The Free Induction Decay (FID) in MRI refers to the exponentially decaying signal emitted by the excited nuclei in a magnetic field after a radiofrequency (RF) pulse is applied.
Understanding Free Induction Decay (FID)
After a patient is placed in a strong magnetic field in an MRI scanner, the protons (specifically, the hydrogen nuclei) align with the field. An RF pulse is then transmitted into the patient. This pulse excites the protons, causing them to absorb energy and tip away from their alignment with the main magnetic field.
When the RF pulse is turned off, the excited protons begin to relax back to their equilibrium state. This relaxation process has two main components:
- T1 Relaxation (Longitudinal Relaxation): The protons realign with the main magnetic field, releasing energy to the surrounding environment (the "lattice").
- T2 Relaxation (Transverse Relaxation): The protons lose phase coherence, meaning they no longer precess in sync with each other.
As the protons return to equilibrium, they emit a radiofrequency signal. This signal is the FID.
Characteristics of the FID
The FID has the following key characteristics:
- Frequency: The frequency of the FID signal is the Larmor frequency, which is proportional to the strength of the main magnetic field.
- Amplitude: The initial amplitude of the FID is proportional to the number of protons that were excited.
- Decay: The amplitude of the FID signal decays exponentially over time, due to T2 relaxation. The rate of decay is determined by the T2 relaxation time of the tissue.
Importance of FID in MRI
The FID is the fundamental signal that is used to create MRI images. The MRI scanner's receiver coil detects the FID signal, and the data is then processed using Fourier transformation to generate an image. By manipulating the timing and characteristics of the RF pulses and gradients, MRI allows us to extract different information from the FID signal, enabling the creation of images with varying contrast and resolution. The faster the decay of the FID, the faster the signal disappears, limiting the possible imaging techniques.
Example:
Imagine a group of runners all starting a race at the same time (aligned protons). An RF pulse is like a starting gun that tells them to begin running around a circular track. As the runners run, some will naturally start to fall out of sync (lose phase coherence) due to individual differences in speed and stamina (T2 relaxation). The "signal" from the running group (FID) gradually fades as fewer runners are running together in sync. Meanwhile, some runners are stopping altogether and getting back in line at the starting line (T1 relaxation).
In summary, the FID is the transient signal generated in MRI due to the relaxation of excited nuclei, and its characteristics are crucial for forming the basis of image construction.
