MOVEMENT AROUND B0
Precession is a circular motion around the main field.
M0 does not simply point along B0. It can be modeled as moving around the B0 axis. This motion is called precession.
The spinning top is a learning analogy for the collective magnetization. Individual proton spin is a quantum property and is not a tiny ball physically rotating.
LEARN THE FREQUENCY WORDS
Frequency tells us how often a cycle repeats.
Hertz
One cycle per second.
Megahertz
One million cycles per second.
Gyromagnetic ratio
A constant that is specific to the nucleus.
Larmor frequency
The precession frequency at a given field strength.
ONE RELATIONSHIP
A stronger field creates a higher frequency.
For hydrogen, γ̄ is approximately 42.58 MHz/T.
0.55 T≈ 23.4 MHz
1.5 T≈ 63.9 MHz
3 T≈ 127.7 MHz
The relationship is direct. If B0 doubles from 1.5 T to 3 T, hydrogen's Larmor frequency also doubles.
The scanner must know hydrogen's frequency to deliver RF energy effectively. Gradients later create small, controlled frequency differences that help locate the signal.
THE SPEED LIMIT ANALOGY
B0 sets hydrogen's pace.
Think of field strength as setting the pace of precession. A higher B0 means a faster pace and therefore a higher Larmor frequency.
Remember: higher field, higher frequency.
CHECK YOUR UNDERSTANDING
Connect the motion to the equation.
What is precession?
It is the modeled motion of magnetization around the B0 axis.
What does the Larmor frequency describe?
It describes the precession frequency for a particular nucleus at a particular magnetic field strength.
If B0 doubles, what happens to hydrogen's Larmor frequency?
It doubles because Larmor frequency is directly proportional to B0.
LESSON 03 COMPLETE
You understand precession and the Larmor frequency.Educational references
This lesson uses a simplified model to support learning and should be studied alongside formal MRI education.