Magnetic Resonance Imaging

Overview

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Magnetic Resonance Imaging (MRI) offers high-resolution, multiplanar views of soft tissues without the use of ionizing radiation. Its superior soft tissue contrast allows for detailed visualization of the brain, spinal cord, joints, organs, and vascular structures.

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Although less portable and more costly than ultrasound or X-ray, MRI is indispensable when fine anatomical detail or soft tissue differentiation is essential. It is especially useful for detecting tumors, inflammation, demyelination, ligament and tendon injuries, and congenital anomalies. Special techniques such as functional MRI (fMRI) and diffusion-weighted imaging (DWI) further expand its utility in evaluating brain activity and ischemic changes.

 

Due to the strength of its magnetic field, MRI requires careful screening for metal implants, pacemakers, and foreign bodies. While safe for most populations, special consideration is required during pregnancy and in patients with renal dysfunction, particularly when contrast agents are needed.

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Basic Principle

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MRI works on the principle of nuclear magnetic resonance. It primarily focuses on hydrogen atoms, the human body is mostly water. Water contains hydrogen protons (H2O).

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Think of each hydrogen proton as a tiny spinning magnet.
Under normal conditions, these mini-magnets spin randomly, but when placed in a powerful magnetic field, they begin to line up in the same direction, just like iron filings align around a bar magnet. The way they align and misalign under the influence of the MRI machine is important for the creation of MRI images.

 

Video Overview: The Basics of Magnetic Resonance Imaging (MRI) - An overview of MRI

https://www.youtube.com/watch?v=JR_5rDDXyUc&ab_channel=CloverLearning

 

Video Overview: The Insane Engineering of MRI Machines

https://www.youtube.com/watch?v=NlYXqRG7lus&ab_channel=RealEngineering

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How MRI Works

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Video Overview: How does an MRI Work? 

https://www.youtube.com/watch?v=FvOezMIL9BU&ab_channel=Dr.PaulienMoyaert

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1. Creating the Magnetic Field

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The powerful magnetic field inside an MRI machine is generated by a superconducting electromagnet. This is essentially a large coil of wire that has been cooled to very low temperatures (using liquid helium) to eliminate electrical resistance. This coil rotates around the person while they are in the machine.
 

According to Ampère's Law, when an electric current flows through this coil, it produces a magnetic field, hence the term electromagnet. The result is a strong, uniform static magnetic field which typically has a strength of 1.5 or 3.0 Tesla (higher the Tesla, the sharper and clearer the images captured).

 

2. Alignment

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This field causes hydrogen protons, which normally spin in random directions, to align either parallel (low energy) or anti-parallel (high energy) to the magnetic field. There is a slight majority in the low-energy (parallel) state, creating a small net magnetization in the direction of the magnetic field.
 

Analogy: Picture hydrogen protons like tiny compass needles. In the MRI scanner, these needles align parallel or antiparallel with the magnetic field, with a slight majority aligning parallel to the field.

 

3. Excitation

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To make these aligned protons give off a signal, the MRI machine sends a radiofrequency (RF) pulse, a short burst of electromagnetic radiation, tuned to the Larmor frequency (imagine this to be the "resonant frequency"  of the hydrogen protons).

 

This RF pulse comes from an RF coil and delivers energy that temporarily knocks the net magnetization of the hydrogen protons away from the axis of the MRI’s magnetic field. Think of the RF as a finger nudging a spinning top to make it wobble sideways.

 

The protons absorb the energy from the nudge and are tipped into the transverse plane (perpendicular to MRI’s magnetic field). While they are spinning together in this plane, they generate a measurable electromagnetic signal.

 

4. Relaxation and Detection

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When the RF pulse is turned off, the protons begin to return to their original alignment. As they relax, they give off energy in the form of electromagnetic waves (their own tiny RF signals), which are detected by receiver coils in the machine. Different tissues relax at different rates, and this forms the basis of image contrast.

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Analogy: Once the RF pulse stops, the tops (protons) wobble and slowly return to their original alignment. As they do, they give off signals. These signals are captured by receiver coils, converted into digital data, and turned into images.

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Types of Relaxation (further explained below):

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Video Overview: What’s the difference between T1 and T2 relaxation

https://www.youtube.com/watch?v=qrR2yoRhAmY&ab_channel=CloverLearning

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• T1 Relaxation (Longitudinal): This is how quickly the protons realign with the magnetic field.
   

Tissues like fat relax quickly and show up bright on T1 images.

 

• T2 Relaxation (Transverse): This is how fast the protons lose their synchronized wobble.
   

Fluids like CSF retain their wobble longer and appear bright on T2 images.
 

The protons, which were spinning in phase after the RF pulse, begin to dephase. This leads to a loss of net transverse (perpendicular to the MRI’s magnetic field) magnetization, reducing the detectable signal.

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Safety Note​

MRI is very safe because it doesn't use X-rays or ionizing radiation. However:

• The strong magnet can attract metal objects and interfere with devices like pacemakers.

• It’s also loud and enclosed, so patients with claustrophobia may need support.

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Key Definitions

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Signal:​

Signal is the radiofrequency energy detected by the MRI receiver coils, produced by the relaxation of hydrogen nuclei after excitation by the MRI system’s RF pulse.

 

Proton Density:​

The more hydrogen atoms in a tissue, the stronger the signal. Tissues like muscles, with lots of water, produce clearer signals than bone, which contain little hydrogen.

 

MRI Gradient:​

An MRI gradient is a controlled and temporary variation in the magnetic field strength applied in a specific direction (x, y, or z axis) within the MRI scanner. These gradients allow the system to spatially encode signals, enabling the machine to determine the exact location of the hydrogen nuclei emitting the MRI signal. 

 

MRI Sequence:​

Specific configuration of radiofrequency pulses and gradients used during an MRI scan to create images of the body. Each sequence determines how the MRI machine manipulates hydrogen protons in the body and how it collects the resulting signal, ultimately affecting image contrast, resolution, and the types of tissues that are highlighted. ​​