Radiofrequency Coils, Hardware, and Signal Reception
Core concepts
RF coils transmit B₁ and receive signal. Transmit coils (body coil) create uniform excitation; receive coils (surface, phased-array) sit close to anatomy for high SNR but limited field of view. Coil SNR depends on proximity, filling factor, and Q. Phased arrays combine multiple small coils for wide coverage with parallel-imaging capability.
Hardware chain
Signal path: coil → preamplifier → receiver → digitizer. The transmit/receive switch protects the receiver during transmission. Coil tuning matches resonance to the Larmor frequency; matching optimizes power transfer. A detuned coil during transmit avoids induced currents.
Traps
- Confusing coil sensitivity (receive) with B₁ homogeneity (transmit).
- Assuming more coil elements always improve SNR everywhere; noise amplification occurs at depth and in parallel reconstruction.
- Forgetting that surface coils give bright signal near the coil and falloff with distance.
Rule of thumb
SNR improves with √(number of elements) only when noise is uncorrelated and geometry is favorable. Always match coil to anatomy: head coil for brain, body array for abdomen.