Physics and Instrumentation
Field, precession, resonance
A static field B0 aligns hydrogen nuclei; an RF pulse at the Larmor frequency tips them. Larmor frequency = 42.58 MHz/T × field strength, so 1.5 T gives about 63.9 MHz and 3 T about 127.7 MHz. Traps: confusing the gyromagnetic ratio with the resonant frequency, and forgetting that both TR and TE determine weighting - short TR and short TE give T1 weighting, long TR and long TE give T2 weighting, long TR and short TE give proton density.
Gradients, k-space, and image formation
Gradients encode position: slice selection during RF, phase encoding by amplitude, frequency encoding by readout. Raw data are stored in k-space; the center holds contrast and the periphery holds detail, and a two-dimensional Fourier transform yields the image. The number of phase-encoding steps sets scan time, so reducing it via a rectangular FOV or parallel imaging saves time at the cost of signal-to-noise.
Instrumentation
Know the main magnet (superconducting, resistive, permanent), shim coils, gradient coils, RF transmit and receive coils, and the cryogen system. Phased-array coils improve SNR near the surface. Receive bandwidth affects SNR and chemical-shift artifact: wider bandwidth reduces chemical shift but lowers SNR.
Rule of thumb
Doubling field strength roughly doubles available SNR but approximately quadruples RF power and SAR, and lengthens T1. A spin-echo sequence refocuses static dephasing and shows true T2, while a gradient echo does not and shows T2*; that is why gradient echo is sensitive to susceptibility and hemorrhage.