FROM SIGNAL TO LOCATION
The receiver hears signal, but gradients identify its origin.
The main magnetic field B0 provides the strong background field. Gradient coils briefly add small, controlled changes to that field across position. Because Larmor frequency depends on magnetic-field strength, position can be encoded into frequency and phase.
The scanner has three physical gradient axes, X, Y, and Z. The system can assign them different logical jobs and combine them to create axial, sagittal, coronal, or oblique imaging planes.
THE THREE JOBS
Select, label, and read.
A gradient makes resonance frequency vary across one direction. An RF frequency band excites the desired slice.
A brief gradient gives spins position-dependent phase shifts along one in-plane direction.
A gradient applied during signal readout gives spins position-dependent frequencies along the other in-plane direction.
For a basic 2D acquisition, one direction selects the slice and two directions encode location within that slice. In 3D imaging, an additional phase-encoding direction helps localize signal through a volume.
LOGICAL ROLES CAN MOVE
X, Y, and Z are hardware directions, not permanent jobs.
Three perpendicular gradient systems built into the scanner.
Jobs assigned by the prescribed plane and sequence.
Changing the phase-encoding direction can redirect certain artifacts. Choosing slice orientation and encoding direction also affects anatomy coverage, resolution, flow behavior, and scan strategy.
THE THEATER-SEAT ANALOGY
Give every signal a three-part address.
Imagine finding someone in a theater. First choose the floor, then the row, then the seat. Slice selection chooses the imaging section. Phase encoding labels one direction within it. Frequency encoding labels the other.
Remember: Slice chooses. Phase labels. Frequency reads.
CHECK YOUR UNDERSTANDING
Match each gradient job to its purpose.
Why can gradients encode position?
They create controlled changes in magnetic-field strength across space. This produces position-dependent precession frequency and phase.
Which encoding gradient is active while the signal is being sampled?
The frequency-encoding gradient, also called the readout gradient.
Is the Z gradient always the slice-selection gradient?
No. X, Y, and Z are physical hardware axes. Logical slice, phase, and frequency roles can be reassigned or combined for the prescribed imaging plane.
LESSON 10 COMPLETE
You understand how gradients encode location.Educational references
This lesson introduces Cartesian spatial encoding. Advanced methods may combine gradient, RF, and coil-sensitivity information in different ways.