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What is the difference between CT, NMR, and B-ultrasound, and what is the working principle?

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Update time : 2023-08-09 10:24:00
To put it simply, CT, MRI, and B-ultrasound are all equipment for diagnosing diseases by seeing through the internal structures and organs of the human body and comparing them with the normal physiological state. The more sophisticated it is, the more important it is to human health and disease.
These types of devices are the results of human discovery and research using modern physics, using certain media to pass through the human body, obtain internal images of the human body, and analyze and diagnose health status through images. The biggest difference between them is that the media used to pass through the human body are different, and the parts and effects of diagnosis are also different.
Now let's take a look at the basic situation of the utilization of these three devices:
CT is the abbreviation of Computed Tomography in English, and its translation means computerized tomography. The main inspection medium used is X-ray, through the penetrating scanning of the body by X-ray, and the image of the scanned part is obtained through a highly sensitive detector, which has the characteristics of fast scanning and clear image.
X-rays are high-energy electromagnetic waves and also a band of light, so they are also called X-rays. Visible light that our human eyes can see has a wavelength of about 380~780nm (nanometers). One hundred thousand times, so it will cause damage to human cells and DNA.
X-ray penetrating power is very strong. When penetrating the human body, it will form different absorption rates according to different tissues and different densities of the human body, leaving black and white images with different grayscales on the photosensitive film. Doctors can observe and analyze these images to get the human body The state of internal tissues, thereby making a diagnosis of the disease.
CT is developed on the basis of X-ray perspective. CT uses a rotating device to perform tomographic scanning of the human body like cutting carrot slices. The highly sensitive detector receives the penetrating rays through the rotating device and will obtain The data is input to the computer, and the image is reconstructed after being decoded by the computer.
 
The main structure of CT equipment has three parts, namely: the scanning part, which is composed of X-ray tubes, detectors and scanning frames; the computer system, which stores and calculates the information and data collected by scanning; the image display and storage system, which will The processed and reconstructed images are displayed on the TV screen or taken by multiple cameras or laser cameras for observation by doctors.
 
The current CT equipment has been updated from the first generation to the fifth generation. From the beginning, the scanning area is small, the scanning time is long (several seconds), the detectors are few (only one or two), and the resolution is very low. Up to now, the scanning area has expanded a lot, the spatial resolution can reach 0.4mm (millimeter), the scanning time is shortened to 40ms (milliseconds), and it only takes 330ms to scan 64 layers of images.
The scanning method can only be translated from the beginning, and now it can do plain scan, enhanced scan and contrast scan, and can also realize three-dimensional dynamic images.
Magnetic resonance imaging is also called spin imaging, also known as magnetic resonance imaging, which is the translation of English Magnetic Resonance Imaging, referred to as MRI. MRI is from the atomic level, through the external gradient magnetic field to detect its physical changes, and draw the image of the internal structure of the object. It is a very complicated process, and it is not so deep as a popular science.
For the sake of easy understanding, some imaging principles are quoted as follows:
 
Nuclei have spin and angular momentum. Since the nuclei are charged, their spin creates a magnetic moment. When the atomic nucleus is placed in the static magnetic field, the bipolar magnet, which is originally randomly oriented, is subjected to the force of the magnetic field and has the same orientation as the magnetic field. Take the proton, the main isotope of hydrogen, as an example. It can only have two basic states: orientations "parallel" and "antiparallel", which correspond to low-energy and high-energy states, respectively. Precise analysis proves that the spins do not perfectly align with the magnetic field, but are tilted by an angle θ. In this way, the dipole magnet begins to precess around the magnetic field. The frequency of precession depends on the strength of the magnetic field. Also depends on the type of nucleus. The relationship between them satisfies the Larmor relationship: ω0=γB0, that is, the precession angular frequency ω0 is the product of the magnetic field intensity B0 and the magnetic rotation ratio γ. γ is a fundamental physical constant of each nuclide. The main isotope of hydrogen, the proton, is most abundant in the human body and its magnetic moment is easily detectable, making it the most suitable for obtaining MRI images from it.
From a macro point of view, in the set of precessing magnetic moments, the phase is random. Their combined orientation forms a macroscopic magnetization, represented by the magnetic moment M. It is this macroscopic magnetic moment that generates the NMR signal in the receiver coil. Of the large number of hydrogen nuclei, slightly more than half are in the lower state. It can be shown that there is a dynamic equilibrium between nucleons in two fundamental energy states, and that the state of equilibrium is determined by the magnetic field and temperature. "Thermal equilibrium" is reached when the number of nucleons transitioning from a lower energy state to a higher energy state is equal to the number of nucleons moving from a higher energy state to a lower energy state. If a radio frequency energy corresponding to the Larmor frequency is applied to the magnetic moment, and this energy is equal to the difference in magnetic field energy between the higher and lower fundamental energy states, the magnetic moment can be jumped from the lower energy "parallel" state To the higher energy "antiparallel" state, resonance occurs.
Let's now understand it in a simple way: the human body contains 60-70% water, which is distributed in every cell and various tissues and organs, and the water content of different tissues and organs is different. Some people liken MRI to a bit like grabbing a bottle of water and shaking it, and then checking the changes in the bubbles that are shaken up.
Under normal circumstances, the direction of the magnetic force lines of each water molecule is random. Under the action of a strong nuclear magnetic resonance magnetic field, the magnetic field lines of these water molecules will show consistency. When the magnetic field disappears, the magnetic force lines of these water molecules will return to randomness. state. Nuclear Magnetic Resonance is to collect data on changes in the magnetic field lines of the human body's magnetic field through the alternating process of emitting a magnetic field and stopping the magnetic field, and reconstruct the image through complex computer operations.
The magnetic resonance equipment is mainly composed of three basic components, namely: the magnet part, which is composed of the main magnet (generating a strong static magnetic field), the compensation coil (correction coil), the radio frequency coil and the gradient coil; the magnetic resonance spectrometer part, mainly including the radio frequency The transmitting part is composed of a set of magnetic resonance signal receiving system; the data processing and image reconstruction part is composed of a signal converter, a temporary register, an image processor, a console, and a display.
The magnetic field used by nuclear magnetic resonance is very strong, generally between 1.5T and 3T. T (Tesla) is a very high magnetic field strength unit of the magnetic field. 1T is equal to 10000Gs (Gauss), while the earth's magnetic field is only 0.3Gs at the equator, 0.6Gs at the north and south poles, and the strongest rubidium magnet has a magnetic field strength of only 300Gs. Therefore, The magnetic field strength of NMR is about 50,000 times that of the earth, and 100 times that of the strongest magnet.
This is why special attention should be paid to the fact that there are no metal objects on the body and no metal instruments in the room during the MRI examination. If there are these things, once the MRI equipment is turned on, accidents will occur. South Korea's "Chosun Ilbo" reported one such accident. On the afternoon of October 14 this year, a patient was suddenly sucked into a metal oxygen cylinder by the strong magnetic field formed by the equipment while undergoing an MRI examination at the Gimhae General Hospital in Gyeongsang-do, South Korea. , Stuck the patient alive.
Magnetic resonance has such a powerful magnetic force, but it has no damage or influence on the human body, so it is the safest examination. This also breaks the superstition that magnets can cure diseases from another angle. Those propaganda that put a few magnets on the soles of shoes or mattresses will cure all diseases are actually gimmicks by charlatans. I hope that after reading this article, Don't be fooled again.
The so-called B-ultrasound is a technology that uses ultrasound as a medium to diagnose diseases through the echo imaging of ultrasound passing through the human body.
All waves have a frequency, and the frequency is the number of vibrations per second. The frequency of sound that can be heard by the human ear is between 20 and 20,000 Hz. The sound waves lower than this frequency are called infrasound waves, and the sound waves higher than this frequency are called ultrasonic waves. Infrasound and ultrasonic waves are inaudible to the human ear, but these sound waves can be made visible through man-made related instruments.
Due to the good penetrability and anisotropy of ultrasonic waves, it is possible to image the interior of objects through absorption, reflection, refraction, and diffraction. In medicine, the working principle of ultrasonography is to transmit ultrasonic waves into the human body. When it encounters various interfaces in the body, it will be reflected and refracted, and will be absorbed and attenuated in different degrees in different tissues. These processes are passed through instruments. Different waveforms, curves and images will be reflected, and doctors can diagnose diseases by analyzing these images.
 
Diagnostic techniques using ultrasound are divided into A, B, C, and D types. Diagnosing diseases in the form of sound wave amplitude is called "one-dimensional display", because the first letter of the amplitude in English Amplitude is "A", also known as A ultrasound; and diagnosing diseases in grayscale brightness mode is called "two-dimensional display". The first letter of Brightness in English is "B", also known as B-ultrasound. The M-type and D-type diagnostic methods are generally used to check the heart and blood flow respectively, and are also called echocardiography and Doppler ultrasound diagnostic methods, which will not be discussed here.
B-ultrasound inspection equipment is mainly composed of probe, host, power supply, display, housing and peripherals. Among them, the probe part is composed of a chip, a sound-absorbing block, a matching layer, and a sound-absorbing block; the host and the display are composed of a computer and a display that process information, and are used to receive the information collected by the probe. Through calculation and processing, various data Convert it into an image, display it on a monitor, or print it out; the power supply and the shell are auxiliary facilities that provide energy and protection for the host and the probe.
B-ultrasound diagnostic technology is now more and more widely used, such as endoscopic ultrasound, contrast-enhanced ultrasound, three-dimensional imaging, elastic imaging, etc., are playing an increasingly important role.
The main pros and cons of the three approaches
Ultrasound examination
It is convenient and fast, relatively cheap, non-invasive and non-radiative, and can be continuously and dynamically repeated scanning. It is the preferred inspection method for solid organs and fluid-containing organs, such as the abdomen, liver and kidney, bladder, and pelvic cavity; however, ultrasound is easily affected by gas. It is blocked from the bone, so it is not suitable for the examination of the lungs, digestive tract, and bones, but the current ultrasonic endoscope can overcome these defects to a certain extent.
Moreover, ultrasonography is greatly affected by the quality, experience, examination skills, and seriousness of the operator, and the certainty of the diagnostic result is affected to a certain extent.
CT scan
The details of the lesion can be seen, the accuracy is high, and the diagnostic results are more certain. It is the first choice for diagnosing diseases in the head, chest, heart, bones, limbs, etc.; but some bones have more artifacts, which affect the display of surrounding soft tissue structures, such as The base of the skull and spinal canal, etc., and are affected by respiratory movement, it is easy to miss small lesions, such as small lesions in the lungs and liver.
Moreover, X-rays are high-energy rays that are harmful to the human body, so long-term or frequent inspections are not suitable. Some patients with serious diseases, such as severe liver and kidney insufficiency, hyperthyroidism, asthma, and certain allergic lesions, are not suitable for this kind of inspection. .
nuclear magnetic resonance
Sensitive to early diagnosis, it can show abnormalities in the early stages of some lesions, and can detect problems earlier than CT and B-ultrasound methods. It is more suitable for examinations of the head, spinal cord, bones, limbs, etc. Therefore, it is especially effective for examination of the skull base and spinal canal. Compared with CT, it also makes up for the defect that it cannot be directly multi-planar imaging. Angiography can be formed without injection of contrast agent, and the lesion can be displayed more clearly.
Disadvantages: The imaging method is complicated, the price is relatively more expensive, and it is generally not the first choice for disease diagnosis; since emergency equipment cannot enter the MRI room, this examination is generally not suitable for particularly critically ill patients; MRI is not good for the fetus, so pregnant women cannot use it This test is also contraindicated in patients with metallic implants (eg, pacemakers, certain stents) in the body; MRI shows poor image quality of calcified lesions and bone skin and is therefore not suitable Imaging diagnosis of fractures and other conditions.
After reading the above introduction, you should have a certain understanding of the characteristics and advantages and disadvantages of CT, MRI, and B-ultrasound examinations. You can choose according to your different needs in the future. Of course, the most important thing is to listen to the doctor, what do you think? Welcome to discuss, thanks for reading.
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