Monte Carlo simulation methods are frequently used to determine light propagation in tissue and x-ray propagation as well as for solving other non-medically related problems. Such techniques are computationally slow, with the signal to noise ratio improving only as the square root of computation time. We present a method for the design of a Monte Carlo program that is capable of running on up to 24 computers simultaneously, with there being very few restrictions on the computer types as long as they run on a common network. This parallel operation is useful when the run time is expected to be long. A mixture of PCs and Sun workstations have been successfully used. The program as described was designed for the simulation of light transport in tissue, but the technique of achieving simple simultaneous execution on a number of different computers could be used wherever Monte Carlo techniques are used.
This paper describes an opto-electronic cross-correlator designed for measurement of the temporal point spread function (TPSF) of light at the bedside. Ultra-short (< 2 ps) pulses of light from a mode-locked laser were used to illuminate a tissue phantom. The light exiting from the tissue phantom was coupled by an optical fibre to a small-area (200 microns diameter, Hamamatsu S2381) avalanche photodiode (APD). The gain of the photodiode was modulated at the repetition rate of the pulsed laser (82 MHz). Usually the gain was approximately 100, but for a period of approximately 130 ps (FWHM) the gain was increased to approximately 105. A lock-in amplifier, which sampled the integrated APD current, gave an output proportional to the difference between the current in the low- and high-gain states. Hence a small section of the TPSF was selectively sampled. An overall temporal resolution of 275 ps FWHM was achieved. As the timing of the gain modulation was controlled by an all-electronic variable-time-delay system, the whole of the TPSF could be sampled without requiring any moving prism or mirror which is typical of many cross-correlators. Hence the system is mechanically very rugged, which enhances its durability in a portable instrument.
An instrument has been designed that should allow the measurement of the temporal point spread function (TPSF) of tissue by a cross-correlation technique using an avalanche photodiode (APD) detector. Although not having the temporal resolution of a streak camera, the system is small, rugged, portable and relatively inexpensive.A laser producing a stream of ultra-short (few ps) pulses of light is used to interrogate the tissue. The scattered light is detected on a small area avalanche photodiode, the gain of which can be rapidly changed, by modulation of the applied DC bias voltage. The gain of the photodiode is usually low, but for a time period of the order of a few hundred picoseconds, the gain is increased. A lock-in amplifier measures the output of the photodiode and displays the difference between the output in the two gain states. Hence the photodiode is made to sample only a small section of the TPSF. A variable time-delay mechanism in the system, allows any section of the TPSF to be sampled, therefore allowing measurement of the whole TPSF.The novel method of generating the modulation voltage for the avalanche photodiode, which uses microwave techniques, means that it is possible to use an electronic phase shifting network, rather than the more commonly used mechanical time-delay mechanism. In principle, this means that the system should be very rugged and reliable, as there are no moving parts. The light source used during the system development is a large mode-locked Ti:sapphire laser, however by replacing this with a picosecond pulsed laser diode, a compact and portable instrument can be built, capable of being used at the bedside. keywords: avalanche photodiode (APD), laser, tissue-optics, step recovery diode (SRD), gain modulation, temporal point spread function (TPSF), cross-correlator, streak-camera. BACKGROUNDWhen light is incident on a scattering medium such as tissue, the optical path taken inside the medium is not a simple straight line, as would be the case in a clear material. Individual photons may take a very convoluted path, resulting in a distribution of transit times for the individual photons that emerge. If the input is a short (few ps) pulse of light, rather than a CW source, it is possible to measure the time for the photons to traverse the tissue, using a streak camera. A graph of intensity vs time is known as the Temporal Point Spread Function (TPSF).This measurement technique, which requires a short-pulsed light source (for example a mode-locked laser) and a streak camera, gives the impulse response of the tissue. Such a technique has been used in a number of research laboratories throughout the world, including UCL, for measuring the TPSF1. This technique is very powerful, as from the impulse response it is principle possible to calculate the optical properties of the tissue (absorption coefficient iaand scattering coefficient )2• An instrument capable of continuous measurement of iaand at the bedside would find many clinical applications. Unfortunately, mode-locked lasers and st...
These are the first absolute measurements of cerebral Hb and HbO2 in human fetuses during labor. The values of total hemoglobin are similar to those obtained in neonates with hypoxia-ischemia and the measurements of fetal cerebral oxygen saturation are similar to previously published values.
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