Nontoxic nanodiamonds (NDs) have proven useful as a vector for therapeutic drug delivery to cancers^1,2 and as optical bioprobes of subcellular processes³. Despite their potential clinical relevance, a means of non-invasively imaging NDs in vivo is still lacking. Extending their diagnostic potential to MRI has proven challenging, with much effort put into making use of intrinsic paramagnetic impurities in the ND to hyperpolarize ¹³C in the ND core^4,5,6. We find however, that paramagnetic impurities in ND will also couple to ¹H nuclei in water. Here, we exploit this coupling and leverage dynamic nuclear polarization (DNP) to acquire high contrast images of aqueous solutions containing ND.

Imaging was performed at 6.5 mT in our ultra-low field MRI scanner⁷ using a highly efficient balanced-SSFP (b-SSFP) Overhauser-enhanced MRI (OMRI) sequence^8,9 at room temperature. The high electronic gyromagnetic ratio (~28 GHz/T) necessitates that OMRI scanners operate at ultra-low magnetic fields (< 10 mT) to maximize RF penetration depth and minimize RF heating. A custom DNP imaging probe was constructed from an Alderman-Grant resonator (ESR: 191 Mhz) and a solenoid (1H: 276 kHz). Spectroscopic measurements were taken in similarly constructed DNP probes with the ESR resonator tuned to f_e = 140 MHz for B₀ sweeps or f_e = 191 MHz for scans at B₀ = 6.5 mT.

Synthetic NDs were purchased from Microdiamant for this study. MSY18 (0-30 nm NDs, median 18 nm) and MSY125 (0-250 nm NDs, median 125 nm) ND/water solutions were prepared using high power probe sonication to disaggregate nanodiamond clusters.

MRI and OMRI images of a phantom containing an aqueous solution of 125 nm ND solution (100 mg/mL concentration) are shown in Figure 1. Images were interpolated from 0.75 mm x 1 mm pixels over a 30 mm x 30 mm FOV. The phantom thickness was 20 mm and the total acquisition time was 110 seconds/image. In Figure 1(a) images were acquired with a conventional b-SSFP sequence before ‘turning on’ the DNP contrast in Figure 1(b) with an OMRI sequence. High contrast is demonstrated between ND solution and water, with an almost 100% reduction in the ¹H polarization with 10 W RF power applied to the ESR resonator. Similar images have been acquired for solutions of significantly smaller 18 nm NDs.

Spectroscopic DNP measurements show that for higher RF powers, the ¹H polarization in a ND solution can be enhanced beyond thermal polarization; an enhancement of -3.2 is observed in Figure 2. Further measurements, shown in Figure 3, show the coupling between electrons and nuclei is largest when the ESR frequency corresponds to g_e = -2.

Our results demonstrate the feasibility of using OMRI to image concentrations of nanodiamonds in aqueous environments. The spectroscopic data indicates that the contrast in these images results from DNP via the Overhauser effect, as illustrated in Figure 4. Low magnetic fields and RF power levels were used to minimize specific absorption rate (SAR) issues and are similar to those used in OMRI studies in vivo^8,10. The ability to generate contrast across a wide range of ND sizes (18 nm vs 125 nm NDs) warrants further investigation. Of particular interest is the possibility of using NDs, tailored to specific sizes, to investigate size dependent transport mechanisms. Our preliminary results also indicate that aqueous solutions of NDs can be used to generate conventional T₁ relaxation contrast in addition to our DNP methodology where the contrast can be turned ‘on’ or ‘off’.We have developed a means of non-invasively imaging concentrations of NDs and presented the first reported images of ND for contrast in OMRI. Our results will drive further research into the use of MRI methodologies as a means of tracking ND and other nanoparticles in vivo.