250 MHz Rapid Scan Cross Loop Resonator

Abstract

A 25 mm diameter 250 MHz crossed-loop resonator was designed for rapid scan electron paramagnetic resonance imaging. It has a saddle coil for the driven resonator and a fine wire, loop gap resonator for the sample resonator. There is good separation of E and B fields and high isolation between the two resonators, permitting a wide range of sample types to be measured. Applications to imaging of nitroxide, trityl, and LiPc samples illustrate the utility of the resonator. Using this resonator and a trityl sample the signal-to-noise of a rapid scan absorption spectrum is about 20 times higher than for a first-derivative CW spectrum.

Figure 1.

25 mm, 250 MHz, Rapid Scan, Cross Loop Resonator Assembly. The mechanical structure of the resonator and scan coil assembly was built with a 3D printer using polylactic acid (PLA).

Figure 2.

Internal construction of CLR.

Figure 3.

Equivalent circuit diagrams for (A) Driven resonator, saddle coil and (B) Sample resonator, LGR.

Figure 4.

Frequency dependence of the amplitude of the mechanical background signal measured with a 70 ms chirp of scan frequencies from about 1.2 to 7.5 kHz. The current in the rapid scan coils corresponds to a 13.7 G sweep width. Each trace was signal averaged for a total of 2.2 min data acquisition time. The incident power on the driven resonator was 65 mW. The background amplitudes were measured for B₀ = 0 G and zero gradient (blue dotted trace), B₀ = 91 G and zero gradient (black trace), and B₀ = 91 G and 10 G/cm z gradient (red trace).

Figure 5.

Rapid scan and CW spectra of 0.1 mM OX63. (A) Sinusoidal rapid scan spectra. (B) Pseudo-modulated (24 mG) rapid scan spectra. (C) CW spectra. The effective filter for the rapid scan spectrum was the 2.5 MHz resonator bandwidth. The detection bandwidth was 5 MHz. The CW spectra were acquired using a Bruker Elexsys signal processing unit (SPU) with 5.2 ms conversion time, and 3-point binomial digital filter (n = 1 selected in the software; the number of points smoothed is 2n+1), which results in a filter time constant approximately equal to the conversion time, roughly 200 Hz. Hence, the CW bandwidth is about 1 percent that of the rapid scan.

Figure 6.

(A) LiPc phantom consisting of two small LiPc samples separated by 1.0 cm. (B) Two LiPc samples, showing dimensions.

Figure 7.

(A) 2-D spectral-spatial image along Z of two LiPc samples (Figure 6A) separated by 1 cm. (B) Spectral slices (red and blue dashed lines) through the centers of the two samples (z = −0.27 and +0.81 cm) superimposed on the nongradient spectrum (black solid line). (C) Spatial slice through the centers of the two samples, B₀ = 91.4 G.

Figure 8.

(A) Plot of observed positions of signals for the two LiPc samples (red and blue solid lines) as a function of gradient. The predicted linear dependence of signal position on gradient is shown with red and blue dashed lines. (B) 2-D spectral-spatial image reconstructed from projections that included the off-axis contributions to the gradients to B₀ as shown in (A). (C) Positions of signals for the two LiPc samples (red and blue solid lines) at the same vertical location in the resonator as for (A), but at each gradient the value of B₀ was offset by an amount that is quadratic in gradient to correct for the effects of the concomitant gradients. The experimental data are superimposed on the predicted linear dependence of signal position on gradient (red and blue dashed lines). (D) 2-D spectral-spatial image reconstructed from projections acquired with the corrected center fields, as shown in (C).

Figure 9.

Aqueous imaging phantom. The wall thickness of each tube is about 1.5 mm. On the left is 0.1 mM OX63. On the right is 1 mM ¹⁴N Tempol (top) and 1 mM ¹⁵ N PDT (bottom) separated along the z-axis.

Figure 10.

(A) 2-D spectral spatial image along Z of aqueous phantom containing 0.1 mM OX63, 1 mM ¹⁴N Tempol, and 1 mM ¹⁵N PDT. The phantom shown in Figure 9. (B) spectral slices through the compartments containing OX63 and ¹⁴N tempol (z = −0.72 cm, red dashed line) and the compartments containing OX63 and ¹⁵N PDT (z = +0.64 cm, blue dashed line) are superimposed on the nongradient spectrum (black solid line).