Resonators for In Vivo Imaging: Practical Experience

Abstract

Resonators for preclinical electron paramagnetic resonance imaging have been designed primarily for rodents and rabbits and have internal diameters between 16 and 51 mm. Lumped circuit resonators include loop-gap, Alderman-Grant, and saddle coil topologies and surface coils. Bimodal resonators are useful for isolating the detected signal from incident power and reducing dead time in pulse experiments. Resonators for continuous wave, rapid scan, and pulse experiments are described. Experience at the University of Chicago and University of Denver in design of resonators for in vivo imaging is summarized.

Figure 1

B₁ map of a trityl-containing phantom 19 mm diameter, and longer than the resonator. This B₁ map is from the detection LGR used in a bimodal Alderman-Grant resonator (AGR), with the AGR grounded.

Figure 2

(a) Example circuit for local coupling using a balun and step-up capacitive impedance transformer. A damping resistor can be used to adjust the resonator Q for pulse operation. (b) The diagram also shows a way to achieve coupling (λ/2 away from the resonator) using a transmission line and varactors for automatic coupling (matching) and frequency (tuning) control (ATC/AFC).

Figure 3

Magnet bench with guide pins and RF shield, showing coupling elements.

Figure 4

19 mm split top resonator. A: The drawing of the resonator structure. B: The assembled resonator. The capacitor and shunt resistor across the gap are visible above the loop. The plastic clamps are used for added mechanical stability. C: Mouse leg installed into the resonator; the top part of the resonator is removed. D. RF shield and coupling element. RF shield surface is slotted for better modulation field penetration. Parts A – C are reproduced from Ref. [53].

Figure 5

250 MHz resonators. A. “general-purpose” LGR 25 mm diameter and 32 mm long is housed in a 4″ diameter shield. B. The 25 mm diameter and 75 mm long resonator used for pulse imaging experiments, with the 500 Ω equivalent shunt resistor (two 1 KΩ resistors in parallel) installed across the resonator gap. Once installed in the magnet, the capacitor plates on top provide coupling to the excitation/detection system. C. Tumor bearing leg in a single-loop-single-gap resonator. The inner diameter was 50 mm, the length 57.4 mm, and the volume 112 ml. Both CW and ESE measurements were performed in the same resonator. The resonator was critically coupled at a frequency of 239.3 MHz with a loaded Q of 11.7 [54].

Figure 6

250 MHz CLR used for mouse imaging. Note the horizontal orientation and the removal of the stacks, relative to the resonator in figure 1 of Ref [29].

Figure 7

Sketch of the orthogonal resonator constructed with a LGR inside an Alderman-Grant type resonator and the equivalent circuit [45].

Figure 8

An assembled version of the resonator is sketched in Figure 7. The LGR is attached to the RF shield while the AGR rocks around the axis orthogonal to B₁^LGR and B₁^AGR, which provides the isolation adjustment [45].

Figure 9

(A) Oximetry of a mouse breast has been performed using a 10 mm diameter surface coil to detect trityl OX63 at 250 MHz. (B) Structure of the surface coil.

Figure 10

700 MHz CLR with 25 mm diameter for rapid scan and pulse. Both capacitively coupled saddle-coil resonators are made from copper tape and chip capacitors attached to the outside of the support tube. Conductors are 6.35 mm wide. Coupling is fixed near critical coupling and the frequency of resonator 1 is adjustable. B₁ is perpendicular to the axis of the sample tube. The inside of the PVC RF shield is painted with conductive silver paint.