- Definition
Magnetic resonance Imaging (MRI) is a functional and structural method of imaging. The MRI uses magnetism and radio waves, combining them into a series of cross-sectional images. Types of MRI views include:
- T1-weighted image: Cerebrospinal fluid (CSF) is dark, fat is bright. White matter in brain (inner tissue) appears bright, and gray matter (outer tissue) appears dark.
- T2-weight image: CSF is bright, fat is bright. White matter is dark, gray matter bright.
- STIR image: CSF is bright, fat is dark.
- FLAIR: CSF is dark, fat is bright. White matter is dark, gray matter bright.
- Discovery
In 1970, Dr. Raymond Damadian discovered that the nuclear magnetic resonance (NMR) signals that come from normal tissue and cancer were very different. Based on this finding, he built a device that can create the magnetic field to induce these signals. While the principle of NMR was already known, he was the first to pioneer a health application for it.
Unfortunately, Dr. Raymond Damadian passed away on August 5, 2022, just a week before this article.
- Mechanism
To understand how MRI works, we need to go down to the atomic scale. Our bodies are made of cells, and each cell is made up of biochemical molecules. Each molecule is comprised of different atoms combined in unique arrangements. Each atom has a positively charged nucleus and a negatively charged electrons. While they mostly add up to a net charge of zero (although many other instances the atom has a net charge), there are instances where electrons could coincidentally be grouped on one side of the atom. This creates a partial charge at the ends of the atom.
- The MRI creates these partial charges via a magnetic field. When the patient goes through the donut-shaped machine, they are basically entering the magnetic field. In our body, the hydrogen atom is everywhere, and they react to the field by aligning themselves like tiny magnets.
- Radio waves are then shot at the patient's body. While this does nothing to the patient at large, it knocks the hydrogen atoms from the alignment they prefer to be in.
- When the waves are turned off, the hydrogen atoms come back into alignment; this releases energy that is captured by the coil inside the machine around the patient as signals.
- Interestingly, the hydrogen atoms return to alignment at different speeds depending on which tissue they are in. This allows high-resolution images to be created (imagine, resolution at an atomic level!)
- Indications
MRI is used to further specify the type of disease, often to evaluate patients with mass/cancer, infection, or inflammation. By the point patient gets the MRI, they would already have received an x-ray, CT, U/S, or other modalities and found to have an abnormality.
It is especially useful for viewing soft tissues:
- Brain and spinal cord
- Breast
- Cardiovascular system
- Abdominal/Pelvic organs
- Musculoskeletal system
Pros: No ionizing radiation! Safe to use for pregnant patients too.
Cons: Metallic objects on or in the body are dangerous, unlike in other imaging modalities. Because MRI is a huge magnet, any tattoos with iron, earrings, piercings, accidentally swallowed keys, and previous surgical clips and pacemakers could fly toward the machine, injuring tissues in the process.
See www.mrisafety.com for more information.
- Terminology