Radiofrequency Fields & Associated Risks

Aurelien Destruel^1,2
¹Aix-Marseille University, CNRS, CRMBM, Marseille, France, ²APHM, CEMEREM, Marseille, France

Synopsis

Keywords: Physics & Engineering: RF Safety, Physics & Engineering: Hardware, Physics & Engineering: High-Field MRI

Radiofrequency (RF) coils are essential components of MR scanners. They can transmit the excitation field B₁, receive the MR signal. They can be birdcages, loops or dipoles (and more), volume or surface, for brain, spine, knee or other,… If left unchecked, RF coils may cause tissue heating, requiring the control of the specific absorption rate (SAR). What exactly is this parameter, and how is it calculated? This talk will give an overview of RF coils and RF safety used in clinical environments, while introducing problematics specific to ultra-high field MRI, such as parallel transmit techniques (pTx).

Introduction

Radiofrequency (RF) coils are essential components of MR scanners. They can transmit the excitation field B₁, receive the MR signal, sometimes even both (although not at the same time). They can be birdcages, loops or dipoles (and more), volume or surface, for brain, spine, knee or other,… If left unchecked, RF coils may cause tissue heating, requiring the control of the specific absorption rate (SAR). What exactly is this parameter, and how is it calculated? This talk will give an overview of RF coils and RF safety used in clinical environments, while introducing problematics specific to ultra-high field MRI.

Radiofrequency coils

RF coils are the hardware components of the MR scanner which stimulate and receive the MR signal. They are typically divided between transmit only coils, receive only coils and transmit/receive (transceiver) coils. Different components are used for the transmit and receive chains, as they have widely different constraints. Transmit coils have to be able to handle large amount of power to excite nuclei across the field-of-view of the MR image, while the receive chain needs to be able to detect small voltages which are amplified before further processing¹.

In most clinical MR scanners, volume coils are used to transmit, using the so-called birdcage design. Two input voltages are typically placed with locations 90° from each other, with the second input being driven with a 90° phase shift. This configuration produces a circularly polarized (CP) RF field, which is the most efficient for exciting large volumes². Power efficiency and homogeneity of the transmit magnetic field (B₁⁺) are important considerations for this type of coils. On the other hand, surface coils consist of an array of single elements, and are used as receive coils in most clinical scanners. Combining the images generated by the different elements (often using loop designs) is critical to achieve high signal-to-noise ratio (SNR), which is one of the main priorities of receive coils³.

Things are a little different for ultra-high field (UHF) MRI, with field strengths (B₀) of 7T or higher⁴. As the resonance frequency of RF coils increases with B₀, the wavelength of B₁ becomes close to the dimension of the imaged body regions (~13 cm at 7T). This causes wave effects that can appear as brightening or darkening of the signal intensity, making whole body coils unsuitable for UHF MRI⁵. Local or surface coils must therefore be used as transmit coils. In recent years, the inhomogeneity of the B₁⁺has led to the development of different types of coil designs, such as dipoles^6,7, improved loops^8–10, or others^11,12, aiming to mitigate UHF issues with better RF coil elements.

Radiofrequency safety

While the magnetic component of the RF field is used to create MR images, its electrical component is only involved in RF safety. A consequence of applying a time varying RF pulse is that it may generate eddy currents in tissue, which consist of charged ions that are displaced and oscillate while electrically and mechanically interacting with their surroundings. This heat generation process is the main source of temperature increase at the frequencies used in MRI, and is monitored by the SAR parameter¹³.

Although tissue temperature is the most relevant parameter¹⁴, accurately and quickly measuring temperature increase in vivo remains complex and is far from being implemented routinely. SAR was introduced to limit RF heating, as an estimate of the amount of RF energy that is deposited in tissue per unit of mass (W/kg), with RF safety guidelines described in the IEC 60601-2-33¹⁵. Whole-body SAR is a measure of the useful forward power (forward minus reflected power) divided by the total mass of the patient. It is the most commonly monitoring parameters used in scanners up to 3T, as whole-body transmit volume coils are typically used. In cases where local transmit coils are used and the RF energy is localized in the head or extremities, head SAR and partial-body SAR are used by dividing the useful forward power by the mass of the exposed body region.

At UHF and when using local transmit coils, the transmit RF field, and therefore the electric field, may be inhomogeneous, with local SAR values sometimes being several times larger than the whole or partial body SAR (apparition of ‘hot spots’)¹⁶. Better knowledge of the local SAR distribution is necessary, and the peak value of SAR averaged over cubes containing 10g of tissue (SAR_10g) is used to limit the forward power¹⁵. Because the electric field cannot be measured in vivo, SAR_10g is calculated from electromagnetic simulations of realistic human models^17,18.

The RF field can also be responsible of heating in case of conductive devices and implants, but this will be addressed in a specific lecture dedicated to devices and implants safety in this session.

Finally, the presentation will briefly introduce methods to reduce whole-body SAR, SAR_10g and mitigate B₁⁺ inhomogeneity, such as parallel transmit (pTx) techniques^19,20.

Acknowledgements

No acknowledgement found.

References

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Proc. Intl. Soc. Mag. Reson. Med. 31 (2023)