Low-contrast Structures Research Articles

Contemporary pediatric thoracic imaging.

children, and its high efficacy and clinical use in the pediatric intensive care setting have been recently confirmed [3]. Unfortunately, chest radiography is also one of the most challenging examinations to perform because of the wide range of tissue densities present in the thorax (especially considering the wide variety of thoracic sizes in pediatric patients), the inherently low-contrast structures of children, and the need to minimize exposure to ionizing radiation. These difficulties imaging children are further complicated by the requirement that images be acquired at peak inspiration and with minimum patient motion, a daunting task when imaging young or uncooperative children. The use of digital radiography is increasing, but the technical basis of the systems may be unfamiliar. In this section, we discuss and illustrate the most common commercial digital systems in addition to conventional radiography suitable for pediatric chest radiography. Conventional Radiography

High Angular Resolution Studies of Coronal Structures with SOHO

Since January 1996, the Solar and Heliospheric Observatory (SOHO)has been providing unprecedented views of the extended solar coronato heliocentric distances of up to 32 solar radii. During the past threeyears we carried out studies of the morphology and dynamical evolutionof various structures in the solar corona using two instruments on boardSOHO: the Large Angle Spectrometric Coronagraph (LASCO) and the ExtremeUltraviolet Imaging Telescope (EIT). We have applied advanced image resolutionenhancement techniques to explore in detail the characteristics of thesmall-scale structures and/or the low contrast structures in the solarcorona. We describe here the results from these high-angular resolutionstudies, including of the kinematics of several Coronal Mass Ejections andpolar jets.

Exploring Coronal Structures with SOHO

AbstractWe applied advanced image enhancement techniques to explore in detail the characteristics of the small-scale structures and/or the low contrast structures in several Coronal Mass Ejections (CMEs) observed by SOHO. We highlight here the results from our studies of the morphology and dynamical evolution of CME structures in the solar corona using two instruments on board SOHO: LASCO and EIT.

Cone-beam filtered-backprojection algorithm for truncated helical data

This paper investigates 3D image reconstruction from truncated cone-beam (CB) projections acquired with a helical vertex path. First, we show that a rigorous derivation of Grangeat's formula for truncated projections leads to a small additional term compared with previously published similar formulations. This correction term is called the boundary term. Next, this result is used to develop a CB filtered-backprojection (FBP) algorithm for truncated helical projections. This new algorithm only requires the CB projections to be measured within the region that is bounded, in the detector, by the projections of the upper and lower turns of the helix. Finally, simulations with mathematical phantoms demonstrate that: (i) the boundary term is necessary to obtain high-quality imaging of low-contrast structures and (ii) good image quality is obtained even with large values of the pitch of the helix.

Elastographic Imaging of Low-Contrast Elastic Modulus Distributions in Tissue

Elastography is a new ultrasonic imaging technique that produces images of the strain distribution in compliant tissues. This strain distribution is derived from ultrasonically estimated longitudinal internal motion induced by an external compression of the tissue. The displayed two-dimensional (2-D) images are called elastograms. In this paper, it is demonstrated that, when signal-to-noise ratio-enhancing techniques are used, elastography is capable of imaging low-contrast elastic modulus tissue structures with high contrast-to-noise ratios. This is demonstrated using both computer simulations and data obtained from 3 days postmortem ovine kidneys in vitro. The elastograms of such organs suggest that the modulus slowly decays from the renal cortex (RC) to the interior of the renal sinus (RS). Such modulus variation is corroborated by independent measurements of the Young’s moduli. It is found that the RC is approximately twice as hard as the interior of the RS. We invoke our previous results on elastographic contrast-transfer efficiency to demonstrate that, at low contrast, the elastogram may be interpreted as a quantitative image of the relative Young’s modulus distribution.

Optimal phosphor thickness for portal imaging.

A theoretical approach known as quantum accounting diagram (QAD) analysis has been used to calculate the spatial-frequency-dependent detective quantum efficiency (DQE) of two portal imaging systems: one based on a video camera and another based on an amorphous silicon array. The spatial frequency-dependent DQEs have then been used to determine indices of displayed and perceived image quality. These indices are figures of merit that can be used to optimize the design of linear imaging systems. We have used this approach to determine which of eight phosphor screen thicknesses (ranging between 67 and 947 mg/cm2) is optimal for the two designs of portal imaging systems. The physical characteristics (i.e., detection efficiencies, gains, and MTFs) of each of the eight x-ray detectors have been measured and combined with the physical characteristics of the remaining components to calculate the theoretical DQEs. In turn, the DQEs have been used to calculate theoretical indices of displayed and perceived image quality for two types of objects: a pelvis object and a pointlike object. The maximal indices of displayed and perceived image quality were obtained with screen thickness ranging between 358 and 947 mg/cm2, depending upon the imaging system design and the object being imaged. Importantly, the results showed that there is no single optimal screen thickness. The optimal thickness depended upon imaging task (e.g., detecting large, low-contrast structures, or detecting edges and small structures). Nevertheless, the results showed that there were only modest improvements in the indices of image quality for phosphor screens thicker than 350-400 mg/cm2.

Physical characteristics of a commercial electronic portal imaging device.

An electronic portal imaging device (EPID) for use in radiotherapy with high energy photons has been under development since 1985 and has been in clinical use since 1988. The x-ray detector consists of a metal plate/fluorescent screen combination, which is monitored by a charge-coupled device (CDD)-camera. This paper discusses the physical quantities governing image quality. A model which describes the signal and noise propagation through the detector is presented. The predicted contrasts and signal-to-noise ratios are found to be in agreement with measurements based on the EPID images. Based on this agreement the visibility of low contrast structures in clinical images has been calculated with the model. Sufficient visibility of relevant structures (4-10 mm water-equivalent thickness) has been obtained down to a delivered dose of 4 cGy at dose maximum. It is found that the described system is not limited by quantum noise but by camera read-out noise. In addition we predict that with a new type of CCD sensor the signal-to-noise ratio can be increased by a factor of 5 at small doses, enabling high quality imaging, for most relevant clinical situations, with a patient dose smaller than 4 cGy. The latter system would be quantum noise limited.

3-D CARDIAC MODELING: A COMPUTER VISION APPROACH IN SPATIO-TEMPORAL IMAGE SEQUENCES

A very active research was conducted on motion analysis. Most of the concepts, methods and assumptions are well established and lead to additional improvements in computer vision applications. Even in medicine where we have to deal with noisy data, low contrast structures and deformable objects, they bring new cues at all the processing stages. This paper emphasizes the specificities of this area and also the potential difficulties. A compilation of results is given aimed at the quantification of heart kinetics in Digital Subtraction Angiography (DSA). They illustrate the benefits of cooperative schemes such as motion based segmentation, moving object identification, three-dimensional reconstruction and interpretation.

Analysis of noise in phase contrast MR imaging.

In this work we analyze the effects of inherent random noise on the detectability of low-contrast vessel structures that possess slow flow. When flow is encoded in more than one direction, the number of independent noise contributions increases in addition to the scan time. In a fast-flow scenario, only the noise contribution from sampling along the direction of flow is of any significance. At slow flow rates, however, it becomes necessary to account for the noise in each encoded Cartesian direction. The degree to which noise affects low-contrast detectability also depends on the method of phase contrast image processing employed. A theoretical analysis of the statistical properties of signal and noise in processed phase contrast magnitude images is presented and verified from experimental MR image data. Results show a progressively increased bias in the processed phase contrast image magnitude at slow flow rates due to contributions from inherent random noise. The amount of this bias increases with the number of physical directions in which flow is encoded and is larger for complex difference processed images than for phase difference processing. Correspondingly, the output signal-to-noise ratio associated with flow is compromised.

1985 [ITAL]Voyager 2[/ITAL] Radio Ranging Measurements of Coronal Density: Asymmetry in the Radial Profiles Explained

An asymmetry in the radial variation of electron density above the cast and west limbs of the Sun was inferred from centimeter wavelength ranging measurements conducted by Voyager 2 during its 1985 solar conjunction. The Voyager 2 ranging measurements, which took place in the heliocentric distance range of 7-40 solar radius, have been compared with the white-light coronagraph measurements of the underlying corona collected by the Mark 3 K-coronameter located at the Mauna Loa Solar Observatory. It is shown that the disparity in radial profiles is not real but is instead caused by longitudinal variations stemming from the probing of significantly different source regions its revealed in the white-light measurements. These results improve our understanding of the probing abilities of ranging measurements and their relationship to white-light measurements. They reinforce the notion that the high-precision and high-sensitivity features of ranging measurements are more fully exploited in the investigation of density variations across the ubiquitous low-contrast raylike structures that permit the corona, rather than in determining radial density profiles.

In vivo, real‐time confocal imaging

We have adapted a tandem scanning confocal microscope for real-time, non-invasive imaging of cells under in vivo conditions. This form of in vivo confocal imaging relies on the optical sectioning abilities of the confocal microscope to obtain en face, sequential, reflected light images of cells at various depths, up to 1 mm, within opaque organs in living animals. Of major consideration in the design of an in vivo confocal microscope is maximizing the real-time detection of signals reflected from low contrast structures which can be affected by the microscope design, objective, and image detector systems. Using an in vivo confocal microscope design with a 20 x BioOptics surface contact objective we have obtained live cellular images from selected tissues including cornea, kidney, liver, adrenal, thyroid, epididymis, and muscle and connective tissue of rabbits and rats. Images were captured, digitized, and processed using a DAGE Mti low light level SIT camera coupled to a Gould IP9527 image processor. In vivo images were also compared with conventional bright field light and scanning electron microscopic images of "dead," fixed tissues. Overall, in vivo confocal imaging can provide remarkable detail of living cells comparable to that of conventional microscopic images of "dead," fixed, and stained tissue. A more unique feature of in vivo confocal imaging is the ability to study cellular structure and function sequentially over time in the same organ or tissue and represents a fundamentally new paradigm in microscopy. With continued refinements in the microscope, objective and detection system designs and their consequent improvements in lateral and axial resolution, in vivo confocal microscopy will enable us as observers to see what no one has been able to see before.

Structure of the Azotobacter vinelandii surface layer.

Electron microscopy of the Azotobacter vinelandii tetragonal surface array, negatively stained with ammonium molybdate in the presence of 1 mM calcium chloride, showed an apparent repeat frequency of 12 to 13 nm. Image processing showed dominant tetrad units alternating with low-contrast cruciform structures formed at the junction of slender linkers extending from corner macromolecules of four adjoining dominant units. The actual unit cell showed p4 symmetry, and a = b = 18.4 nm. Distilled water extraction of the surface array released a multimeric form of the single 60,000 molecular-weight protein (S protein) which constitutes the surface layer. The molecular weight of the multimer was estimated at 255,000 by gel filtration, indicating a tetrameric structure of four identical subunits and suggesting that this multimer was the morphological subunit of the S layer. Tetrameric S protein exhibited low intrinsic stability once released from the outer membrane, dissociating into monomers when incubated in a variety of buffers including those which served as the base for defined media used to cultivate A. vinelandii. The tetramer could not be stabilized in these buffers at any temperature between 4 and 30 degrees C, but the addition of 2 to 5 mM Ca2+ or Mg2+ completely prevented its dissociation into monomers. Circular dichroism measurements indicated that the secondary structure of the tetramer was dominated by aperiodic and beta-sheet conformations, and the addition of Ca2+ did not produce any gross changes in this structure. Only the tetrameric form of S protein was able to reassemble in vitro in the presence of divalent cations onto the surface of cells stripped of their native S layer.

Observations of very low-contrast white-light solar structures utilizing differential photometry

The spatial derivative of the intensity structure of the photosphere has been investigated utilizing a differential photometer system capable of recording intensity variations on the solar disc of 0.05% per arc sec. Time evolution images of the differential intensity profile for a 562.5 arc sec segment of the solar photosphere show a high degree of structure. To a large extent the observed structure persists for times greater than one hour and is rather highly ordered in spatial distribution. Power spectra of the spatial structure averaged over one hour show maxima at 28000 km and 16000 km, close to the spatial scale expected for intensity perturbations from supergranulation.

Boxy isophotes, discs and dust lanes in elliptical galaxies

CCD images of 42 elliptical and S0 galaxies are examined for low-contrast structures or subtle distortions of the isophotes from perfect ellipses. 75 percent of the galaxies have isophotes completely describable as concentric ellipses to within the photometry errors. 'Boxy' isophotes, stellar discs, and dust lanes are detected in the remaining 25 percent of the sample. The boxy elliptical galaxies appear dynamically indistinguishable from normal ellipticals and are therefore different from boxy bulges, which rotate rapidly. Most of the galaxies with faint discs, however, appear dynamically similar to S0 galaxies. Nearly edge-on dust lanes are found in four galaxies, which suggests that dust lanes may commonly occur in elliptical galaxies.

Clinical and experimental investigation of a smoothed CT reconstruction algorithm.

The usefulness of a smoothed reconstruction CT algorithm was studied using raw data from the EMI Mark I head scanner. The reconstruction algorithm operated on an off-line computer, independent of the EMI algorithm. This technique greatly reduces image noise and improves the visibility of very low-contrast structures, but at the cost of reduced spatial resolution. Phantom tests with contrasts as low as 0.14% demonstrated the validity of the images. Clinical results showed greatly improved visualization of gray and white matter with no increase in dose. It was necessary to expand the CT density scale so that the range from air to water was divided into 2,000 parts.

On optimizing the xeromammographic image

This work undertakes a detailed system-based analysis of the xeromammographic process starting from basic considerations. Both the edge enhancement and wide-recording latitude, the two principal characteristics of xeroradiography, are shown to bear an intimate relationship to the electric-field distribution. Criteria and methods are formulated for optimizing xeromammographic image quality and a procedure is developed for calculating the white gap. Densitometric curves are derived for both positive- and negative-mode xeroradiography and found to be in excellent agreement with experiment. The question of image linearity is examined carefully and a threshold value of the electrostatic contrast is established, which sets a natural criterion for the application of Fourier analysis. Furthermore, it is shown that, in xeromammography with its inherently low-contrast structures, an optimal exposure exists which optimizes simultaneously all low-contrast edges. This last finding, coupled with experimental results, suggest immediately the possibility of an automatic exposure termination in xeromammography. Beam hardening is also investigated and it is shown that increased filtration combined with a lower bias potential leads to substantial dose reduction without significant loss of image quality. The paper concludes with a discussion of scattered radiation and how it affects xeromammographic image quality.

Evaluation of xeroradiographic image quality.

A theory of edge enhancement has been developed to describe xeroradiographic images of linear step objects. The theory is shown to give an accurate description of the relationship between edge enhancement, subject contrast, radiation exposure, selenium charging potential and developer bias potential. An optimal transmitted x-ray exposure exists which gives the maximum edge enhancement. For low contrast structures, this same optimal exposure was found to yield the most acceptable images as judged subjectively. This finding has led to the successful application of automatic exposure termination in mammographic examinations using xeroradiography.