KEYWORDS: Light, Modulation, Photon counting, Signal detection, Field programmable gate arrays, Digital electronics, Signal to noise ratio, Time division multiplexing, Optical testing, Analog electronics
Relying on deeper penetration of light in the tissue, Diffuse Optical Tomography (DOT) achieves organ-level
tomography diagnosis, which can provide information on anatomical and physiological features. DOT has been widely
used in imaging of breast, neonatal cerebral oxygen status and blood oxygen kinetics observed by its non-invasive, security
and other advantages.
Continuous wave DOT image reconstruction algorithms need the measurement of the surface distribution of the
output photon flow inspired by more than one driving source, which means that source coding is necessary. The most
currently used source coding in DOT is time-division multiplexing (TDM) technology, which utilizes the optical switch to
switch light into optical fiber of different locations. However, in case of large amounts of the source locations or using the
multi-wavelength, the measurement time with TDM and the measurement interval between different locations within the
same measurement period will therefore become too long to capture the dynamic changes in real-time.
In this paper, a frequency division multiplexing source coding technology is developed, which uses light sources
modulated by sine waves with different frequencies incident to the imaging chamber simultaneously. Signal corresponding
to an individual source is obtained from the mixed output light using digital phase-locked detection technology at the
detection end. A digital lock-in detection circuit for photon counting measurement system is implemented on a FPGA
development platform. A dual-channel DOT photon counting experimental system is preliminary established, including the
two continuous lasers, photon counting detectors, digital lock-in detection control circuit, and codes to control the
hardware and display the results. A series of experimental measurements are taken to validate the feasibility of the system.
This method developed in this paper greatly accelerates the DOT system measurement, and can also obtain the multiple
measurements in different source-detector locations.
Optical diagnostics has the potential to provide real-time diagnosis of tissue noninvasively, and many optical
diagnostic techniques are receiving extensive attention and being developed. Frequency domain (FD) near-infrared
diffuse spectroscopy (NIRS) is one of the three common techniques in NIRS field. Generally, a FD system modulates
the light intensity in radio frequency and measures the amplitude attenuation and phase delay of the diffused light using
heterodyne detection.
This article deals with the method for eliminating or calibrating both coupling factor and the intrinsic parameters of
the measurement system, which include the intrinsic amplitude attenuation and intrinsic phase delay. Several calibration
methods are proposed, namely, calibration with standard phantom, calibration based on multiple source-detector
separations (SDS), and calibration with the combination of standard phantom and multiple SDS.
Two solid tubular phantoms with known optical properties are adopted to evaluate the proposed calibration
methods. Endoscopic measurements on the phantoms were carried on to obtain the amplitude attenuation and phase
delay while Monte Carlo simulation was employed to calculate the "real" ones. Results show that the calibration
method with the combination of standard phantom and multiple SDS gets the minimum relative error of amplitude.
As a typical 3D image algorithm, Feldkamp-David-Kress reconstruction algorithm is a filtered backprojection very
similar to the 2D algorithm. Recent years, it has been widely used for its easy implementation and acceptable
reconstruction precision for small cone-beam angle. For big cone-beam angles, however, the reconstruction exactness of
horizontal planes will dramatically decrease with the distance from the central plane increases. Therefore, for improving
the reconstruction precision in vertical direction, a similar reciprocal Gaussian function as the weighted factor was
introduced into FDK algorithm in the article. The validity of the improved FDK algorithm was verified and evaluated
through both the computer numerical simulation and the phantom model experiment. The quantitative analysis for the
reconstruction results demonstrated that the reconstruction image using the improved FDK algorithm could primely
revise the original reconstruction images and restore well closely to the tomography image of the prototype object.
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