Linear, reversible magnetic-resonance frequency shifts
Aqueous solutions from nitrate or halide salts of alkalimetal ions, transition-metal ions, and group-II and -III metal due to applied pressure and elevated temperature are observed at 2 T for simple, hydrated closed shell ions across ions were prepared in the concentration range of 0.1-1 M.
Solutions of ions with high charge density were made acidic the periodic table. NMR frequencies of nontransition-metal ions in general are found to increase with pressure and de-to avoid hydrolysis. Solutions of 0.1 M aq Co(II)R 6 with R ร
NH 3 , Phen, CN, Cl, and ethylenediamine (en) were crease with temperature by an amount proportional to the size of the solute ions. NMR frequencies for transition-metal prepared for studying the ligand-dependent Co(III) frequency shifts. ions, in general, shift in the opposite directions as compared to the closed-shell ions. A unified theoretical approach in-
The center frequency of the dispersion mode NMR signal was measured in a homebuilt CW NMR spectrometer at a volving solute-solvent interaction is underway. This work demonstrates feasibility of multinuclear MR thermometry field strength of 2.08 T ( 1 H ร
90.0 MHz) and a field homogeneity of 0.2 ppm over the 2 cm sample tube length. Each during online process control and interventional MR imaging.
measurement required 1-16 signal averages (a few seconds), depending upon the sensitivity and T 1 of the nuclei. Spectroscopy of solvated ions is challenging due to the unstable solvent structure for most of the loosely solvated Field drifts were corrected by an external 7 Li lock signal at 35.000 MHz. ions (1-3). Until now, nuclear magnetic-resonance experiments (3) have furnished limited although valuable struc-Ti-Al nonmagnetic alloy was used to build the high pressure/temperature probe chamber. It enclosed the glass sam-tural information about the interactions in ionic solutions. Benedek et al. (4) observed the NMR frequency of Co(III), ple tube with movable pistons to transfer pressure from an and Jameson et al. (5) observed that of Pt(III) complexes oil line, as well as an appropriate solenoidal RF coil in the to shift with pressure and temperature. Wilbur and Jonas (6) required frequency range and a Cu-Constantan thermocouhave noted 19 F NMR frequency shifts due to pressure in ple. The probe assembly was immersed in a temperaturebenzofluoride solutions. This work and previous experiments controllable oil bath. across the periodic table (7) demonstrate a linear, reversible
The NMR central frequency for each of the solutes of relation between frequency shifts and pressure or temperainterest was recorded as a function of pressure in the range ture as well as a direct proportionality of the frequency shifts of 0.1 to 250 MPa followed by a repeat run to assess hystereto the crystal ionic volume of the solvated ions. No structuresis while decreasing the pressure. The variable temperature breaking or susceptibility role of the bulk solvent is obdata at 0.1 MPa were acquired in the range of 25 to 95ะC served.
during the heating and cooling cycle. The total time for a Recent studies in the interventional situation involving complete pressure or temperature run was 3-4 h for each MR imaging or MR-guided ultrasound therapy (8-10) have sample. used the proton frequency shift to monitor tissue tempera-NMR frequency shifts with respect to pressure and temture. However, in a complex medium, solvent-proton resoperature for several hydrated ions are reported in Table 1. nance is very sensitive to the local environment seen as The frequencies of nontransition-metal ions increase with susceptibility variation (8), and nonlinearity and hysteresis increasing pressure and decrease with increasing temperature (9). The linear, reversible shifts observed here may offer (range 1 to 165 parts per billion/MPa or /ะC; positive presreliable multinuclear alternatives to proton MR thermometry sure and negative temperature coefficients for closed-shell for mid-to-high field interventional MRI.
ions, except for 207 Pb and 209 Bi). For Na / and Cl 0 ions, the pressure experiments were repeated at two different temperatures (25ะC, 60ะC