Magnetic Resonance Imaging, commonly known as MRI, is a key medical diagnostic tool. This non-invasive technology allows medical professionals to visualize the internal structures of the human body with great clarity. MRI scanners use powerful magnets and radio waves to generate detailed images of organs, soft tissues, bone, and other internal body structures. This capability makes MRI valuable for identifying conditions not visible with other imaging methods.
Harnessing Magnetic Fields
The operation of an MRI scanner begins with a strong, static magnetic field. This field is generated by large superconducting magnets within the cylindrical bore of the machine. The strength of these magnets is measured in Teslas (T), with scanners often operating at 1.5 T or 3 T, which is thousands of times stronger than the Earth’s magnetic field.
The human body is mostly water, containing hydrogen atoms. Each hydrogen atom has a single proton with a property called spin. This spin causes the proton to behave like a tiny magnet, randomly oriented in the absence of an external magnetic field.
When a person enters the MRI scanner, these trillions of hydrogen protons respond to the static magnetic field. They align with the main magnetic field, much like small compass needles pointing north. This alignment creates a net magnetization within the body, forming the foundation for image acquisition.
The Resonance Principle
Once the hydrogen protons are aligned within the static magnetic field, the MRI scanner introduces a radiofrequency (RF) current. This current generates a brief, secondary magnetic field perpendicular to the main magnetic field. This RF pulse is tuned to the natural frequency at which hydrogen protons precess, similar to how a tuning fork resonates only with a specific sound frequency.
When the RF pulse is on, the protons absorb energy, causing them to temporarily tip out of their alignment with the main magnetic field. This energy absorption and orientation change is known as resonance. The duration and strength of the RF pulse determine how much the protons are tilted.
Once the RF pulse is off, the protons begin to “relax” and return to their original alignment with the main magnetic field. During this process, they release the absorbed energy as radio signals. The rate at which these protons relax and release energy varies depending on the type of tissue they are in. For example, fat and water-rich tissues relax at different rates, producing distinct signals.
From Signals to Detailed Scans
The radio signals emitted by the relaxing hydrogen protons are detected by receiver coils around the patient. These coils act like antennas, capturing the radio waves from the body. The strength and timing of these signals are unique, reflecting the varying relaxation properties of different tissues.
To create a spatial map, the MRI system uses additional, smaller magnetic fields called gradient coils. These coils produce slight, controlled variations in the main magnetic field across dimensions. By changing the magnetic field strength systematically, the precession frequency of protons varies slightly depending on their location, allowing the system to pinpoint signal origin.
A computer processes these complex and spatially encoded radio signals. Using mathematical algorithms, the computer converts the raw signal data into detailed cross-sectional images of the body. These images are displayed as thin slices, but the data can also be used to reconstruct three-dimensional views of organs and structures. Distinct signal characteristics from different tissues, combined with precise spatial location, enable the computer to generate high-resolution, contrast-rich images characteristic of MRI scans.