Imaging method and apparatus

A method of determining a location of an optical fibre that is positioned at least partially inside a human or animal body, the method comprising the steps of:

• transmitting, from a light source, pulsed light into a proximal end of the optical fibre such that the pulsed light is guided along the optical fibre and emitted from the optical fibre into tissue of the human or animal body;
• transmitting, by the light sources, to a time-resolved single photon detector positioned outside the human or animal body, an electrical pulse when the pulsed light is transmitted into the proximal end;
• receiving, by the time-resolved single-photon detector positioned outside the human or animal body, photons of the pulsed light that have passed through the tissue of the human or animal body after having been emitted from the optical fibre;
• receiving, at the time-resolved single-photon detector, the electrical pulse from the light source;
• producing, by the time-resolved single-photon detector, signals corresponding to the received photons, wherein, for each of the received photons, the signal representative thereof includes a position and a time of arrival, wherein producing said signals comprises starting or stopping timing based on the reception of the electrical pulse so that the time of arrival represents a difference between a time of transmission of a pulse in which the photon originated and a time at which the photon was received, and wherein the produced signals represent at least ballistic photons, snake photons, and highly scattered photons each originating from the pulsed light emitted from the optical fiber and having passed through the tissue of the human or animal body;
• receiving, by a processor, the signals from the time-resolved single-photon detector;
• selecting, by the processor, signals corresponding to some of the received photons wherein the selecting comprises:
• selecting first signals having a time of arrival within a first time interval, the first selected signals representing ballistic photons or a combination of ballistic photons and snake photons,
• selecting second signals having a time of arrival within a second, different time interval, the second selected signals representing snake photons, and
• discarding signals representing at least the highly scattered photons;
• and
• determining a location of the optical fibre within the human or animal body based on the selected signals, wherein the determining of the location of the optical fibre based on the selected signals comprises:
• forming a first image using the first selected signals,
• forming a second image using the second selected signals, and
• determining the location of the optical fibre within said human or animal body based on a combination of the first image and the second image.