2.3 Radiofrequency (B1) Field Hazards

Radiofrequency (B1) Field Hazards

In magnetic resonance imaging, the radiofrequency (RF) field, designated as the B1 field, is an electromagnetic wave applied perpendicular to the static magnetic field (B0). The B1 field oscillates at the Larmor frequency of hydrogen protons (e.g., 63.86 MHz at 1.5T and 127.7 MHz at 3.0T) to tip the net magnetization vector into the transverse plane. However, the patient's body acts as a conductor, and the majority of the RF energy transmitted during excitation is absorbed by the patient's tissues, dissipating as heat. This energy deposition presents significant physiological hazards, primarily in the form of thermal heating and proximity burns, requiring strict adherence to Specific Absorption Rate (SAR) limits, conductive loop management, and RF shielding principles.

---

Thermal Heating and Specific Absorption Rate (SAR)

The rate at which RF energy is deposited in body tissue is measured as the Specific Absorption Rate (SAR), expressed in units of Watts per kilogram (W/kg).

The deposition of RF energy and the resulting SAR are governed by several key variables: $\text{SAR} \propto B_0^2 \cdot \theta^2 \cdot D \cdot f$

Where:

• $B_0$ is the static magnetic field strength. Because SAR is proportional to the square of the static magnetic field ($B_0^2$), scanning a patient at 3.0T deposits four times the RF energy of an identical scan at 1.5T.
• $\theta$ is the RF flip angle. A 180° refocusing pulse (commonly used in fast spin-echo sequences) deposits four times the RF energy of a 90° excitation pulse.
• $D$ is the duty cycle, representing the number and frequency of RF pulses applied per unit time.
• $f$ is the patient's body weight and conductivity.

FDA and IEC SAR Limits

To prevent core body temperature elevation, the FDA and the International Electrotechnical Commission (IEC) enforce strict limits on SAR during clinical imaging:

Scanner Operating Modes

The MR system operates under three defined safety modes:

1. Normal Operating Mode: SAR is kept below limits that could cause physiological stress (e.g., < 2.0 W/kg whole body). Safe for all patients.
2. First Level Controlled Operating Mode: One or more parameters (such as SAR up to 4.0 W/kg whole body) can cause physiological stress, which may manifest as mild core heating. The technologist must actively monitor the patient and provide clinical justification.
3. Second Level Controlled Operating Mode: SAR exceeds First Level limits. This mode is restricted to institutional research protocols under strict IRB oversight, as there is a significant risk of tissue damage.

---

Proximity Burns and Conductive Loops

The most common patient injuries in the MR environment are RF-induced thermal burns. These burns occur when the oscillating B1 field induces electrical currents within the patient's tissues or in external conductive materials (such as cables or wires).

Mechanism of RF Burns

If the patient's skin touches another part of their own skin (e.g., hands clasped together, thighs touching, or arms resting against the torso), a closed electrical circuit—or conductive loop—is formed. The RF field induces a current that flows through this loop. At the point of contact, the surface area is extremely small, creating high electrical resistance. According to Joule's Law ($P = I^2 R$), this resistance causes rapid, intense localized heating, leading to severe, full-thickness (third-degree) burns.

Similarly, if the patient's skin directly touches the inner bore wall of the scanner (where the transmit RF body coil is located), the high local RF field can induce currents directly in the skin, causing severe contact burns.

---

Conductive Loop Management

To prevent RF-induced heating and burns, the technologist must implement strict positioning and cable routing protocols:

• Insulation Pads: Always place non-conductive, MRI-safe foam pads or blankets between the patient and the scanner bore, and between any skin-to-skin contact points. These pads must be at least 1 to 2 cm (0.4 to 0.8 inches) thick to provide adequate physical and thermal insulation.
• Positioning: Ensure the patient's arms are kept at their sides and do not cross. The ankles and knees must be separated by foam pads. The patient's hands should never touch or clasp.
• Cable Routing: Any cables from surface coils, ECG leads, or pulse oximeters must be inspected for insulation damage. Cables must be routed straight down the center of the table, directly away from the scanner bore wall. They must never be looped, crossed, or allowed to touch the patient's bare skin.
• Internal Implants: Patients with internal conductive implants (such as pacemakers, deep brain stimulators, or orthopaedic hardware) can experience localized heating at the tips of the implants. Technologists must follow the specific MR Conditional guidelines for each device, which often limit scan times, require specific coils, or mandate reduced-SAR sequences.

---

RF Shielding (Faraday Cage)

The radiofrequency signals emitted by the patient's precessing protons are extremely weak. To detect these signals without corruption, the scanner room (Zone IV) must be isolated from the external electromagnetic environment.

The Faraday Cage

The scanner room is lined on all six sides (walls, floor, ceiling) with sheets of copper or aluminum, forming an electromagnetic shield known as a Faraday Cage. The viewing window is constructed with a double layer of glass sandwiching a fine copper mesh screen.

Purpose of RF Shielding

1. Excluding External Noise: It prevents external radiofrequency noise (such as radio broadcasts, cellular networks, television stations, and electrical equipment) from entering the scanner room. If these external signals enter, they are picked up by the receiver coils and create image artifacts, particularly a line of noise known as a zipper artifact.
2. Containing Scanner RF: It prevents the high-power RF excitation pulses generated by the scanner from escaping and interfering with nearby hospital equipment.

Maintenance of Shielding Integrity

The weakest point of the Faraday cage is the magnet room door. The perimeter of the door is lined with copper spring contact strips, known as RF fingers.

• Technologist Duty: The door seal and RF fingers must be kept clean, free of debris, and undamaged. If the door is left slightly ajar or if the RF fingers are damaged, external RF noise will leak into the room, resulting in a persistent zipper artifact aligned with the phase-encoding axis of the image.