Time-correlated single photon counting imager for simultaneous magnetic resonance and near-infrared mammography

We describe a near infrared (NIR) imager for mammography, designed to work simultaneously with a magnetic resonance (MR) scanner. The imager employs two pulsing laser diodes, with average power of 25 μW, at 780 and 830 nm. The two wavelengths are time multiplexed into 24 source fibers. The detection part consists of eight parallel time-correlated photon-counting channels with overall counting capacity of ${10}^6\hspace{0.17em}\mathrm{photons/s}.$ We use long optical fibers to avoid interference with the magnetic field. Specially designed coupling plates, for breast soft compression, bear both the MR radio-frequency coils and the optical source and detector fibers. Capillaries containing water and copper sulfate mark the position of the plates on the MR images for accurate coregistration of NIR and MR images. Instrument compatibility has been successfully tested with volunteers in the MR scanner. The use of gallium arsenide photomultiplier tubes has allowed penetration depths of 10 cm in the human breast. Imaging algorithms, based on the analytical modeling of photon propagation in inhomogeneous media, have been applied successfully to image 0.8-mm-diam absorbing and scattering cylindrical perturbations in transmittance geometry of breast-like phantoms.