What is magnetic resonance imaging? | Explained

An MRI procedure reveals an image of a body part using the hydrogen atoms in that part

Published - May 06, 2024 04:15 pm IST

A person (lying down) prepares to undergo an MRI scan while an operator (standing) prepares for the test.

A person (lying down) prepares to undergo an MRI scan while an operator (standing) prepares for the test. | Photo Credit: DarioGaona/Getty Images

The story so far: For those trying to look inside the human body without surgery, magnetic resonance imaging is an indispensable tool. The underlying techniques were worked out in the early 1970s; later the same decade, Paul Lauterbur and Peter Mansfield refined them to pave the way for their commercial use. For these efforts, they were awarded the medicine Nobel Prize in 2003, speaking to the significance of the technique and its place in modern medical diagnostics.

What is magnetic resonance imaging?

Magnetic resonance imaging (MRI) is used to obtain images of soft tissues within the body. Soft tissue is any tissue that hasn’t become harder through calcification. It is a non-invasive diagnostic procedure widely used to image the brain, the cardiovascular system, the spinal cord and joints, various muscles, the liver, arteries, etc.

Its use is particularly important in the observation and treatment of certain cancers, including prostate and rectal cancer, and to track neurological conditions including Alzheimer’s, dementia, epilepsy, and stroke. Researchers have also used MRI scans of changes in blood flow to infer the way the activity of neurons is changing in the brain; in this form, the technique is called functional MRI.

Because of the MRI technique’s use of strong magnetic fields, individuals with embedded metallic objects (like shrapnel) and metallic implants, including pacemakers, may not be able to undergo MRI scans. In fact, if they have a credit card in their pocket, the magnetic fields will wipe its magnetic strip!

How does MRI work?

An MRI procedure reveals an image of a body part using the hydrogen atoms in that part. A hydrogen atom is simply one proton with one electron around it. These atoms are all spinning, with axes pointing in random directions.  Hydrogen atoms are abundant in fat and water, which are present almost throughout the body.

An MRI machine has four essential components. The machine itself looks like a giant donut. The hole in the centre, called the bore, is where the person whose body is to be scanned is inserted. Inside the donut is a powerful superconducting magnet whose job is to produce a powerful and stable magnetic field around the body. Once the body part to be scanned is at the centre of the bore, the magnetic field is switched on.

Each hydrogen atom has a powerful magnetic moment, which means in the presence of a magnetic field, the atom’s spin axis will point along the field’s direction. The superconducting magnet applies a magnetic field down the centre of the machine, such that the axes of roughly half of the hydrogen atoms in the part to be scanned are pointing one way and the other half are pointing the other way. This matching is almost exact: in around a million atoms, only a handful remain unmatched — i.e. a small population of ‘excess’ atoms pointing one way or the other.

The machine’s third component is a device that emits a radiofrequency pulse at the part under the scanner. When the pulse is ‘on’, only the small population of ‘excess’ atoms absorbs the radiation and gets excited. When the pulse goes ‘off’, these atoms emit the absorbed energy and return to their original, lower energy states. The frequency of pulse the ‘excess’ atoms have to absorb is called the Larmor frequency. Its value depends on the strength of the magnetic field and the type of tissue in which the atoms are present.

The fourth and final component, a detector, receives the emissions and converts them to signals, which are sent to a computer that uses them to recreate two- or three-dimensional images of that part of the body.

What are the pros of MRI?

After the big, powerful magnetic field comes on, the MRI machine activates three magnets that produce smaller magnetic fields that are weaker than the main field by about 80-times, if not more. These fields also have a gradient, i.e. they’re not uniform. These fields interfere with the main field at the part to be scanned such that the resulting field highlights very specific portions, which can be the focus of the scan.

By turning the gradient magnets on and off in specific sequences, the MRI machine can thus scan portions that are just a few millimetres wide. The sequences can also be organised such that the machine scans different parts of the individual’s body without asking them to move inside the bore.

In fact, because of the way the machine is built and the magnets are organised inside it, an MRI scan can practically image the body from all useful directions and, if required, in very small increments.

When the ‘excess’ atoms emit the energy they’d absorbed to return to their lower energy states, the return happens over a duration called the T1 relaxation time. The hydrogen atoms in water have different values of T1 depending on the tissue in which they’re present. An MRI machine exploits this fact to show different tissues in different shades of grey. Clinicians may also inject an individual with a contrast agent — typically a gadolinium-based compound — that lowers the T1 time in some tissues, improving their visibility in an MRI scan.

Finally, researchers have deeply investigated the effects of strong magnetic fields on the body. MRI scans don’t pose any threats; once the magnetic fields are taken away, the atoms in the scanned part don’t remain affected. There is no long-term harm associated with scans. However, a scan’s effects on pregnant women aren’t as well-studied, so many scanning facilities simply refuse such appointments.

What are the cons of MRI?

MRI machines are expensive: depending on the specifications, including the strength of the magnetic fields and the imaging quality, they cost from a few tens of lakh rupees to a few crores. Diagnostic facilities pass this cost on to its patients. Based on the clinical requirements, scans often cost Rs 10,000 or more each — a sizeable sum in India, especially for those without insurance, and more so for those required to get multiple MRI scans.

These costs are compounded by the discomfort of using the machine. While it’s an advantage that an individual inside the bore doesn’t have to move for the machine to scan different parts, the individual is actually expected to lie still for tens of minutes, until the scan is complete. If the individual moves, the resulting image will be distorted and the scan will have to be repeated. The problem is exacerbated if the individual is claustrophobic (although some ‘open-bore’ MRI machine designs can alleviate this issue).

Generating a magnetic field of strength 1 tesla or more — as the main magnet does — is no mean feat. To do so, a heavy current is passed through coils of wire made of a superconducting material. When the setup is cooled with liquid helium, the wires become superconducting and the current passing through them plus the geometry of the wires produces a strong magnetic field. While the wires don’t lose any energy as heat — which a non-superconducting material would — maintaining the setup is energy-intensive, which is expensive.

Further, the switching of such heavy currents within the machine, as the gradient coils are operated in sequence, means the machine produces loud noises when operating. This can be an additional source of discomfort for the individual.

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