What exactly is the difference between X-rays, CTs and MRIs?


X-ray, CT and MRI are the three major “Key Instruments” in the radiology department of a hospital. As a healthcare professional, I am often asked these questions: MRI is the most expensive and is it the clearest examination? Why is it necessary to do CT after X-ray, and then finally recommended to do an MRI? Wouldn’t it be better to do MRI directly? Is the radiation level of these three kinds of examinations high and are they harmful to the body?
First. X-ray (roentgen rays)
X-ray examination has been used in medicine for about 100 years, belonging to the “oldest” in the imaging industry. x-ray through the human body, due to the absorption of different processes by the body, such as the amount of x-rays absorbed by the bones than the amount absorbed by the muscles, and thus shows a different density of shadows, leaving shades of black and white, equivalent to a plane photo of the internal body. This is equivalent to taking a flat picture of the inside of the body. However, because of the overlap between the front and back of the ordinary X-ray film, it is sometimes difficult to detect lesions in the organs that overlap with the front and back of the X-ray film.The advantage of X-ray examination is that it is fast and inexpensive. The radiation dose of an X-ray is similar to that of a 20-hour flight.

Wilhelm Conrad Röntgen or Roentgen First x-ray image

Second, CT (computed tomography)
Fifty years after the invention of X-ray examination, electronic computed tomography (CT) imaging means came into being in the 1970’s. CT is also the use of radiographic imaging, but its imaging technology is completely different from that of the X-ray machine, and it can be regarded as an upgraded version of the X-ray examination. CT is capable of tomographic interval-free imaging, and it is also capable of reconstructing the image in any direction. The body can be “sliced” from head to toe. There is less overlap in the tissue structure of the body, resulting in higher resolution and clearer images, which is ideal for observing small lesions. However, the radiation dose from a CT scan is higher than that from an X-ray, with a low-dose CT scan of the chest emitting about 10 times as much radiation as an X-ray.


Third. Magnetic resonance
MRI involves placing the patient in a special magnetic field and exciting the hydrogen nuclei in the body with radiofrequency pulses, causing the hydrogen nuclei to resonate and absorb energy, releasing the absorbed energy after the radiofrequency pulses, which is picked up by the receiver to obtain an image. Different tissues of the human body have different hydrogen content, which will show different black and white contrast on the MRI. In simple terms, it is equivalent to shaking your hand to make the hydrogen nucleus vibrate and then calming it down to feel the vibrations inside. Since lungs contain more air, bones more calcium, and both have less hydrogen, MRI is less able to detect their lesions and is not dominant; whereas it is more able to explore soft tissues, muscle tissues, and nerve tissues. So MRI is significantly better than CT scan for diagnosing lesions in soft tissue, brain, spinal cord, nerves, blood vessels, intervertebral discs, articular cartilage and so on. Advantages: Compared with CT, soft tissue is displayed more clearly and is non-radioactive. Disadvantages: Higher cost and more contraindications. When the MRI equipment is working, the strong magnetism will make metal objects go crazy, a small coin will fly up like a bullet, not to mention wheelchairs, infusion stands and trolleys. Strong magnetic fields can demagnetise credit cards, deform eyeglass frames, disable pacemakers, and bend nerve electrodes around the bend …… Isn’t that scary? That’s why it’s important not to bring any metal objects with you when you have an MRI!

Magnetic resonance

If we look at the human body as a piece of sandwich bread, X-ray is to squash the bread and look at it, CT is to cut it open and look at it, and MRI is to make use of the different components, analyse the differences and cut it open in multiple directions. Doctors will choose or even stack different auxiliary tests according to the different needs of the patient’s condition. Each of the three tests has its own advantages, and patients must follow the doctor’s instructions and listen to the professional doctor’s advice, so as not to delay the condition.

Is the radiation of X-ray and CT examination harmful to the body?
Radiation is everywhere in our daily life. The radiation dose of a banana is 0.0778μSv; the radiation dose of a 20-hour aeroplane flight is 0.1mSv; the radiation dose of 20 cigarettes a day is 2mSv a year; the radiation dose of a CT scan in a normal lifetime is within the safe range, so there is no need to be worried about the radiation dose of X-rays and CT scans in a normal medical checkup.


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