Novel 1H/19F double-tuned coil using an asymmetrical butterfly coil

Abstract

Background: Fluorine-19 (¹⁹F) magnetic resonance imaging (MRI) is a non-invasive imaging tool for the targeted application of fluorinated agents, such as cell tracking, and for the demonstration of oximetry. However, as the SNR of ¹⁹F is significantly weaker than that of proton (¹H) imaging, the ¹⁹F coil must be combined with ¹H coils for anatomical co-registration and B₀ shimming. This is difficult due to the strong coupling between the coils when they are in proximity, and is problematic since the Larmor frequency of ¹⁹F is 94% that of ¹H, further increasing the potential for coupling between the ¹H and ¹⁹F elements.

Purpose: Conventional double-tuned coil methods tend to generate loss compared to single-tuned reference coils. The asymmetrical butterfly coil has a split resonance peak, which can cover frequencies of ¹H and ¹⁹F without losses arising from lossy traps or switching circuits. In this study, the use of an asymmetrical butterfly coil was evaluated for ¹H/¹⁹F applications.

Methods: To increase quadrature efficiency at both the ¹H and ¹⁹F frequencies, the left and right loops of the butterfly coil were tuned asymmetrically. The coil's tuning and performance were evaluated in simulations and MR measurements, and the results were compared to a dimension-matched single-tuned loop coil.

Results: The split resonance peak of the asymmetrical butterfly coil successfully spanned the ¹⁹F to ¹H frequency. It operated with higher quadrature efficiency at both ¹H and ¹⁹F frequencies and demonstrated superior receive sensitivity and SNR compared to the dimension-matched single-tuned loop coil.

Conclusions: The split resonance peak of the asymmetrical butterfly coil supported both ¹H and ¹⁹F frequencies, delivering a higher SNR than that of the single-tuned loop coil. Since the asymmetrical butterfly coil can cover ¹H and ¹⁹F frequencies without loss and provides higher efficiency than the reference single-tuned coil, it can be effectively utilized for ¹H/¹⁹F MRI applications.

Keywords: 19F; MRI; RF coil.

FIGURE 1

Schematic of the loop (a) and butterfly coils (b), the CST simulation model of a 10 × 10 cm² butterfly coil (c) and the interface of transmit‐only coil (d) and receive‐only coil (e).

FIGURE 2

S‐parameter of the single‐tuned loop coil, LP butterfly coil, and asymmetrical butterfly coil. The green and blue lines indicate that the coils were tuned at the ¹⁹F and ¹H frequencies, respectively. The dashed lines indicate the location of the ¹⁹F (282 MHz) and ¹H (300 MHz) frequencies.

FIGURE 3

Simulated transmit efficiency, receive sensitivity, and quadrature (QD) efficiency of the single‐tuned loop coil and asymmetrical butterfly coil at the ¹H and ¹⁹F frequencies, respectively.

FIGURE 4

Measured SNR maps of the single‐tuned loop coil and asymmetrical butterfly coil at the ¹⁹F and ¹H frequencies. The square boxes show the locations of the ROIs. The mean values were calculated in the whole phantom area.