Spatial Frequencies Of Interest Research Articles

Using composite sinusoidal patterns in structured-illumination reflectance imaging (SIRI) for enhanced detection of apple bruise

Structured-illumination reflectance imaging (SIRI) provides a new means for enhanced detection of defects in horticultural products. Implementing the technique relies on retrieving amplitude images by illuminating the object with sinusoidal patterns of single spatial frequencies, which, however, are limited in interrogating the tissues at different depths. This study presented a first exploration of using composite sinusoidal patterns that integrated two and three spatial frequencies of interest, in SIRI for enhanced detection of defects in food (e.g., bruises in apples). Three methods based on Fourier transform were proposed to retrieve amplitude images at different frequencies by using either phase shifting with or without spiral phase transform (SPT) or frequency-domain filtering. The phase-shifting method involves solving a linear system that is composed of multiple phase-shifted pattern images in the Fourier space, and SPT that acts as a two-dimensional quadrature transform operator is used to reduce the images needed for amplitude retrieval; while the filtering-based method directly extracts different frequency components from only one pattern image that are then subjected to SPT processing. The three methods were tested for dual-frequency and triple-frequency patterns through numerical simulations and experiments on the detection of fresh bruises in apples by SIRI. The phase-shifting methods showed good performance in terms of small demodulation errors and strong image contrast for bruise detection; the filtering-based method, although viable in numerical simulation, needed improvement due to the worst practical performance. In addition, more frequency components introduced in the pattern would deteriorate the performance of these methods, and grid composite patterns were superior over the fringe ones due to reduced interactions between different frequency components.

Task-driven imaging in cone-beam computed tomography.

Conventional workflow in interventional imaging often ignores a wealth of prior information of the patient anatomy and the imaging task. This work introduces a task-driven imaging framework that utilizes such information to prospectively design acquisition and reconstruction techniques for cone-beam CT (CBCT) in a manner that maximizes task-based performance in subsequent imaging procedures. The framework is employed in jointly optimizing tube current modulation, orbital tilt, and reconstruction parameters in filtered backprojection reconstruction for interventional imaging. Theoretical predictors of noise and resolution relates acquisition and reconstruction parameters to task-based detectability. Given a patient-specific prior image and specification of the imaging task, an optimization algorithm prospectively identifies the combination of imaging parameters that maximizes task-based detectability. Initial investigations were performed for a variety of imaging tasks in an elliptical phantom and an anthropomorphic head phantom. Optimization of tube current modulation and view-dependent reconstruction kernel was shown to have greatest benefits for a directional task (e.g., identification of device or tissue orientation). The task-driven approach yielded techniques in which the dose and sharp kernels were concentrated in views contributing the most to the signal power associated with the imaging task. For example, detectability of a line pair detection task was improved by at least three fold compared to conventional approaches. For radially symmetric tasks, the task-driven strategy yielded results similar to a minimum variance strategy in the absence of kernel modulation. Optimization of the orbital tilt successfully avoided highly attenuating structures that can confound the imaging task by introducing noise correlations masquerading at spatial frequencies of interest. This work demonstrated the potential of a task-driven imaging framework to improve image quality and reduce dose beyond that achievable with conventional imaging approaches.

WE-C-304A-08: The Instrumentation Noise Equivalent Exposure (INEE): Including Conversion, Secondary Quantum, Structure and Electronic Noise

Purpose: To provide a quantitative and clinically relevant measure of the instrumentation noise of digital x‐ray imagers, using the Instrumentation Noise Equivalent Exposure (INEE). Method and Materials: The NPS was “decomposed” into constituent elements of primary quantum noise,Poisson excess noise (Swank noise), secondary quantum noise, structure noise and additive electronic noise using a plot of the total measured NPS versus exposure and using known relationships based on a generalized parallel cascade model analysis. The INEE was determined by measuring the exposure at which the total instrumentation noise and primary quantum noise were equivalent. Exposures at the INEE correspond to a drop in DQE of one‐half. To demonstrate the usefulness of this new metric, the spatial‐frequency‐dependent INEE was measured on a clinical (indirect) dynamic flat panel detector(FPD) with a pixel pitch of 194 μm and a 600 μm thick CsI:Tl phosphor.Results: The INEE was measured to be 2.0, 7.1 and 75 μR at 0, 1 and 1.2 cycles/mm, respectively. Above 1.2 cycles/mm the detector was instrumentation‐noise‐limited at all exposures, resulting in a degradation in image quality beyond that dictated by the noise in the incident x‐ray quanta alone. Hence, above 1.2 cycles/mm the DQE was always less than half the maximum value. Conclusions: In this work, all sources of instrumentation noise were incorporated into the INEE formalism, consisting not only of the additive electronic noise, but also including the Poisson excess noise (i.e. noise resulting from the phosphor), secondary quantum noise (i.e. noise resulting from the conversion and transmission of quanta) and structure noise (i.e. noise resulting from variations in the detector sensitivity). The INEE can be used to assess quantum‐noise‐limited operation at all spatial frequencies of interest by characterizing the total instrumentation noise in terms of equivalent detector entrance exposure. (Partial support: NIH grants R01EB008425, R01EB002873, R01NS43924)

Comparison of linear and rotationally shearing interferometric layouts for extrasolar planet detection from space.

We use the point spread function and the modulation transfer function (MTF) as two figures-of-merit to evaluate the performance of the multiaperture interferometric configurations for the detection of a faint planet in the vicinity of its bright star. We design nonredundant interferometric layouts that provide satisfactory coverage of the spatial frequencies of interest. We propose a design incorporating a rotating, rotationally shearing interferometer in a gravity-free environment and compare its performance with the Earth-based, fixed, linear configurations. The side peak of its MTF may be centered on the coordinate associated with a likely planet spatial frequency, resulting in planet signal enhancement and isolation.

Holographic 4x4 star-coupler in silver-halide sensitized gelatin

Star couplers are fanin-fanout devices with each input beam split into several output beams with arbitrary intensities traveling at different angles. We demonstrate a 4 x 4 holographic star coupler fabricated as multiplexed holographic gratings in silver-halide sensitized gelatin (SHSG) derived from Agfa-Gevaert plates. The processing formula is modified to achieve high diffraction efficiency and an acceptable modulation transfer function within the region of spatial frequencies of interest. The holographic characteristics of the processed SHSG are measured. The required gratings are recorded simultaneously to avoid reciprocity law failure, which occurs in SHSG as well. The gratings are recorded to ensure equal grating strength, which gives an acceptable intensity ratio between the output beams. The star coupler is examined for use as a broadcast medium in fiber-based local area networks (LAN5), where relatively high loss and inhomogeneity between the outputs could be tolerated.

Effect of Mid- and High-Spatial Frequencies on Optical Performance

In recent years there have been many optical system designs requiring high optical quality (sharp point spread function) as well as low scatter performance on aspheric surfaces. Because of the fabrication differences between the processes of figuring and smoothing on aspheric surfaces, it is important that the optical engineer understand how to trade off these processes during the manufacturing cycle. To describe these processes, the residual wavefront error (departure from the Gaussian reference sphere) is evaluated over a very wide spatial frequency band. From a physical point of view, the spatial frequencies of interest are grouped into three general categories: (1) the classical aberrations, which are low frequency in character and describe what might be called the figuring errors; (2) surface ripple or residual tooling errors, which arise in the fabrication of the optical elements; and (3) microroughness, the very high frequency structure on the surface, generally responsible for surface scattering. In this paper, the methods pioneered by Hopkins for aberration tolerancing and Barakat for random wavefront errors are extended to the discussion of mid- and high-spatial frequency surface errors. Some examples will be given to illustrate some general system properties such as the nature and role of the surface autocorrelation function.

Fast automatic lens testing for extended-field image quality

Monitoring of high-volume lens manufacture for Polaroid Land photography requires an acceptance criterion that is both relevant to picture quality and suitable for automatic testing. In a hardware test based upon the modulation transfer function (MTF), the problem is to choose what not to measure, while obtaining enough information about a lens to assign it a useful figure of merit. The predominant defects of manufacture are field tilt, resulting from tilting or decentering of components, and field curvature, from small errors in spacing, thickness, radius, or index. Field tilt and curvature are not local defects of the image surface, insofar as they may be removed by a local refocus. We therefore need a test procedure that can (perhaps simultaneously) examine image quality over an extended field, but is principally called upon to evaluate focus differences. At multiple field locations a single-frequency modulation measurement adequately defines the MTF for the low spatial frequencies of interest and can be interpreted easily in terms of the rms blur of the point spread function. We will discuss the logic of 100% lens acceptance testing and two types of instrumentation we have used.