UCL Dept of Medical Physics & Bioengineering

We have developed an opto-electronic cross-correlator to determine the optical properties of tissue. A short (< 2 ps) pulsed mode-locked laser is used to supply pulses of light to tissue. The light exiting the tissue is temporally broadened up to a few nanoseconds in length. The exact characteristics of the light exiting the tissue depends upon the optical properties of tissue. The optical properties of tissue, characterised by an absorption coefficient and a transport scattering coefficient, are frequently needed when planning treatments that use optical radiation. By making measurements of the response of the tissue to a short pulse of light, we are able to determine the impulse response of the tissue. Computer models using the Monte Carlo technique are then used to determine the optical properties that would produce the measurements made with the cross-correlator. A search procedure is under development to enable the optical properties of the tissue to be determined, by making comparisons between the measured data and the theoretical predictions from the Monte Carlo model.

The opto-electronic cross-correlator uses a silicon avalanche photodiode (APD) as the detector. This is gain modulated using a microwave step recovery diode (SRD). The electrical pulses generated by the SRD are coupled via a bias T to the SRD so that the APDs bias voltage is rapidly changed. This increases the APDs gain for a period of the order of 200 ps, allowing sampling of the temporal profile of the light exiting the tissue. By varying the phase of the radio frequency drive to the SRD, we are able to sample any section of the TPSF. The phase is altered electronically, rather than the mechanical methods usually used in the design of cross-correlators. A lock-in amplifier is used to measure the APD output current. The lock-in amplifier is connected to a computer, which also controls the control voltage fed to the phase shifter. A simplified block diagram of the system is shown below, where the small inserted graphs show idealised optical intensities and electrical waveforms. Thin lines are used to indicate electrical connections, but thicker lines are used to indicate optical fibre connections.

The cross-correlator has a dynamic range of 40 dB, but the signal to noise ratio is currently 10 dB. The temporal response is 275 ps FWHM. Attempts are continuing to improve this, which has the rather odd characteristic that the noise increases in direct proportion to the signal. The source of the noise is believed to be the SRD diode. A number of references describing the system have been published [1,2,3].

In order to reduce the time required for the Monte Carlo models to execute, we have developed a parallel program that is able to execute on a diverse network of computers, including Sun workstations and PC's. A paper describing this has been published [4], and the final PhD thesis is available for download by clicking here.

Kirkby, DR, and Delpy, DT (1995): Measurement of tissue temporal point spread function by use of a cross-correlation technique with an avalanche photodiode detector, Proc. SPIE 2389, 190-197.
Kirkby, DR, and Delpy, DT (1996): Measurement of tissue remporal point spread function by use of a gain-modulated avalanche photodiode detector, Physics in Medicine and Biology 41(5), 939-949.
Kirkby, DR, and Delpy, DT (1996): An optoelectronic cross-correlator using a gain modulated avalanche photodiode for measurement of tissue temporal point spread function, OSA TOPS on Advances in Optical Imaging and Photon Migration 3, 108-112.
Kirkby, DR, and Delpy, DT (1997): Parallel operation of Monte Carlo simulations on a diverse network of computers. Physics in Medicine and Biology 42(6), 1203-1208.