Design of catheter radio frequency coils using coaxial transmission line resonators for interventional neurovascular MR imaging

Abstract

Background: It is technically challenging to design compact yet sensitive miniature catheter radio frequency (RF) coils for endovascular interventional MR imaging.

Methods: In this work, a new design method for catheter RF coils is proposed based on the coaxial transmission line resonator (TLR) technique. Due to its distributed circuit, the TLR catheter coil does not need any lumped capacitors to support its resonance, which simplifies the practical design and construction and provides a straightforward technique for designing miniature catheter-mounted imaging coils that are appropriate for interventional neurovascular procedures. The outer conductor of the TLR serves as an RF shield, which prevents electromagnetic energy loss, and improves coil Q factors. It also minimizes interaction with surrounding tissues and signal losses along the catheter coil. To investigate the technique, a prototype catheter coil was built using the proposed coaxial TLR technique and evaluated with standard RF testing and measurement methods and MR imaging experiments. Numerical simulation was carried out to assess the RF electromagnetic field behavior of the proposed TLR catheter coil and the conventional lumped-element catheter coil.

Results: The proposed TLR catheter coil was successfully tuned to 64 MHz for proton imaging at 1.5 T. B₁ fields were numerically calculated, showing improved magnetic field intensity of the TLR catheter coil over the conventional lumped-element catheter coil. MR images were acquired from a dedicated vascular phantom using the TLR catheter coil and also the system body coil. The TLR catheter coil is able to provide a significant signal-to-noise ratio (SNR) increase (a factor of 200 to 300) over its imaging volume relative to the body coil.

Conclusions: Catheter imaging RF coil design using the proposed coaxial TLR technique is feasible and advantageous in endovascular interventional MR imaging applications.

Keywords: Interventional MR; MR sensitivity; RF coil; catheter coil; coaxial transmission line; endovascular imaging; transmission line resonator (TLR).

Figure 1

Circuit diagram of a coaxial transmission line resonator with a short-ended circuit. The resonant frequency can be tuned by using a tunable capacitor (C_t) connected in parallel at the other end of the transmission line resonator. One of the approaches to feeding the resonator is to use a capacitor connected in series as indicated in this figure. This resonator can be fed by using a capacitor (C_m) connected in series. In the circuit, Z₀ is the characteristic impedance of the coaxial transmission line, β is the propagation constant, and L is the length of the transmission line.

Figure 2

Schematic (A) and photograph (B) of the prototype catheter RF coil using transmission line resonator design for endovascular interventional MR imaging at 1.5 T. RF, radio frequency.

Figure 3

Schematic diagram of the imaging experiment setup of body coil transmit and receive (A) and body coil transmit and catheter coil receive (B). During body coil transmitting, the catheter coil is detuned while during catheter coil receiving, the body coil is detuned. RF, radio frequency.

Figure 4

Vascular phantom used for the imaging validation experiments at 1.5 T.

Figure 5

1D plots of B₁ fields calculated along the dashed white line in the image for TLR catheter coil (left) and a conventional lumped circuit catheter coil (right). The proposed TLR catheter coil achieved 6-fold B₁ gain over the conventional lumped element catheter coil. TLR, transmission line resonator.

Figure 6

Vascular phantom imaging study using body coil transmission with presence of the TLR catheter coil (A) shows excellent decoupling of the catheter coil. By using the TLR catheter coil to receive and body coil transmission, high SNR imaging can be obtained (B). TLR, transmission line resonator; SNR, signal-to-noise ratio.

Figure 7

Sensitivity profiles (1D) of the body coil (A) and the TLR catheter coil (B). Note that the scale on the right profile is 100 times greater than the left. TLR, transmission line resonator.

Figure 8

The top inset is a plot of the SNR comparison between the transmission line resonator catheter coil and the body coil. The TLR catheter coil provides a significant SNR increase (a factor of 200 to 300) over its imaging volume relative to the body coil. The bottom inset is a representative image acquired by using the proposed TLR catheter coil in a vascular phantom with nominal vessel diameter of 1 cm. TLR, transmission line resonator; SNR, signal-to-noise ratio.

Figure 9

Two images acquired from body coil and TLR catheter coil at different positions along Z-direction, i.e., catheter axis direction (Z1 and Z2). With body coil reception (left column), nearly no elevated signal is observed in the catheter coil area (orange arrows), indicating acceptable decoupling performance between the body coil and the TLR catheter coil. Nevertheless the decoupling performance can be further improved by better design of the detuning circuit on the catheter coil. TLR, transmission line resonator.