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You might have seen this on TV, or some of you might have even experienced this strange procedure first-hand. MRI scans are used in the medical field for diagnosis. In cases such as a brain hemorrhage, a blood clot occurs deep inside the brain, where doctors can't see. An MRI scan produces a very detailed 2D image of the brain, allowing doctors to analyze the damaged tissue and then perform their targeted surgery. Let's take a deep dive into how MRI imaging works and the principle behind it.

MRI is a technique that uses varying magnetic fields and radio pulses to create detailed images of the organs and tissues in the human body. MRI machines have tube-shaped strong electromagnets, gradient coils, and RF coils to generate strong varying magnetic fields and radio frequency pulses. Uniform magnetic fields are produced inside the bore. Now, let's focus on the effect of magnetic fields and radio frequency signals on the body. The human body is made up of 60% of water, and water composition surprisingly plays an important role in this technique. When you lie inside an MRI machine, the magnetic field generated by electromagnets and gradient coil temporarily realigns most of the water molecules in your cells in the direction of the magnetic field. The scanner operator commands RF coils to send radio pulses, causing these aligned atoms to flip. Within a few seconds, the atoms realign with the magnetic field, emitting an RF signal. This RF signal is used to create cross-sectional 2D MRI images. We’ll expand on this brief explanation of MRI imaging technology later on. First, let's explore the difference between healthy and unhealthy tissues.

In the human body, each tissue has its own unique water composition. When a blood clot happens in a certain tissue, it changes the water composition of that area. This allows us to clearly see the difference between healthy and damaged tissues.

Let’s narrow our focus on the water molecules inside our tissues. The water molecule has two hydrogen atoms. This hydrogen atom has a small magnetic field, and it acts as a tiny bar magnet. However, this magnetic field keeps on spinning since the atom is spinning. Naturally, the axis of the spinning magnetic field is oriented randomly in the human body. When this hydrogen atom comes into contact with an external magnetic field, the orientation of the magnetic field of the hydrogen atom changes and aligns with the direction of the field. Additionally, the rate of magnetic field spin also changes. In this case, the rate of the spin is known as the resonance frequency, and it varies according to the strength of the external magnetic field. This resonance frequency is quite important in MRI and is determined using the Larmor equation.

Let’s move on to the next step. So far, we have aligned all the hydrogen atoms in the direction of the magnetic field. Let's strike a radio pulse on a hydrogen atom. As a result, the hydrogen atom changes its orientation and flips 90 degrees with respect to the main magnetic field. It is important to remember that the frequency of an RF signal should be the same frequency as the hydrogen atom’s resonance frequency. Only then will the hydrogen atom flip. When we remove the RF pulses, the atom loses its energy and returns to its normal orientation. The energy it emits is in the form of RF pulses. However, the time it takes each atom to return to normal is different, meaning that the duration of emission of RF signals is also different. The computer receives these signals and converts them into images. Since the unhealthy tissue has more hydrogen atoms, it generates different RF signals. But how exactly can we generate a detailed image? The secret lies in phase and frequency encoding with the help of Fourier transformation.