Lithium visibility in rat brain and muscle in vivo by 7Li NMR imaging

Hyperpolarized lithium-6 as a sensor of nanomolar contrast agents

Lithium is widely used in psychotherapy. The (6)Li isotope has a long intrinsic longitudinal relaxation time T(1) on the order of minutes, making it an ideal candidate for hyperpolarization experiments. In the present study we demonstrated that lithium-6 can be readily hyperpolarized within 30 min, while retaining a long polarization decay time on the order of a minute. We used the intrinsically long relaxation time for the detection of 500 nM contrast agent in vitro. Hyperpolarized lithium-6 was administered to the rat and its signal retained a decay time on the order of 70 sec in vivo. Localization experiments imply that the lithium signal originated from within the brain and that it was detectable up to 5 min after administration. We conclude that the detection of submicromolar contrast agents using hyperpolarized NMR nuclei such as (6)Li may provide a novel avenue for molecular imaging.

Biomedical applications of 7Li NMR

The biomedical applications of 7Li MRS and MRI have been progressing slowly. The interest derives primarily from the clinical use of Li to treat bipolar disorder. One area of concern is the nature of ionic transport and binding, so as to elucidate the mechanism(s) of therapeutic action and toxicity. Another is the development of a non-invasive, in vivo analytical tool to measure brain Li concentration and environment in humans, both as an adjunct to treatment and as a mechanistic probe. Here we review the most recent progress toward these goals.

Localized7Li MR spectroscopy: In vivo brain and serum concentrations in the rat

The brain concentration of lithium (Li) in treated rats was measured using a recently developed method based on in vivo 7Li PRESS localized MRS. Comparison was made to the corresponding serum concentration at two treatment durations. The brain and serum Li concentrations were highly correlated with each other, more so than found previously for humans. The brain and serum Li concentrations also correlated with dose. Both the brain Li concentration and the serum concentration at 16.1 days of treatment correlated with the corresponding measure at 6.6 days. After correction of the brain Li concentrations for reduced 7Li MRS visibility, the mean brain/serum Li ratio for rats was close to unity, unlike most previous values found for humans. However, in some individual cases the ratio deviated substantially from the mean, suggesting that serum Li is not always a reliable indicator of brain Li.

Localized 7Li MR spectroscopy and spin relaxation in rat brain in vivo

Localized 7Li MR point-resolved spectroscopy (PRESS) was developed as a technique to measure lithium (Li) concentration in rat brain in vivo. Localized 7Li spectra could be obtained at 4.7 T in a 0.7-ml voxel in rat brain over the entire therapeutic range of serum Li for humans. Localized 7Li spin-lattice (T1) and spin-spin (T2) relaxation times were measured. Measured intensities were corrected for spin relaxation effects and 7Li MR visibility in vivo. The average T1 was 3.3 +/- 0.9 sec, and the average T2 was 82 +/- 20 ms. Neither T1 nor T2 correlated with brain concentration. No statistically significant change was found in either T1 or T2 from approximately 7-17 days of Li dosing.

Brain lithium concentrations in bipolar disorder patients: preliminary (7)Li magnetic resonance studies at 3 T

BACKGROUND

This study was conducted to investigate the feasibility of human brain (7)Li MRS investigations at a high magnetic field (3 T), and to further explore the relationship between brain and serum lithium measures in lithium-treated bipolar patients.

METHODS

Eight bipolar disorder type I patients (5 males, 3 females; mean age +/- SD = 33 +/- 9 years) were studied. A 3-T scanner, using a dual-tuned ((1)H and (7)Li) echoplanar imaging (EPI) compatible radiofrequency (RF) birdcage coil was used. (7)Li magnetic resonance spectroscopy (MRS) signal was acquired at the frequency of 49.64 MHz using an imaging selective in vivo spectroscopy (ISIS) sequence (TR = 15 sec, 128 averages), and quantitation was obtained in reference to an external standard.

RESULTS

The mean +/- SD oral lithium dose was 1265 +/- 442 mg/day, and the mean +/- SD 12-hour serum level was 0.69 +/- 0.19 mEq/L. The measured brain lithium concentrations varied from 0.23 to 0.55 mEq/L (mean +/- SD = 0.35 +/- 0.11 mEq/L). The brain-serum ratios varied from 0.30 to 0.80 (mean +/- SD = 0.52 +/- 0.16). Subjects on single daily doses of lithium at bedtime (n = 5) had higher brain-serum lithium ratios compared with those on twice-a-day schedules (n = 3) (0.61 +/- 0.12 and 0.37 +/- 0.07, respectively; Mann--Whitney U test, Z = -2.24, p =.03).

CONCLUSIONS

This study demonstrated for the first time the feasibility of (7)Li MRS human studies at 3 T. Future studies should examine a possible role for this methodology in investigations of lithium refractoriness and prediction of treatment outcome in bipolar patients.

Brain lithium measurements with (7)Li magnetic resonance spectroscopy (MRS): a literature review

7Li magnetic resonance spectroscopy (MRS) has been successfully used in recent years as a new tool to measure brain tissue lithium concentrations in vivo. After demonstration of its feasibility in animal studies over a decade ago, human investigations have characterized the brain pharmacokinetics of lithium. Preliminary studies have investigated brain pharmacokinetic correlates of clinical response in the treatment of bipolar disorder patients, with indication of possible clinical relevance of 7Li MRS measures. In this paper we reviewed the accumulated literature in this area, and discuss possible directions for this research in the context of preliminary studies conducted by our group that demonstrated the feasibility of 7Li MRS at 3 T.

Applications of (7)Li NMR in biomedicine

The applications of (7)Li NMR spectroscopy and imaging in biology and experimental medicine have been progressing steadily. The interest derives primarily from the clinical use of Li salts to treat mania and manic-depressive illness. One area of investigation is ionic transport across the cellular membrane and compartmentation, so as to elucidate the mechanism(s) of therapeutic action and toxicity in clinical practice. The second is the development of a noninvasive, in vivo analytical tool to measure brain Li concentrations in humans, both as an adjunct to treatment and as a mechanistic probe. Here we review progress to date in this area.