Introduction

Responsive contrast agents (CAs) that can activate by a specific biochemical marker have the potential to improve MRI sensitivity and specificity to certain pathologies associated with those biomarkers. Most CAs lack such a targeted response and affect only one nuclei signal, such as proton (¹H) T₁ relaxivity. Recent CA research has expanded to multinuclear agents that can give rise to multiple signals from multiple nuclei, such as ¹H and fluorine (¹⁹F) ^1-3.

We developed a multinuclear ¹H/¹⁹F agent: Fe(II)-F₂H₄qp4 - an iron(II) complex with a fluorinated quinol-containing macrocyclic ligand (Figure 1A). The Fe(II)-F₂H₄qp4 molecule is designed to activate only in the presence of hydrogen peroxide (H₂O₂) to shorten the T₁ relaxation times for both ¹H and ¹⁹F signals. This makes Fe(II)-F₂H₄qp4 a potentially versatile assay tool for studies involving H₂O₂ and similar reactive oxygen species.

For phantom validation of Fe(II)-F₂H₄qp4, we constructed a 7T ¹H/¹⁹F RF coil with custom interface-preamplifier (Figure 1B) and developed a flexible ¹H/¹⁹F frequency-selectable Inversion Recovery Look-Locker (IRLL) sequence to quantify both ¹H and ¹⁹F T₁ relaxation rates (Figure 2). Although ¹H IRLL is common, the application of ¹⁹F IRLL is sparse in the literature and has not been applied to study CAs as now ^{1, 2}.

Methods

Our 7T ¹H/¹⁹F RF Tx/Rx coil with interface-preamplifier (Figure 1B) was tuned for ¹⁹F peak performance with sodium fluoride (NaF+H₂O) phantoms. We then developed a ¹H/¹⁹F frequency-selectable IRLL sequence that employed a non-selective adiabatic inversion followed by multiple fast low-angle shot (FLASH) readouts of one kspace line with 20 preplanned inversion times (TI) (Figure 2). Additional ¹⁹F T₁-weighted FLASH at flip angles 10 and 30 deg further demonstrated ¹⁹F signal T₁ contrast between phantoms.

Six 7T Fe(II)-F₂H₄qp4 phantoms were prepared with 10, 15 and 20 mM concentration and each concentration prepared with or without 10 mM hydrogen peroxide (H₂O₂) – activated versus non-activated respectively. Similar to these 7T phantoms, ten additional Fe(II)-F₂H₄qp4 phantoms were prepared for 3T ¹H IRLL scans to determine T₁ values at 3T. Since the Fe(II)-F₂H₄qp4 agent is much more T₁ responsive with ¹H at 3T, the 3T phantoms were prepared at lower concentrations of 0, 0.1, 0.4, 0.7 and 1.0 mM, with and without 0.1 mM H₂O₂ for activation. All ¹⁹F MRI was performed on a 7T human-bore scanner and ¹H MRI performed on both 7T and 3T scanners (Siemens, Erlangen, Germany). All 7T and 3T analyses were performed on custom Matlab programs (Mathworks, Natick, MA) using the Neider-Mead Simplex method, and background noise compensation, to curve-fit the IRLL magnitude data into T₁ values.

Common MRI parameters: FOV = 64x64 mm; Matrix = 32x32; Slice-thickness = 10 mm; Pixel-size = 2.0x2.0x10 mm. Specific for 7T ¹H: TR = 10 sec; 20 TI times ranging from of 10 to 2600 ms; FLASH flip-angle = 3 deg; Manual TxV_ref = 50 V; Averages = 4. Specific for 7T ¹⁹F: TR = 10 sec; 10 TI times ranging from 10 to 2000 ms; Manual TxV_ref = 50 V; FLASH flip-angle = 5 deg; Averages = 9. Specific for 3T ¹H: TR = 10 sec; 20 TI times same as 7T; FLASH flip-angle = 2.5 deg; Averages = 6.

Results

In all ¹⁹F scans, the 7T ¹H/¹⁹F RF coil demonstrated good ¹⁹F sensitivity with SNR values ranging from 20 to 150 and ¹H sensitivity was acceptable when using multiple signal averages. Figure 3 presents the ¹⁹F IRLL T₁ recovery graphs. Figure 3A presents a composite of all six ¹⁹F T₁ curves from all six ¹⁹F phantoms. Figure 3B-C-D breakout the ¹⁹F T₁ curves for each Fe(II)-F₂H₄qp4 concentration with and without H₂O₂ activation. Figure 4-Table 1 summarizes the 7T ¹H and ¹⁹F T₁ values and Figure4-Table 2 summarizes the 3T ¹H T₁ values. The ¹⁹F T₁ curves and corresponding T₁ values clearly show the significant change of ¹⁹F T₁ with Fe(II)-F₂H₄qp4 during H₂O₂ activation. The ¹H T₁ values also indicate significant change of ¹H T₁ at both 7T and 3T. Figure 5 presents the ¹⁹F T₁-weighted FLASH showing the flip angle control for best T₁ contrast in activated versus non-activated Fe(II)-F₂H₄qp4 ¹⁹F signal.

Discussion/Conclusion

Our ¹H/¹⁹F RF coil combined with ¹H/¹⁹F IRLL quantified the changes of both ¹⁹F T₁ and ¹H T₁ with Fe(II)-F₂H₄qp4 agent being H₂O₂ activated versus non-activated. These methods were consistent across both ¹H and ¹⁹F nuclei at both 7T and 3T. This application of multinuclear ¹H and ¹⁹F MRI validated the performance of the multinuclear Fe(II)-F₂H₄qp4 agent under a range of varied concentration and H₂O₂ activation states. Future efforts will expand to in vivo rodent application of these methods.