Introduction

RF array coils with a local transmitter can perform MRI scan with significantly less power and SAR compared with a whole body coil (WBC) as a transmitter. Two approaches may be used. One is to build a local transmitter outside receiver array coils, ^{1, 2} and the other is to use diodes, T/R switches and combiners to connect the receiver array coils to make a transmitter in Tx mode.^{3, 4} Both require complicated electronics and are difficult to implement if the number of receiver channels is high, such as a 48-ch head array coil at 3T.⁴ Furthermore, the additional electronics may introduce extra coupling and require additional spacing. This abstract proposes a novel Tx/Rx coil concept to create a local uniform Tx field by letting receiver array coils inductively coupled to the WBC simultaneously in Tx mode.

Theory

When a small resonant loop is inside a WBC in Tx mode, it couples to the WBC uniform Tx B₁ field and the induced current generates stronger local B₁ field. If another similar loop is also inside the WBC and face directly to the first loop, then the two loops can generate two eigenmodes. One is that both currents flow in the same direction (co-rotating) and the other is opposite (counter-rotating). Since the WBC Tx field is uniform, only the co-rotating mode has large net coupling magnetic flux with WBC field and the other mode net flux is very small. Thus, co-rotating mode is dominant. If N resonant loops are around a subject, similarly they generate N eigenmodes. The most uniform eigenmode has the largest net flux from the uniform WBC field and is dominant while other eigenmodes’ are weak. The uniform mode frequency may be lower than the working frequency but its induced field is still stronger than the WBC’s due to smaller size. If N identical loops are around a cylinder uniformly in one row and inside a WBC in Tx mode, the induced current distribution in loop arrays is the same as an N-rung birdcage coil. Thus, the combined induced Tx field from the same receiver array coils may be used as a local Tx coil.

Methods

Two steps were taken to prove the concept. The first step is EM simulation using xFDTD (REMCOM). Eight 20cm-long loops are built around a 20cm diameter cylinder in simulation. Each coil is independently tuned to 1.5T with high Q (>200). All loops are allowed to couple each other and the WBC. A 16-rung birdcage with 70cm diameter was created to generate uniform B₁ field. A cylindrical phantom (16cmx16cmx30cm) was put at the iso-center of the coils. The setup is shown in Fig. 1. The second step is to test a two-row 8-ch inductively coupled Tx/Rx coil at 1.5T (Fig. 2). In Tx mode D1 is forward biased (short) and L1 and C5 resonate each other. This creates high impedance to isolate preamplifier from the coil element at the working frequency. This allows the coil element to resonate at high Q and couple to the WBC. In Rx mode D1 is backward biased (open) and the coil works as the phase array coil.5 The coil is 18cm long and 16cm in diameter. Siemens 1.5T AERA MRI system and T1_fl2d sequence were used to scan. A two-liter phantom (5gNaCl+3.25g CuSO4 ∙5H2O/liter H2O) was scanned. A 1.5T commercial T/R 15-ch coil with identical mechanical former was also scanned for comparison. The 15-ch coil has a 12-rung birdcage coil as its local transmitter.

Results

EM simulation results are shown in Fig. 3. As expected the collectively induced B1 field inside 8-loop is uniform and 5.5 times stronger than the WBC’s field. Fig. 4 shows that the 8-ch inductively coupled Tx/Rx coil has similar uniformity in both axial and sagittal as the 15-ch T/R coil. For the same phantom the system transmitter voltages are 43V for the 8-ch coil, 73V for the 15ch coil and 311V for the WBC only. 43V is 2% power of 311V.

Discussions

Both EM simulation and experiments demonstrated the combined induced Tx field from the same Rx array coil is uniform and can be used for the local Tx field purpose. This new concept can be used to add local Tx function to highly parallel receiver array coils. This additional local Tx function may enable more SAR demanding applications.

Conclusions

We demonstrated a novel inductive coupled array Tx/Rx coil concept. This concept can be used to transform a receiver array coil with additional local Tx function without additional local Tx coil.

Acknowledgements

No acknowledgement found.