A time-domain measurement can be equivalently expressed in the frequency domain. Some researchers have developed imaging systems which acquire frequency-domain data directly using a source that is amplitude modulated at a high frequency (a few hundred MHz), and measuring the reduction in amplitude and phase shift of the transmitted signal. While time-resolved measurements have the advantage of acquiring information at all frequencies simultaneously, frequency-domain systems can employ light sources and detectors which are significantly less expensive than those required for time-resolved systems.
The science and technology of phase measurement for NIRS and imaging has been comprehensively reviewed by Chance et al. (1998). Systems require a radio-frequency (RF, typically a few hundred MHz) oscillator to drive a suitable laser diode and to provide a reference signal for the phase measurement device which receives the detected signal from an appropriate high-bandwidth detector (e.g. PMT or APD, depending on the desired sensitivity). Heterodyning is commonly performed to convert the RF to a few kHz prior to phase detection. The detected signal is digitized over an appropriate period of time, and phase and amplitude are computed.
The transillumination and tomographic approaches described in section 2.3 for time-resolved systems are equally applicable for frequency-domain devices, and both have been widely explored. In the mid-1990s two major companies in Germany reported development of breast imaging systems based on frequency-domain measurement of transmitted light. These prototypes, constructed by Carl Zeiss (Kaschke et al. 1994, Moesta et al. 1996) and by Siemens (Götz et al. 1998), both involved rectilinear scanning of a single source-detector pair over opposite surfaces of a compressed breast, resulting in single-projection images at multiple NIR wavelengths. Unfortunately the performance of both systems during quite extensive trials fell below that required of a method of screening for breast cancer, although various improvements to the Carl Zeiss system and their reconstruction method have since been implemented (Franceschini et al. 1997, Fantini et al. 1998).
As summarised in section 4, a variety of frequency-domain systems have been developed for both optical topography (Danen et al. 1998, Franceschini et al. 2000) and optical tomography (Pogue et al. 2001). Culver et al. (2003) have built a hybrid CW/frequency-domain device for optical tomography which combines the benefits of speed and low cost of CW measurements with the ability to separate scatter and absorption available from the amplitude and phase of frequency-domain data. The system employs four amplitude-modulated laser diodes operating at different wavelengths (690 nm, 750 nm, 786 nm, and 830 nm) which are rapidly switched between 45 optical fibres on a 9 x 5 array. The array is positioned against one side of a compressed breast, while light emerging on the opposite side is focussed on to a CCD camera. Meanwhile, diffusely reflected light is detected simultaneously by APDs via nine fibres interlaced among the source fibres. The amplitude and phase of the APD signals are determined using a homodyne technique.
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