3.12 Coil Selection and Positioning

Radiofrequency (RF) Coil Principles and Classification

In magnetic resonance imaging (MRI), radiofrequency (RF) coils act as the antenna of the system, serving as the essential interface between the patient's tissues and the scanner's electronics. RF coils operate on the principle of electromagnetic induction (Faraday's Law of Induction). They can function as RF transmitters (depositing electromagnetic energy into the patient's tissues to excite hydrogen nuclei), RF receivers (detecting the weak voltage induced by the precessing transverse magnetization), or both, which are classified as Transmit/Receive (T/R) coils. Achieving high-quality diagnostic images requires selecting the appropriate coil type and ensuring precise physical positioning and electrical decoupling.

Classification of RF Coils

RF coils are classified based on their geometry, configuration, and sensitivity profiles. The three primary categories are volume coils, surface coils, and phased-array coils.

Volume Coils

Volume coils are designed to completely surround the target anatomy, providing a highly homogeneous RF excitation and signal detection profile throughout the enclosed volume.

• Main System Body Coil: This is the largest volume coil, built directly into the scanner gantry. It typically functions as the primary RF transmitter for most imaging sequences, while smaller local coils act as receivers.
• Local Volume Coils: These include rigid, cylindrical, or semi-rigid structures designed for specific anatomy, such as head coils, extremity coils (e.g., knee or wrist), and birdcage coils. Many of these function as T/R coils. Using a local T/R coil restricts RF deposition to the target area, which decreases the patient's overall Specific Absorption Rate (SAR).
• Trade-Offs: The primary advantage of volume coils is their uniform spatial sensitivity. The main disadvantage is their lower Signal-to-Noise Ratio (SNR) for localized structures compared to surface coils. Because a volume coil is large, it detects noise from the entire volume of tissue it encompasses, raising the noise floor.

Surface (Local) Coils

Surface coils (or local coils) are flat or shaped single-loop receiver coils placed directly on or very close to the patient's skin over the area of interest.

• Sensitivity Profile: Surface coils provide extremely high SNR for tissues immediately adjacent to the coil. However, their sensitivity is highly inhomogeneous and drops off rapidly with depth. As a clinical rule of thumb, the effective sensitivity depth of a simple circular surface coil is approximately equal to the coil's radius (or half of its diameter). Beyond this depth, the signal degrades below diagnostic levels.
• Clinical Applications: Surface coils are ideal for small, superficial structures requiring high spatial resolution, such as the temporomandibular joints (TMJ), orbits, shoulder joints, and the spine.

Phased-Array Coils

Phased-array coils combine the high SNR of small surface coils with the large coverage area of volume coils. They consist of multiple small surface coil loops (referred to as elements) arranged adjacent to one another.

• Mechanism: Each element in the array is connected to its own independent receiver channel and preamplifier. The scanner receives and processes signals from all channels simultaneously, combining them to form a single image.
• Parallel Imaging: Phased-array coils are the hardware foundation for parallel imaging techniques (e.g., SENSE, GRAPPA). These techniques use the distinct spatial sensitivity profiles of individual coil elements to reduce the number of phase-encoding steps, accelerating scan times. The acceleration factor ($R$) is limited by the number of coil elements and the geometry factor (g-factor), which measures the noise amplification associated with parallel imaging reconstruction.

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RF Coil Positioning and Alignment

Proper coil selection must be paired with accurate positioning to optimize image quality and prevent artifacts.

Center of Interest and Isocenter

For all exams, the target anatomy must be placed in the exact center of the RF coil, and the coil itself must be positioned at the magnetic isocenter of the scanner. The main static magnetic field ($B_0$) and the transmit RF field ($B_1$) are most homogeneous at the isocenter. Positioning the anatomy off-center leads to:

1. Geometric distortion due to gradient non-linearity at the edges of the magnet.
2. Poor fat saturation, as fat-suppression pulses rely on a uniform resonance frequency across the field of view.
3. Reduced SNR and signal shading near the edges of the coil's active range.

Orientation Relative to B0

According to Faraday's Law, a precessing magnetic vector induces an electromotive force (voltage) in a conductor only if the magnetic flux through the loop changes over time.

• The Orientation Rule: The transverse magnetization vector ($M_{xy}$) precesses in the transverse plane (XY plane), which is perpendicular to the main magnetic field ($B_0$, along the Z-axis). To detect this precession, the active loop of a surface coil must be oriented parallel to the main magnetic field ($B_0$).
• Physics Mechanism: When the loop is parallel to $B_0$ (placed in the XZ or YZ plane), the rotating transverse magnetization cuts across the loop's plane, creating a changing magnetic flux that induces a voltage. If the loop is placed perpendicular to $B_0$ (in the XY plane), the transverse vector rotates within the plane of the loop itself. The net magnetic flux passing through the loop remains constant (zero change), resulting in complete signal loss.

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RF Coupling and Decoupling Mechanisms

When multiple RF coils are present within the magnet bore, they can interact through mutual inductance, a phenomenon known as RF coupling.

Hazards of RF Coupling

During the transmit phase of a sequence, the high-power RF pulse from the body coil can induce large currents in the nearby receiver coil. This coupling is dangerous for three reasons:

1. It distorts the $B_1$ transmit field, causing severe image artifacts and non-uniform excitation.
2. It can burn out the sensitive receiver preamplifiers.
3. It concentrates RF energy in the receiver coil conductors, creating local heating that can cause RF burns on the patient's skin.

To prevent this, receive coils must be electrically isolated, or decoupled, from the transmit field during RF excitation.

Decoupling Methods

Scanners employ three primary methods to decouple coils during operation:

• Active Decoupling: This is the primary electronic safety mechanism. It uses electronic switches, specifically PIN diodes, controlled by the scanner's RF control system. During the RF transmit pulse, a bias voltage is sent to the receive coil, forward-biasing the PIN diode. This detunes the receive coil's resonant circuit, making it non-resonant (invisible) to the transmit RF frequency. During the receive window, the bias voltage is removed, retuning the coil to detect the echo.
• Passive Decoupling: This acts as an automatic hardware backup. It utilizes crossed diodes (antiparallel diodes) connected across the coil's tuning circuit. When a high-amplitude RF pulse (the transmit pulse) is present, the diodes automatically conduct and detune the circuit. The weak voltages induced during the receive phase are too small to activate the diodes, allowing the coil to detect the MR signal.
• Geometric Decoupling: Within a phased-array coil, adjacent elements must be decoupled from one another to prevent mutual inductance and noise correlation. This is achieved by overlapping adjacent coil loops by a critical distance—approximately 10% to 15% of their diameter. This specific overlap cancels out the mutual inductance between the two loops. Non-adjacent elements are decoupled using low-input-impedance preamplifiers or inductive decoupling circuits.

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Signal-to-Noise Ratio (SNR) Optimization

Technologists can directly optimize SNR by adhering to three core clinical rules:

1. The Proximity Rule: Position the coil as close to the target tissue as possible. The MR signal strength drops off rapidly with distance from the coil surface, whereas the ambient noise detected by the coil remains constant.
2. The Size Rule: Select the smallest coil that provides adequate coverage of the anatomy of interest. A larger coil detects noise from a larger volume of tissue, which increases the noise floor and decreases the SNR for small, localized structures.
3. Quadrature vs. Linear Detection: Linear coils use a single receiver channel and detect signal along a single axis. Quadrature (circularly polarized) coils use two channels positioned 90 degrees apart. By combining the signals from both channels, quadrature coils increase the SNR by a factor of $\sqrt{2}$ (approximately 41%) and reduce the required transmit RF power by 50%.

RF Coil Comparison Table