The Dr. Bicuspid (registration required) article linked here discusses some recent research into how effective the Canary System from Canada is. I saw the system in Chicago and mentioned it here. Canary detects demineralization and decay up to 5mm below the surface of the tooth. It uses a different scientific method than existing systems like Spectra. It is still awaiting FDA approval.
By simultaneously measuring the luminescence of the reflected light and the heat created by the absorption of the laser energy by the tooth, the system is able to provide a user with information on the presence and extent of tooth decay at its surface and to a depth of 5 mm below
via DrBicuspid Imaging.