What's the difference between X-ray, CT, MRI and B-ultrasound? How to choose? What the doctor won't tell you

The more tests you do, the more you worry that radiation may be harmful to your health.

"X-ray, CT, nuclear magnetic resonance and B-ultrasound, these tests have radiation? What is the difference? Is it harmful to the body? "

Today, we will answer questions about radiation in image inspection.

Medical history data, physical examination and imaging diagnosis can be said to be the three magic weapons for orthopedic doctors to see a doctor. With the continuous emergence of new technologies, the importance of imaging examination is self-evident, and even plays a decisive role in the diagnosis of many diseases.

When we were in the hospital, we sometimes just had an X-ray examination, but when we took the film to the doctor, the doctor asked us to have another CT or MRI examination.

This makes many friends very confused: why take X-rays and CT? Why do you sometimes have to do expensive MRI? What is the purpose of ultrasound examination?

Don't worry, a series of questions, I believe that after reading the following content, you can solve most of your doubts.

X-ray photography includes general X-ray photography, computer X-ray photography system (CR) and digital X-ray photography system (DR), commonly known as X-ray photography.

When taking X-rays, the machine will emit X-rays. When X-rays pass through human bones, muscles, air and other different media, they will have different attenuation due to different absorption degrees. Using this penetrability of X-ray, a plane image with different black and white levels will be formed on the negative film, so it is also called plain film inspection.

This is like flattening the structure of human body at all levels on the two-dimensional plane according to the absorption characteristics of tissue to rays.

X-ray exposure imaging time is very short, which greatly shortens the examination and waiting time of patients. Therefore, it is used to judge whether there are enough fractures, tissue calcification and changes in bone tissue structure in many early orthopedic clinics. The structure of human body is three-dimensional. Because it is plane imaging, the images of various deep and shallow parts overlap with each other, and the effect may not be very good for some tissues with poor resolution.

CT examination, also known as computerized X-ray tomography, also uses rays to scan and form images. However, unlike X-ray plain films, it emits rays from different directions to form plane images with different angles. After computer processing, three-dimensional structural images of different parts can be obtained.

It is said that CT can see more clearly than X-ray. For example, comparing the human body to bread, CT scanning is like cutting bread layer by layer, and each layer is like X-ray plain film. Therefore, CT is like an enhanced version of X-ray, which is more accurate than X-ray and has better imaging effect on different levels of structures.

Therefore, CT uses X-ray beam to scan a certain part of the human body to obtain a cross-sectional or three-dimensional image of the detected part of the human body. It can provide complete three-dimensional information of the examined parts of human body, make organs and structures clearly developed and clearly show lesions.

CT examination has important diagnostic value for bone injury, chest diseases, heart and great vessels, and has been widely used in clinic.

In addition to more detailed observation of skull, spine and limbs, CT has incomparable advantages in screening lung and trachea, nodules, cancer, imaging of great vessels and screening diagnosis of coronary heart disease.

It can be seen that the imaging principle of X-ray film and CT is the same, which uses the penetrability of X-ray to form images with different attenuation intensity after passing through human bodies with different tissue densities.

So, can CT completely replace X-ray? Why do we often check by taking an invisible X-ray plain film instead of directly using CT? There is radiation in this examination, will it be harmful to our health?

We all know that both CT and X-ray are imaging by ionizing radiation, which can cause cancer in human body.

In fact, there are a lot of ionizing radiation in our natural living environment. According to the data of the Nuclear Regulatory Commission of the United States, the average annual radiation of a person under natural conditions is about 3. 1 millisieverts. Abdominal X-ray is about 0.7 millisieverts, which is equivalent to 4 months of natural radiation, and skull CT scan is about 2 millisieverts, which is close to 1 year of natural radiation.

Talking about the safety of any scanning ray, ignoring the dose and frequency is "hooliganism". The amount of radiation absorbed by each organ in CT and X-ray scanning is different, which mainly depends on the number of scans, the size of the scanned part of the patient, the power of the equipment used and the exposure time.

100 CT scans will produce about 600 millisieverts of radiation, which will lead to an increase in cancer incidence. According to the data of American Society of Health Physics, the health risk of 50- 100 millisieverts can be ignored, so the radiation absorbed by physical examination at ordinary times has little effect on us.

However, due to the risk of radiation teratogenesis, pregnant women should avoid CT or X-ray examination.

In the case of radiation, the radiation dose of CT is larger than that of ordinary X-ray machine, the radiation dose received by patients will be larger, and CT examination is more expensive, so CT examination is not suitable for routine diagnosis in daily diagnosis and treatment.

If suspicious pathological images are found on X-ray films and cannot be diagnosed, then we will recommend CT examination as an intensive supplement.

Magnetic resonance imaging does not use radiation, but uses large magnets, radio transmitters and computers to reconstruct human structures and form images. Its essence is to use the magnetic field to make the magnetic field lines of all water molecules in the body in the same direction. At this time, the magnetic field of the magnetic resonance machine suddenly disappeared, and the magnetic lines of water molecules in the body suddenly returned to the original random arrangement.

After imaging, the difference between normal tissue and diseased site can be observed, so magnetic resonance vibration is also called shaking inspection.

Nuclear magnetic resonance vibration has no radiation damage and ionizing radiation, which has become a common image inspection method.

For the brain, spinal cord and other parts that need to observe the soft tissue structure, it is better than CT examination. Similarly, for the examination of joints, muscles and adipose tissues, because CT only has good resolution on bone structure, nuclear magnetic resonance is also the first choice.

However, MRI is not as effective as ct in the manifestations of lung and bony structures, so MRI cannot be regarded as a universal examination.

Moreover, it is not suitable for people with dentures, pacemakers and other metal objects to do MRI examination, because under strong magnetic field, these magnetic metals fixed in the body will shift and cause danger.

However, for many new internal fixation materials such as non-magnetic metal fixed steel plates, magnetic vibration inspection can actually be carried out.

However, for safety reasons, if there is a metal internal fixator in the body, you must inform the doctor in advance and make sure the metal characteristics before checking.

Ultrasound examination, that is, we often say B ultrasound and color ultrasound. Ultrasonic waves are used to penetrate the human body. When sound waves meet human tissues, they will produce reflected waves, and the reflected echoes are B-ultrasound portraits.

It's like knocking and listening while picking watermelons to experience the situation inside.

In our traditional sense, B-ultrasound examination uses gray values to display the examination results. Nowadays, many places say that it is to do color ultrasound examination. So, are they an exam?

In fact, in most cases, color Doppler ultrasound is what we call B-ultrasound, but color Doppler ultrasound is an imaging technology based on gray-scale ultrasound and blood flow imaging, which can distinguish local blood flow distribution. It is only an additional function of traditional B-ultrasound, and it is also ultrasound in essence.

Ultrasound examination is harmless and painless to human body, with various display modes, especially suitable for detecting human soft tissues and observing the hemodynamics of cardiovascular organs, such as superficial masses, blood vessels and puncture sites.

It also has irreplaceable advantages in the observation of liver, gallbladder, pancreas, spleen, kidney, uterus and other substantive organs.

However, due to the weak penetration of ultrasound, it is difficult to detect gas-containing organs, such as lungs and intestines, and it is also difficult to show them on the spectrogram when the lesions are small or the acoustic impedance difference is not great, so the detection accuracy of tiny tumors is also limited.

X-ray, CT, MRI and B-ultrasound have their own advantages and limitations in practical application because of their different principles. On the premise of correct diagnosis of diseases, choose simple, convenient, safe and low-cost examinations and follow the principle of gradual screening.

In some cases of difficult diseases and differential diagnosis, it may be necessary to combine several different tests to make the same judgment, so don't be superstitious about one test.