Development of an L-band resonator optimized for fast scan EPR imaging of the mouse head

Abstract

Purpose: To develop a novel resonator for high-quality fast scan electron paramagnetic resonance (EPR) and EPR/NMR co-imaging of the head and brain of mice at 1.25 GHz.

Methods: Resonator dimensions were scaled to fit the mouse head with maximum filling factor. A single-loop 6-gap resonator of 20 mm diameter and 20 mm length was constructed. High resonator stability was achieved utilizing a fixed position double coupling loop. Symmetrical mutually inverted connections rendered it insensitive to field modulation and fast scan. Coupling adjustment was provided by a parallel-connected variable capacitor located at the feeding line at λ/4 distance. To minimize radiation loss, the shield around the resonator was supplemented with a planar conductive disc that focuses return magnetic flux.

Results: Coupling of the resonator loaded with the mouse head was efficient and easy. This resonator enabled high-quality in vivo 3D EPR imaging of the mouse head following intravenous infusion of nitroxide probes. With this resonator and rapid scan EPR system, 4 ms scans were acquired in forward and reverse directions so that images with 2-scan 3,136 projections were acquired in 25 s. Head images were achieved with resolutions of 0.4 mm, enabling visualization of probe localization and uptake across the blood-brain barrier.

Conclusions: This resonator design provides good sensitivity, high stability, and B₁ field homogeneity for in vivo fast scan EPR of the mouse head and brain, enabling faster measurements and higher resolution imaging of probe uptake, localization, and metabolism than previously possible.

Keywords: electron paramagnetic resonance imaging; instrument development; microwave resonator; mouse head imaging; paramagnetic probes.

Figure 1.

Mechanical layout of the resonator. A. Assembled resonator. B. Expanded view. 1a. Front lid; 1b. Housing; 1c. Rear lid; 2a. Cylindrical shield (plastic reinforcing shell is not shown); 2b. Ring shield (reinforcing plastic support is not shown) 3a. Gap filling separator (one out of six pulled out shown); 3b. Resonator segment; 3c. Conductive loop segment; 4a. Modulation coil; 4b. Modulation coil frame; 5a. Double coupling loop (shown with part of the coaxial RF cable) 5b. Coupling loop housing.

Figure 2.

Photo of the 1.25 GHz 6-gap single loop resonator before assembly. 1. Resonator; 2. Cylindrical shield; 3. Coupling loop with housing and coaxial cable; 4. Coupling capacitor soldered to RF coaxial elbow.

Figure 3.

A. Photo of the 1.25 GHz 6-gap single loop resonator assembly positioned in the holder for solenoid magnet. B. Opposite view. 1. Resonator; 2. Holder; 3. Coupling capacitor. 4. Coaxial RF cable; 5. Coupling rod; 6. Animal holder support. C. Resonator loaded with the mouse before positioning in the solenoid magnet. Mouse is substituted by stuffed animal.

Figure 4.

Lumped element presentation of the schematic diagram of the resonator. C -capacitance of the individual gap. L - inductance of the individual loop. L_CL - inductance of the coupling loop. C_C -- adjustable coupling capacitor. Arrows indicate the direction of the current in the coupling loop. Dash lines represent magnetic flux.

Figure 5.

EPR imaging of the mouse head phantom. Slices are shown from 3D EPR image (cropped from FOV 35 x 35 x 35 mm) of plastic 5 cc conical tube filled with 1 mM TEMPONE in 0.9 % saline. Top - coronal, middle – sagittal, bottom - axial slices from the 3D data set. EPR images are reconstructed from 16,384 projections with 10 x 3.8 msec scans per projection. The parameters used for the EPRI acquisition were as follows: frequency 1.25 GHz, microwave power ~200 mW, modulation amplitude 0.1 mT, field gradient 60 mT/m, field sweep 2.4 mT.

Figure 6.

In vivo EPR imaging of the mouse head. Slices from the 3D EPR image (cropped from FOV 30 x 30 x 30 mm) of mouse head after IV infusion of nitroxide (A) 3CP or (B) MCP: Top - coronal, middle – sagittal, bottom - axial slices from the 3D data set. EPR images are reconstructed from 3,136 projections with 2 x 3.8 msec scans per projection. The parameters used for the EPRI acquisition were as follows: frequency 1.26 GHz, microwave power ~200 mW, modulation amplitude 0.1 mT, field gradient 66 mT/m, field sweep 2 mT.

Figure 7.

In vivo EPR imaging of the mouse head with proton MRI coimaging. Axial slices from Figure 5 (bottom) enlarged and superimposed with the corresponding proton MRI slices acquired at 9.4 T.