Actively Decoupled Transmit-Receive Coil-Pair for Mouse Brain MRI

Abstract

A low-cost, high performance RF coil-pair for MR imaging of mouse brain is described. A surface receiving coil is used for high spin-sensitivity, while a larger transmit coil, located outside the mouse holder, delivers good B₁ uniformity across the brain with reasonable efficiency. The volume coil is constructed with an open architecture, making experimental setup easy and providing clear access to the head of the mouse. Each coil is switched between active and inactive modes using PIN diodes driven by a small amplifier external to the spectrometer. Because of this active decoupling, there is no requirement for orthogonal orientation of the coils. The coil pair is platform independent, requiring only a transmit/receive (T/R) signal to switch the amplifier that drives the PIN diodes, and can therefore be used with virtually any commercial or home-built MR scanner.

Figure 1

Overall connection diagram for the actively decoupled transmit (volume) and receive (surface) coils. The bias-Tees add the PIN diode DC bias to the RF lines (T, R) that run from the scanner to the transmit and receive coils.

Figure 2

Receive coil. Actual circuit (a) and RF-only equivalent circuits ((b) transmit mode; (c) receive mode) for the receive surface coil. The L₂C₂ combination is a parallel trap to block current on the surface coil L₁ during transmit.

Figure 3

Transmit coil. The starting two-turn geometry in (a) is modified in (b) to the actual coil shape by curving the vertical legs to provide unobstructed access for the mouse and the mouse holder. The full circuit in (c) shows the multiple stations of tuning capacitors C₁-C₅. The RF-only equivalent circuits in (d) and (e) refer to transmit and receive modes, respectively.

Figure 3

Transmit coil. The starting two-turn geometry in (a) is modified in (b) to the actual coil shape by curving the vertical legs to provide unobstructed access for the mouse and the mouse holder. The full circuit in (c) shows the multiple stations of tuning capacitors C₁-C₅. The RF-only equivalent circuits in (d) and (e) refer to transmit and receive modes, respectively.

Figure 4

RF bias-Tee (two required) for adding PIN DC bias to RF line. The capacitance value of C is chosen to series-resonate the stray inductance of the RF path.

Figure 5

PIN driver amplifier accepts TTL input from the T/R switch and provides two separate outputs of +5 volts (current limited to 25 mA) during transmit to turn on each PIN diode and −18 volts during receive to turn off the PIN diodes. The power supply is shown below; it is powered by an AC adaptor wall transformer.

Figure 6

(a) Sagittal view, gradient-echo image of mouse brain collected in vivo using the actively decoupled coil pair. (b) Four slices, transaxial view, from a multi-slice, T₂-weighted spin-echo of mouse brain in vivo. Slices are 0.5 mm thick and are separated from one another by 3 mm.

Figure 6

(a) Sagittal view, gradient-echo image of mouse brain collected in vivo using the actively decoupled coil pair. (b) Four slices, transaxial view, from a multi-slice, T₂-weighted spin-echo of mouse brain in vivo. Slices are 0.5 mm thick and are separated from one another by 3 mm.