200 ps FWHM and 100 MHz repetition rate ultrafast gated camera for optical medical functional imaging

Abstract

The paper describes the realization of a complete optical imaging device to clinical applications like brain functional imaging by time-resolved, spectroscopic diffuse optical tomography. The entire instrument is assembled in a unique setup that includes a light source, an ultrafast time-gated intensified camera and all the electronic control units. The light source is composed of four near infrared laser diodes driven by a nanosecond electrical pulse generator working in a sequential mode at a repetition rate of 100 MHz. The resulting light pulses, at four wavelengths, are less than 80 ps FWHM. They are injected in a four-furcated optical fiber ended with a frontal light distributor to obtain a uniform illumination spot directed towards the head of the patient. Photons back-scattered by the subject are detected by the intensified CCD camera; there are resolved according to their time of flight inside the head. The very core of the intensified camera system is the image intensifier tube and its associated electrical pulse generator. The ultrafast generator produces 50 V pulses, at a repetition rate of 100 MHz and a width corresponding to the 200 ps requested gate. The photocathode and the Micro-Channel-Plate of the intensifier have been specially designed to enhance the electromagnetic wave propagation and reduce the power loss and heat that are prejudicial to the quality of the image. The whole instrumentation system is controlled by an FPGA based module. The timing of the light pulses and the photocathode gating is precisely adjustable with a step of 9 ps. All the acquisition parameters are configurable via software through an USB plug and the image data are transferred to a PC via an Ethernet link. The compactness of the device makes it a perfect device for bedside clinical applications.

A time-gated near-infrared spectroscopic imaging device for clinical applications.

“…The image obtained with a conventional tube is a representation of the wave propagation of the electrical pulse inside the tube, whose velocity is very slow as compared to the light velocity c in the vacuum. To overcome this limitation, a new design of photocathode shape has been realized 14 . The aperture of the image intensifier is now completed in less than 70 ps, instead of more than 300 ps otherwise.…”

High quantum efficiency S-20 photocathodes in photon counting detectors

“…The latter is a direct consequence of the photocathode deposition process optimized to get higher red-sensitivity. Such photocathodes, as well as new developed Hi-QE photocathodes, can be provided for fast gating applications with a mesh underlay, which allows for a fast gating time down to below 1 ns [4], but it simultaneously reduces average QE by about 10-15%.…”

Sub-nanosecond gated photon counting for high spatial resolution CMOS imagers

“…To our knowledge, it is the first large size sub-ns gated 2D image sensor reported. CMOS ultra-fast gated image sensors could be an alternative for the current state of the art, ultra-fast gated intensified cameras using vacuum tube technology [16].…”