mri level 2

B Sub One: Radiofrequency Pulse

  • Definition: B sub one represents the RF pulse, depicted as a blue sine wave in diagrams. It is perpendicular to the static magnetic field, denoted as v sub zero.
  • Purpose: The RF pulse is used to excite hydrogen nuclei in a process known as resonance.
  • Frequency Details:
    • RF pulses are typically transmitted in the frequency range of megahertz (MHz).
    • For a 1.5 Tesla MRI scanner, the transmit frequency is approximately 64 MHz.
    • For a 3 Tesla MRI scanner, the transmit frequency is approximately 128 MHz.
  • Health Risks: RF is associated with the potential for heating and burns in patients.

Measurement Units for B Sub One

  • Specific Absorption Rate (SAR): A measurement of the RF energy absorbed during each pulse sequence, varies by manufacturer and body parts scanned, determined by weight and height, reported in watts per kilogram (W/kg).
  • Specific Energy Dose (SED): Another term that could be referenced with regards to energy absorption.
  • B One Plus RMS: A measure that represents the magnitude of the positively rotating component of the RF pulse averaged over a duration of ten seconds, reported in microtesla (µT).

Operating Modes of MR Scanners

  • Normal Mode:

    • Core Body Temperature Increase: Increases temperature by 0.5°C.
    • Health Risk: No significant health risk or physiological stress on the patient.
    • Limits by IEC and FDA: 2 W/kg for whole body and 3.2 W/kg for the head.

  • First Level Mode:

    • Core Body Temperature Increase: Increases temperature by 1°C.
    • Health Risk: There is a significant risk of physiological stress; caution required.
    • Limits by IEC and FDA: 4 W/kg for whole body and 3.2 W/kg for the head.

  • Second Level Mode:

    • Core Body Temperature Increase: Increases temperature by more than 1°C.
    • Health Risk: This level indicates significant risks and should only be used in research settings, not in clinical practice.

Heat Dissipation Mechanisms in Patients

  • Evaporation: Body cools through sweat evaporation, lowering core temperature.
  • Convection: Heat is moved away via flowing cool air (e.g., fan turned on by technologist).
  • Conduction: Blood and capillaries are cooled by an external medium (ice pack or heat sink) and circulate the cooled blood throughout the body.
  • Radiation: Heat radiates out of the body to maintain core temperature, occurs between pulse sequences.

Factors Affecting Thermoregulation

  • Patient Factors:
    • Age, body habitus, medications (like beta-blockers and vasodilators), existing medical conditions (e.g., diabetes, congestive heart failure).
    • Body habitus: the ratio of surface area to body mass, significantly influences heating; obese patients tend to have more heat generation, while infants possess a large surface area relative to body mass, reducing heating concerns.
  • Environmental Controls: MRI suites should maintain a temperature below 25°C (77°F) and humidity levels below 60%.

Techniques to Minimize RF Heating

  • Technologist Strategies:
    • Turn on fan inside scanner to enhance airflow.
    • Allow time for heat dissipation between pulse sequences.
    • Sandwich gradient echo sequences between spin echo sequences.
    • Modify repetition time (TR) and echo spacing, reduce receiver bandwidth and flip angle, decrease the number of slices in imaging, and avoid rapid spin echo pulses.
    • Consider half Fourier case-based filling method, reduce phase encoding steps, increase acquisitions, and utilize soft pulses among others.

RF Burns and Prevention Strategies

  • Common Causes of RF Burns:
    • Proximity Burns: Occur when tissue makes direct contact with the scanner bore. More common in larger/obese patients; prevention includes a 0.5 to 1 cm foam pad barrier.
    • Induced Loop Burns: Result from tissue contact forming a loop, causing induced currents; prevention involves using foam pads and ensuring limbs do not cross.
    • Reflective Burns: Caused by conductive materials on the patient's body (e.g., patches, clothing); awareness and avoidance are key.
    • Resonant Burns (Antenna Effect): Linked to focused electric fields on implants, influenced by the implant’s location, shape, orientation, and size (implants > 2 cm have a higher risk).

Heating Dynamics of Implants

  • Implant Location and Heating:
    • Near field (close to transmit coil): Higher RF energy exposure.
    • Shape: Sharp edges concentrate electric fields, increasing heating potential.
    • Orientation: Implants parallel or perpendicular to B sub one will experience the most significant heating.
    • Size: Implants larger than 2 cm pose a risk as heating correlates with their length/dimensions in relation to RF wavelength;
    • At 1.5 Tesla: RF wavelength ~ 500 mm, significant heating occurs when implants reach or exceed 250 mm.
    • At 3 Tesla: RF wavelength ~ 240 mm, significant heating at 120 mm.

RF Drop-Off Plot

  • Purpose: Calculates the RF deposition in a patient at specific distances from isocenter; maximum RF deposition occurs at isocenter.
  • Visualization: Typically represented graphically, illustrating how RF energy decreases with distance from the center of the scanner.

Types of RF Coils

  • Transmit Only Coils: (e.g., body coil/birdcage coil) produces larger area RF energy deposition.
  • Transmit and Receive Coils (TR Coils): Can transmit and receive RF energy; produce localized smaller area RF deposition.
  • Receive Only Coils: (e.g., surface coils); these are positioned directly on the region of interest to receive RF energy effectively.