Transmit Arrays for UHF Body Imaging
Stephan Orzada1,2
1Erwin L. Hahn Institute for MRI, Essen, Germany, 2High-Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany

Synopsis

As the main magnetic field strength increases, the corresponding RF wavelength is shortened. This leads to pronounced wave effects in the transmit field, causing inhomogeneous excitation. Multi-channel arrays provide additional degrees of freedom to mitigate such effects and to manipulate (or to tailor) RF transmission. Roughly these can be divided in 3 types, namely local arrays, remote circumferential arrays and travelling wave arrays. Examples of these arrays are presented in this educational talk.

Target Audience

MR engineers, scientists, and technicians who have an interest in understanding the basics of transmit coil arrays for UHF large FoV body imaging

Objectives

Illustrate different types of body arrays and basic understanding of difficulties in transmission at field strengths of 7T and above.

Purpose

Ultrahigh field (UHF) magnetic resonance imaging (MRI) at field strengths of 7 T or higher is very challenging. As the resonant frequency increases linearly with main magnetic field strength, the wavelength in tissue drops below the diameter of the examined body region. While at 1.5 T the mean wavelength in tissue is more than 50 cm and therefore much larger than the diameter of on axial torso slice, at 7 T the mean wavelength is only approximately 13 cm. This leads to visible wave effects that can cause complete signal dropouts within the field of view (1-3) and makes common birdcage coils unsuitable for body imaging at UHF. To get rid of this problem, a multitude of methods have been proposed: static methods like RF shimming (4, 5), semi static methods like TIAMO (6), or dynamic methods like transmit SENSE (7, 8). Common to all of these methods is the use of multiple transmit channels.

Arrays for imaging of the torso and large fields of view can roughly be divided into three groups: local arrays (9-15), remote circumferential arrays(16-18) and travelling wave antennas/arrays (19, 20), though there is a smooth transition between these groups.

Local arrays achieve a high transmit efficiency and if used as a transmit/receive array they also provide a high SNR at least in the periphery. On the downside these arrays can take up a lot of space within the small bore diameter of about 60 cm due to the necessary thickness of the elements. This makes it difficult to include subjects with a larger body physique and reduces patient comfort.

Remote circumferential arrays or integrated arrays have the advantage of leaving more space for the patient inside the bore, but they are less power efficient and exhibit a lower SNR at least in the periphery when used as transmit/receive arrays. Furthermore, integrating such arrays is time consuming and quite intricate because a lot of the system has to be disassembled. Figure 1 shows an example of such an integrated 32-channel body coil. The freed space inside the bore can be used to accommodate subjects with a larger body physique, or to include other equipment like dedicated local receive arrays.
When installing remote circumferential coil arrays, it may be necessary to also include extra peripheral components like controllers for T/R-switches and detuning as well as switches for the receive chain to ensure other experiments with local transmit coils are not influenced and that not too many system resources (PIN-diode currents, receive channels etc.) are used up since the coil arrays cannot be easily removed.

Travelling wave concepts are easy to implement, but their transmit efficiency can be very low. When imaging the center of the body, half of the body tissue in one z-direction has to be penetrated to reach the FOV. Concerning space inside the bore, this concept shows the same advantages as the remote circumferential arrays.

Results & Discussion

The existence of travelling wave concepts shows that an important issue to consider in all types of arrays is wave propagation. At higher field strengths the ratio of the size of the transmit elements to the wavelength increases, leading to potentially higher radiation, especially since waveguide modes can propagate along the bore. Concepts known from lower field strengths like estimating the SNR efficiency from the ratio of loaded Q to unloaded Q are not valid anymore since the losses in the “unloaded” case include radiation losses that are not necessarily equal to the radiation losses in the loaded case where more of the power is radiated towards the sample. On the other hand, the radiation has to be taken into account since it has an impact on field distribution inside the bore and power can be radiated out of the bore, potentially causing unwanted interaction with magnet room peripheral components.
Furthermore one fundamental difference between transmit arrays and receive arrays is the most important figure of merit of each case. While for receive elements it is $$$ \frac{B^{-}_{1}\big(\overrightarrow{r}\big)}{\sqrt{P}} $$$ as a measure of SNR at the point of interest, for transmit arrays the more important value is $$$ \frac{B^{+}_{1}\big(\overrightarrow{r}\big)}{\sqrt{SAR_{most\ critical}}} $$$ since in the transmit case the limit for the most critical SAR aspect is a hard limit while more transmitter power is just a matter of amplifier costs.

Acknowledgements

No acknowledgement found.

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Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)