Connecting the 3D physical evolution and relaxation of the nuclear magnetic moment — spin tipping, Larmor precession, isochromat dephasing, and T1 recovery — directly to the voltage signal induced in an RF receiver coil via Faraday's Law of Induction.
Physical Principles & Excitation Memory:
1. Remembering Mz & T1 Saturation: An RF pulse can only tip whatever longitudinal magnetization (Mz−) is currently present in the system at the moment of excitation. If you fire a 90° pulse shortly after a previous excitation before T1 recovery is complete, Mz− is small, so the resulting transverse magnetization Mxy(0+) = Mz− · sin(90°) and the generated FID are proportionally small. This progressive reduction of signal with rapid pulsing is T1 saturation.
2. Inversion Recovery & The Null Point: Fire a 180° pulse to invert Mz to −1.00. As Mz(t) recovers towards +1.00, it passes through zero (the null point at tnull = T1 ln 2 ≈ 0.693 T1). If you apply a 90° pulse exactly when Mz ≈ 0, Mxy(0+) = 0 × sin(90°) = 0 — no FID signal is produced! This is the exact principle used in FLAIR (suppressing CSF) and STIR (suppressing fat).
3. Faraday's Law of Induction: An RF receiver coil detects the time-derivative of magnetic flux: Ε(t) = −dΦB/dt ∝ −dMx/dt. Only transverse magnetization (Mxy) produces an alternating flux through the transverse coil loop.