MRI Physics and Instrumentation
Core physics
Candidates must be fluent in the proton as the MR signal source: net magnetization, precession at the Larmor frequency (ω = γB₀, with γ/2π ≈ 42.58 MHz/T for hydrogen), excitation by an RF pulse, and relaxation via T1 (longitudinal recovery) and T2/T2* (transverse decay). Know the spin-echo and gradient-echo sequences, TR/TE effects on contrast, and how flip angle and echo time drive gradient-echo weighting. Understand spatial encoding: slice selection with a gradient plus RF bandwidth, frequency encoding (readout), and phase encoding that determines scan time (number of phase-encoding steps × TR, adjusted by parallel imaging and partial Fourier).
Instrumentation
The static field (superconducting magnet, shim coils, gradient coils, RF transmit/receive coils) and how gradient slew rate and amplitude limit spatial resolution and speed. Higher field (1.5 T vs 3 T) improves SNR but worsens B0/B1 inhomogeneity, chemical shift, and SAR.
Traps
Questions conflate T1 with T2, or ask which parameter lengthens scan time (phase-encoding steps, not TE). Remember that T2* includes static field inhomogeneity while T2 does not, and that spin echo refocuses static dephasing but not true T2 decay. Do not confuse the Larmor frequency scaling (linear with B₀) with relaxation times (largely field-independent for T2, longer T1 at higher field).
Rule of thumb
Scan time ≈ TR × phase-encoding steps × number of averages ÷ (parallel imaging factor). Doubling phase-encoding resolution doubles time unless acceleration is added.
Worked example
At 3 T, hydrogen precesses at ≈127.7 MHz (42.58 × 3). A spin-echo with TR 600 ms and 256 phase-encoding steps, one average, no acceleration: 0.6 s × 256 ≈ 154 s.