Modulation Transfer Function Characterization Research Articles

Reference-free determination of tissue absorption coefficient by modulation transfer function characterization in spatial frequency domain

BackgroundSpatial frequency domain (SFD) measurement allows rapid and non-contact wide-field imaging of the tissue optical properties, thus has become a potential tool for assessing physiological parameters and therapeutic responses during photodynamic therapy of skin diseases. The conventional SFD measurement requires a reference measurement within the same experimental scenario as that for a test one to calibrate mismatch between the real measurements and the model predictions. Due to the individual physical and geometrical differences among different tissues, organs and patients, an ideal reference measurement might be unavailable in clinical trials. To address this problem, we present a reference-free SFD determination of absorption coefficient that is based on the modulation transfer function (MTF) characterization.MethodsInstead of the absolute amplitude that is used in the conventional SFD approaches, we herein employ the MTF to characterize the propagation of the modulated lights in tissues. With such a dimensionless relative quantity, the measurements can be naturally corresponded to the model predictions without calibrating the illumination intensity. By constructing a three-dimensional database that portrays the MTF as a function of the optical properties (both the absorption coefficient μa and the reduced scattering coefficient mu^{prime}_{s}) and the spatial frequency, a look-up table approach or a least-square curve-fitting method is readily applied to recover the absorption coefficient from a single frequency or multiple frequencies, respectively.ResultsSimulation studies have verified the feasibility of the proposed reference-free method and evaluated its accuracy in the absorption recovery. Experimental validations have been performed on homogeneous tissue-mimicking phantoms with μa ranging from 0.01 to 0.07 mm−1 and mu^{prime}_{s} = 1.0 or 2.0 mm−1. The results have shown maximum errors of 4.86 and 7% for mu^{prime}_{s} = 1.0 mm−1 and mu^{prime}_{s} = 2.0 mm−1, respectively. We have also presented quantitative ex vivo imaging of human lung cancer in a subcutaneous xenograft mouse model for further validation, and observed high absorption contrast in the tumor region.ConclusionsThe proposed method can be applied to the rapid and accurate determination of the absorption coefficient, and better yet, in a reference-free way. We believe this reference-free strategy will facilitate the clinical translation of the SFD measurement to achieve enhanced intraoperative hemodynamic monitoring and personalized treatment planning in photodynamic therapy.

Linear chirped slope profile for spatial calibration in slope measuring deflectometry.

Slope measuring deflectometry is commonly used by the X-ray optics community to measure the long-spatial-wavelength surface figure error of optical components dedicated to guide and focus X-rays under grazing incidence condition at synchrotron and free electron laser beamlines. The best performing instruments of this kind are capable of absolute accuracy on the level of 30-50 nrad. However, the exact bandwidth of the measurements, determined at the higher spatial frequencies by the instrument's spatial resolution, or more generally by the instrument's modulation transfer function (MTF) is hard to determine. An MTF calibration method based on application of a test surface with a one-dimensional (1D) chirped height profile of constant amplitude was suggested in the past. In this work, we propose a new approach to designing the test surfaces with a 2D-chirped topography, specially optimized for MTF characterization of slope measuring instruments. The design of the developed MTF test samples based on the proposed linear chirped slope profiles (LCSPs) is free of the major drawback of the 1D chirped height profiles, where in the slope domain, the amplitude strongly increases with the local spatial frequency of the profile. We provide the details of fabrication of the LCSP samples. The results of first application of the developed test samples to measure the spatial resolution of the BESSY-NOM at different experimental arrangements are also presented and discussed.

Modeling and Characterization of MTF and Spectral Response at Small Pitch on Mercury Cadmium Telluride

Space applications are challenging infrared (IR) technologies, demanding the best system performance achievable. This requires covering the entire IR spectrum from short-wavelength infrared (SWIR) to very long-wavelength infrared (VLWIR) for various pixel sizes, which is possible thanks to a well-mastered mercury cadmium telluride technology. Because of its adjustable gap, it can be operated in all the IR bands. Nevertheless, technology optimization requires deep understanding of physical mechanisms. This paper presents computations by finite-element modeling of two aspects of electrooptical performance: spectral response and modulation transfer function (MTF). Computations and characterizations for all IR bands demonstrate the accuracy of our simulations and the state-of-the-art nature of our technology, which performs according to theory. This paper also highlights the capability to measure MTF at very small pitch (10 μm) by a nondestructive method.

Random laser speckle based modulation transfer function measurement of midwave infrared focal plane arrays

Direct measurement of the modulation transfer function (MTF) of focal plane arrays (FPAs) using random laser speckle approaches for the visible/near-infrared wavelength band has been well documented over the last 20 years. These methods have not transitioned to the midwave infrared (MWIR) primarily because other techniques have been sufficient and MWIR laser sources with sufficient output power have been unavailable. However, as the detector pitch decreases, MTF measurements become more difficult due to diffraction, while potential MTF degradation due to lateral carrier diffusion crosstalk makes accurate MTF characterization critical for sensor system design. Here, a random laser speckle FPA MTF measurement approach is adapted for use in the MWIR that utilizes a quantum cascade laser coupled with an integrating sphere to generate the appropriate in-band random speckle. Specific challenges associated with the technique are addressed including the validity of the Fresnel diffraction assumptions describing the propagation of the random speckle field from the integrating sphere to the FPA. Improved methods for estimating the power spectral density (PSD) of the measured speckle that reduce data requirements are presented. The statistics and uniformity of the laser speckle are presented along with PSD measurements and estimated MTFs of a MWIR FPA.

Modulation transfer function characterization and modeling of a Scophony infrared scene projector

A Scophony-configuration infrared scene projector, consisting of a raster-scanned CO₂ laser and an acousto-optic (AO) modulator, was characterized for modulation transfer function (MTF) performance. The MTF components considered in the model were the Gaussian beam input to the AO cell, the finite aperture of the scan mirror, the width of the detector in the image plane, the transfer function of the amplifier electronics, and a term caused by Bragg-angle detuning over the bandwidth of the amplitude modulation (AM) video signal driving the AO cell. The finite bandwidth of the input video signal caused a spread in the Bragg angle required for maximum diffraction efficiency. In the Scophony configuration, a collimated laser beam enters the AO cell at only one particular angle, so a falloff of diffraction efficiency (and hence MTF) resulted as the modulation frequency was increased. The Bragg-angle detuning term was found to dominate the measured system MTF.