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Description
INTRODUCTION
Inductive RF coils provide a cost-effective and simple approach for creating wireless RF coils in MRI1-5. They streamline MR scan setup and enhance patient comfort by eliminating the need for bulky components like cables, baluns, preamplifiers, and connectors. However, volume-type wireless coils are usually operated in transmit/receive mode due to their complex structure and multiple resonant modes. Adding multiple detuning circuits to these coils would decrease the SNR and increase costs. In this work, we proposed an innovative inductive wireless volume coil based on the Litzcage6 design for 1.5 T head imaging.
METHODS
A uniquely designed wireless birdcage coil was constructed for head imaging, incorporating a Figure-of-Eight (Fo8) conductor pattern within its 16 rungs, each measuring 26.5 cm, the diameter of the cylindrical tube is 26 cm. Eight passive detune circuits were employed (Figure1) and equivalent circuit of the wireless coil as shown in Figure2.
During the receive phase, the cross-diodes remain OFF, the wireless coil operates in the Litzcage volume resonator mode, as shown in Figure 2b.
In the transmit phase, uniform transverse magnetic field flux passing through the upper and lower segments of the Fo8 loops induces counteracting currents, successfully achieving geometric decoupling from the body coil. Furthermore, passive detune circuits are utilized for decoupling the remaining sections of the coil, as shown in Figure 2c.
To quantitatively evaluate the extent of RF transparency of the wireless coil to the body coil, a set of EM simulations was performed using FEM-based Maxwell solver (Ansys HFSS) 7
The wireless Birdcage and Litzcage coils were simulated on a cylindrical surface rather than replicating the complex domed structure for simplicity. To evaluate detuning performance, the B1+ of the body coil was compared in scenarios with and without the detuned wireless coils, and the coil configurations were documented in Table 1.
Table1
EM simulation coil configuration Diameter / Hight rungs detune circuits added
Body coil only (a) 60 / 60 cm 16 N/A N/A
Body coil + wireless birdcage 26.5 / 26 cm 16 16 (b) 8 (d)
Body coil + wireless Litzcage 26.5 / 26 cm 16 16 (c) 8 (e)
Note : (a) to (e) corresponding to the scenarios of the simulation results in figure 6
All MR measurements were performed using a 1.5T whole-body scanner (Siemens Sempra).
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RESULTS
The wireless coil’s operating frequency was at 63.67 MHz. The unloaded Q-factor was ~350 and was ~35 with a human head.
The system's RF power calibration shows a minimal 0.2% difference with and without the wireless Litzcage coil, indicating its near invisibility in the transmit phase. This aligns with the simulation results in Figure 3e.
SNR maps were generated by processing gradient-recalled echo (GRE) images reconstructed from raw data. Individual receive channel images were combined using the commonly used "Sum-of-Squares" (SoS) technique. The wireless coil exhibited approximately 3.9 times higher SNR compared to the body coil. Notably, there was a 10% increase in SNR in the central region and a 21% decrease at the surface when compared to a 12-channel receive array, as depicted in Figure 4a-c.
Figure 5 shows T1/T2-weighted and FLAIR images for the same healthy female volunteer. The wireless Litzcage provided similar image quality when compared to the commercial 12-channel wired local array. The high degree of image uniformity could also validate that the wireless coil was adequately detuned during the transmit phase, ensuring the uniform transmit field of the body coil remained unaffected
DISCUSSION
The wireless coil is suitable for most applications without compromising patient safety in Rx-only mode, For specific areas like the knee and other body parts where phase wrap needs to be avoided, the Tx/Rx mode (which does not detune during the transmit phase) is appropriate. The wireless Litzcage coil has limitations for parallel imaging with the current MRI system setup. Alternative approaches such as compressive sensing or deep learning techniques can be explored in such cases.
CONCLUSION
The domed wireless Litzcage coil offers comparable image quality to a wired receive array while being simple, lightweight, and cost-effective in design. This technology can be extended for application in MRI systems of 0.55T, 3.0T, and 7T. It is applicable for extremity, breast, and body imaging, enhancing patient comfort and allowing more flexible patient positioning. Different types of inductive wireless coils might outperform wired coils in MRI-guided intraoperative and interventional procedures, including laser and microwave ablation surgeries.
REFERENCES
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| Keyword-1 | MRI |
|---|---|
| Keyword-2 | birdcage |
| Keyword-3 | RF coil |
