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Abstract
Purpose
In an attempt to further improve the radiofrequency (RF) magnetic (B1) field strength in temporomandibular joint (TMJ) imaging, a 4-channel spiral-loop coil array with RF circuitry was designed and compared with a 4-channel single-loop coil array in terms of B1 field, RF transmit (B1+), signal-to-noise ratio (SNR), and applicability to TMJ imaging in 7T MRI.
Materials and Methods
The single- and 4-channel spiral-loop coil arrays were constructed based on the electromagnetic (EM) simulation for the investigation of B1 field. To evaluate the computer simulation results, the B1 field and B1+ maps were measured in 7T.
Results
In the EM simulation result and MRI study at 7T, the 4-channel spiral-loop coil array found a superior B1 performance and a higher B1+ profile inside the human head as well as a slightly better SNR than the 4-channel single-loop coil array.
Conclusion
Although B1 fields are produced under the influence of the dielectric properties of the subject rather than the coil configuration alone at 7T, each RF coil exhibited not only special but also specific characteristics that could make it suited for specific application such as TMJ imaging.
Figures and Tables
Fig. 1
Geometry of (a) single-channel single-loop coil and (b) single-channel spiral-loop coil, respectively. The spiral-loop coil was modeled with 4 turns of the coil with conductor trace width of 2 mm and trace spacing of 2 mm.
Fig. 2
Schematic representations of each 4-channel TMJ coil arrays with relevant dimensions. Coil layout for (a) 4-channel single-loop coil array and (b) 4-channel spiral-loop coil array, respectively Between the elements, 4 mm spacing was made. To eliminate mutual coupling between loops, a decoupling capacitors (Cd) was inserted between the loop coil elements.
Fig. 3
RF signal pathway for two 4-channel TMJ coil arrays using the power divider (PD) and phase shifter (PS), and Tx/Rx switches. The RF signal comes from the (a) single-channel RF amplifier and is delivery to a (b) 2-way Wilkinson power dividers with -3dB power split and no phase shift. The final RF power signal is shifted via a (c) phase shifter using a coaxial cable delay. For the Tx/Rx operation, (d) Tx/Rx switches were inserted as a lumped element. The 8-output RF ports, two sets of coils with 4 each side, were connected to (e) each 4-channel TMJ coil arrays.
Fig. 5
Computer simulation of relative (a, b) B1 field distribution and (c, d) B1+ maps for single-channel single-loop and spiral-loop coil was acquired by FDTD calculation. The B1+ maps of each coil were compared after normalization with 3 µT in a location 20 mm apart from the coil plane. The color scale is expressed in terms of a fraction of the maximum scale value. (e) The B1+ profiles on the line of P1 and P2 (in Fig. 5c, d).
Fig. 6
Computer simulation results of (a) the 4-channel spiral-loop coil array and (b) 4-channel single-loop coil array, respectively in a human head model. (c) The B1+ maps and (d) B1 field in the central transverse plane (dotted line in (a, b)) at 300 MHz. (e) The B1 profile shown with blue line (P1-P2 line in (d)) indicates the corresponding SNR map.
Fig. 7
Two types of 4-channel TMJ coil arrays; (a) the spiral-loop coil array and (b) the single-loop coil array. For the experimental setting, both coils are attached onto the distilled water phantom (composition: 1.25 gNiSO4 × 6H2O + 5 gNaCl per 1000 H2O, size 20 cm × 20 cm × 30 cm in the x, y, and z directions). (c) is the experimental setting at 7T MRI.
Fig. 8
Upper part; B1 field distributions in phantom obtained by GRE imaging for each 4-channel TMJ coil arrays for different FA values; 10°, 20°, 30°, 40°, and 60°. Lower part; B1+ maps obtained for different values of FA. FA was normalized for the central transverse plane at 7T.
Fig. 9
MRI images obtained from the TMJ region using 3D FLASH sequence (TR / TE / α= 16 / 3.42 ms / 10°, field-of-view = 200 × 200 mm, matrix size = 320 × 320, slice thickness = 0.6 mm, bandwidth = 200 Hz, and total acquisition time = 6 min 10 s). SNR maps in the TMJ image were obtained from both (a-i) 4-channel spiral-loop coil array (c-i) 4-channel single-loop coil array, respectively, while in the middle (b-i), a corresponding transverse slice are shown. In the bottom row (iii), SNR maps of the 4-channel spiral-loop coil array and the 4-channel single-loop coil array are shown. The numerical values shown are the SNR values. As shown 4-channel spiral-loop coil array has better image intensity profile than the single-loop coil for specific SNR value (dotted line of box indicate SNR of 30 and over).
Acknowledgements
This work was supported by the National Research Foundation (NRF), the Ministry of Education, Science and Technology (2008-2004159) and supports from the Gil foundation.
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