magnetic **** vibrational imaging. In measurements, pulses emitted at various points on a surface with the same magnetic field strength all have the same frequency. So one imaging method of nuclear magnetic *** vibration is the same as the projection method described earlier. We can use different magnetic field gradients in the observation, so that the measured amount is the distribution of hydrogen nuclei in the human body in different directions of the projected value. Specifically the measured pulses are Fourier transformed, when the value of the intensity in the frequency spectrum corresponds to the sum of the energies of the nuclei spectra on all equal magnetic field intensity surfaces in different directions. In nuclear magnetic **** vibration we can also measure a specific region in the body, when we make that specific region have a specific range of magnetic field strength by adjusting the three components of the gradient field. When we introduce an excitation field, we can make the frequency width of the varying pulse that excites this field very narrow, so that only the magnetic field of the nuclei in this particular region can be excited. In this way, after the magnetic field is removed, the received pulse signal is generated only by the distribution of hydrogen nuclei in this small region. There is also a method of regional local measurement in the measurement of nuclear radiation in other regions using alternating gradient magnetic field, so that in addition to the measurement value is stable in the specified region, the intensity of the measurement value of other regions are up and down swing, so that after the pulse is received can be used to electrically compare the stability of the signal, to remove the information of the frequency is constantly changing, and to retain only the frequency of the specified region of the constant frequency. Information. In fact, modern nuclear magnetic **** vibration scanners for human body scanning is generally a two-dimensional Fourier transform method. With this method the human body can be imaged in profile quickly and with high efficiency. The two-dimensional Fourier transform method introduces a change in the temporal gradient of the magnetic field strength in addition to the spatial gradient of the magnetic field strength. The specific method is as follows: 1) the gradient of the axial magnetic field is introduced at the same time when the magnetic field is excited, so that the direction of the direction of the restriction of the range of nuclear magnetic information generated; 2) when the excited magnetic field is turned off, the gradient of the magnetic field in the direction of the time domain is introduced firstly during the first hourly period. Such a magnetic field gradient corresponds to a difference in frequency. The integration of pulses with different frequencies over time introduces a phase difference in the axial direction in the magnetic field of the nucleus, which is called phase encoding. Note that this phase encoding is repeated during the measurement, so that the phase changes are evenly distributed between degrees; 3) After this period of time, the magnetic field gradient in the direction is closed and the coil starts to receive the pulse signal, while at the same time the instrument introduces a spatial magnetic field gradient in the direction, which lasts until the time of integration. During this period, the magnetic field gradient in space introduces a frequency code in the direction. Therefore, the induced signals recorded by the NM***Vibration instrument are not only frequency coded, but also behaviorally coded. The measured pulse signal is subjected to the first Fourier transform to obtain the frequency intensity distribution at that phase encoding. As a result, the phase and frequency are encoded in both directions at the same time. Repeat steps 2) and 3) to obtain a different phase encoding of the frequency intensity distribution of the curve, and finally to obtain the distribution of values in the plane in the direction of its phase axis were carried out several times the Fourier transform, so that the nuclear magnetic **** vibration can be obtained in two dimensions to obtain a complete image of the intensity distribution. Of course as with other measurements, it is sometimes necessary to repeat the same quantity several times, averaging to minimize the contribution of error. This two-dimensional imaging method can also be generalized to the three-dimensional case, when in step 2) another gradient magnetic field should be introduced in the direction of the axis, and the value of the gradient should be changed accordingly in this direction, in order to obtain a three-dimensional frequency projection. Finally, a series of Fourier transforms are performed in the direction of the axis to obtain the three-dimensional distribution of the intensity. It should be noted that the distribution of hydrogen nuclei in various organs in the human body is different, they are distributed in the body in large quantities in the soft tissues and liquids, so compared with the CT scanner for nuclear magnetic **** vibration is more practical for imaging the soft tissues of the human body. In the human skeleton, there are basically no hydrogen nuclei, so it can not understand the details of the skeleton. NMR***Vibration is a very important measurement method, which can be used not only to measure the hydrogen nucleus, but also other nuclei such as carbon, phosphorus, sodium, potassium, etc. It can be used in medical imaging, as well as in the imaging of soft tissues in the human body. It can be used not only for medical imaging, but also for material science, geological prospecting and other fields. When used for the detection of water and oil resources, the earth's magnetic field can be used as the main magnetic field, and a large coil is used on the ground to generate an additional magnetic field. At the same time, this coil is used to detect the magnetic response of the hydrogen nuclei in the water or oil in the ground. Nuclear magnetic **** vibration is a very important high-tech, the above introduction is only the most basic principles and methods of it. At the end of this article we would also like to mention the application of CT scanning methods in geological surveying. Seismic waves have different propagation speeds and absorption characteristics in different media. When an earthquake occurs at a certain point on the earth, the projection of a certain area in the stratum in a certain direction can be obtained by measuring at different points on the ground. If many of the effects of the earthquake at various points on the surface can be obtained, data similar to the information obtained from CT scans are obtained. With these data, the density distribution and structural distribution within the formation can also be obtained by Fourier transform and inverse transform. Seismic waves consist of both longitudinal and transverse waves, of which the transverse wave is difficult to pass through the structure of liquids and gases, so the use of this method can also be used to measure the oil and gas fields as well as the investigation of groundwater.