MRI Principles: Imaging the Body

in medical •  10 days ago  (edited)

[3 min read]

Author: Naim uddin forhad

In healthcare industry, MRI plays an important role in disease screening and imaging. But what's the underlying mechanism of MRI? Let's find out.


If you’ve been to the hospital due to chest pain or headache, you may have already experienced an MRI. MRI is the short form of Magnetic Resonance Imaging. First-ever full-body MRI was invented back in 1977. It is an imaging technology that is now being used widely in medical settings by healthcare professionals to screen patients.

How does MRI work?

MRI machines can vary in shape and size, but the essential components are the same in every machine. The machine hosts a patient table, a radio frequency coil, a scanner, gradient coils and a magnet. The magnet is the most crucial part of an MRI machine. It is used to create a powerful, stable magnetic field. It can be more than 4 times as powerful as the earth's magnetic field. Most modern MRI machines can produce from 0.2 Tesla to 3.0 Tesla magnetic field. Tesla is the unit we use to measure the strength of a magnetic field. Okay, enough of the small talk, let’s deep dive into the geeky aspect of MRI and find out the basic principle based on which all MRI machines work.

Our body is comprised mostly of water. Thus hydrogen, one of the building blocks of water, is abundant in our whole body. Hydrogen is a single atom, and its nucleus is called “proton.” In our body, the hydrogen atoms spin on its axis and act like tiny magnets where their axes are aligned randomly.  When the MRI magnet is on and the patient is sent through the tube, the strong magnetic field created by the MRI magnet aligns all the protons in the direction of the magnetic field. Since the MRI magnet runs straight down the center of the machine, the protons divide into almost equal two groups, facing upwards and downwards. So they cancel each other out. But a few of the protons are still left out.

H atoms before and after applying magnetic field.

Then the MRI machine sends an RF (radio frequency) pulse in the targeted area of the body. This extra energy is absorbed by the protons and causes them to spin again in a different direction. That’s what we call “resonance.”

By using the gradient coilsthe main magnetic field can be altered in our favor. This way, we can picture the specific body part as per our needs. This particular part is called “slice.” When the RF pulse is switched off, the protons return to their original state by emitting the extra energy absorbed earlier. This is called T2 relaxation. The radio wave energy signal is picked up by receiver coils and sent to the computer to generate an image.

The computer uses a sophisticated algorithm called Fourier transform to create image from the received signals. The MRI slices can be both 2D or 3D depending on the patient's need. Also, by tuning the gradient magnets, imaging can be done in any depth of the body giving physicians the flexibility to understand the position of potential abnormality.

Okay, now we understood the basics of MRI. But how do we detect abnormalities? The cellular activity in defected tissue is different than that of healthy tissue. Thus the cells with defect exhibit different signals than that of healthy tissues when subjected to RF pulse.

Unlike X-ray and many other popular imaging methods, MRI does not use ionizing radiation to screen the patient. As of now, MRI has no known health hazard. Making it the safest way to screen for disease. Also, unlike an x-ray, which produces a 2D image of the body, MRI can be used to generate a 3D image from any side of the body without requiring the patient to move inside the tube. This is achieved by calibrating the magnetic gradient. Thus, MRI can provide much more precise information about abnormalities. MRI is widely used to screen soft body parts, such as organs that can be damaged by ionizing radiation.

Even though MRI has no known health effects, it is strongly recommended to consult a physician before being exposed to MRI since MRI utilizes a strong magnetic field. Physicians usually don’t ask for an MRI right away. People who use medical implants such as a pacemaker, heart extent inside their body should consult with their doctor before being subjected to an MRI examination.

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