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PubMedDate not supplied

Image resolution of a microchannel plate x-ray image intensifier.

The resolution has been evaluated for an experimental x-ray image intensifier employing three microchannel plates (MCPs) as the photon absorber and electron multipliers. The line spread function (LSF) was measured and used for determination of the modulation transfer function (MTF). The MTF was found to be independent of the incident photon energy from 20 to 150 keV. An additional measurement using a lead-bar test pattern showed that the resolution exceeded 7 line pairs (lp)/mm. The factors influencing the resolution capabilities of the intensifier are discussed. The resolution is limited primarily by the 53-micron center-to-center separation of the channels of the MCPs.

Energy Transfer↗

PubMedDate not supplied

Evaluation of the spatial resolution of a CT scanner by direct analysis of the edge response function.

A new approach to the measurement of the spatial resolution of a computed tomography (CT) scanner system is presented. The method is based on a direct least-squares fit of an analytical expression to a set of data obtained from a CT image of the interface between two materials. The implementation of the method in connection with the G.E.RTPLAN computer configuration is described. The method has been applied in determining the resolution of an EMI-7070 scanner and it is shown that the assumption of uniformity of the system resolution across the CT image is fulfilled within the accuracy of the present method. The reproducibility of the method has been estimated from a series of spatial resolution determinations performed on ten images taken with identical scan parameters. The standard deviation of this series was 3.2%.

Evaluation Studies as Topic↗

PubMedDate not supplied

Improvement of spatial resolution properties of image intensifier-TV digital systems with a multiple-narrow-slit beam imaging technique.

Multiple-slit beam imaging technique with an image intensifier (II)-TV digital system has been developed to remove scatter and veiling glare while high x-ray beam utilization is maintained. Although the contrast and signal-to-noise ratio (SNR) are improved with this technique, the overall image quality obtainable with an II-TV digital system is still limited due to low spatial resolution, which is mainly caused by the large pixel size, i.e., by the small matrix size used. In order to overcome the limitation of the pixel size, we have developed a new method of improving the resolution properties of the II-TV digital system by use of a multiple-slit assembly (MSA) having a narrow slit width. When the slit width of the MSA is narrower than the pixel size of the II-TV digital system, two signals from a given slit due to different MSA placements may be detected by the same pixel in different image frames, and the detected signals of the slit images are mapped to a large matrix. In this way, the spatial resolution in the direction perpendicular to the slit openings can be improved along with the increased contrast and SNR as the scatter and veiling glare can be removed. Experimental results are presented, and the effect of an anisotropic resolution property on the overall image quality is discussed.

Hand↗

PubMedDate not supplied

Investigation of basic imaging properties in digital radiography. 12. Effect of matrix configuration on spatial resolution.

We investigated the effects of three matrix configurations (a conventional 512 X 512 matrix, a double-sampling matrix, and a high-resolution 1024 X 1024 matrix) on the resolution properties of an image intensifier (II)-TV digital radiographic imaging system. Expressions for the optical transfer function were derived theoretically for each of the three matrix configurations, and the corresponding digital modulation transfer functions (MTF's) were calculated using the measured MTF's of the presampling analog components and the parameters of the digital system. In addition, digital images of a star pattern phantom and a hand phantom were obtained with the II-TV system for each matrix type. From our results, we conclude that, in order to increase the resolution of an II-TV digital imaging system, it is necessary not only to increase the matrix size, but also to improve the resolution capabilities of analog components such as the II and/or TV camera. In addition, if the system cannot be upgraded to a 1024 X 1024 matrix configuration, a double-sampling technique with an 512 X 512 matrix may be employed.

Hand↗

PubMedDate not supplied

A generalized formulation of diffusion effects in micron resolution nuclear magnetic resonance imaging.

A generalized formulation of the diffusion related nuclear magnetic resonance (NMR) signal is derived from a random walk model. Previous analyses performed in the NMR spectroscopy were the formulations of the diffusion related signal amplitude at a specific time, such as the spin echo formation time. They are, in general, not applicable to continuous time domain analyses. In this paper, we have extended the theory to the two-dimensional imaging case and derived an analytical formula useful for the computation of the diffusion affected signal as a function of continuous time for a time variant gradient. This formulation will be useful in NMR imaging, especially in NMR microscopy where the diffusion associated signal attenuation is serious due to the strong gradient fields (100-1000 G/cm), and at the same time data are acquired continuously for the acquisition period. In addition to the loss of the resolution and signal-to-noise ratio due to the random phase fluctuation by diffusion, the variation of the intensity during the data acquisition period introduces a line broadening whose full width at half-maximum is found to be much larger than the bandwidth-limited resolution or diffusion related intrinsic resolution. This line spreading effect is integrated in a computer simulation and is evaluated as an integral part of the overall diffusion effects in micron resolution NMR imaging or NMR microscopy.

Computer Simulation↗

PubMedDate not supplied

Evaluating the spatial resolution performance of a time-resolved optical imaging system.

An imaging system is being developed as a possible means of screening for breast cancer using harmless doses of visible or near-infrared radiation. This system produces transmission images of highly scattering objects by recording and discriminating between the times-of-flight of transmitted photons. Experiments have been performed to evaluate the likely spatial resolution performance of such a system. This involved measuring the edge profile produced by an opaque mask embedded in a highly scattering medium, and evaluating the spatial resolution as a function of the period of time over which transmitted light was integrated. The results suggest that a resolution of a few millimeters is achievable with a system with a temporal resolution of about 10 ps.

Breast Neoplasms↗

PubMed1995

An investigation of the effect of finite system resolution and photon noise on the bias and precision of dynamic cardiac SPECT parameters.

The exchange of a radionuclide (and similarly of oxygen) between blood and the myocardium is a dynamic process. Dynamic SPECT offers the possibility of directly quantifying the kinetic parameters which describe this process. In this paper we investigate and quantify the effect of typical SPECT system resolution and photon counting statistics on the bias and precision of dynamic cardiac SPECT parameters. System resolution and photon noise are only two of the image degrading processes which occur in SPECT. Therefore, the bias and precision quoted should be viewed as a lower limit on those which could be expected with an experimental study. Dynamic SPECT projection data are simulated using a realistic human torso phantom. Data are simulated assuming both perfect system resolution and a system resolution typical of a clinical SPECT system. A triple detector SPECT system which acquires a full set of projection data in 10 s using continuous detector motion is modelled. Kinetic parameters are estimated using a number of myocardial regions of interest. The results show that the rate constant characterizing the washing of activity into the myocardium is more sensitive to region of interest position than is the washout rate constant. The bias and precision of the dynamic parameters are estimated from multiple realizations of projection data exhibiting various noise levels. The main effect of increased photon noise in the projection data is to decrease the precision of the estimated parameters. However, there is also some evidence that for high noise levels the bias of the parameters may also be affected.

Biophysical Phenomena↗

PubMed1995

The spatial resolution performance of a time-resolved optical imaging system using temporal extrapolation.

Optical imaging methods are being explored as a potential means of screening for breast cancer. Previous investigations of time-resolved imaging techniques have suggested that due to the lack of photons with sufficiently small pathlengths, the spatial resolution achievable through a human breast would be unlikely to be better than a centimeter. Experimental results presented here indicate, however, that higher resolution may be achieved by extrapolating the measured temporal distribution of transmitted photons. This is performed using a least-squares fit between data and an analytic model of photon transport. The spatial resolution of a time-resolved imaging system was evaluated by measuring the edge response produced by an opaque mask embedded in the center of a 51-mm-thick, very highly scattering medium. The limiting spatial resolution was improved from about 13 mm to about 5 mm.

Breast Neoplasms↗

PubMed1997

Low-contrast resolution in volumetric x-ray CT--analytical comparison between conventional and spiral CT.

Spiral/helical computed tomography (CT) was introduced to scan an anatomical volume in a single breath-hold for better temporal resolution as compared to conventional CT. Recently, it was established that given an x-ray dose, spiral CT also allows better longitudinal high-contrast resolution due to retrospective reconstruction. In spiral CT, full scan with interpolation (FI) and half-scan with interpolation (HI) are limited by the degraded slice sensitivity profile and increased image noise, respectively. We combined these two interpolation methods for a desirable balance. This new interpolation method, FI + HI, was shown to produce 4% lower image noise standard deviation than conventional CT, without loss of longitudinal bandwidth according to one-tenth-cutoff and mean-square-root measures. The analytic model of image noise was validated in a water phantom experiment. Our findings suggest the superiority of spiral CT over conventional CT in terms of both low-contrast resolution and high-contrast resolution.

Biophysical Phenomena↗

PubMed2000

High resolution computed tomography and MRI for monitoring lung tumor growth in mice undergoing radioimmunotherapy: correlation with histology.

A model lung tumor system has been developed in mice for the evaluation of vascular targeted radioimmunotherapy. In this model, EMT-6 mammary carcinoma tumors growing in the lung are treated with 213Bi, an alpha particle emitter, which is targeted to lung blood vessels using a monoclonal antibody. Smaller tumors (< 100 microm in diameter) are cured, but larger tumors undergo a period of regression and then regrow and ultimately prove lethal. The goal of this work was to determine if external imaging with MRI or CT could be used routinely to monitor the growth/ regression of lung tumors in live mice. To attempt to evaluate individual tumors in vivo, animals were initially imaged with magnetic resonance imaging (MRI). High resolution MRI images could be obtained only after sacrifice when lungs were not moving. In contrast, high resolution computed tomography (CT) produced evaluable images from anesthetized animals. Serial CT images (up to 5/animal) were collected over a 17 day period of tumor growth and treatment. When tumored animals became moribund, animals were sacrificed and lungs were inflated with fixative, embedded in paraffin, and then sectioned serially to compare the detection of tumors by high resolution CT with detection by histology. CT proved most useful in detecting lung tumors located in the hilar area and least useful in detecting serosal surface and anterior lobe tumor foci. Overall, CT images of live animals revealed tumors in approximately 2/3 of cases detected in histologic serial sections when relatively few tumors were present per lung. Detection of lesions and their resolution post therapy were complicated due to residual hemorrhagic, regressing tumor nodules and the development of lung edema both of which appeared as high density areas in the CT scans. We conclude that the microCT method used could identify some lung tumors as small as 100 microm in diameter; however, no concrete evaluation of therapy induced regression of the tumors could be made with CT analyses alone.

Animals↗

PubMed2002

Super-resolution in time-reversal acoustics.

The phenomenon of super-resolution in time-reversal acoustics is analyzed theoretically and with numerical simulations. A signal that is recorded and then retransmitted by an array of transducers, propagates back though the medium, and refocuses approximately on the source that emitted it. In a homogeneous medium, the refocusing resolution of the time-reversed signal is limited by diffraction. When the medium has random inhomogeneities the resolution of the refocused signal can in some circumstances beat the diffraction limit. This is super-resolution. A theoretical treatment of this phenomenon is given, and numerical simulations which confirm the theory are presented.

Acoustics↗

PubMed2005

Voice gender identification by cochlear implant users: the role of spectral and temporal resolution.

The present study explored the relative contributions of spectral and temporal information to voice gender identification by cochlear implant users and normal-hearing subjects. Cochlear implant listeners were tested using their everyday speech processors, while normal-hearing subjects were tested under speech processing conditions that simulated various degrees of spectral resolution, temporal resolution, and spectral mismatch. Voice gender identification was tested for two talker sets. In Talker Set 1, the mean fundamental frequency values of the male and female talkers differed by 100 Hz while in Talker Set 2, the mean values differed by 10 Hz. Cochlear implant listeners achieved higher levels of performance with Talker Set 1, while performance was significantly reduced for Talker Set 2. For normal-hearing listeners, performance was significantly affected by the spectral resolution, for both Talker Sets. With matched speech, temporal cues contributed to voice gender identification only for Talker Set 1 while spectral mismatch significantly reduced performance for both Talker Sets. The performance of cochlear implant listeners was similar to that of normal-hearing subjects listening to 4-8 spectral channels. The results suggest that, because of the reduced spectral resolution, cochlear implant patients may attend strongly to periodicity cues to distinguish voice gender.

Adult↗

PubMed1984

Temporal gap resolution in narrow-band noises with center frequencies from 6000-14000 Hz.

Temporal resolution was examined in normal-hearing subjects using a broadband noise and five narrow-band noises with center frequencies (fc) spaced 2 kHz apart between 6 and 14 kHz. Bandwidths of the narrow-band signals were equal to 0.16 fc, and broadband noise maskers with spectral notches were used to restrict the listening bands. Subjects used a Békésy procedure to track the minimum signal level required to keep a periodic temporal gap of fixed duration at threshold. Gap durations from 25 ms to the smallest trackable value were tested with each signal to generate performance curves, which showed the relationship between gap resolution and signal level in the low-to-moderate intensity range. Results showed that gap resolution improved progressively with increased signal level to about 35 dB SL, where minimum gap thresholds of about 3 ms were observed for all signals. These results, when combined with previous low-frequency data, indicate that gap threshold decreases systematically with increased signal frequency to about 5 kHz, and asymptotes at 2-3 ms for higher frequencies. In the context of functional models, the frequency effect is qualitatively consistent with the notion that both the auditory filter and a sensory integrator operate in series to govern temporal resolution in audition.

Auditory Perception↗

PubMed1984

Temporal gap resolution in masked normal ears as a function of masker level.

Recent studies of temporal resolution in hearing-impaired listeners indicate that many ears with cochlear damage exhibit elevated temporal gap thresholds. The deviations from normal may be large, for equivalent-SPL comparisons, but are often small or absent for equivalent-SL comparisons. In the present experiment, we examine the premise that SL effects in gap resolution are independent of sound pressure level. Normal hearing subjects used a Békésy procedure to track the minimum level of an octave-band signal at 1 kHz needed to keep a periodic temporal gap at threshold. Performance functions were generated to show stimulus intensity as a function of gap duration, for gaps from 25 ms to the smallest detectable value. In separate conditions, the level of a white-noise background was varied to shift the subject's audibility threshold. Results indicate that the limits of resolution are largely invariant with SPL, but criterion sensation levels for resolution are progressively reduced at higher SPL. It is concluded that gap thresholds from normal and impaired ears should be compared for equivalent SPL; comparisons made with SL equated are likely to underestimate the consequence of cochlear damage on resolving capacity.

Audiometry, Pure-Tone↗

PubMed1987

Temporal gap resolution in listeners with high-frequency sensorineural hearing loss.

Temporal gap resolution was measured in five normal-hearing listeners and five cochlear-impaired listeners, whose sensitivity losses were restricted to the frequency regions above 1000 Hz. The stimuli included a broadband noise and three octave band noises centered at 0.5, 1.0, and 4.0 kHz. Results for the normal-hearing subjects agree with previous findings and reveal that gap resolution improves progressively with an increase in signal frequency. Gap resolution in the impaired listeners was significantly poorer than normal for all signals including those that stimulated frequency regions with normal pure-tone sensitivity. Smallest gap thresholds for the impaired listeners were observed with the broadband signal at high levels. This result agrees with data from other experiments and confirms the importance of high-frequency signal audibility in gap detection. The octave band data reveal that resolution deficits can be quite large within restricted frequency regions, even those with minimal sensitivity loss.

Audiometry, Pure-Tone↗

PubMed1994

Frequency and frequency-ratio resolution by possessors of absolute and relative pitch: examples of categorical perception.

The methodology derived from the trace-context theory of intensity resolution (Durlach and Braida, 1969; Macmillan et al., 1988) was applied to resolution over an octave range along two continua: a sequential-frequency-ratio continuum for possessors of relative pitch (RP), and a pure-tone frequency continuum for possessors of absolute pitch (AP). The performance of both RP and AP possessors was exceptional in that total identification sensitivity along both continua was much greater than identification sensitivity along unidimensional psychophysical continua characterized by the 7 +/- 2 rule. In addition, the performance of RP possessors was exceptional in that, on average, total sensitivity for identification resolution was greater than sensitivity for resolution in discrimination. Finally, identification sensitivity between category prototypes (chromatic semits) along both continua was approximately the same as identification sensitivity between phonemic category prototypes along speech continua, despite the fact that both the discrimination ranges and the total number of categories are much larger for the two pitch continua.

Humans↗

PubMed2000

Analysis of resolution for an amplitude-steered array

In 1976, Hughes and Thompson introduced the idea of steering the maximum response of a linear array by amplitude weighting the output signals of the elements, thus eliminating the need for time delays or phase-shift networks. Currently that amplitude-steered array concept is being extended to a broadband two-dimensional array that can be used for real-time three-dimensional imaging. In shifting the use of the amplitude-steered array from underwater acoustic communications to imaging, we must consider different issues of the array's performance such as lateral and axial resolution. For the linear amplitude-steered array, we show that both lateral and axial resolution are limited by the length of the array. The dependence of axial resolution on the length of the array is a unique feature of the amplitude-steered array, leading to an interesting tradeoff between lateral and axial resolution. A theoretical basis for the dependence is developed and simulation results are given.

Journal Article↗

PubMed2002

Nitrogenase MoFe-protein at 1.16 A resolution: a central ligand in the FeMo-cofactor.

A high-resolution crystallographic analysis of the nitrogenase MoFe-protein reveals a previously unrecognized ligand coordinated to six iron atoms in the center of the catalytically essential FeMo-cofactor. The electron density for this ligand is masked in structures with resolutions lower than 1.55 angstroms, owing to Fourier series termination ripples from the surrounding iron and sulfur atoms in the cofactor. The central atom completes an approximate tetrahedral coordination for the six iron atoms, instead of the trigonal coordination proposed on the basis of lower resolution structures. The crystallographic refinement at 1.16 angstrom resolution is consistent with this newly detected component being a light element, most plausibly nitrogen. The presence of a nitrogen atom in the cofactor would have important implications for the mechanism of dinitrogen reduction by nitrogenase.

Azotobacter vinelandii↗