Session Title: RF Engineering (Coils)

Speaker Name: Yeunchul Ryu, yeunchul.ryu@gachon.ac.kr Department of Radiological Science and Neuroscience Research Institute, Gachon University, Incheon, Korea

Highlights:

● Transmit arrays enable finer control over the RF field distribution in exciting the MR signals

● Prerequisites include knowledge of the field distributions (magnetic and electric) and the complete scattering matrix (matching/tuning/decoupling).

● Potential applications include enhanced uniformity of the RF excitation field, subvolume excitation, and SAR management.

Program Name: RF Arrays (Transmit Arrays & Circuitry)

Target audience: MR engineers, scientists, and technicians who have an interest in understanding the basic of transmit coil arrays

Objectives: Illustrate the principles of operation and basic applications of transmit RF coil arrays

Principles: From the beginning of MRI, homogeneous exciting the spins over the FOV (Field of View) with RF excitation (B1+) field is the most important role. The birdcage coil with single-port excitation (or two-port excitation) has been realized that role of transmt (Tx) coil faithfully in general diagnostic MRI (especially in the lower field magnetic field strength)1. However as the main field strength goes up for some advantages such as better signal-to-noise ratio (SNR), the B1+ field uniformity and the specific-absorption-rate (SAR) goes worse due to their shortened radio frequency (RF) wavelength. To provide more control over the B1+ fields, arrays of Tx coil elements have been introduced that allow independent driving of each coil element for more degree of freedom. One of the most basic geometries is the degenerate birdcage 2, which basically decouples all of the circular subunits formed by the legs and end-rings from one another. Many of the Tx arrays currently under investigation are, however, based on the combination of multiple, individual transmission line elements, each of which supports a transverse electromagnetic (TEM) field pattern 3.

Due to the increasing number of Tx channel, several issues need to be considered in the design of a Tx array. The matching and tuning with less interference between elements are one of the major issue. Also, the Tx pattern of each element should be known; this knowledge is often obtained in vivo with B1+ mapping techniques, but unfortunately these techniques do not provide sufficient accuracy for all applications within appropriate time. Unlike a volume coil which usually has a fixed Tx mode, Tx arrays can be driven with variable magnitude and (or) phase relations between the Tx coil elements. Especially at higher magnetic fields, this mode may not lead to the desired circular polarization (CP) inside the tissue. By adjusting the phase differences and element magnitudes, the B1+ field uniformity can be greatly improved. With similar algorithm to this B1 shimming, the electrical field distribution and hence the SAR distribution can be more finely controlled for decreasing RF absorption in the tissue. To fully describe a Tx array and assess the RF power deposition (local SAR) of the subject, the field distributions of the individual elements must be known for a suitable inhomogeneous body model, and these field distributions must be superimposed for each applied given by B1 shimming techniques. Concerning RF safety, this makes the use of Tx arrays more complicated compared to Tx arrays with fixed excitation vectors. The Tx array technology have shown that it is possible to achieve a uniform B1+ field even in an ultra high field MRI. However such arrays also can be used to apply unique RF pulses on each element as required in Transmit SENSE (T-SENSE)4. By changing RF frequency, phase, magnitude, and RF shape on each element in the pulse sequences, and also spatial placement of the individual elements, the degrees of freedom to control B1+ fields can be maximized. T-SENSE can facilitate the use of multi-dimensional RF pulses by shortening them to practical durations. Such pulses can be used to excite arbitrary subvolumes, correct for artifacts due to non-uniformities in both the B0 and B1 fields, and manipulate the electric field (local SAR)

Conclusion: Due to the shortened RF wavelength effects at higher magnetic field strengths, Tx arrays are likely a necessity to perform even basic imaging in anatomy with large cross-section. But even at the conventional MRI system of 1.5T and 3T, Tx arrays open up many opportunities to be taken advantage of manipulation of complex RF field. Due to these multifaceted uses, Tx arrays can be expected to play an increasing role in MRI in the coming years and replace conventional volume resonators, helping to improve MR image quality, promote safety, and increase application versatility.