Spatial Frequency Region Research Articles

A non-negative fast multiplicative algorithm in 3D scatter-compensated SPET reconstruction

Single-photon emission tomographic (SPET) reconstruction can be improved, especially for noisy images, by using the iterative expectation-maximization of the maximum-likelihood (EM-ML) algorithm. Its application to clinical routine is, however, hampered by the high number of iterations necessary to achieve acceptable results. Therefore various methods have been developed to accelerate the EM-ML algorithm. In this paper a new accelerated EM-ML-like multiplicative algorithm is proposed for SPET reconstruction. Contrary to some other accelerating methods, it preserves two of the most important properties of the EM-ML, namely pixel positivity inside the patient body and null activity outside. The convergence speed is improved by a factor which can reach 100 in high spatial frequency or low count regions. Good estimates in the low count region are obtained without any smoothing, even at typical routine clinical count rates. The algorithm used in conjunction with the 3D effective one scatter path model provides high-quality SPET images and accurate quantitation.

Modulational instability of quasi-plane-wave optical beams biased in photorefractive crystals

The modulational instability of quasi-plane-wave optical beams in biased photorefractive media is investigated under steady-state conditions. The spatial growth rate of the sideband perturbations is obtained by globally treating the space-charge field. Our analysis indicates that the growth rates depend on the strength of the externally applied electric field and, moreover, on the ratio of the optical beam's intensity to that of the dark irradiance. Our results are then compared to previous local treatments of the space-charge field equation. The two approaches are found to be in good agreement in the low spatial-frequency regime provided that the external bias field is sufficiently high. Conversely, in the high spatial-frequency region notable differences may exist. Relevant examples are provided.

Grating Optical Low-Pass Filter

The spurious signals appearing in the picture image from a charge-coupled semiconductor device (CCD) camera were removed by means of a novel grating optical low-pass filter (GOLF). By placing the GOLF near the imaging plane of the CCD and adjusting the effective grating period projected onto the imaging plane with respect to the array period, or its integral multiplicative inverse of the picture elements formed in the imaging plane, it was verified that the GOLF eliminated the grating image superimposed on the picture image, which had been a long-standing problem. Photochemical reaction was useful for forming a sinusoidal grating crosssection which increased the MTF value in the region of low spatial frequencies to realize a clear-cut picture image. The GOLF fabricated in the form of a thin plate was promising to realize a new CCD with a built-in optical low-pass filter applicable for downsizing of camera optics.

A study on three-dimensional (spatio-temporal) noise characteristics of letterbox EPITV

TheF difference between the three-dimensional noise characteristics of the current NTSC and that of the letterbox EDTV has been studied. Hereafter, we use "three-dimensional" for spatio-temporal. We have shown that when the letterbox EDTV pictures (main panel) are displayed in the 16:9 aspect area with the same picture height as that of NTSC pictures (4:3), and when the signal levels arc the same, the three-dimensional noise power spectrum of the main panel of letterbox EDTV is transformed into a low spatial frequency region with high density, and the total noise power remains unchanged both in NTSC and EDTV. The helper signals which are multiplexed onto the black bands in the top and bottom portions of the screen, generally, increase the total noise power of decoded EDTV signals even if the amplitude of the helper signals is large enough. By applying a three-dimensional weighting function, it is shown that the weighted noise power of the letterbox EDTV becomes larger than that of NTSC. Hence, in order to retain the existing service area after introducing the EDTV broadcasting service, it will be necessary to install noise suppressing measures in the EDTV system.

Vector-quantization-based video codec for software-only playback on personal computers

This paper discusses a video compression and decompression method based on vector quantization (VQ) for use on general purpose computer systems without specialized hardware. After describing basic VQ coding, we survey common VQ variations and discuss their impediments in light of the target application. We discuss how the proposed video codec was designed to reduce computational complexity in every principal task of the video codec process. We propose a classified VQ scheme that satisfies the data rate, image quality, decoding speed, and encoding speed objectives for software-only video playback. The functional components of the proposed VQ method are covered in detail. The method employs a pseudo-YUV color space and criteria to detect temporal redundancy and low spatial frequency regions. A treestructured-codebook generation algorithm is proposed to reduce encoding execution time while preserving image quality. Two separate vector codebooks, each generated with the treestructured search, are employed for detail and low spatial frequency blocks. Codebook updating and sharing are proposed to further improve encoder speed and compression.

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.

Coding characteristics of the pixel-adaptive DPCM utilizing the variable-length-codes

The coding characteristics of a modified DPCM (differential pulse code modulation) method for data compression of still images for digital still cameras are described. In pixel adaptive DPCM, the pixels are classified into eight groups according to the magnitude of the prediction error of preceding pixels, and the prediction error signals of each group are quantized using eight nonlinear quantizers, each of which is designed optimally to each group. The performance of the pixel adaptive DPCM can be improved by utilizing the variable bit-rate scheme. It is shown in an example that the signal-to-noise ratio can be improved to 48 dB by 6 dB at 3.0 b/pixel, that this improvement is due to the relief of the slope overloading at the edges, and that the spectrum of the coding errors is distributed uniformly in the 2-D spatial frequency region.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Spatial and temporal frequency in figure-ground organization.

Figure-ground organization of an ambiguous pattern can be manipulated by the spatial and temporal frequency content of the two regions of the pattern. Controlling for space-averaged luminance and perceived contrast, we tested patterns in which the two regions of the ambiguous pattern contained sine-wave gratings of 8, 4, 1, or 0.5 cycles per degree (cpd) undergoing on:off flicker at the rates of 0, 3.75, 7.5, or 15 Hz. For a full set of combinations of temporal frequency differences, with each spatial frequency the higher temporal frequency was seen as background for more of the viewing time. For two spatial frequency combinations, 1 and 4 cpd, and 1 and 8 cpd, tested under each of the four temporal frequencies, the lower spatial frequency region was seen as the background for more of the viewing time. When the effects of spatial and temporal frequency were set in opposition, neither was predominant in determining perceptual organization. It is suggested that figure-ground organization may parallel the sustained-transient response characteristics of the visual system.

Representation of edges of variable blur by neuronal responses in the lateral geniculate body and the visual cortex of cats: Limits of linear prediction

We have measured the responses of cells in the cats lateral geniculate body and the visual cortex to edges which were blurred to various degrees (cosinusoidal blur). For the same cells also the responses were determined to sinusoidal gratings of various fundamental frequency and to slits of various blur and width. All stimuli were moved across the receptive fields at various speeds. The responses of most cells increased with increasing edge sharpness, but usually reached a maximum at a blur corresponding to a high frequency cutoff at 0.6–1.2 c/deg. The responses to the sharpest edges were usually smaller than those to a blurred edge (up to −50% in individual cells and −15% in the average). After normalization, the responses predicted from the spatial frequency tuning curves and the Fourier transform of the edge stimuli corresponded well to the measured blur functions up to the maximum of the edge response which varied considerably between cells, however. At edge sharpness beyond that maximum, the predicted curves rose up to edge sharpness with high frequency cutoff 1.6–1.8 times above that which produced the experimental neuronal response maximum. On the other hand, responses could increase with edge sharpening in spatial frequency regions, in which no or only small responses were seen with sinusoidal gratings (e.g. at lower spatial frequencies in “band pass neurons”). Geniculate X- and cortical simple cells as well as those geniculate Y-cells which showed phase locked grating responses behaved similarly in all respects. We concluded that edge sharpness is not represented by response amplitude of individual neurons but by the spatial distribution of excitatory peaks across the representation of the retinotopic cortical map. Our findings further indicate that spatial models of receptive fields assuming linear signal summation have only a limited value for predicting edge sharpness.

Further evidence for a broadband, isotropic mechanism sensitive to high-velocity stimuli.

Spatial frequency and orientation selectively, the most prominent properties of image-processing in the striate cortex, are not uniform throughout the spatiotemporal frequency domain. Some current models include one "transient" mechanism at very high velocities (i.e. low spatial and high temporal frequencies), and multiple "sustained" mechanisms elsewhere in the spatiotemporal frequency domain, but they do not consider the parameter of orientation. On the basis of earlier, orthogonal masking experiments, we concluded that the high-velocity mechanism is sensitive to a broad band of spatial frequencies, and has little or no orientation selectivity. In the present study we use pattern adaptation to measure the spatiotemporal properties of this mechanism. In other experiments, we attempt to relate it to the direction-selective motion detectors that also respond at high velocities. Finally we compare the pattern-adaptation results to the results of orthogonal subthreshold summation experiments in the same region of high temporal and low spatial frequencies.

On the spatial resolution of the IR remote sensing systems

According to the close relation between spatial resolution and radiant resolution and utilizing the experimental equation of MRTD, it is proved that there is a selectable optimum value of IFOV for particular spatial frequency or spatial frequency region which is interesting. The convolution process of line scanner is also analyzed, it is concluded that the spatial resolution can greatly exceed the limit of IFOV as long as the contrast is large enough, especially for linear or spot targets. Thus, the relative phenomena of the satellite image can be explained effectively. It is indicated that the spatial resolution can further be improved, if the processing of solving convolution operation is carried out in the digitized image processing.

Periodicity of striate-cortex-cell receptive fields.

If striate cells had the simple bipartite or tripartite receptive fields (RF's) classically attributed to them, they should be quite broadly tuned for spatial frequency. Most striate-cortex cells, however, are fairly narrowly tuned and would be expected to have more-periodic RF's. We have examined this question in recordings of the responses of cat and monkey striate-cortex cells to gratings of increasingly large number of cycles, all centered on the cells' RF's. Simple cells narrowly tuned for spatial frequency were found to increase their responses with increasing numbers of stimulus cycles beyond the 1 1/2 cycles expected from the classical RF shape. Broadly tuned simple cells were found to have less-periodic RF's. Whereas narrowly tuned complex cells were also found to respond maximally to many stimulus cycles, other more broadly tuned complex cells did as well (possibly reflecting summation across many broadly tuned simple cells without regard to phase). A suppressive region was often seen just outside the excitatory two-dimensional spatial-frequency region, at off orientations and/or off spatial frequencies and around the whole RF in space. Most striate cells can thus be described as having periodic RF's in the space domain such that they fire just to patterns whose local spatial-frequency spectra fall within a compact, restricted, roughly circular two-dimensional spatial-frequency region, with an encircling suppressive region in both the space and the frequency domains.

Relationship between spatial-frequency and orientation tuning of striate-cortex cells.

If striate cells had the receptive-field (RF) shapes classically attributed to them, their preferred spatial frequencies would vary considerably with orientation. Other models of RF shape would predict a greater independence between orientation and spatial-frequency tuning. We have examined this by recording the responses of cat striate-cortex cells to a wide range of different spatial-frequency and orientation combinations. In almost all cells studied, peak orientation did not consistently vary with spatial frequency, but the majority of cells showed some change in peak spatial-frequency tuning with orientation. The amount of change in peak spatial frequency tended to be greater for cells that were narrowly tuned for orientation. However, cells narrowly (and also very broadly) tuned for spatial frequency tended to show considerable independence of spatial-frequency and orientation tuning, and in all but a few cells the degree of change was less than predicted by the classic RF model. Such cells were found to fire only to patterns whose local spatial spectra fell within a compact, restricted, roughly circular two-dimensional spatial-frequency region. We conclude that the two-dimensional RF shape of striate cells more closely approximates that predicted by a two-dimensional Gabor model or by a Gaussian-derivative model than it does the classic shape based on the output of geniculate cells with aligned RF's.

Temporal frequency discrimination in human vision: Evidence for an additional mechanism in the low spatial and high temporal frequency region

Temporal frequency discrimination at and above the detection threshold has been studied using gratings of low (0.2 c/deg) and medium (2 c/deg) spatial frequencies. At 2 c/deg the results of previous investigators are confirmed: The results being consistent with the existence of two broadly tuned and directionally selective temporal mechanisms (up to 32 Hz). For the lower spatial frequency an additional temporal frequency discrimination at threshold can be made between 4 and 32 Hz and enhanced temporal frequency discrimination at suprathreshold levels occurs above 24 Hz. One interpretation of this result is the existence of one or more additional temporal mechanisms with restricted spatial acuity responding to higher temporal frequencies.

Reduction of Noise Interference from METEOSAT Water Vapor Image Data by Means of Fourier Transform and Frequency Domain Filtering

Images provided by the water vapor channel data of meteorological satellites are suitable for the determination of wind vectors in the midtroposphere. Preprocessing the image data affects the quality and quantity of the derived wind vectors. Standard space domain filtering of image pixels with suitable algorithms gives rise to water vapor images which are usable for wind determination. However, if the superimposed noise shows obvious spatial frequency components, the image spectra make it much easier to attenuate those spatial frequency regions than is possible with the image itself. Then, interactive spatial frequency domain filtering after a Fourier transform of the original images allows better results for less effort. A space domain filter algorithm to tackle such a problem is, of course, conceivable. However, on the interactive system we use, constructing a space domain filter is much more difficult than constructing a spatial frequency filter.

Development of temporal contrast sensitivity

A prolonged exposure to foveal defocus is well known to affect the visual functions in the fovea. However, the effects of peripheral blur adaptation on foveal vision, or vice versa, are still unclear. In this study, we therefore examined the changes in contrast sensitivity function from baseline, following blur adaptation to small as well as laterally extended stimuli in four subjects. The small field stimulus (7.5° visual field) was a 30 min video of forest scenery projected on a screen and the large field stimulus consisted of 7-tiles of the 7.5° stimulus stacked horizontally. Both stimuli were used for adaptation with optical blur (+2.00 D trial lens) as well as for clear control conditions. After small field blur adaptation foveal contrast sensitivity improved in the mid spatial frequency region. However, these changes neither spread to the periphery nor occurred for the large field blur adaptation. To conclude, visual performance after adaptation is dependent on the lateral extent of the adaptation stimulus.

On the number of intensity levels discriminated in textures.

The number of intensity levels detected in briefly displayed textures was studied as a function of two related spatial parameters: granularity and spatial frequency. A texture-discrimination paradigm was used to determine the discriminability of an original texture and of a version of it reduced (further "quantized') in intensity levels over a range of texture granularities and frequency bandwidths. Results show specific sensitivities to intensity level reductions as a function of the spatial-frequency composition of the textures, with particular loss of sensitivity in the higher spatial-frequency regions.

Comparison of topographic and spatial-frequency characteristics of the lateral suprasylvian and striate cortex in cats

Differences in spatial frequency and topographic characteristics of receptive fields of neurons in two visual projection areas (striate cortex and lateral suprasylvian region) were analyzed on the basis of results of the writer's previous and present investigations. Receptive fields of neurons in the suprasylvian cortex are large; their size increases with eccentricity and the transmission band occupies a region of low and intermediate spatial frequencies. Receptive fields of striate cortical neurons, on the other hand, are small; they increase in size with eccentricity and the transmission band lies in the region of intermediate and high spatial frequencies. The latent period of evoked potentials is longer in the striate than in the suprasylvian cortex. Analysis of spatial low-frequency information is processed in the suprasylvian cortex a little earlier than high-frequency information is processed in the striate cortex. Retinotopically organized interaction of projection zones enables the two descriptions of a perceived scene to be correlated.

Spatial frequency characteristics of receptive fields of neurons in the lateral suprasylvian area of the cat cortex

Two-dimensional spatial frequency characteristics of receptive fields of 46 neurons in the lateral suprasylvian area of the cat cortex were obtained. These receptive fields possessed orientation anisotropy. Peak frequencies lay in the frequency region below 1.5 cycles/deg. The transmission band width was measured during optimal orientation of test gratings in 21 neurons. It averaged 1.47±0.6 octave. In the remaining neurons the lower boundary frequency was shifted into the region of spatial frequencies below the range used. During nonoptimal orientation of test gratings, inhibition of the discharge was observed in 17 neurons. The inhibitory spatial frequency characteristics of six neurons were of the narrow band type, and averaged 1.1±0.6 octave.

A “stand alone” image processing system for STEM images

A “stand alone” image processing system for STEM images is discussed. The system employs a minicomputer (32 K capacity, using BASIC language) and digital memory (640×512 pixels, 16 bits) as well as a black and white and a programmed-color display monitor. The various image processing programs were applied to STEM low dose (2 electrons / Å 2) images of the giant hemoglobin of Lumbricus terrestris. The images were either rotation (6-fold or double reflection) averaged or unrotated, then summed to obtain statistically high quality images. These summed images were then spatially filtered and inverse-transformed. In addition, some images on transforms can be subtracted from one another to investigate structure in selected spatial frequency regions.