The spin echo sequence in MRI is a fundamental technique used to generate signals from hydrogen nuclei (protons) and create detailed images of the body's internal structures. Here's a summary of the sequence:

Excitation Pulse:

• A brief radiofrequency (RF) pulse is applied, which tips the protons away from their equilibrium alignment with the magnetic field.

Precession

• After the excitation pulse, the protons begin to precess, or rotate, back into alignment with the magnetic field. However, due to natural variations in the magnetic field, the protons precess at different rates, leading to signal dephasing.

Refocusing Pulse

• A second RF pulse, known as the refocusing pulse or echo pulse, is applied at a specific time after the excitation pulse. This refocusing pulse reverses the dephasing of the protons, causing them to emit a synchronized signal known as the spin echo.

Signal Detection

• The spin echo signal is detected by receiver coils in the MRI scanner.

Image Formation

• The detected signal is processed using mathematical algorithms to reconstruct detailed images of the body's internal structures. The spin echo sequence helps improve image contrast and reduce artifacts, making it a valuable tool in MRI imaging for diagnosing various medical conditions.​​

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Free Induction Decay (FID) is a phenomenon in MRI that occurs after the cessation of a radiofrequency (RF) pulse. Here's a summary:

Excitation Pulse

• When an RF pulse is applied during MRI scanning, it tips the protons in the body away from their equilibrium alignment with the magnetic field.

Signal Emission

After the RF pulse is turned off, the protons begin to return to their equilibrium alignment. During this process, they emit radiofrequency signals as they release the energy absorbed from the RF pulse.

Free Induction Decay

• The emitted signals form a decaying signal known as Free Induction Decay (FID). FID represents the sum of signals emitted by the protons in the body.

Signal Detection

• FID is detected by receiver coils in the MRI scanner. The detected signal is then processed to create images of the body's internal structures.

Tissue Contrast:

• The characteristics of FID, such as its duration and decay rate, provide information about tissue properties, contributing to the contrast observed in MRI images.

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