AFTER RF EXCITATION
A moving magnetic field can create an electrical signal.
After RF excitation, transverse magnetization precesses around B0. From the receiver coil's point of view, this produces changing magnetic flux. According to Faraday's law of induction, that changing flux induces a voltage in the coil.
FOLLOW THE SIGNAL PATH
The scanner turns magnetic behavior into digital data.
Transverse magnetization moves around B0.
Changing magnetic flux induces voltage in the receive coil.
The signal is amplified, sampled, and converted into digital data.
A coil may transmit RF, receive signal, or do both, depending on its design. During reception, the coil acts like a sensitive antenna tuned for the MR signal.
MEET THE FID
The first signal begins strong and fades.
The voltage detected immediately after excitation oscillates near the Larmor frequency and decreases with time. This decaying signal is called the free induction decay, or FID.
The FID becomes smaller as transverse coherence is lost. In practice, field differences also contribute, so the observed FID commonly reflects T2 star decay.
A signal is not yet an image. Gradients must encode location, many measurements must be collected, and reconstruction must organize the data into pixels or voxels.
THE MICROPHONE ANALOGY
The coil listens to a fading chorus.
Imagine many voices beginning together after a cue. A microphone detects their combined sound. As the voices lose timing and spread apart, the combined sound fades. The receiver coil similarly detects the changing magnetic effect of transverse magnetization as coherence decreases.
Remember: precession changes the flux, the coil detects voltage, and the FID fades with time.
CHECK YOUR UNDERSTANDING
Follow the signal from the body to the scanner.
What produces voltage in the receive coil?
Changing magnetic flux from precessing transverse magnetization induces voltage in the coil.
What does FID mean?
Free induction decay. It is the oscillating MR signal detected after excitation that decreases with time.
Is one detected signal already an MR image?
No. Spatial encoding, repeated data collection, and reconstruction are still needed to create an image.
LESSON 05 COMPLETE
You understand how the first MR signal is created.Educational references
This lesson uses simplified models. Signal acquisition and reconstruction contain additional engineering and mathematical detail introduced later in the learning path.