Optimal detection of the neutron capture therapy agent borocaptate sodium (BSH): A comparison between H-1 and B-10 NMR

Boron Neutron Capture Therapy (BNCT), an experimental binary cancer treatme nt modality, requires selective targeting of B-10 containing compounds to t umors. One of the compounds under evaluation in an EORTC phase I trial, and used in Japan for patient treatments for many years, is borocaptate sodium (BSH, also known as sulfhydril boron hydride). To optimize the clinical ap plications, a noninvasive method is needed to monitor the distribution of t he boron compound, and NMR may offer such a possibility. A comparison betwe en the relative sensitivities for detecting BSH by B-10 or H-1 NMR was cond ucted at two magnetic field strengths: 2 and 4.7 T. At each field Strength, similar-sized radio frequency (rf) coils were used for both nuclei. Theore tical predictions for the intrinsic signal to noise (S/N) advantage of H-1 over B-10 detection vary between a factor of 5.4 and a factor of 28.9, depe nding on whether the effective resistance is dominated by coil losses or sa mple losses. Our tests, conducted on relatively small aqueous samples, whic h loaded the coils less than expected for animal or human subjects, resulte d nevertheless in advantage factors close to the lower limit of this range. The measured S/N detection advantage factors for H-1 were about 5.2 at 4.7 T, using a dedicated H-1 coil, and 7.7 at 2 T, where the measurements were conducted with a double-tuned coil. However, when predicting the expected performance for in vivo MRS or MRI, one should bear in mind that proton det ection has to be conducted by spectral-editing pulse sequences with an inhe rent S/N loss by at least a factor of 2, and that the T-1 relaxation time f or B-10 in BSH is about 30 times shorter than the H-1 T-1 value. In view of these considerations, direct B-10 detection could well be the preferred st rategy for MRI/MRS of BSH in vivo. (C) 2001 American Association of Physici sts in Medicine.