Basic MRI Physics

Basic MRI Physics
MRI uses a magnet, a radio pulse, and a receiver coil to turn signals from hydrogen in the body into pictures.
The big picture
Magnet lines up protons → RF pulse tips them → Protons relax → Coil hears the signal → Computer creates the image
1. Your body has tiny magnets
Your body contains lots of hydrogen protons, mostly in water and fat. Think of each proton as a tiny magnet. Before the MRI magnet is applied, they point in many different directions.
2. The main magnet lines them up
The scanner’s main magnetic field is called B₀. It makes slightly more protons point with the field than against it. That small difference creates one combined magnetic direction called net magnetization.
3. An RF pulse tips them
The scanner sends a short radiofrequency, or RF, pulse. It gives the protons energy and tips the net magnetization away from B₀. The amount of tipping is called the flip angle.
4. The protons relax
When the RF pulse stops, the protons return toward their resting state. As they relax, they produce a very small electrical signal in the receive coil. This is the MR signal.
5. T1 means recovery
T1 describes how quickly magnetization grows back along B₀. An easy memory aid is: T1 = standing back up. Different tissues recover at different speeds, which helps create T1 contrast.
6. T2 means fading
T2 describes how quickly protons lose teamwork in the sideways, or transverse, plane. As they get out of step, the signal fades. An easy memory aid is: T2 = falling out of sync.
7. Gradients locate the signal
Gradient coils slightly change the magnetic field from place to place. This lets the scanner determine which slice was selected and where each signal came from.
8. The computer builds the picture
The scanner stores the collected information in k-space. K-space is raw image information, not the final picture. A computer organizes it to create the MRI image.
9. Detail versus signal
Small voxels show finer detail but collect less signal and may look noisier. Large voxels collect more signal but show less detail. MRI protocols balance these two needs.
10. More data usually takes more time
More phase steps, more averages, and more slices usually make the scan longer. Faster echo trains and acceleration can shorten it, but every speed change can affect detail, signal, contrast, or artifacts.
MRI safety is part of the physics
MRI does not use ionizing radiation, but the static magnetic field, time-varying gradients, and RF energy create projectile, implant, hearing, nerve-stimulation, and heating risks. Screen every person and object, use hearing protection, prevent conductive loops, and follow the exact MR Conditional limits for identified devices.
Educational summary only. Follow your site’s MR safety policies, scanner documentation, supervising radiologist, and MR safety or medical-physics personnel.