Introduction:

Directions that address the challenges of field inhomogenieties involve the development of new RF coil element concepts, where each concept is designed and optimized for a specific application and setup. Hence, many different antennas were developed in the past years. Additionally, the evaluated performance benchmarks of the antennas can vary. To date, no common standard on performance benchmarks nor on evaluation setups of Tx arrays exist. This makes the cross paper comparison of transmit elements difficult.

Therefore, this page aims to provide an overview on multiple transmit antennas developed for 7T MRI for standardized phantom and selected performance benchmarks.

You will find data of each antenna for the power and SAR efficiency, as well as for the intrinsic decoupling and the load dependence.
Additionally, CAD data of each antenna is provided to you for download so that you can simulate the antenna for your use case by yourself. Furthermore, information about the standardized setup as well as on the simulation settings is provided.

We will continously add further transmit elements and are open to feedback and comments. Also feel free to send us your antenna as a .stp or CST file so it can be added to the list. Use: antennacomp@gmail.com for any requests.

The Performance Benchmarks:

Power Efficiency

The power efficiency is one of the most used performance benchmarks for transmit elements and indecates how much B1 field is generated by a certain amount of power. For the comparison the power efficiency is normalized to 1W of stimulated power and is displayed in µT/√W.

SAR Efficiency

The SAR efficiency takes the patient safety into account and is displayed by the power efficiency normalized to the square root of the peak SAR in the subject. It is an important measure since it describes how much power can be transmitted without substantial subject heating and is displayed in µT/√(W/kg).

Load Dependence

Many transmit antennas are used in several applications with varying subjects. For the imaging workflow it is important to have a reliable antenna. Therefore, the load dependence indicates how much the tuning of the antenna changes due to load variations. It is displayed as a deviation in % from an initial tuning to -50dB.

Decoupling

The intrinsic decoupling describes how strong the crosstalk between adjecent transmit elements is. A strong crosstalk results into less power transmission into the patient, hence lower transmit efficiency and it reduces the pTx capability. Therefore, a low intrisic decoupling is desired. It is represented by S21 in dB.

The Setup:

Phantom Properties and Simulation Setup:

Test setup for the antenna comparison

a) For the evaluation of the power and SAR efficiency, the antennas are centrally placed 2cm above the phantom (PVP Solution (yellow): 40cm x 30cm x 15cm, relative permittivity = 60, conductivity = 0.8S/m, density = 1058kg/m^3; PMMA (turquoise): permittivity = 3.4, conductivity = 0.0043S/m, density = 1180kg/m^3), the blue lines indicate the center and offcenter measurement (at 1/4th of the antenna length from the center)

b) For the evaluation of the intrinsic decoupling, two antennas are placed next to each other and the decoupling depicted by S21 evaluations is performed for 3 distances between the antennas (5cm, 10cm and 15cm)

c) The load dependence is evaluated by changing the distance between the phantom from 2cm to 6.5cm, whereby all settings remain the same. The remaining S11 value after the change in distance is evaluated as a measure for the load dependence

Simulation Settings:

• Solver:            Time domain solver
• Frequency:     297.2MHz
• Conductors:    PEC
• Accuracy:        -60dB
• Meshing:
  • Hexahedral
  • at least 2 mesh cells per substrate
  • cell ratio: 1.5
  • port: at least (2,2) cells
  • traces: at least 2 cells per width
  • phantom: 4mm isotrop
• Lumped Elements:
  • Capacitances: first order model, ESR = Datasheet value
  • Inductors: Q model inductance (Q=230)

The Results:

Antenna

Geometries

CAD-Data

Power and SAR Efficiency

in µT/√W and µT/√(W/kg)

Load Dependence [dB]

Decoupling [dB]

[5cm | 10cm | 15cm]

Symmetrically Fed Microstrip With End Line Meanders [1]

	Power		SAR
	Center	Offcenter	Center	Offcenter
0cm	1.02	0.99	0.64	0.62
10cm	0.09	0.08	0.06	0.05

-2.1

– | -17.2 | -24.4

Self-Grounded Bow-Tie Antenna [2]

	Power		SAR
	Center	Offcenter	Center	Offcenter
0cm	1.45	1.13	0.36	0.28
10cm	0.11	0.10	0.03	0.02

-11.3

-30.1 | -43.9 | -56.4

Leaky Wave Antenna [3]

	Power		SAR
	Center	Offcenter	Center	Offcenter
0cm	0.34	0.25	0.78	0.58
10cm	0.06	0.06	0.13	0.13

-21.9

– | -11.9 | -18.4

Snake Antenna [4]

	Power		SAR
	Center	Offcenter	Center	Offcenter
0cm	0.68	0.61	0.61	0.55
10cm	0.10	0.08	0.09	0.07

-3.1

– | -9.4 | -27.7

Fractionated Dipole [5]

	Power		SAR
	Center	Offcenter	Center	Offcenter
0cm	0.86	0.68	0.62	0.49
10cm	0.10	0.08	0.07	0.05

-4.7

-10.0 | -17.2 | -23.9

Rectangular Loop

	Power		SAR
	Center	Offcenter	Center	Offcenter
0cm	1.30	1.27	0.85	0.83
10cm	0.05	0.04	0.03	0.03

-4.5

– | -13.9 | -26.7

Integrated Multi-Modal Antenna With Coupled Radiating Structures [6]

	Power		SAR
	Center	Offcenter	Center	Offcenter
0cm	0.89	0.62	0.79	0.55
10cm	0.09	0.07	0.08	0.06

-5.1

-10.3 | -19.1 | -26.3

Passively Fed Fractionated Dipole [7]

	Power		SAR
	Center	Offcenter	Center	Offcenter
0cm	0.81	0.57	1.04	0.73
10cm	0.07	0.05	0.09	0.07

-9.7

-14.0 | -23.0 | -30.5

Efficiency Images:

Power Efficiency Center in µT/√W within the phantom in mm

SAR Efficiency Center in µT/√(W/kg) within the phantom in mm

Power Efficiency Offcenter in µT/√W within the phantom in mm

SAR Efficiency Offcenter µT/√(W/kg) within the phantom in mm

1. Rietsch, Stefan H. G.; Quick, Harald H.; Orzada, Stephan (2015): Impact of different meander sizes on the RF transmit performance and coupling of microstrip line elements at 7 T. In: Medical physics 42 (8), S. 4542–4552. DOI: 10.1118/1.4923177

2. Eigentler TW, Kuehne A, Boehmert L, Dietrich S, Els A, Waiczies H, Niendorf T (2021) 32-Channel self-grounded bow-tie transceiver array for cardiac MR at 7.0T. Magn Reson Med 86 (5):2862-2879.

3. Solomakha, Georgiy; Svejda, Jan; Rennings, Andreas; Erni, Daniel (2020): A new RF coil for UHF MRI based on a slotted microstrip line. In: Journal of Physics Conference Series 1461, p 1-3. DOI: 10.1088/1742-6596/1461/1/012168.

4. Steensma, Bart; van de Moortele, Pierre-Francois; Ertürk, Arcan; Grant, Andrea; Adriany, Gregor; Luijten, Peter et al. (2020): Introduction of the snake antenna array: Geometry optimization of a sinusoidal dipole antenna for 10.5T body imaging with lower peak SAR. In: Magnetic Resonance in Medicine 84 (5), S. 2885–2896. DOI: 10.1002/mrm.28297.

5. Raaijmakers, Alexander J. E.; Italiaander, Michel; Voogt, Ingmar J.; Luijten, Peter R.; Hoogduin, Johannes M.; Klomp, Dennis W. J.; van den Berg, Cornelis A T (2016): The fractionated dipole antenna: A new antenna for body imaging at 7 Tesla. In: Magnetic Resonance in Medicine 75 (3), S. 1366–1374. DOI: 10.1002/mrm.25596.

6. Li, Mingyan; Jin, Jin; Weber, Ewald; Engstrom, Craig; Destruel, Aurelien; Crozier, Stuart; Liu, Feng (2022): Prostate imaging with Integrated Multi-modal Antenna with coupled Radiating dipoles (I-MARS) at 7T. In: International Society of Magnetic Resonance in Medicine, Article 3228.

7. Zivkovic, Irena; Castro, Catalina Arteaga de; Webb, Andrew (2019): Design and characterization of an eight-element passively fed meander-dipole array with improved specific absorption rate efficiency for 7 T body imaging. In: NMR in biomedicine 32 (8), e4106. DOI: 10.1002/nbm.4106.

Thank you for visting this  website.

Bilguun Nurzed and Max Joris Hubmann