RF Coils in MRI

Study Notes

RF coils are crucial components affecting image quality in magnetic resonance imaging (MRI).
They transmit radiofrequency (RF) pulses and receive the RF signal.
RF coils can be classified into several types:
- Transmit-only coils: These coils only transmit RF signals.
- Receive-only coils: These coils only receive RF signals.
- Transmit-receive coils: These coils both transmit and receive RF signals.

Volume coils: Designed for uniform RF excitation across large volumes, suitable for whole-body imaging. Volume coils generally have better magnetic field homogeneity than surface coils. A larger body coil transmits the RF pulse while a smaller coil receives the signals.
- Types of volume coils include: Circularly Polarized, Quadrature, Bird Cage, Crossed, Helmholtz Pair, Paired Saddle, Single Turn Solenoid (most coils today).
Surface coils: Simple coil design (loop of wire), placed over a specific region of interest on a patient for enhanced magnetic sensitivity. Surface coils typically have higher signal-to-noise ratios (SNR) for tissues close to the coil compared to those further away. They are used for regions like spines, shoulders, jaw joints, and smaller body parts.
- Types of surface coils include: Array, Body Wrap Around, Linearly Polarized, Saddle.
Phased Array coils: Combine benefits of smaller coils (high signal-to-noise ratio) and larger coils (large measurement field). These often comprise multiple smaller coils that can be used individually or together for targeted areas of interest. Some phased array coils are coupled, isolated, and also include the patient couch as a coil.
Quadrature coils: Are commonly used volume coils. They use two loops of wire perpendicular to one another, producing more signal than single-loop coils.