Theory of point-spread function artifacts due to structured mid-spatial frequency surface errors

Optical design and tolerancing of aspheric or free-form surfaces require attention to surface form, structured surface errors, and nonstructured errors. We describe structured surface error profiles and effects on the image point-spread function using harmonic (Fourier) decomposition. Surface errors over the beam footprint map onto the pupil, where multiple structured surface frequencies mix to create sum and difference diffraction orders in the image plane at each field point. Difference frequencies widen the central lobe of the point-spread function and summation frequencies create ghost images.

Generalized spatial filtering velocimetry and accelerometry for uniform and nonuniform objects

We present a technique to measure velocity, acceleration, and higher order derivatives of motion using periodic and nonperiodic spatial filters. The technique can be applied using a single detector or an array of detectors. In one configuration, the velocity distribution of an object such as a fluid can be measured by imaging the object onto an array of detectors. In another configuration, multiple projections of an object are used to reconstruct a cross-sectional velocity distribution using a tomography algorithm. The advantages and disadvantages of our technique applied to uniform and spatially varying motions are described.

Quantitative assessment of angiographic right ventricular wall motion in arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C)

INTRODUCTION

Angiography of the right ventricle (RV) is a standard, reference technique to diagnose wall motion abnormalities in arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C). RV wall motion is usually assessed by qualitative, visual impression, and has lacked a quantitative basis for defining abnormalities. Since the normal RV has a markedly asymmetric movement, angiographic interpretation can differ, even among experienced clinicians. The purpose of this study was to quantify RV wall motion based on contrast ventriculography in patients with ARVD/C and to specify the severity and location of wall motion abnormalities, as compared with normal subjects.

METHODS AND RESULTS

We analyzed the angiographic contours of the RV in three views from 19 normal subjects and 23 subjects with ARVD/C. Contour area movement during contraction was calculated circumferentially and further analyzed in nine zones. RV ejection fraction was also computed. Wall motion in ARVD/C was depressed by more than 30% at the tricuspid valve and inferior wall regions (P < 0.001) and significantly reduced at the apex (P = 0.003). However, the RVOT and anterior wall motion were not significantly reduced. RV ejection fraction was depressed from 60 +/- 11% in normal subjects to 41 +/- 12% in ARVD/C patients (P < 0.001).

CONCLUSION

Wall motion abnormalities in ARVD/C can be quantified and compared with normal controls, showing primarily reduced movement in the tricuspid and inferior wall regions. This study delineates objective measurements that can be used to aid in the diagnosis of ARVD/C. In addition, they may be incorporated in future refinements of criteria to diagnose ARVD/C.

Do patients with right ventricular outflow tract ventricular arrhythmias have a normal right ventricular wall motion? A quantitative analysis compared to normal subjects

BACKGROUND/AIM

Patients with ventricular ectopy from the right ventricular (RV) outflow tract (RVOT) are often referred for RV angiography to exclude disorders such as arrhythmogenic RV cardiomyopathy/dysplasia (ARVC/D). This is usually based on a qualitative assessment of the wall motion. We present a method to quantify the wall motion and to apply this method to compare patients with RVOT ectopy to normal subjects.

METHODS

RV angiograms were analyzed from 19 normal subjects and 11 subjects with RVOT ventricular arrhythmias (RVOT arrhythmia subjects) who had no other clinical or other evidence for ARVC/D. By a newly developed computer-based method, RV contours were first traced from multiple frames spanning the entire cardiac cycle. The fractional change in area between contours was then calculated as a serial function of time and location to determine both total contour area change and timing of contour movement. Contour area strain, defined as the differential change in area between nearby regions, was also computed.

RESULTS

The contour area change was greatest in the tricuspid valve region and least in the RVOT and midanterior regions. The onset of contraction was earliest in the RVOT region and latest in the apical, inferior, inferoapical, and subtricuspid valve regions. The contour strain was largest in superior tricuspid valve and inferior wall and near zero within the lateral tricuspid valve region. There were significant pairwise differences in contraction area, timing, and strain in the various regions. There were no significant differences between normal subjects and RVOT arrhythmia subjects.

CONCLUSIONS

The RV wall motion is nonuniform in contour area change, strain, and timing of motion. Patients with RVOT ventricular ectopy demonstrate wall motion parameters similar to those of normal subjects. This technique should be applicable in analyzing RV wall motion in patients suspected of having ARVC/D.

Evaluation of and compensation for spatial noise of LCDs in medical applications

Recent developments in liquid crystal display (LCD) technology suggest that this technology will replace the cathode ray tube (CRT) as the most popular softcopy display technology in the medical arena. However, LCDs are far from ideal for medical imaging. One of the principal problems they possess is spatial noise contamination, which requires accurate characterization and appropriate compensation before LCD images can be effectively utilized for reliable diagnosis. This paper presents some work we have conducted recently on characterization of spatial noise of high resolution LCDs. The primary purpose of this work is to explore the properties of spatial noise and propose a method to reduce it. A high quality CCD camera was used for physical evaluation. Spatial noise properties were analyzed and estimated from the camera images via signal modeling and processing. A noise compensation algorithm based on error diffusion was developed to process images before they were displayed. Results shown in this paper suggest that LCD spatial noise can be effectively reduced via appropriate processing.

Using a human visual system model to optimize soft-copy mammography display: influence of MTF compensation

RATIONALE AND OBJECTIVES

The investigators developed an efficient method for optimizing cathode ray tube (CRT) monitor performance for digital mammography, based on the correlation between the performance of human observers and the performance of a mathematical computer model of the human visual system. The investigators examined observer performance on soft-copy display of mammographic images that were either unprocessed or processed to compensate for modulation transfer function (MTF) deficiencies in the CRT display. The results were used to validate the human visual system model.

MATERIALS AND METHODS

Six radiologists viewed a series of 250 mammographic images with microcalcification clusters with different contrast levels on a CRT monitor. The images were viewed twice: once without image processing and once with processing designed to compensate for MTF deficiencies in the CRT monitor. The images were analyzed with the JNDmetrix Visual Discrimination Model, which is based on the principles of just-noticeable difference measurement and frequency-channel vision modeling. Receiver operating characteristic (ROC) curves were generated for the human observers and compared statistically with the model observers' performance.

RESULTS

Both human and model performance was better overall with the MTF-compensated images, especially for microcalcifications in the midlevel contrast range. There was a very high correlation between human and model observers.

CONCLUSION

The use of image-processing methods to compensate for limitations in the MTF of CRT monitors can improve the detection performance of radiologists searching for microcalcifications in mammographic images, and a model based on characteristics of the human visual system can be used to predict human observer results accurately.

Searching for nodules: what features attract attention and influence detection?

RATIONALE AND OBJECTIVES

The goal of the study was to determine whether there are certain physical features of pulmonary nodules that attract visual attention and contribute to increased recognition and detection by observers.

MATERIALS AND METHODS

A series of posteroanterior chest images with solitary pulmonary nodules were searched by six radiologists as their eye-position was recorded. The signal-to-noise ratio, size, conspicuity, location, and calcification status were measured for each nodule. Dwell parameters were correlated with nodule features and related to detection rates.

RESULTS

Only nodule size (F = 5.08, P = .0254) and conspicuity (F = 4.625, P = .0329) influenced total dwell time on nodules, with larger, more conspicuous nodules receiving less visual attention than smaller, less conspicuous nodules. All nodule features examined influenced overall detection performance (P < .05) even though most did not influence visual search and attention.

CONCLUSION

Individual nodule features do not attract attention as measured by "first hit" fixation data, but certain features do tend to hold attention once the nodule has been fixated. The combination of all features influences whether or not it is detected.

Linear estimation theory applied to the reconstruction of a 3-D vector current distribution

Linear estimation theory incorporating statistical a priori knowledge is applied to the inverse problem of reconstructing a static 3-D vector source field from another 3-D vector measurement field. The motivation for this development is to reconstruct 3-D electric current distributions from a set of magnetic measurements. Such a capability would be useful for the clinical determination of neural currents, for example. A simulation is presented to demonstrate the reconstruction of a class of simple nonbiological source objects, and to show the dependence of these reconstructions on the data taking configuration and the statistical a priori knowledge that is incorporated into the reconstruction process.

Design and testing of artifact-suppressed adaptive histogram equalization: a contrast-enhancement technique for display of digital chest radiographs

One of the goals of our research in the field of digital radiography has been to develop contrast-enhancement algorithms for eventual use in the display of chest images on video devices with the aim of preserving the diagnostic information presently available with film, some of which would normally be lost because of the smaller dynamic range of video monitors. The ASAHE algorithm discussed in this article has been tested by investigating observer performance in a difficult detection task involving phantoms and simulated lung nodules, using film as the output medium. The results of the experiment showed that the algorithm is successful in providing contrast-enhanced, natural-looking chest images while maintaining diagnostic information. The algorithm did not effect an increase in nodule detectability, but this was not unexpected because film is a medium capable of displaying a wide range of gray levels. It is sufficient at this stage to show that there is no degradation in observer performance. Future tests will evaluate the performance of the ASAHE algorithm in preparing chest images for video display.

Evaluation of the ACR-NEMA standard for communications in digital radiology

An implementation and evaluation of a prototype multivendor communications system which complies with the American College of Radiology (ACR) and the National Electrical Manufacturers Association (NEMA) standard for communications in digital radiology is discussed. The system allows communications between interfaces from different manufacturers within a networked environment. The implementation includes network software compatible with the International Standards Organization's Open Systems Interconnect standard. The experience of the implementation effort and the evaluation of the system provide the basis for a critique of the ACR-NEMA standard. It is concluded that the ACR-NEMA standard is not well suited for application to the networked environment of picture archiving and communications systems. Two possible solutions are recommended for this problem. The first is a major revision of the existing standard. The second is the development of a family of network communications standards for digital radiology.

Correlelogram stereo display: computer simulations

Computer simulations are presented of a correlelogram stereo-display screen. First, an underlying physical model of the correlelogram is developed. A program for generating stereo pairs is then described. Next, the calculation of the layer transmissions for the correlelogram is discussed. Finally, several synthesized image pairs are presented.

Tomosynthesis and computer tomography: a continuous description with examples

A continuous, as opposed to sampled, mathematical description applicable to 3-D image reconstruction from 2-D projections is applied to three examples. These examples are computer tomography, tomosynthesis, and coded-source tomosynthesis. The effects of various projection filtering procedures on reconstructed image quality are discussed for each example.

Multispectral size-averaged incoherent spatial filtering

A method of 2-D size-averaged incoherent recognition filtering is described. White light input illumination is used to multiplex chromatically a continuum of inputs differing in their spatial Fourier spectral magnifications. A two-grating dichromated-gelatin lateral-dispersion compensating device is inserted to correct chromatic dispersion introduced by a computer-generated hologram filter. The resulting registered recognition patterns are added to form a single effective filter size-averaged recognition peak. Using high efficiency compensating gratings and a bleached binary hologram allow the use of a relatively low-power light source.

Simplified production of spatial inverse filters

A logarithmic nonlinearity in the transmittance-versus-exposure curve of a photographic material can greatly simplify holographic fabrication of inverse filters. We present here two filter-production methods utilizing this non-linearity, which produce relatively high-dynamic-range inverse filters. Confirming computer simulations and coherent-optical experiments are presented.

Volume holograms constructed from computer-generated masks

A new type of hologram that combines computer-generated holograms with volume holograms is described. This hologram allows arbitrary selection of the location and color of a computer-generated image when white light illumination is used. Potential applications include optical information processing, holographic optical elements, multicolor displays, and lens testing. Calculations are made to determine the range of wavelengths possible for image reconstruction. Experimental results are given and discussed.

Magnitude-coupled phase quantization

A method is described for minimizing Fourier-domain phase-quantization noise in the image reconstructed from a computer-generated hologram. This method uses manipulation of the hologram magnitude to counteract the deleterious effects of phase quantization.

Deterministic diffusers for holography

We present here a new family of diffusers suitable for use in holography. We first exhibit a method for obtaining mathematical descriptions of diffusers that give nearly uniform amplitude at the hologram before insertion of an object. From these diffusers we select a few that redundantly map information into the hologram. Finally, we present a method of comparing the performance of different diffusers using a computer simulation technique that has a simple experimental analog.

Phase quantization in holograms-depth effects

A three-dimensional holographic image is deteriorated due to quantization of the phase in the hologram. As in two-dimensional Fourier holograms, the deterioration is exhibited as a superposition of false images. However, in the three-dimensional case, the false images fall at depth positions other than the plane of the image to which they correspond. If far out of focus, these false images are harmless.