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Boron‐11 NMR of borocaptate: Relaxation and in vivo detection in melanoma‐bearing mice

Longitudinal and transverse relaxation rates for the 11B resonances in sodium borocaptate (BSH) at varying concentrations were measured in undiluted horse serum in a 4.7 Tesla field. The results could be fit by a model that assumes fast exchange of the BSH molecule between a free and a bound state, using values of 0.77 ± 0.7 MHz for the 11B quadrupole coupling constant and (6.3 ± 0.9) × 10−9 s for the rotational correlation time in the bound state. These results were used as a basis for assessing the requirements and limitations of quantitative determination of BSH concentrations in vivo, using 11B NMR. Surface coil 11B NMR spectroscopy was performed on a total of 14 mice injected with BSH. Some of the animals (n = 9) had implanted M2R melanoma tumors grown to various sizes in the rear thigh, in which case the surface coil was placed against the tumor, whereas for the other animals (without tumor), the coil was placed against the rear thigh muscle. NMR spectra were acquired under fully relaxed conditions. The spectra were quantitated by peak integration; apparent absolute BSH concentrations were derived by comparison with spectra from a phantom with known BSH concentration, using extrapolation of the time‐domain data to zero preacquisition delay. The results indicate significantly higher 11B BSH signal intensities in tumors, compared with muscle tissue, whereas the uptake and clearance kinetics were similar.