Improvement In Lateral Resolution Research Articles

Super-resolution in confocal scanning microscopy. V. Axial super-resolution in the incoherent case

For pt.IV see ibid., vol.8, p.1 (1992) One of the basic properties of confocal scanning laser microscopy (CSLM) is the possibility of high axial resolution in the case of fluorescent objects. As a consequence 3D images of 3D objects can be obtained. In the previous papers of this series it has been demonstrated that an improvement of lateral resolution in CSLM (lateral super-resolution) can be obtained if, at each step of the scanning procedure, the full image is detected in the image plane and these data are inverted to estimate the object at the confocal point. In this paper we prove that the same data contain sufficient information for improving also the axial resolution of CSLM (axial super-resolution). The analysis is performed in the case of a simplified model, i.e. two-dimensional objects (one lateral plus one axial dimension) and lenses with a small numerical aperture.

Improvement of lateral resolution in far-field fluorescence light microscopy by using two-photon excitation with offset beams

A method is described of increasing the resolution in far-field fluorescence light microscopy by a factor of 2. Calculations show that a lateral resolution of 75 nm is achieved with a lens of numerical aperture of 1.4 (oil immersion) by a two-photon excitation mode with each photon stemming from displaced focused light beams of λ = 500 nm and overlapping main maxima.

Enhanced imaging of biomolecules with electron beam deposited tips for scanning force microscopy

Tip/sample interaction on the scanning force microscope (SFM) is a particularly difficult problem with biological materials. One major factor affecting image quality is the tip shape. Improved electron beam induced deposition technique with a scanning electron microscope (SEM) was used for the reproducible fabrication of carbon sharp tips on the end of commercially available silicon nitride cantilevers for scanning force microscopy. By aligning a fine focused beam of 20 nm diameter directly down the axis of the pyramidal tip at electron energy of 20 kV, carbon deposits grow with full cone angle of about 25°, cone length of 2 μm, and radii of curvature down to 10 nm, making these e-beam tips suitable for biomolecules imaging. The tip dimensions also were controlled by adjusting the beam parameters. Three different types of SFM tips were used to image tobacco mosaic virus (TMV). Conventional pyramidal tips appeared generally worse for imaging helical particles of TMV than SEM-deposited tips which were found more robust than commercially available conical tips. The use of sharper tips for SFM imaging of protein DNA revealed a 25% improvement in lateral resolution.

A discussion of two wavefront aberration correction procedures.

This review paper discusses the basic properties of two adaptive signal processing procedures for dealing with weak scattering in a phased array transducer system. A fundamental improvement in the lateral resolution of ultrasonic echo scanners will result if the weight vector of a large phased array transducer can be modified to account for distortion in the propagation medium. Lateral resolution in most tissue is limited to a few mm by wavefront-distortion-induced sound-speed variations. One important wavefront-distortion source is scattering from local speed variations within large and reasonably homogeneous tissue beds such as the liver. Scattering disperses some energy from the beam and perturbs the wavefront, thereby distorting the image and limiting the resolution to the scale of the distortion. Often, such scattering is weak, meaning that most of the energy in the beam is unscattered. The total field at the receiving transducer is the vector sum of the unscattered and scattered fields. In weak scattering the unscattered field is dominant and the resultant field can be treated as the unscattered field plus a perturbation. The net effect is primarily a distorted phasefront, while the amplitude or modulus of the wavefront remains reasonably intact. Refraction and strong scattering effect the wavefront more severely and are less responsive to these algorithms.

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.

A lithium liquid metal ion source suitable for high voltage terminal applications

Liquid metal ion sources offer potential improvement in lateral resolution over conventionally used gaseous sources for MeV microprobe applications because of their intrinsic brightnesses. The use of a Li liquid metal ion source is particularly attractive because of the increased sensitivity of lithium for detecting hydrogen (hydrogen profiling) and for detecting near-surface contaminants through high-resolution Rutherford backscattering spectrometry. However, the technical difficulty of occasional sparking between the needle and extraction electrode requires that the needle of the Li liquid metal ion source be rewetted before reignition can be effected; this problem makes Li liquid metal ion sources of the usual design risky for operation in the insulated high-voltage terminals of accelerators used in such applications. We have designed a reliable, long-lived, Li liquid-metal ion source which has provisions for overcoming this limitation. The design features and performance characteristics of the source are described in this article.

High spatial resolution at very high sensitivity using synchrotron radiation excited XPS

A dedicated surface analysis beamline with ‘‘small spot’’ XPS facilities is currently under construction at BESSY. When completed in 1989, it will serve as a primary means for the study of metal, semiconductor, and insulator surfaces via laterally resolved XPS and photoyield measurements. This report discusses the first results of this system as attached to a BESSY plane grating monochromator beamline. The data obtained to date clearly demonstrate the potential these two analytical techniques offer for the study of materials science. A further improvement of lateral resolution (now ≊150 μm) into the 1 μm or even submicron range is discussed.

Comparison of SAFT and two-dimensional deconvolution methods for the improvement of resolution in ultrasonic B-scan images

This Paper compares the properties of the well known synthetic aperture focusing technique (SAFT) with those of two deconvolution methods, which have previously been less widely applied to digitized ultrasonic data. The linear deconvolution method of Wiener filtering and the non-linear maximum entropy method are applied to two-dimensional unrectified (RF) B-scan data, to improve the resolution simultaneously in the lateral and range directions. With SAFT, only an improvement in lateral resolution is possible. Data from linear scans of several unfocussed transducers over machined reflectors at different depths in steel were collected and used to quantify the resolution obtainable by the three processing methods. Wiener filtering is shown to give higher lateral resolution than SAFT processing, by factors of between 1.3 and 2, depending on the depth of the reflector. Maximum entropy gives even higher apparent lateral resolution, but has a tendency to underestimate reflector size. The improvement in range resolution obtainable with Wiener filtering was approximately a factor of 1.5, whereas maximum entropy gave corresponding factors of between 1.7 and 5. For far-field reflectors, deconvolution of rectified (envelope-detected) data is shown to give inherently lower lateral resolution than the deconvolution of the equivalent RF data.

On Ultrasound Transducers with Curved Surface for Improvement of Lateral Resolution

Abstruct-In order to produce high-quality images in medical ultrasonics, high lateral resolution is required over a large depth. The properties of the acoustic field maintained by a transducer with an arbitrary gently curved surface and an arbitrary surface velocity distribution is discussed. The acoustic field pressure is calculated by means of the impluse response method. It is shown, that the impulse response in every space point is given by an integral, the integrand of which contains the curvature and velocity distribution of the transducer. The integral is evaluated numerically, and images of the acoustic field pressure are presented. These images show how the curvature and velocity of the surface influence the lateral resolution. The behavior of the pressure in the far field is also studied.

Lateral deconvolution of ultrasonic beams

A number of papers have appeared recently describing methods of lateral resolution improvement in ultrasonic images by deconvolution of the amplitude detected cross-beam characteristic. It is the intention of this paper to demonstrate that such methods of resolution improvement are appropriate only for a limited class of targets. In general, the effect of interference between the r.f. signals from multiple targets at differing distances is such that the amplitude beam width obtained for these targets cannot be considered as the linear combination of shifted replicas of the amplitude beam width for an individual target. This is a consequence of the non-linear character of the detection process. In these cases the method of deconvolving the detected responses, although capable of giving reduced spot sizes, makes errors in the position of the images and is susceptible to false images and sidelobes. The phenomenon is demonstrated with computer and experimental data on point targets.

Lateral deconvolution of ultrasonic beams.

A number of papers have appeared recently describing methods of lateral resolution improvement in ultrasonic images by deconvolution of the amplitude detected cross-beam characteristic. It is the intention of this paper to demonstrate that such methods of resolution improvement are appropriate only for a limited class of targets. In general, the effect of interference between the r.f. signals from multiple targets at differing distances is such that the amplitude beam width obtained for these targets cannot be considered as the linear combination of shifted replicas of the amplitude beam width for an individual target. This is a consequence of the non-linear character of the detection process. In these cases the method of deconvolving the detected responses, although capable of giving reduced spot sizes, makes errors in the position of the images and is susceptible to false images and sidelobes . The phenomenon is demonstrated with computer and experimental data on point targets.

LINEAR SYNTHETIC APERTURE FOCUSING USING ULTRASONIC CONTACT TRANSDUCERS

Abstract Synthetic aperture processing performed on linear scans with contact ultrasonic transducers can yield useful improvements in lateral resolution of deeply buried defects. Degradation in resolution caused by adverse surface conditions can be minimised by a simple pre-processing normalisation technique.

Instrumentation for Ultrasonic Transkull Visualization

A~$stmct-lnstrumentation operating in the 0.5 to 1.0 MHz frequency range specifically designed for acoustic visualization of intracranial anatomy through the intact adult human skull is described. The instrumentation consists of a commercial B-mode scanner which has been extensively modified to enhance the reduced sensitivity and resolution inherent in such visualization schemes, as well as aid in the identification of intracranial anatomy. The instrumentation is described in three parts: a) the analog video system including transducers, pulsers, receiver, and associated circuitry; b) the digital video system which includes the analog-to-digital converter, signal processing for longitudinal and lateral resolution improvement, and the interface with the commercial unit; and c) the system for storing the scans and for overlaying the anatomical atlas information. Preliminary results of the use of this instrumentation to obtain transkull ultrasonic images are also presented.

Signal processing in acoustic imaging

A review of signal processing methods which can be used to improve the effectiveness of systems designed for acoustic imaging and bearing estimation is presented. Topics covered include a) signal processing for increased resolution, b) the processing of stochastic acoustic signals, c) image processing, enhancement, and pattern recognition. The discussion of resolution processing includes lateral resolution improvement by both superresolution techniques and aperture synthesis, and improvement of both range and Doppler resolution. The stochastic signal-processing section addresses adaptive processing, as well as methods of imaging in the case of incoherent, noisy signals.